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THE IMPACT OF AC POWER LINE DISTURBANCES ON TELECOMMUNICATIONS RECTIFIER TECHNOLOGIES AND POWERING ARCHITECTURES.

Bo C. L. Lindemark Ericsson Components AB,

S-164 81 Stockholm, Sweden

Background

When electronic equipment was introduced in the telephone switches, it was necessary to consider the DC distribution characteristics and phenomena that may occur and damage the new equipment. New design and careful engineering of the DC distribu- tion was one method to get control over such char- acteristics and phenomena.

When engineering DC distribution for a switch twenty years ago, voltage drop and proper fuse sizing were the main dimensioning factors.

Today we experience a similar situation when switch mode rectifiers (SMR:s) are introduced and widely spread. This means that we now need to engineer and design for appropriate AC distribution as well. In a way we can say that the situation is similar to that we experienced 20 years ago. The same electric laws (Ohm’s, Kirckhoff’s, etc.) are valid also on the AC side, so there is nothing unknown.

Traditionally we only considered the AC voltage window and appropriate fuses and wire dimensions when dimensioning. Now we need to consider other factors such as voltage sags and surges, power line notches and voltage transients, including their causes and effects. Influence from lightning has to be more carefully considered. Also the utility load and grid switching, as well as load changes in the AC distribution network close to SMR power plants, are now important considerations.

DC distribution in CO

Within Ericsson, 10 - 15 years ago, an extensive research was done in order to be clear of how future architecture for DC distribution within CO equipment would evolve. We found it that a stabile 48 VDC distribution would be the platform from where the CO shall be fed. All other

alternatives doesn’t show the same flexibility for new coming CO architectures and technologies.Also since 20 years ago, DC distribution has been carefully engineered and most phenomena which may occur is known and protected against. One example is Ericsson’s Transient Limiting Distribution System. Fig. 1 shows the difference between transients occurring in a transient limited DC system(high resistance) compared with a conventional DC distri- bution. The transient limiting function is derived from the know-how about transient occurrence, impedance and reliability requirements from the individual DC consumers.

“O-1

Inductive spikes

h J Resistivevoltagedrops

Figure 1. Other manufacturer use other means to get control of shown transients. Thus the 48 VDC system is a proven to be a very good platform from which telecom and computer equipment (i e mu1timedia)can evolve and it is also a virtual barrier to all external disturbances. The 48 VDC is also prescribed in a number of national and international standards, for example ETSI.

Change in Rectifier Technology

The change in CO HW technology from electromechanical to electronic take place 20 years

@780320344/94/ $4.00 1994 IEEE 41 3

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ago. Now is the time when thyristor and ferroresonant rectifiers are substituted with Switch Mode Rectifiers (SMR) in large scale. They are now widely spread globally, as many operators take advantages of the SMR:s characteristics.Those may include low

Linear Loads and Non-Linear Loads.

The AC mains was originally constructed for linear loads such as motors and incandescent lamps. Practical rules and norms reflect this situation

weight,small size and footprint, less audible noise, sinusoidal current draw and low THD and EMC. However, reduction in size 4 times and in weight 3 times has a trade-off, and that is that the robust barrier towards the AC mains is physically reduced. Still, the AC mains is the same and has its characteristics. The AC mains can not be considered as the stabile platform we have discussed for the 48 VDC. Thus we need to get control over the AC mains phenomena (as we did for the 48 VDC 20 years ago) in order to maintain Telecom Reliability. As with CO of electromechanical type , transients in the 48 VDC was not a big issue, AC mains transients was not a big issue when using rectifiers with a robust barrier.

throughout the electrical installation industry. For example in a normal AC mains it is customary to estimate the current in the neutral conductor to be 30% of the phase conductor current because of the load distribution. However at installation with a considerable proportion of information processing equipment the current in the neutral conductor is often greater than in the phase conductor, because the loads are mainly unlinear or electronic. An example of this situation is shown in Fig. 2.

Non-Power Factor Corrected

The Development of the Utility Mains.

The flow of electricity is not as simple as its use. The output from a socket is not always what it should be. In an AC system both current and voltage oscillate, and they are meant to do so. However currents pas- sing any coil generates impedance. Most electrically powered products includes coils. When any of such products is switched on the effect of its impedance

Phase b = 100 A RMS-

Phase c =IOOARMS+

travels through the electricity grid. Voltages vary, showing sudden spikes and slower swells and sags.

I neutral Such phenomena may reduce performance or even = 170 A RMS-

damage equipment connected at the end of the power lines.

0 360

According to studies by Westinghouse Electric Corp. ~ ~ $ ~ l ~ ~ g ; , i ~ ; o ~ ~ ~ ~ e such problems may cost only the industry in Ame- ineutrai i n e u t r ~ = ~ 1 ~ x 1 w + 1 w x 1 ~ + 1 w x 1 0 0 ) 0 . 5 = 1.2 + ibZ + i~2)o"

rica $ 3 - 5 billion dollars each year. Computer cen- ineutra = 173.2 A

ters and financial trading operations suffer most, with a momentary glitch costing as much as $2 million dollars. Manufacturers suffer too. A semiconductor manufacturer might lose $ 500*000 from a five-minute power cut. Other installations lose about $200.000 a time. The change from typical electromechanical devices to digital electronic ones is reflected in the above problems. 40 years ago 60 cycles per second was accurate enough to control electric clocks. The clock in today's 486 computer chip generates one cycle every 30 billions of a second - half a million times more often. Some kind of power conditioning or control of phenomena from the AC mains is required for loads with large economica~ importance Or loads with advanced functions and technology.

Figure 2

The disturbance characteristics of the neutral conductor current are further magnified by the high peak-to-peak factor and the harmonic com- ponent. To avoid this situation many telecom installations instructions recommend or pre- scribe the use Of an AC network Of a TN-S type for their Power installations-

The above illustrate situations where power dis- turbances are natural byproducts when using electrical loads. Furthemore the generation and distribution may also contribute to disturbances.

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High Medium Sub station - voltage - voltage -

transmission transmission

Lightning for example, has normally a big effect and is propagated throughout the distribution system. Thus the conclusion we can make so far is that the AC power path to the load and the load itself are parts of one system. spike).

The following AC power disturbances are common: Undervoltage(sag); overvoltage (surge); notch; black- out (AC power failure); and transient (impulse or

Local distribution

UNDERVOLTAGE (SAG) The AC Distribution System

Figure 3 shows a typicalAC distribution system.The system begins with the generators whose voltage is stepped up to a very high voltage level for long dis- tance transmission. Then there are intermediate dis- tribution steps such as routing power into a town from a main line. There are also substations that feed power into the local distribution system. Step-down trans- formers are used to bring the substation voltage down to the level required in a building. Finally , the load is connected to form the final part of the system. The interconnected various loads will interact; this interaction can affect a rectifierkelecom or compu- ter load, thus causing significant disturbances. Such disturbances may have negative effect on the reli- ability of a connected rectifier andor computer load.

There are both temporary and long-term under voltages. Short term under voltages is normally caused by starting a load connected to the same power distribution system. Electric motors for example, have high starting currents which could result in a reduction of line voltage. The duration of this type of sag is typically nit longer than 10 seconds. The sag get worse if the distribution system is close to, or slightly, overloaded. SMR:s are normally designed to handle short-term sags, normally without shut- ting down. In any case the batteries will take over in case of shut down, and there will be no damage to the rectifier. Long- term under voltage is normally caused by a permanently or temporarily overloaded power distribution system common in areas with under investment in infrastructure. The problem could also be local; for example, the building may have outgrown the capacity of its distribution system.

AC Power Disturbances

l$$ical AC distribution system

Other utility customers I N-G Bond

~ & Building distribution - - -

I Building

Rectifier

DC Filter 0 Telephone

switch

Figure 3. 41 5

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OVERVOLTAGE (SURGE)

Surges are sometimes caused by a common remedy to alleviate sags, namely, adjusting the voltage taps of the distribution transformer. A voltage-tap adjust- ment will work fine when the overloaded transform- er’s load condition is stabile but a temporary change downward in load will cause the transformer’s out- put voltage to rise, sometimes significantly. The cy- cling of an air-conditioning or refrigation compres- sor is often enough. A long term overvoltage often results after the transition from peak to off-peak hours either for the larger power distribution system or in the local building. Utility grid-switching is an other source of surges because switching high-voltage power lines, even at zero-cross-over, can increase volt- age amplitudes significantly. When outside the SMR:s input envelope, surges are potentially very damaging. Fig 4 describes the voltage envelope. Care must be taken to ensure that a rectifier not frequently is exposed to either short or long time surges.

230 VRMS 264 VRMS 184VRMS 141 VRMS

also be caused by utility lightning arresters momen- tarily shorting the power line during thunder storms.

TOTAL POWER OUTAGE.

An interruption of the flow of AC power for more than one full-line cycle is defined as a black-out. Black-outs are caused by the failure of substation and/or distribution transformers as well as various overcurrent protection devices. A black-out can also result when the utility shuts down one part of the distribution system to preserve generating capacity during peak-load hours.

TRANSIENTS

Transients as shown in Fig. 6 are caused by the on/ off cycling of loads connected to the power-distribu- tion system. A transient is a short-duration surge. Normally a fraction of a cycle. A transient results when there is an abrupt change in current. The tran- sient becomes greater if the series inductance of the

353 VRMS feeder is high. (Ref to Fig 1). A load with a high in- ductance magnifies a tran- sient. Transients often result from lightning, utility grid- switching, contactors, weld- ers, power supplies, copiers and cycling of equipment. Transients can also be caused by loose wiring. Transients have varying lev- els of energy content but can cause anything from system degradation to destruction.

Figure 4.

NOTCH AC Grounding

A notch is a short-duration loss of power during a half cycle as shown in Fig 5 . Notches can be caused by the switching of power-factor- correction capaci- tors which momentarily shorts the line. Notches are often accompanied by impulses and can cause dam- age to weak input circuits as well as problems for certain types of regulation transformers, Notches can

Many different grounding systems are found world- wide, but this paper deal with them-S system com- monly used in many European countries as well as in North America. It is important to note that there are other grounding system used in other parts of the world.

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p

!

i i i i I

k--------*-

r

Audit of AC Mains

a w - L2 - w L3

a - N - -

. ----?-- PE - T----- - i i

i i i I

I I .- .-I

-..-..-..-. 4 r .-.-..-.-. 4

NOTCH

Figure 5

TRANSIENT (IMPULSE)

Figure 6

A typical TN-S system is shown in figure 7. Only one neutral-to-ground connection is allowed in a building power distribution system unless a separately derived source is established. The consequence of establishing additional neutral-to-ground connections would be the ground wire sharing the load current with the neutral wire, thus violating safety codes.

In line with the situation 20 years ago, the total con- trol of the 48 VDC distribution was obtained. To achieve the same situation for theAC mains, is much more complex. One method to get control over the AC mains is to measure a number of power quality parameters including disturbances. Modem power analyzers have graphic presentation of data, from where the source of disturbances or other data can be found. With this information, the load to be connected can be engineered to deal with the situation. More such data from audits gives the designers of SMR power plants a very good platform from where they can construct proper surge immunity based on facts. However facts and conditions may differ from site to site, so a lot of economical versus technical considerations must be done to achieve “cost-effec- tive reliability” on SMR products.

International and National Norms

A lot of research has been done by a number of bodies, such as IEEE, IEC, VDE, Bellcore etc. There- fore there are a number of predicted disturbances (surges) to be used as design rules for designers. The most frequently ones referred to in technical specifi- cations are:

EN 50082-1

European Standard, Electromagnetic Compatibility - Generic Immunity Standard (Draft, February 1991, the draft was established by CENELEC)

Open circuit surge voltage 1.2150 is Common mode amplitude of 4 kV Differential mode amplitude of 2 kV

Short circuit current 8/20 microseconds

Note the higher voltage for common mode versus normal mode. These differences result from the as- sumption that neutral is grounded only at the sub- station, as is the case in some European countries.

Note, the above pulses are similar to ANSI/IEEE C62.41-1991 Category B unidirectional impulse be- low.

Figure 7 41 7

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ANSyIEEEC62.41-1991 (formerlv IEEE 587-1980)

Category B

Unidirectional impulse with a 1.2 microseconds rise time to 6 kV and an exponential decay to half volt- age in 50 microseconds. Short circuit capability is 3,000 A, with a current rise time of 8 microseconds, decaying by half in 20 microseconds.

The above pulses are assumed to appear in the nor- mal mode as the U.S. National Electric Code requires neutral to be bonded to ground at every building serv- ice entrance. The reason, presumably, is to minimize the difference in potential between power-system wiring and other wiring-telecom, cableTV, etc., par- ticularly during fault conditions-by bonding them all to ground at the same point, at the entry to each building.

The US. approach to grounding the neutral results in common-mode surge voltages that are no greater than normal-mode surge voltages.

The 6 kV maximum versus the IEC’s (EN 50082) maximum has no ready explanation-maybe it is just different degrees of conservatism.

wide. Furthermore we have made a research and study on available literature about surges and AC distribu- tion for telecom. Also change of experience with the utilities, medical and military industries has been made. Our research shows that external factors such as lightning activity, slow surges(VDE 0160) and significant overvoltages as well as out-of-the-range input voltages are phenomena where severAC power conditions may occur.The following are examples of sites where a potential risk for degradation or dam- age of rectifiers, if not proper engineered:

Regions with high isokeraunic levels.

Equipment installed in containers that are fed from a drop from an overhead line.

Equipment fed by non-TN-S AC topology

Equipment that is located too far from the neutral- to- ground bond.

Sites with abnormally high nominal voltage.

Conclusion and Discussion

We have identified three types of surges which we classify as follows:

DIN VDE 0160

Pulse that begins at the supply voltage peak, with a maximum value of 2.3 times the peak voltage, a rise time of 0.1 ms, half amplitude duration of 1.3 ms, repetition three times each on the positive and nega- tive peaks.

All these well documented predictability standards are based on real facts and scientific research.

Field Research.

However, even if individual rectifiers pass tests ac- cording to above standards in laboratory environment, it has happen that rectifiers are returned from the field. Why is that some rectifiers that are returned from the field have totally burnt out boards ? Ericsson has therefore decided to look beyond the obvious. Something more sinister must be happening.

We have therefore since some years ago made AC mains audits and measurements with power line analyzers at a large number of telecom sites world-

LINE OVERVOLTAGE SLOW SURGE FAST SURGE

Line overvoltage is identified as more than 1 cycle, slow surges as a duration of milliseconds and fast surges as a duration of microseconds.

We can also conclude that the purpose of grounding is safety-not equipment performance. However aTN- S system is recommended as to establish controlled ground path for rectifiers. According to the laws of physics, low-resistive grounding wont help much against fast surges as seen in the following example: “Consider a wire that is 10 m long, resistance is ap- proximately 0.2 ohm while its inductance is approxi- mately is 10 microH. Thus its reactance will be 6.28 ohm for a pulse duration of 20 microseconds, while its impedance will be 6.283 ohm. The effect of try- ing to shunt a surge current of 3000A back to ground via the wire will be a voltage of 1500V.

SMR:s are designed to handle reasonable input volt- age envelope. It is not economical to design all rec- tifiers to withstand greater deviations as most sites

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falls within the curve shown in fig 8. Our conclu- sion is that a variety of equipment for enhanced surge immunity shall be offered for proper engineering of SMR installations.

U

- 245v

230 v

184v

Ericsson’s Engineering Rules

* The DC power Plant shall be located close to the neutral-to-ground bond.

* Surge diverters shall be installed in conjunction with the neutral-to-ground bond or a separately de- rived source.

* All grounds shall be tied back to the same point

* “Improved ground” does not help against light- ning, load switching and other fast events-inductance counts.

* High-energy MOVs are effective against slow surges.

* Multi-cycle overvoltages need to be interrupted.

* All AC power distribution branch circuits shall be fed from the same point.

* Voltage range extension devices must be compat- ible with rectifiers.

Finally, an appropriate coordination of all included immunity devices within a telecom site is essential. This is the advice Ericsson can contribute with for obtaining optimal reliability of telecom services.

References

1. The Economist February 19th 1994, “Smoother flowing”.

2. Eberhard Munchow, Safe power distribution and ), Telescon 1994

3. E E E Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, C62.41-1991

* Filters have limited impact against slow high-en- ergy surges.

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