IEEE HVDC Guide Outline-July22-2012

7/27/2019 IEEE HVDC Guide Outline-July22-2012

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Proposed Outline of HVDC Overhead Line Design Guide

July 2012

HVDCTransmission Systems

Intention of Guide (For AC designers already know how to design lines)

Introduction

HVDC Line Performance

DC versus AC ratings

Reliability

Single Pole (Monopole) operation

Overhead line versus underground cable

System performance Wayne Galli

System Design:

Insulation Coordination (tower clearance, clearance to ground) Next presentation (backup)

Switching surges

Contamination

Lightning

Tower clearances

Clearances to ground

Lightning and Grounding

Acceptable performance

Calculation Methods

Electrical Effects Next Chapter (Winter meeting presentation)

Corona & Field Effects

Calculation Methods (AN, RI)

Inductions

Rules and Regulations (Maximum values and limit at edge of ROW)

Co-use of Towers/ROW

Conductor System - Next Chapter (Winter meeting presentation)

Current Rating Determination

o Selection of weather conditions for ratings Hardware Testing

o Selection of criteria

o Corona Performance


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Types

Selection of foundation

Electrode

Types of electrodes

Comparison suitable type of electrodes

Location

Components

Performance criteria

Electrode line

Voltage requirement

Current requirement

Line configuration

Reliability

Metallic ground return


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Introduction

Transmission of electric power by high voltage direct current(HVDC) was launched in 1954 when the first

commercial transmission link between the Gotland and the mainland of Sweden was put into

service.Since then, HVDC technology has advanced dramatically, and close to 100 HVDC transmission

systems have been installed around the world. HVDC is better known for its capability of transferring

large amount of power between two points and its potential for other applications has been overlooked

until recent years.

Today the most common application of HVDC is bulk power transfer over a long distance. HVDC

transmission has the advantage of lower line losses and lower transmission line costs. These savings areoff-set by the costs of conversion stations and the conversion losses. For a 500 kV HVDC transmission

system carrying 3 GW of power to be competitive with the AC option, the distance had to be longer than

500 miles in the 1970s. Due to relatively lower converter costs and higher line loss costs comparing to

the other costs, this breakeven point is now around 300 miles. The highest voltage for DC transmission

was at 600 kV until recently when an ultra-high-voltage direct-current (UHVDC) 800 kV transmission

system was built in China. In countries such as China, India, Brazil and South Africa where large amounts

of power are regularly transferred over long distances, HVDC could become the backbone of their

transmission systems.

The second most common application of HVDC has been back-to-back asynchronous interconnection.

Two power systems operating at different frequencies are tied together using two HVDC converter

stations and a short or no overhead line between them. This scheme can also be adopted to isolate

systems within the power grid, when required, from affecting one another and thus provide stability to

the grid. There are several of these types of back-to-back interconnections in US, not just for

asynchronous frequencies, but also for reasons of reliability.

With the advent of Voltage Source Converter (VSC) based technology, opportunities for HVDC

application have opened up. Conventional HVDC converters make use of phase commutated converter

(PCC) based technology. The advancement of solid state devices has increased the power capability of

ratings but not controllability. VSC technology uses gate turn off devices, like IGBTs (Insulated Gate

Bipolar Transistors), which have higher switching frequencies, therefore allowing for better voltage

control. This technology is used for the following applications.

Urban in-feeds

Constrained right-of-ways

Improved voltage stability

Underground and sea cable transmission

Wind farm integration


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Some recent examples are:

2002: Cross Sound Cable HVDC Light for controlling power exchange

2007: Sharyland Link between ERCOT and Mexico for asynchronous networks

2009: NordE.ON1 linking an offshore windfarm to the German mainland

There is an increasing amount of wind farms that need to be connected to the AC transmission network.

Integration of wind farms using HVDC Voltage Source Converters (VSCs) have emerged as the

technology of choice.Power can be fully controlled using VSC based technology so that the intermittent

electricity supply from a wind farm cannot disrupt the grid. At the same time, the wind farm is isolated

from the ac network by the dc link. The voltage in the wind farm is therefore not affected by changes of

the voltage in the ac network that may be caused by switching actions or remote faults.

Other than bulk power transfer, HVDC technology can be applied to improve power system reliability, to

connect an AC system to renewable power sources, to control and improve power flows. The maximumcapacity of a VSC is currently at about 1,000 MW. The improvement of the capability of VSC will open

up even more opportunities for the application of HVDC technology.


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HVDC Line Performance

The conventional wisdom concerning HVDC line design is that it is a simple extension of ACdesign. In a physical sense, HVDC lines are similar to AC transmission lines in that both use

bare stranded conductors and both must meet or exceed minimum specified electrical clearanceto ground and to other conductors at operating voltage.

A direct comparison of performance between HVDC and AC should consider the level of

maintenance required to maintain acceptable outage rates. HVDC lines are made of essentially

the same materials as AC lines of a comparable voltage level. Normal weathering of

components of foundations, structures, conductors, splices, clamps, insulators, and shield wires

should be similar in a 400 kV HVDC line and a 345500 kV HVAC line.

However there are several differences between AC and DC lines, generally DC insulators require

greater leakage lengths, DC lines have higher power flow capability and controllability, DC alsoallows for the flexible operation of lines with various ground return arrangements. The cost of

construction and the power flow on HVDC circuits is, however, much higher than with AC

circuits so that design mistakes are amplified. Figure 1 shows an AC and DC line running

parallel to each other.

Figure 1: DC line on the left, AC line on the right

DC line outage rates also tend to be lower than that of AC lines in general. In specific instances

where the DC line outage rate was initially higher than a comparative AC line due to component

or equipment issues, once the issue was corrected the DC line outage rates dropped to below the

AC line outage rates.

Some of the differences of an HVDC circuit compared to an HVAC circuit are:


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Failure of an HVDC line can be quite different from that of an HVAC line. Flashover

of any of the three-phase conductors of a single-circuit AC line causes breaker operationand takes the line completely out of service. A bi-pole HVDC line is somewhat like a

double-circuit AC line in that the poles of the HVDC line can be operated independently,just like the AC circuits. The failure of one pole or circuit allows continued operation

although at a reduced power level.

A bi-pole HVDC line needs only two conductors, instead of the three required by asingle-circuit AC line or the six conductors in a double-circuit AC line. This normallyrequires a smaller right of way and a less visually obtrusive tower. Figure 2 showsschematically the tower configurations for 1200 MW (two circuits AC, bipolar HVDC)

and 15002000 MW transmission at EHV AC single circuit or monopolar HVDC byalternative tower designs.

Figure 2: Comparison of physical size for HVAC and HVDC circuits with comparable power flow

capacity [1]

The conductors in an HVDC line do not experience skin effect, so electrical losses arelower for a given power flow.

DC lines offer power flow control, whereas HVAC lines are electrically passive.Depending upon the placement of the HVDC circuit within the AC transmission system,

this advantage can be critical to system reliability during system emergencies.

Monopole

HVDC

Single AC

circuit

Bipole

HVDCTwo AC

circuits


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HVDC cables have no length limit as do AC cables, so HVDC cables can be used for

long-distance transmission of power, either underground or undersea (submarine). There is no need for frequency synchronization between systems linked by DC, so

HVDC circuits can be used to link systems that are not necessarily in frequencysynchronization. The line portion of the circuit is not necessary to claim this advantage

since a back-to-back converter station works similarly.

The addition of an HVDC circuit does not increase the short circuit currents on existingAC switchgear.

An HVDC link can be relied upon as part of a systems generation reserve.

HVDC performance studies have often focused on the converter station, information on DC line

performance is therefore rarely available. However the performance levels of DC lines andconverter stations needs to be separated in order to better understand the issues which impact

upon DC line performance levels.

HVDC Line Performance Survey

In 2010 EPRI conducted a survey regarding the performance of various HVDC schemes aroundthe world, the results were published in 2011 [2]. The aim of the survey was to get an initial

understanding of various issues related to HVDC transmission line performance. A secondaryaim was to determine areas where further research and study is warranted. A more detailedsurvey done in conjunction with CIGRE study Committee B2 is to be formulated based on this

initial survey. Highlights of the observations of the survey responses are shown below.

Design ParametersThe voltages ranged from 400 to 600 kV with one and two pole designs and two, three, or fourconductor bundles. All the lines are greater than 440 miles (708 km) long with no interveningtaps. Most of the lines have extensive end-point grounds, figure 3 shows a) a horizontal ground

electrode configuration and b) a vertical ground electrode configuration.

Figure 3: Ground electrode configurations: a) Horizontal and b) - Vertical

a) b)


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Planning IssuesFrom a planning perspective, the ability of HVDC lines to operate temporarily with a single poleis helpful. For one HVDC line, single-pole operation has in use for over 10 years.

In HVDC lines with separate structures for each pole, it appears that most consider the loss of

both poles as an n-2 contingency. In most cases, the operation of the HVDC line with a singlepole using earth return was anticipated. This is consistent with normal planning criteria for two

parallel single-circuit AC lines where the loss of both lines (on separate structures) is usuallyseen as an n-2 contingency.

In the case of an HVDC line with both poles on a common structure, the loss of both poles would

be considered an n-1 contingency. Given the much higher power flow on HVDC lines and their

limited presence in most AC systems, this may be a very important limitation on the use of

HVDC.

OperationsThe maintenance costs for HVDC lines appear to be similar to HVAC lines. In the case of oneutility, where there are three 765-kV HVAC lines roughly paralleling the two HVDC bi-pole

lines for 800900 km, the outage rate for the HVDC lines is about half that of the HVAC lines.

DC line outage rates are in general lower than that of comparable AC line outage rates. Oneutility responded that the DC line outage rates monitored over a 14 year period was about a 1/5

of a comparable AC lines outage rates over the same time period.

Perception and AnnoyanceMost of the problems with noise, interference, and human discomfort appear to have beenthoroughly reviewed before construction of the lines. A few cases of audible noise problems

have occurred. There were no reports about human perception of ion currents. EPRI is currentlyundertaking studies on this topic under its HVDC program.

Development of Very Reliable HVDC Lines

Historically, the high cost and unreliable nature of converter stations dominated any concerns

about the reliability of HVDC lines. The literature and manufacturers data indicate that DCconverters are becoming more reliable and less expensive. As this occurs, overhead HVDC

circuit reliability will come to be driven by line reliability, much as it is for AC circuits.


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HVDC Line Design

Similar to the design of AC overhead lines, the design of a HVDC overhead line can be

separated into two main categories:

1. System Designs

2. Component Designs

The system design covers the performance of the overall line or system and includes the

following topics:

Insulation Coordination

Lightning and Grounding

Electrical Effects (Rules and Regulations) Other Electrical Designs (Ampacity)

Mechanical Performance (Weather loading, vibration, galloping)

Conductor Selection (Life Cycle Costing Method)

The component design covers the performance of the individual components. The performance

of each component is coordinated with one another and must meet the system design

requirements. The component design covers the following topics:

Insulators

Spacer Dampers

Other Hardware

Conductor (Phase & Ground Wire)

Structure

Foundation

Electrode

Electrode line

The guide covers voltages up to 600kV. Information above this voltage is not readily available.


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Insulators

Effects of Contamination on HVDC Insulators

One of the key issues that impacts the insulation design of HVDC lines and substations is thecontamination performance of the external insulation. Since the beginning of overhead powertransmission it was noted that the performance of external insulation is adversely affected whenthe insulating surface is contaminated with airborne deposits such as marine salt or industrialpollution. These deposits may form a conductive or partially conductive surface layer on theinsulator when wet, resulting in discharges and in severe cases flashover of the insulators.

Under DC energization, it is important to properly design the external insulation to withstand thecontamination conditions. This is illustrated inFigure Error! No text of specified style indocument.-1Figure Error! No text of specified style in document.-1Figure 1-1which shows acomparison of the required insulation lengths for HVAC and HVDC systems to withstandlightning and switching overvoltages as well as the effects of insulator contamination.

Figure Error! No text of specified style in document.-1A Comparison of the Requirements for Switching Lighting and Contamination for HVAC andHVDC Systems. SeeTable Error! No text of specified style in document.-1Table Error! No text ofspecified style in document.-1Table 1-1for the Assumptions Used.

It is apparent fromFigure Error! No text of specified style in document.-1Figure Error! Notext of specified style in document.-1Figure 1-1that on HVAC systems, the insulation lengths

are in most cases determined by either switching or lighting overvoltages. Insulators with asuitable leakage distance to arcing distance ratio are then selected to meet any contaminationperformance requirement. This is in contrast with HVDC systems whereFigure Error! No textof specified style in document.-1Figure Error! No text of specified style in

0

1

2

3

4

5

6

7

8

300 400 500 600 700 800 900 1000 1100 1200

Maximum System Voltage (kV)

Req

uiredStrikingDistance(m)

0

1

2

3

4

5

6

7

8

300 400 500 600 700 800


Req


Lightning

1.4p

.u.

1.8p.u.

2.6

p.u.

Very

Heavy

Heavy

Mediu

m Ligh

t

VeryL

ight

Lightning

VeryL

ight

Light

Mediu

m

Heavy

1.75

p.u.

HVAC HVDC

0

1

2

3

4

5

6

7

8

300 400 500 600 700 800 900 1000 1100 1200


Req


0

1

2

3

4

5

6

7

8

300 400 500 600 700 800


Req


Lightning

1.4p

.u.

1.8p.u.

2.6

p.u.

Very

Heavy

Heavy

Mediu

m Ligh

t

VeryL

ight

Lightning

VeryL

ight

Light

Mediu

m

Heavy

1.75

p.u.

0

1

2

3

4

5

6

7

8

300 400 500 600 700 800 900 1000 1100 1200


Req


0

1

2

3

4

5

6

7

8

300 400 500 600 700 800


Req


Lightning

1.4p

.u.

1.8p.u.

2.6

p.u.

Very

Heavy

Heavy

Mediu

m Ligh

t

VeryL

ight

Lightning

VeryL

ight

Light

Mediu

m

Heavy

1.75

p.u.

HVAC HVDC


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document.-1Figure 1-1shows that the insulation design is clearly dominated by the

contamination performance requirement.

Table Error! No text of specified style in document.-1Assumptions Used in the Calculation of the Clearance Curves Presented inFigure Error! No text ofspecified style in document.-1Figure Error! No text of specified style in document.-1Figure 1-1

L

ightning

Flashover rate of 0.6 flashover/100 km/years.

Tower footing resistance of 20 ohms with a soil resistivity of 400 ohm-meters.

Calculations based on a ground flash density of 6.0 flashes/km2/year.

Note that the lightning curves are relatively flat, since the lightning requirements should be relativelyconstant with system voltage. Tower heights increase and coupling factors decrease with increasingsystem voltage. These effects, along with the increase in power frequency voltage, combine to producea gentle increase in the curve.

SwitchingSurge

For HVAC a Gaussian stress distribution and for statistical overvoltages E2 of 2.6, 1.8, and 1.4 perunit. (E2 of 2.6 per unit represents a typical value for high-speed reclosing of breakers without a pre-insertion resister; 1.8 per unit represents a typical value for high-speed reclosing with a single pre-insertion resister; and 1.4 per unit represents a value for a breaker with possibly one or two pre-insertion resisters or with controlled closing.). A line with 500 towers is assumed.

For HVDC a typical slow front stress level of 1.75 p.u. is assumed. This is associated with theovervoltages occurring on the un-faulted pole of a bipolar HVDC line during a ground fault on theother pole.

Each of the curves sweeps sharply upward, portraying the plot of the strike distance as a function ofthe CFO.

PowerFrequency

The power frequency voltage requirements are shown as a function of the IEC contamination levels of:Contamination level ESDD (NSDD = 0.1 mg/cm2) Unified Specific Creepage Distance

Very Light < 0.004 mg/cm2 22 mm/kV

Light > 0.004 mg/cm2 28 mm/kV

Medium > 0.02 mg/cm2 35 mm/kV

Heavy > 0.1 mg/cm2 44 mm/kV

Very Heavy > 0.5 mg/cm2 55 mm/kV

For HVAC the use of ceramic insulators with a 170 mm spacing and 380 mm creepage is assumedfor Very Light and Light contamination levels. For Medium to Very Heavy the use of ceramicinsulators with a 170 mm spacing and 350 mm leakage are assumed.

For HVDC the use of ceramic HVDC insulators with a 170 mm spacing and 545 mm creepage isassumed for all contamination levels.

The use of V-strings is assumed.


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Note: The Unified Specific leakage Distance is defined as the leakage distance of the insulator divided by

the maximum voltage which will be applied across the insulator. For AC cases this would be themaximum phase to ground systems voltage, i.e.

3

mU

.

Figure Error! No text of specified style in document.-1Figure Error! No text of specifiedstyle in document.-1Figure 1-1also shows that while the insulation length requirements forswitching and lightning is comparable between HVAC and HVDC, the resulting insulationdistances to satisfy the contamination performance requirement is significantly higher for HVDCthan for HVAC. In areas with significant contamination levels this may require large insulationdimensions, which may influence, and in some cases dictate, the conceptual design of theproject. Choices that may be impacted include:

The routing of the lines and siting of the converter station, to avoid contamination conditions

The use of underground cable instead of overhead lines to minimize the exposure of external

insulation surfaces to contamination. Utilizing indoor switchyards and converter stations to protect the external insulation surfaces

from contamination and/or wetting.

The choice of particular insulator assemblies or conductor configurations for the transmissionline or special layouts of the converter stations to accommodate long insulation distances orspecial insulation solutions.

It is clear that these choices may have a significant cost impact and it is therefore important totake the necessary care when doing the insulation design. The importance of this subject isunderlined by the fact that there are now both Cigr and IEC working groups which aim toformalize an approach for the selection and dimensioning of high-voltage insulators for use incontaminated conditions. Although this work is not complete yet, there have been significantprogress and the aim with this report is to provide an overview of the present state of the art.

This report therefore contains sections dealing with: Differences in the behavior between HVDC and HVAC insulation in contamination

conditions. With special emphasis on why there is a need for longer insulation lengths whenenergized with HVDC.

Experiences with composite insulators, which should include aspects regarding aging and thepossible use of lower insulation distances when the insulation surface is hydrophobic.

Developments in Cigr and IEC with regards to the establishment of international guidelinesfor the selection and dimensioning of insulation.

An overview of subjects and research questions where more work is needed.

The report is structured around the elements of the contamination process. Briefly, insulatorsinstalled in contaminated environment, e.g. close to the ocean, factories, or other sources of saltsuch as agricultural activities or roads (due to salting in winter time), may be covered with a

conductive surface layer during wet conditions. This could result in discharge activity, known asdry band arcing, which, in severe cases, could lead to flashover. Important parameters thatdefine the insulator performance are:

The contamination deposition process


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Insulator flashover characteristics

Long-term (aging) performance


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All these aspects are brought together to derive a simplified dimensioning process, which is

presented later.


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Performance of Insulators of Different Materials

The electrical stresses that HVDC places on the electrical insulation are very different to thatunder AC energization. Electrical discharges on DC energized insulators tend to be more severebecause of the absence of voltage zeros of the applied voltage. This results in a requirement forlonger insulation lengths. Also the electric field (E-field) along the DC insulators can be verydifferent to that of the AC. Under DC energization the voltage distribution along a clean and dryinsulator is determined by the internal resistance along the insulator. This voltage distribution iseasily disturbed by a change in surface conditions, unlike the condition under AC where thecapacitive influence of the E-field is much stronger. It is therefore found that small changes inthe surface conditions of DC insulators may result in large changes in the E-field along theinsulator string. On disc insulator strings, for example, the occurrence of single disc

flashovers has been reported. This is when a single disc in an insulator string experiencesflashover while the rest of the insulators are under stressed. On composite insulators the E-fieldalong the insulator may also be severely distorted due to the presence of space charge.

From the above it becomes clear that AC practices and experiences cannot be directly applied toDC insulator designs which therefore require specific consideration. In this section the focus ison the long-term performance of DC insulators and the issues that need to be considered whenselecting and dimensioning DC insulators.

Porcelain Disc Insulators

Most recorded in-service failures of DC porcelain disc insulators have been caused by corrosionof the zinc-alloy sleeve in the insulator pin which causes it to swell. This in turn subjects theporcelain dielectric to hoop stresses, eventually resulting in cracks and a mechanical failure ofthe insulator. This phenomenon is however restricted to areas with a high contamination severityand continual high humidity. On modern DC porcelain insulators this problem has been solvedby employing a pure zinc sacrificial sleeve and by the application of a thin polymer coating tothe pin in the area where it is in contact with the cement to block circulating electrolytic current.

Other possible failure mechanisms such as thermal runaway or ion migration in the disc has notbeen observed under normal service conditions.

Toughened Glass Disc Insulators

Spontaneous bursting of glass discs on HVDC lines resulting in significant failure rates on pre1988 installations. These failures were associated with ion movement in the glass material,especially around inclusions. The ion concentrations in the disc results in a distortion of themechanical stresses in the glass and ultimately in a spontaneous shattering of the glass shell.

These failures prompted the development of a special high purity glass for HVDC applications.The glass used in these insulators is characterized by minimal in homogeneities and much higherresistivity glass.

In locations with severe contamination it was also found that glass insulators may be subjected toerosion from arcing activity. In some cases these erosion tracks may be deep enough toprecipitate the shattering of the glass disc. Laboratory tests have shown that glass insulators aremore prone to such erosion than porcelain. Erosion of glass insulators are a sign that the


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insulator strings are under dimensioned for the conditions present and it can therefore be solved

by re-insulating with appropriately dimensioned insulators.DC glass insulators are also normally fitted with zinc sleeves on both the cap and pin, and the pincement interface is usually protected by a non conductive coating to block electrolytic current.

Polymer Insulators

Service experience collected during the early 1990s has shown the service experience oncomposite insulators on HVDC systems have generally been good. This results should, however,be seen against the very limited number of insulators (i.e. less than 1000 units) contained in thesample and their relatively short service life (i.e. less than 10 years) at the time of the survey.This survey highlighted however some instances of severe erosion in high contamination areasand corrosion of the end fittings. In more recent reports excellent service experience of HVDCcomposite insulators after 25 years of service has been claimed.

Aging in Contaminated Condi t ionsFrom service experience it is known that erosion damage has only been observed at sites with ahigh contamination severity. Most DC systems with external insulation are however located inareas with a moderate to low contamination severity, thereby avoiding excessive erosion stresses.In more recent applications of composite insulators the trend is to reduce the leakage distanceutilized on polymer insulators to about 75% of that which would be specified for glass andporcelainError! Reference source not found.. This would increase the stress on the materialwith regards to its erosion performance. In China this has been recognized to such an extent thata new multi-stress test has been developed to replicate Chinese contamination conditions.

In a comparison of the tracking performance of polymer insulators under AC and DCenergization when subjected to single and multi-stress tests has shown that under the same stress(i.e. DC voltage equal to the r.m.s. AC voltage) that the erosion and deterioration incurred ismore severe (both in extent and erosion depth) under DC energization than for AC. This finding

has also been confirmed by inclined plane tests.

From the collected service experience, however, it is not apparent that polymer insulators run agreater risk of erosion damage as only a very few cases have so far been recorded. This maypartly be explained by the longer leakage distance required on DC systems as compared with ACto obtain an acceptable performance, which would also reduce the electrical stress on thematerial. On the other hand the applicability of especially the inclined plane test to DCinsulators has been questioned. During the inclined plane test the insulators hydrophobicity isintentionally broken down, while in service this is a primary factor which limits erosion damageby blocking leakage current.

Aging in Areas with Li t t le to No Contamination

On AC systems it is now well established that the primary aging mechanism on polymerinsulators under clean conditions (i.e. little to no contamination) is corona in combination with

water on the insulator surface. It is believed that the ionized air combine with the water to forma weak acid that can either directly attack the material or cause it to loose its hydrophobicity.This normally results in cracks in the housing material and eventually the insulator may failmechanically due to a brittle fracture. Also corona plumes (under dry conditions) from the end


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fittings or grading rings, when in direct contact with the housing material may lead to the

premature aging and cracking of the housing material. One of the primary ways to inhibit suchaging is the application of corona rings to limit the E-field to below the threshold value.

To date this aging mechanism has not received much attention on HVDC systems but it has beenconfirmed that water induced corona occur on HVDC insulators and that it results in a loss ofhydrophobicity. Based on this evidence it can be concluded that there is also a need to grade theelectric field along DC insulators with corona rings to below the threshold for aging due to waterinduced corona. Only the design of such corona rings is not as simple as the E-field along theinsulator is determined by the resistivity of insulator materials, the resistivity of the surface layerand it may be temporarily distorted by the presence of space charge.

Knowledge Gap

The above provided a broad overview of the aging effects present on all types of insulation. Forporcelain and glass insulators the problems are well known and to a large extent already covered

by existing standards. The situation is different for polymer insulators where service experiencewith this insulator type is not well publicized. It is therefore difficult to draw firm conclusion onthe most appropriate ways to test the insulators to ensure its long term performance. Thefollowing aspects should be further investigated:

Research is needed to better understand the aging mechanisms on polymer insulators underDC energization to determine appropriate design limits.

There is a need to develop a methodology for the design of corona rings for HVDC polymerinsulators.


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Dimensioning Process

This section provides an overview of the insulator design process for HVDC insulation withrespect to contamination. It is recommended that an exhaustive approach be followed tominimize uncertainties of the input data and its impact on the final design. Consequently such anexhaustive design approach may need to include the following activities for each different set ofconditions and constraints (e.g. different sections of the line, pollution environments, etc.). Ageneral overview of the process is provided in Figure 2-1.

There are four activities that must be performed:

1. Candidate insulator selection

2. Site severity determination

3. Selection of the leakage distance

4. Qualification

Selection of candidate insulating solutions: The process is initiated by identifying possibleinsulation solutions. In the initial phase this may be based on a simplified design procedure as isexplained later in this section or on past experience. The term insulating solution is used here

in its broadest sense to include:

1. The selection of insulator material and profile,

2. the choice of outdoor or indoor substation configurations,

3. the implementation of mitigation measures (e.g. washing, or coating) as part of theinsulation design or,

4. setting operational constraints such as operating temporarily at reduced voltages duringcritical pollution or climatic events.

Site severity assessment: The aim with contamination severity assessment is to obtain anaccurate picture of the contamination severity of area concerned based on data collected over arelatively long period. The initialassessment is usually based on:

1. Collected performance data on existing lines or substations, preferably DC energized, butAC data could also be useful.

2. Identification of the type (i.e. Type A or B as defined in IEC 60815-1) and composition(i.e. type of salts, non soluble deposits etc.),

3. Measurement of the quantity of the pollution present,

4. Characterization of the climate, specifically identifying if there is a prolonged dry season.

5. An assessment of geographical, topological and geological features to identify possiblecontamination sources and

6. A survey of present and foreseeable future pollution sources and land use.


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Figure 2-221An Overview of the Insulator Design Process for HVDC Insulation with Respect to Contamination [Cigr Forthcoming].

1Candidate insulator selection (eg, selection of material,

profile)

Decreasing confidence

2 Design severity determination for the candidates

Information from exisiting d.c.

installations in the area (or

similar)

or Test station data from d.c.energised insulators

or

Extrapolation of data from a.c.

installations or test station or

pollution monitoring

or Qualitative severity estimation

3 Selection of creepage distance

On the basis of existing

applicable insulator data from

the field

or

On the basis of existing

applicable insulator data from

laboratory

or

Evaluation by testing where

previous data is not

available/applicable

4 QualificationPrequalified by previous

experienceor Full scale test or

Agreement to use dimensional

interpolation/extrapolationor

Agreement to use severity

interpolation/extrapolation

Note: Phases 1-3 may need to be iterated

Insulator design process


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For critical installations (e.g. converter stations) the above information may not be accurate

enough, thus resulting in a need for a more detailed assessment. This preferably includes settingup experimental stations at representative locations with a selection of DC energized insulatorsto get an estimate of long term pollution accumulation.

Selection of leakage distance: The required insulator dimensions (notably the leakage distance)are determined from available service experience or test results. If that is not availablerepresentative tests may be performed on the candidate insulator types to determine the statisticalflashover properties. Under representative test is understood any laboratory test, which isdesigned to imitate the natural contamination conditions as closely as possible by replicating the(1) pollution severity (i.e. ESDD and NSDD) (2) its composition (i.e. type of salt, and nonsoluble components) (3) the uniformity of the deposit and (4) wetting conditions.

The above information is then used to do a statistical risk evaluation of the insulation design,including any mitigation measures, to verify compliance with the required performance criterion.This should include an assessment of the impact of (1) the number of insulators exposed to thesame conditions, (2) the frequency of pollution events and (3) may include additional safetyfactors to cater for any uncertainties in the input data.

Qualification of the insulation design: This is the last step in the process whereby the choseninsulation design is evaluated either by testing or by a comparison with past experience. Animportant part of this process is to obtain agreement and approval of the insulation design fromall stakeholders.

The Simplified Design Method

The simplified design method can provide useful orientation at the start of a project to identifythe range of preliminary solutions. It can also be an effective tool to analyze the outageperformance, and the adequacy of the insulation solutions of existing systems. It is howeverimportant to note that the simplified method has serious limitations which may result in either an

over- or under dimensioned insulation. Aspects that may affect the accuracy of the design are: For DC the contamination performance is the dominating factor determining the size (i.e.

axial length of the insulators). Thus any uncertainty in the estimation of the pollutionseverity may directly impact the required insulator length. This is fundamentally different toAC where insulator lengths are rarely impacted by the required contamination performance.

Higher contamination levels call for a greater increase in the leakage distance on DCinsulators than for AC. Any error in the severity estimate has therefore a larger impact on theDC insulation dimensions than for AC.

A further complication in the DC design process is the effect of the electrostatic attractionwhich can result in significantly higher levels of pollution accumulation on the DC energizedinsulators when compared with AC energized or non-energized insulators. This ratio mayrange between 1 and 10 and therefore introduces a large uncertainty in the estimation of thepollution severity when general environmental or AC-specific site severity methods are used.

Other uncertainties in the input data which may affect the outcome of the design process arethe non-uniformity of the pollution accumulation on the insulators (e.g. top to bottom ratio,radial differences and distribution along the insulator) as well as the type and composition ofthe contamination present.


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An overview of the simplified dimensioning method is given inFigure 2-3Figure 2-3Figure 2-2.In the following paragraph, each step is briefly discussed for sites characterized by pre-deposited contamination.

The first step in the simplified dimensioning process is to determine the reference sitecontamination severity. This is preferably done by measuring the contamination severity on DCenergized insulators to obtain the most representative results. The measurements should includeEquivalent Salt Deposit Density (ESDD) and Non-Soluble Deposit Density (NSDD)measurements, which are performed so that the top to bottom ratio and distribution ofcontamination along the insulators is also quantified. It is also valuable to perform a chemicalanalysis on the contamination present to determine the dominant salts present.

As an alternative, it is also possible, but not recommended, to base the site severity assessmenton measurements on AC energized or non-energized insulators. In such a case it is necessary to

estimate the contribution of the electrostatic field on the accumulation on DC energizedinsulators. This is done with the DC/AC accumulation factorKp. This factor is selected byconsidering the generation and transportation of contaminants around the DC station. Generalguidance on the selection ofKp is as follows:

Kp is between 1 and 1.2 in areas where the wind speed is the dominant factor that determinesthe amount of pollution carried in the air or in areas where high wind speeds prevail.Examples of such areas are those with pre-deposited contamination which is close to thecoast or in the desert.

Kp range from 2 to 3 in areas characterized by human activity such as mining, industry, roadsetc.

Kp falls in the range 1.3 to 1.9 in areas which falls between the above two categories andwhich is characterized with extended wind still or low wind conditions. These areas may besome distance from the coast or from pollution sources associated with human activity.

Kp is between 1 and 1.2 in areas where the contamination deposit and wetting occurssimultaneously.

It should be noted thatKp can be higher than the values given above when the site location ischaracterized by extended dry periods and can be lower when there are frequent natural cleaningevents such as rain.


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Figure 2-332An Overview of the Simplified Insulator Design Process for HVDC Insulation with Respect toContamination [IEC/Cigr Forthcoming]

Once the site severity measurements are available then these natural contamination level needsto be converted to an equivalent laboratory test severity. With this correction it is recognizedthat artificial testing differs from natural contamination in a number of important aspects, whichis:

Type of Salt: Laboratory testing is mostly performed with marine salt (NaCl) whereasnatural contamination layers may often contain less soluble salts such as gypsum (CaSO4).Presently, however, there is no generally applicable method to quantify this effect other thanperforming specific flashover testing on insulators with natural contamination.

Amount of non-soluble material present in the contamination layer: The standardizedlaboratory test usually subjects the insulator to a contamination layer with an NSDD=0.1mg/cm2. In service the NSDD levels may typically vary from 0.0110 mg/cm2. Through

Measurements from

AC installations or

on non-energized insulators

as per IEC 60815

Conversion of values to

equivalent dc energized**

Correct the severity for the non-

uniformity of the pollution layer

Correct for

electrostatic attraction

Correct the severity for NSDD

to a reference value of 0.1 mg/cm2

Correct the severity for type of salt

(Not practical at present)

Correct the severity for the

diameter effect on accumulation

Measurements from

DC test site or installation

ESDDdcTop/bottom

ESDD/NSDD

Pollution composition

Correct the severity for any

Statistical data correction

** Eventually taking

account of climate data

(e.g. wind velocity)

Site DC severity

Number of events

Number of insulators

Type of insulator

For each candidate

Insulator type

Required/Design DC severity

. .

Preliminary estimation of the

required USCDDC based on

insulator type and material

Correct the USCD for the

effect diameter on flashover

Design USCDDC

Measurements from

AC installations or

on non-energized insulators

as per IEC 60815

Conversion of values to

equivalent dc energized**

Correct the severity for the non-

uniformity of the pollution layer

Correct for

electrostatic attraction

Correct the severity for NSDD

to a reference value of 0.1 mg/cm2

Correct the severity for type of salt

(Not practical at present)

Correct the severity for the

diameter effect on accumulation

Measurements from

DC test site or installation

ESDDdcTop/bottom

ESDD/NSDD

Pollution composition

Correct the severity for any

Statistical data correction

** Eventually taking

account of climate data

(e.g. wind velocity)

Site DC severity

Number of events

Number of insulators

Type of insulator

For each candidate

Insulator type

Required/Design DC severity

. .

Preliminary estimation of the

required USCDDC based on

insulator type and material

Correct the USCD for the

effect diameter on flashover

Design USCDDC


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mathematical manipulation the ESDD level can be corrected as follows:

33.0

106.0

1.0

NSDDK

NSDD

These corrections result in an estimate of the equivalent site severity at a NSDD=0.1 mg/cm2,which is more-or-less independent of the insulator type which will be utilized.

The next step is to adjust this basic site severity characteristic for each of the candidate insulatorsconsidered. This is done as follows:

The non-uniformity of the contamination layer: The equivalent ESDD for a uniformlycontaminated insulator can be derived from field measurements and the top to bottom ratio(T/B) as follows:

33.0

1

10//38.01

/59.01

59.1

BTLOG

BTK

BT

The non-uniformity of the contamination deposit along the string is taken into account byutilizing the average value of the ESDD/NSDD measurements taken along the string.

Insulator diameter: Larger diameter insulators collect less contamination than smalldiameter insulators. The amount of contamination on insulators with a large diameter (i.e. anaverage diameter, D larger than 115 mm) can be estimated from measurements on discinsulators as follows:

35.0

115

DK

D

Statistical considerations: This correction factor is chosen to obtain a sufficiently low riskfor flashover. This takes account of the number of insulators simultaneously exposed to thesame contamination event, the frequency of contamination events etc. into account.For line insulators: 4.1

SK and

For substations or installations with less than 50 insulators: 0.1SK

The correction factors mentioned above are utilized as follows to determine the required designDC severity (SDD):

MeasuredSDBTNSDDPESDDKKKKKSDD

/

The design DC severity corresponds to the contamination severity at which representativelaboratory tests can be performed. At this stage it is also possible to make a first estimate of theleakage distance (unified specific leakage distance: USCD) that will be required for the project.The following equations, derived from previous laboratory results, can be used:

For all hydrophilic insulators:33.0

115 SDDUSCD BasicDC


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For all hydrophobic insulators:25.0

65 SDDUSCD BasicDC These equations are valid for line insulators and substation insulators with a relatively smalldiameter. Insulators with a large diameter generally have a lower flashover voltage thaninsulators with a smaller diameter, thus requiring longer leakage distances. The followingcorrection factor can be used to correct for the effect of the insulator average diameter onhydrophilic insulators, D [mm]:

30.0

115

DCD

At present it is not considered necessary to correct for the effect of diameter on hydrophobicinsulators.

For installations at high altitude an additional correction factor can be considered to adjust theleakage distance for the lower flashover voltage under low air density conditions. The followingequation is proposed for hydrophilic insulators and a height above sea level, H[m]:

815035.0

H

aeC

For hydrophobic insulators this effect can conservatively be estimated as:

8150H

aeC

Finally the required unified specific leakage distance is then determined as:

BasicDCaDDCUSCDCCUSCD

In areas where the contamination and wetting occurs simultaneously the contaminationseverity is determined by monitoring the leakage current activity on the insulators and tocompare peak values with those obtained from Salt-Fog tests. The severity of the site isexpressed as the laboratory severity (in terms of the salinity of the salt water used [kg/m3]) whichwould result in the same level of leakage current which is observed at the site. This is named theSite Equivalent Salinity (SES).

Since there is a direct link between the site severity assessment and the laboratory test (i.e. Salt-Fog) the simplified design process becomes even simpler as there is no need to correct for thetype of salts, NSDD and uniformity of the contamination deposit. At present it is proposed tocorrect for:

Diameter effect: Where the same equation as for pre-deposited contamination is utilized.

Statistical considerations: Again the same correction as for pre-deposited contamination isutilized.

The equivalent salinity (ES) is thus calculated as follows:


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SESKKES SD

The basic required unified specific leakage distance can be estimated as follows:

For all hydrophilic insulators:33.0

15 ESUSCDBasicDC

For all hydrophobic insulators:25.0

15 ESUSCDBasicDC

As for pre-deposited contamination it is also necessary to adjust the basic required specificleakage distance for the effect of diameter and altitude. The same equations can be used thus:

BasicDCaDDCUSCDCCUSCD

Other ConsiderationsIn the foregoing section an overview is given on how to determine the insulator dimensionsbased on theflashover performance of the insulators. This does not take into account of anylong-term aging effects if present. At present there is not enough documented service experienceavailable to give general guidance. There can be large variations in the composition of insulatorhousing materials on offer from the different manufacturers and these materials may have verydifferent abilities to withstand the service stresses placed on it by the DC energization. It istherefore advisable to collect as much relevant service experience as possible on the particularinsulator make that is a candidate for installation on the HVDC system. In addition it is alsonecessary to consider E-field grading along the insulator to avoid the aging effects observed onAC insulators in clean areas. Indications are that similar aging mechanisms may be at work onDC energized insulators.


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Electrode

HVDC lines can operate in bipolar or monopolar mode. When a DC line is operating inmonopolar mode, a return path is required. The return path can either be a dedicated metallicreturn or an electrode. HVDC systems utilizing electrodes have been successfully designed andput into commercial operation. Some of the earlier HVDC systems utilizing electrodes havebeen operated for extended periods of time using the electrode return with no adverse effects.However, in recent years, concerns regarding the operation of electrodes have been raised.

This guide reviews the latest techniques in establishing a ground electrode, the environmentalconstraints and regulations in establishing ground electrodes, challenges and advantages of landand ocean termination as well as condition assessment.

Introduction

The most cost-effective way of designing a monopolar HVDC systemis to construct it with ahigh-voltage conductor and an earth return or sea return, depending on the application. In such asystem,the dc currentpath is through the high-voltage conductor, and the return path is throughthe earth or sea return.In monopolar HVDC links, an electrode connection is required at eachterminal. The electrode either carries dc current into the earth or receives dc current from theearth, so in principle, the earth is utilized as the current return path for the dc current. If earth orsea return is not desired or acceptable, then a metallic conductor dedicated for current returnreferred to as metallic returnisimplemented.

Current return through the earth saves the extra cost of the metallic return and reduces the powerlosses as a result of the smaller ground resistance path when compared to the resistance of themetallic return. The cost of adding an extra conductor for metallic return in the case of anoverhead line differs from the cost of adding a dedicated metallic return cable for monopolarcable applications. The cost impact for each case needs to be evaluated during the planning phase

of the project.

In the normal operation of a bipolar HVDC transmission system, currents in the positive andnegative poles are equal and in opposite directions, therefore current between converter stationsis essentially zero and practically limited to the tolerances in the control and measuring systemsof the two poles, typically in the range of 10 amperes. During single-pole operation, earth returncould be utilized, or the conductor of the out-of-service pole can be utilized as a metallic returnfor the return current. However, to ensure uninterrupted power transfer in one pole during asudden block or trip in the other pole, a bipolar system should be equipped with an earthelectrode or a dedicated metallic return.

Earth electrodes perform an important function for either monopolar or bipolar HVDC systems.Almost 30 years ago, EPRI produced a detailed design manual [1] for high-voltage direct current(HVDC) earth electrodes. Since then, changes and improvements have been made in earth

electrode design and operation. This guide provides an overview of HVDCground electrodesincluding the following specific topics related to design and operation of ground electrodes.

Latest ground electrode techniques

Environmental constraints and applicable regulations for ground electrode operation


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Challenges and advantages of ground electrode ocean termination

Condition assessment of ground electrodes

Converter Configurations

Monop olar HVDC System Configu rat ions

Monopole with Electrode Return

A monopole system with earth electrodes is shown inFigure 1-1. In this configuration, there isonly one high-voltage conductor, and the current return path is through the electrodes. However,this configuration may not be acceptable in some situations due to environmental concerns. Thisconfiguration has been applied in some older cable systems.

Figure 1-441Monopolar HVDC with Earth Return

Monopole with a Dedicated Metallic Return

This type of system avoids the concerns raised due to permanent earth electrode current. In thiscase a second conductor of the same current rating of the main conductor but at much lower dcvoltage is needed as shown in Figure 1-2.

Figure 1-552Monopolar HVDC with Dedicated Metallic Return

Bi-polar HVDC System Config urat ions

1. In a bipolar HVDC system, there are two poles of opposite polarity, and the typical operationmode is with equal current between the two poles, which means no current will flow in theelectrodes, as shown in Figure 1-3.


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Figure 1-663Bipolar HVDC System with Ground Electrodes

2. In the event that one pole is out of service, the operation of the second pole can continueeither using the electrode as a return current path, or by using the metallic conductor of theout-of-service pole as a return path, which is referred to as metallic return operation. Themetallic return operation is the preferred mode of monopolar operation, because no currentwill then be carried by the earth electrode.

3. Shown in Figure 1-4 is the starting point of a bipolar system with balanced current operation.Any current in the electrodes would be due to a very small amount of unbalance between thetwo poles. Note that the circuit breakers shown in red color are closed.

Figure 1-774Bipolar System with Balanced Current Operation

Id

Id

Id

+


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Figure 1-5 shows the same system with one pole taken out of service, while the remaining pole

continues its operation with the full dc current flowing in the electrodes.

Figure 1-885Monopolar Operation Using Ground Return

4. The Ground Return Transfer Switch (GRTS) is then closed; this then connects themetallic return in parallel with the electrode path, as shown in Figure 6.

Figure 1-996

Monopolar Operation Using Ground Return and Metallic Return

MRTB

GRTS

Id

Id1

Id2

Id = Id1 + Id2

MRTB

GRTS

Id

Id


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5. The next step is to open the Metallic Return Transfer Breaker (MRTB) and complete the

transfer of current from the electrodes to the metallic path, completing the process as showninFigure 7.

Figure 1-10107Monopolar Operation Using Only Metallic Return

The MRTB is a special commutating breaker that can force the current from the electrodes to the

metallic return (see Figure 1-8).

Figure 1-11118Components of MRTB

Operation of the MRTB is as follows:

1.

As the contact S opens, and develops an arc voltage, it generates an oscillation in the circuitL,C, and S at the natural frequency of the loop, which is known and is part of the design.

2. As the current in S decreases by being shunted to the L and C branch, the arc voltage of Sincreases due to its negative current voltage characteristic.

S

L C

I

Zno

I1

MRTB

GRTS

Id

Id


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3. A current oscillation is set in L and C, which grows in magnitude with time until its

magnitude equals the current to be interrupted, creating a current zero in S, and allowing it toextinguish and recover.

Electrode Rating

Existing electrodes in various parts of the world are rated between 800 A and 4000 A [2,3].However, there is no definition available for the current rating of the electrode. Depending on therequirements, the electrode rating could either be:

the maximum current that the electrode could handle under various operating scenarios, or

the continuous rating of the electrode, so that maximum current for temporary overload couldbe higher than the rating.

Reversible Electrodes

Current transfer in a monopolar system is always in one direction. Therefore, the electrode thatinjects current into the earth (anode electrode) and the electrode that collects current from earth(cathode electrode) are fixed.

Current transfer in a bipolar system is also in one direction. However, when a bipolar systemoperates in monopolar mode, the electrode that injects current into the earth and the electrodethat collects current from the earth depend on whether the positive pole or the negative pole is in-service. Electrodes connected to bipolar systems should be able perform as an anode as well as acathode depending on the polarity of the out-of-service pole. Electrodes that could perform asanode or cathode are referred to as reversible electrodes.

Electrode Line and Reliability Consideration

A survey on existing earth electrodes shows that the length of electrode lines varies from 8 km to

85 km [3]. The lower limit of the length of electrode line is dictated by the influence of theelectrode electrical field on the ac grid at the converter station and the upper limit of the length ofthe electrode line is influenced by many factors, including finding a suitable area with lowresistivity, proximity to infrastructure such as the ac grid and other metallic structures, and theavailability of suitable land sites. Figure 1-9 shows a bipolar system in steady-state operationwith equal dc currents in the two poles. The current flow into the electrode line at steady state isessentially zero and practically limited to the tolerances in the control and measuring systems ofthe two poles, typically in the range of 10 amperes. Upon the failure of the electrode line (opencircuit) during bipolar operation, the system can continue operating with the converter stationhigh-speed neutral bus ground switch (NBGS) closed. Operation with the station ground mat isshown inFigure 1-13Figure 1-13Figure 1-10. This mode of operation can continue until anyevent that leads to the loss of a pole takes place. Under this condition, the healthy pole must beremoved from service to avoid any dc currents in the station ground mat. Electrode lines aregenerally reliable and designed with two separate conductors in order to avoid this situation.


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Figure 1-12129Bipolar System in Steady-state Operation with Healthy Electrode Line

Id

Id

Electrode

Open NBGS

Electrode

Station ground

Id

Id

Open circuited Electrode

Closed NBGS

Station ground


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Figure 1-131310Bipolar System in Steady-state Operation with Broken Electrode Line

Electrode Classification

Electrode stations can be categorized into three categories based on the location of the electrodeas shown inError! Reference source not found..

Table 221Classification of Electrodes Based on Location [7]

Type of Electrode Description

Land Electrode Located on the land away from the sea or freshwater lakes

Shore Electrode Located on a shore against (salt) seawater. Shore electrodes can belocatedeither on the beach without direct contact with seawater at a short distance (< 50m) from the waterline or in the water, but protected by a breakwater

Sea Electrode Located (typically on the seabed) in the water at some distance (> 100 m) fromthe coastline

Current Blocking Devices

DC currents between ground electrodes cause potential gradients on the surface of the earth. As aresult of these earth potential gradients, dc currents may enter the neutrals of transformers.Current in transformer neutrals will lead to core saturation and may also cause corrosion. Suchdc currents must be minimized or eliminated. Transformer neutral blocking devices can beapplied, as shown inFigure 1-14Figure 1-14Figure 1-11; however,these solutions can beexpensive depending on the number of transformers involved.

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Figure 1-141411Schematic of Diagram of Transformer Neutral Blocking Device [9]

References

[1]HVDC Ground Electrode Design, EL-2020, Research Project 1467-1.

[2]International Engineering Company Inc. for Electric Power Research Institute(EPRI), 1981.

[3]Summary of Existing Ground Electrode Designs, 1998.

[4]M.R. Nielsen on behalf of CIGR Working Group 14.21.

[5]Compendium of HVDC Schemes Throughout the World.

[6]CIGRE Advisory Group B4.04, 2005.

[7]HVDC Ground Electrodes Technical Report S90-003.

[8]Swedish Transmission Research Institute (STRI), 1990.

[9]J. C. Gleadoe, B. J. Bisewski, and M.C. Stewart, DC Ground Currents andTransformer Saturation on the New Zealand HVDC link, International Colloquiumon High Voltage Direct Current and Flexible AC Power Transmission Systems,October 1993, New Zealand.


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Advancement in Present Ground Electrode Technologies

Electr ical Resist ivity and Electr ic al Field Calculat ion

The selection of a proper ground electrode site is of paramount importance for the reliableoperation of HVDC systems. A properly selected ground electrode site can not only ensuresmooth system operation but also rule out the need for mitigation measures against adverseground current effects. It is prudent to carry out a thorough soil investigation at the proposedlocation. For such investigations, it is practical for the power utility to make use of theexperience of other organizations / research institutes involved in geological explorations such asoil and gas exploration companies and other organizations involved in geophysical research andmapping.

In selecting a particular site, the utility should perform geographical and geophysical surveys thatconsider various factors, namely

Electrical resistivity of earth and water Thermal properties such as thermal conductivity and thermal capacity

Porosity (water and gas permeability)

Penetration of moisture and influx of water

Buried and earthed metallic structures within the area of influence

Electrical infrastructure within the area of influence

Environmental/land use/landowner considerations

Accessibility

The current practices of geographical and geophysical surveys are discussed in detail in the EPRIground electrode design manual [1]. This section highlights several technological advancementsin the area of electrical resistivity measurements and electrical field calculations.

Low surface electrical resistivity in the local electrode area (i.e., earth or seabed and sea) isimportant in order to keep the step-and-touch voltage in the close vicinity of the electrode withinsafe limits. The deep earth layers in a larger area are more important in reducing the electricalfield distribution, which in turn, keeps the voltage gradient in the general area of groundelectrode within acceptable limits. A low-voltage gradient ensures that the ground current doesnot cause corrosion of buried metal structures and does not enter the neutrals of transformersinstalled in the area of the electrode station. For this reason and also to optimize the cost of theground electrode, it is important to know the electrical resistivity of the earth up to a depth of 20km, covering an area with a radius of 10- 20 km from the ground electrode site.

Galvanic and inductive methods have been used to estimate the ground resistivity since the earlydays of the ground electrode design [1, 2]. The modern versions of these techniques are:

1. High-resolution multielectrode DC resistivity imaging techniques suitable for shallow

resistivity measurements2. Magnetotelluric techniques suitable for deep resistivity measurements.These methods were

used in recently completed projects [3, 4].


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High-resolution Multielectrode DC Resistivity Imaging Techniques

The high-resolution multielectrode DC resistivity imaging method is an active source methodand provides high-resolution images of the electrical resistivity structure up to depths of severalhundreds of meters. Obtaining the resistivity structure up to few kilometers using this method ispossible, but typically not used, because the large amount of current that needs to be injected intothe ground, employing large electrode separations, is unrealistic. Multielectrode DC resistivityimaging is a fully automated technique that uses a linear array of multiple current and potentialelectrodes connected to a multicore cable. The current and potential electrodes are organizedaccording to a preprogrammed electrode array configuration. A sequence of measurements ofpotential difference are measured and recorded for a number of ordered combinations of currentelectrodes injecting a preprogrammed amount of current. Commercially available software isused to process these measurements into an image of ground resistivity-depth profile.

High-resolution multielectrode DC resistivity imaging methods can also be used for shallowresistivity measurements [3]. One candidate electrode site occupying an area of approximately

600 meters by 600 meters was covered with seven multielectrode DC profiles at 100 metersseparation to image the shallow structure up to a depth of 150 meters. A multielectroderesistivity imaging system with 80 electrodes at 10-meter intervals was used, and this setup gavea total profile length of 790 meters and a penetration depth of more than 120 meters. A typicalresistivity image of shallow ground obtained during the data processing is shown inFigure Error!No text of specified style in document.-15Figure Error! No text of specified style indocument.-15Figure 2-1. In these measurements, a Wenner-Schlumberger configuration wasemployed.


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Figure Error! No text of specified style in document.-15151Typical Resistivity Image Obtained from High-resolution Multielectrode DC Resistivity ImagingMethod [3]

Magnetotelluric Technique

The magnetotelluric (MT) technique is also a fully automated technique.Two basic types of MTmethods available for deep resistivity measurements arenatural source MT and controlled sourceMT.

In the natural source MT technique, natural electromagnetic (EM) waves are used to determinethe earth's electrical resistivity structure from a few hundred meters to several hundredkilometers deep, depending on the frequency of the signal. These natural sources are mainlylocated in the magnetosphere and ionosphere, separated from the earths surface by thenonconductive atmosphere. Because the earth is a conductor, these natural sources inducesecondary fields in the earth.

In the controlled source MT technique, a current source of variable frequency is used to injectthe current into the ground. Depending on the frequency of the injected current, different depthsof penetration of the current into the earth takes place. The basis for the MT theory is providedby Maxwells equations, which relate electrical and magnetic fields. The deep resistivitystructure is determined by measuring five components of time series data consisting of threemagnetic field and two electric field components [3].


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Deep resistivity investigations were carried out using wide-band natural source MT equipment in

India [3]. This equipment consisted of a six-channel data acquisition unit, three highly sensitivemagnetic induction coils, and a GPS module. Commercially available software was used toanalyze the time series data and generate an image of the ground resistivitydepth profile. In onecandidate site, an area with a 10 km radius from the earth electrode site is covered by about 13MT measurements, each with a time series recording of one day to achieve the desired depth ofinvestigation, assuming moderately conducting ground conditions. This was done to ensure thatresistivity data of up to a 5-second period with a good signal/noise ratio is obtained. A typicalresistivity image of deep ground obtained during the data processing is shown inFigure Error!No text of specified style in document.-16Figure Error! No text of specified style indocument.-16Figure 2-2.

Figure Error! No text of specified style in document.-16162Typical Deep Earth Resistivity Image Obtained from MTResistivity Imaging Method [3]

The ground electrode site selection process for the Caprivi Link in Namibia [4] also employedthe multielectrode DC resistivity imaging technique for shallow earth resistivity imaging and theMT technique for deep resistivity imaging. The three-dimensional cube shown inFigure Error!No text of specified style in document.-17Figure Error! No text of specified style indocument.-17Figure 2-3represents a 27x27x20 km earth resistivity model developed for theelectrode site.

Once the ground resistivity data is available, as shown inFigure Error! No text of specified stylein document.-17Figure Error! No text of specified style in document.-17Figure 2-3,

Good Conductivity up to the

depth of 4150m except the

top layer of about 100m


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commercially available finite element programs could be employed to estimate the potential

distribution in the area of the ground electrode station due to the dc current injected/received atthe electrode station.

Figure Error! No text of specified style in document.-17173Example of Three-dimensional Earth Resistivity Model That Was Employed to Estimate PotentialDistribution due to the Earth Electrode in the Area. The Color Scale Shows the Logarithm ofResistivity in Units of m [4]

Figure Error! No text of specified style in document.-18Figure Error! No text of specified style in

document.-18Figure 2-4depicts the iso-potential contours for a current injection of 1000 amperes

at the ground electrode [4].

Figure Error! No text of specified style in document.-18184Contours of Potential Distribution Estimated for a 1000 Amperes of Hypothetical Current Injectionat the Ground Electrode Station Using Three-dimensional Resistivity Model [4]

Finite Element Modeling Software Programs

Modern software programs such as the Current Distribution Electromagnetic InterferenceGround and Soil Structure Analysis (CDEGS) program [5, 16] have interfaces to accept field

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measurements of resistivity data or resistivity models compiled by other software programs.

Once the resistivity model is available, a three-dimensional finite element model of the electricaland magnetic field is developed. Quantities such as electric and magnetic field strengths, as wellas step-and-touch potentials required for electrode design, could be obtained from theseprograms. Additionally, modules are also available for electrode size optimizations.

In addition to estimating the resistivity profile and electromagnetic field distribution, softwareprograms are utilized to calculate chlorine distribution caused by a sea or beach electrode.FigureError! No text of specified style in document.-19Figure Error! No text of specified style indocument.-19Figure 2-5shows the predicted chlorine concentration distribution for an upgradedSACOI beach electrode at Punta Tramontana, Italy.

Figure Error! No text of specified style in document.-19195Representation of Chlorine Concentration Distribution [6]

Design of Ground Electrodes

The design criterion for electrode stations is typically 50 years. A CIGRE technical brochure forground electrodes design published in 1998 [7] and the EPRI ground electrode design manualpublished in 1982 [1] provide detailed explanations of ground electrode design. This subsectionon design of ground electrodes provides a summary of ground electrode design including thelatest developments in the area of electrode design.

Design of Land Electrode

The design aspects of land electrodes have not changed much since the early days of theirapplication. Some of these design aspects are common for all land electrodes, and some arespecific for given location and electrode design. This subsection discusses five aspects listed asimportant in [7].

Heating of Soil

Heating of soil close to the electrode surface is an important quantity to consider in electrodedesign. The current industry practice [7] is to design the electrode so that the maximumtemperature is limited to the boiling temperature of the water.


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The boiling temperature of water depends on the water pressure; therefore, the boiling

temperature depends on the height of the water column and the altitude of the electrode site.Typically, a soil temperature of 85C is considered as the threshold, and temperatures in excessof this could cause steam formation [7]. The danger of steam formation is that the steam trappedinside the soil might develop excessive pressures that may cause the electrode to explode. Ingeneral, the overheating of soil reduces the moisture content, and drip irrigation techniques maybe adopted to maintain the soil moisture level for prolonged ground return mode ofoperation.However, if the electrode is buried deep below the earth surface and there is watercolumn above it, the boiling temperature increases and a suitable correction to the maximumtemperature must be made.

The temperature rise of the electrode surface is calculated by using a formula first derived byKimbark [8]. This formula assumes a uniform heat conductivity and earth resistivity, and relatesthe temperature rise at the electrode surface to the potential of the electrode using earth electricalresistivity and thermal conductivity. The adopted industrial practice to estimate earth resistivity

and thermal conductivity is to use a soil sample extracted close to the surface of the electrode.A CIGRE brochure [7] shows that this formula provides a pessimistic estimation of thetemperature rise due to the simplified assumptions, and it suggests the use of a correction factorof 5 for continuous operation of ground electrode with burial depth less than 3 meters, and acorrection factor between 5 and 1 for deeply buried electrodes with burial depth between 50meters and 500 meters.

Moisture Content of Soil/Electric Osmosis

Electrodes cannot be successfully operated in dry lands such as dry sand or hard rock areas.Watering (irrigation) systems to maintain moisture content have been applied to ground electrodesites. Such practices have been successfully applied in the Nelson River scheme in Canada andRihand-Delhi scheme in India.

The current density at the surface of a land electrode must be limited to 1 A/m2 to avoid electro-osmosis (i.e., movement of water in the direction of electrical field) [9]. The Rice Flats electrodeof Pacific Intertie used a current density of 0.5 A/m2, whereas in the case of Danish shoreelectrodes, a current density of 5 to 8 A/m2 is used because the presence of water is ensured bythe location of electrodes (below the sea level, 20 meters towards the beach) [7].

Geometric Layout

The resistivity profile at the ground electrode station indicates what geometric type of electrodeis suitable and whether one single electrode or a parallel combination of a number ofsubelectrodes is required. Land electrodes could be categorized as vertical or horizontalelectrodes based on their geometric layout.

Vertical Arrangement (Borehole Electrodes)

A vertical electrode design is used for electrode stations with conducting layers at some depth.

In addition, vertical electrodes require less land space, and hence this design is an attractiveoption for HVDC schemes associated with high currents. This is because a maximum currentdensity associated with the soil properties and material is used in electrodes. In the case ofvertical electrodes, longer electrodes buried deep into the soil could be used to maintain current


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density, whereas horizontal electrodes need a higher cross section, which means a greater land

area to limit the current density.Vertical electrodes consist of several subelectrodes with each subelectrode having a depth of 50to 200 meters. Each subelectrode has an inner conductor surrounded by a backfill of aconductive layer.Figure Error! No text of specified style in document.-20Figure Error! Notext of specified style in document.-20Figure 2-6 shows a typical cross section of a verticalelectrode.

Figure Error! No text of specified style in document.-20206Cross section of Typical Vertical Electrode Arrangement [11]

Horizontal Arrangement

Similar to the vertical arrangement, a horizontal arrangement also has an inner conductorsurrounded by backfill of a conductive layer. Typically, the active part (inner conductor) of thehorizontal electrode is buried about 2 meters below the earth surface. A cross section of a typicalhorizontal electrode is shown in Figure Error! No text of specified style indocument.-21Figure Error! No text of specified style in document.-21Figure 2-7.

Figure Error! No text of specified style in document.-21217

Cross section Through Horizontal Land Electrode [11]

Many different configurations, depending on the specific design, are possible for horizontalelectrodes.Refer to Appendix A for different configurations. In general, the configurations canbe linear, ring-shaped, or star-shaped arrangements. Symmetrical ring configurations have the


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advantage of even current density along the electrode circumference, provided that the ground

resistivity in the area of ground electrode is uniform. All the other configurations have unevencurrent distribution and hence unequal current density. Linear electrodes have a higher currentdensity at the two ends. High current density in an area increases the step voltage along thesurface of the earth. This could be mitigated by burying a part of the electrode at a greater depthcompared to the other parts of the electrode. For example, outmost tips of a star-shaped landelectrode at Bog Roy are buried to a greater depth than the other parts of the electrode to mitigatehigh step voltages [12].

In addition to their different shapes, electrodes can be categorized as continuous andnoncontinuous configurations based on the continuity of the inner conductor. Continuousconductor configurations have continuous backfilling trenches.

Noncontinuous conductor configurations have continuous or noncontinuous backfilling trenchesdepending on whether a clear separation of the electrode is required. The inner conductor mustbe connected to a common feeding point such as a bus bar where the ground electrode line isterminated. Typically, the inner conductor is connected to the common point at a number ofequally spaced points along the circumference of the electrode. The idea of noncontinuousconfigurations is to subdivide the electrode into separate parts so that irregularities in the currentfeeding into each segment can be controlled separately. In some schemes, a small resistance isadded in series with the segments of electrode to reduce the current density around certainsegments. It is also possible to have irregular current distribution as a result of local resistivityvariations. This situation could cause high current densities, causing electric osmosis or heatingof soil in local areas. Segmenting of electrodes is also useful for maintenance where the segmentunder maintenance could be disconnected while other segments are in-service.

Material Selection

Almost all the land electrodes built so far have an inner conductor surrounded by some form ofbackfill containing carbon to give good contact between soil and the electrode. A CIGRE

brochure that provides guidelines on ground electrode design [7] describes inner conductormaterial and backfill material containing carbon, as shown in Error! Reference source notfound..

Table Error! No text of specified style in document.-331 Inner Conductor and Backfill Material [7]

Conductor Material

1 Steel or "mild" steel rods or tubes, 30-40 mm in diameter. The steel conductor is mostly coveringthe length of the electrode continuously.

2 SiCrFe rods, commonly 45 mm in diameter, length 1.25-1.75 m. These electrode bars are normallyonly part covering the total length of the coke filling, 30-50 percent, which means that the cokecolumn is used also for longitudinal flow of current. There is a certain risk of an unequal currentdensity on the outside of the coke filling.

3 Graphite rods, commonly 100 mm in diameter, 1.2-2.4 m in length. As for the SiCrFe rods, thegraphite rods only cover part of the length or depth covered by the coke.

Backfill Material


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1 Coke breezesmall particle solid residue left by the cracking process of petroleum refining.

2 Coke Result of distillation of bituminous raw coal.

3 In one electrode station the carbon material is described as graphite powder emulsion.

Step and Touch Voltages

Step voltage is the voltage difference across a step of humans or animals. Touch voltage isdefined as the voltage between the ground surface and any object such as a fence that might betouched by a person standing close to the object. The permissible touch and step voltages [13] towhich an individual may be subjected to when the electrode is carrying DC current aredetermined, based on standards such as IEC standards 60479-1 and 60479-2 and IEEE standard80. Typical values of potential gradient and touch and step potential are given below. It isimportant to note that, during the design and operation stages, due consideration should also begiven to local regulations regarding safety

Documents

IEEE HVDC Guide Outline-July22-2012