PRF - PV - Technical Note on Solar Grid Integeration Ver 1 0

8/11/2019 PRF - PV - Technical Note on Solar Grid Integeration Ver 1 0

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TECHNICAL NOTE

Solar Grid Integration

Nov 2012


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TABLE OF CONTENTS

1 Executive Summary 4

2 Introduction 4

3Grid Interconnection

Standards5

3.1 Safety Standards 6

3.1.1 Fire Safety 6

3.1.2 Islanding 6

3.1.3 Over/Under voltage 7

3.1.4 Grounding 8

3.1.5 Over/Under Frequency 8

3.1.6 Manual Disconnect Switch 9

3.1.7 Short Circuit Capacity 9

3.2 Quality Standards 10

3.2.1 Harmonics 10

3.2.2 Power Factor 11

3.2.3 Flicker 12

3.2.4 Voltage Imbalance 12

3.2.5 DC Injection 13

Appendix 14-16


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1.Executive Summary

The note summarizes the technical guidelines that need to be followed for integrating a PVsystem to grid, problems accompanied with higher penetration of PV system and possible

solutions to overcome it. With the increased Levelized cost of electricity from conventional

energy sources such as coal, petroleum etc, its evident that solar energy would achieve grid

parity sooner. The best part of PV systems is that they can be installed on every roof top that

has minimum shading. So the penetration level of solar in supplying power to the grid is huge.

It offers several advantages such as reduction in transmission losses; reduction in carbon foot-

print and uninterrupted power supply.

With the advancement in inverter technologies many more things are to come such as

power factor control, voltage stabilization and remote energy management system. With such a

kind of opportunity available, one must plan the distribution network in order to account for this

increased penetration. The document discuss the precautions one need to take for connecting

PV to grid, based on current standards and so the user has to be aware of the change in

standards with time and region. The document assumes that the reader is fully aware of the

technical concepts related to grid infrastructure and its protective equipments. Moreover a

basic knowledge of electrical engineering is a must.

2. Introduction

The concept of synchronization is a recent trend evolved in 19thcentury when large sized

Diesel Generators were added to the grid by many industries in Europe and America.

The conditions for grid synchronization are that the

1) The phase angle of voltage source must be within 15 with that of the grid voltage.

2) The frequency of the sources must exactly match.

3) In case of three phase synchronization the phase sequences must also match.

The modern technical standards for grid connection are based on the above ground rules

and other power quality issues. Each region either follow their own standards (like


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Australia) or they specify the either of the three such as IEEE, NEC or UL standards to be

followed.

The basic structure for grid tied solar photovoltaic system is shown in Figure 1.

The system comprises of an inverter which usually is a grid tie inverter. These inverters

should have automatic synchronizing mechanisms and should be able to provide islanding

protection. There are a lot of technical and safety standards that a grid tied inverter must

satisfy to be employed in this kind of system.

3. Grid Integration Standards

The grid integration requires some safety and power quality standards to be met. The

following section outlines the standards that the user has to be aware of before installing a

grid connected PV systems.

Figure [1]


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3.1 SAFETY STANDARDS

3.1.1 Fire Safety

The potential of fire hazard is greater for DC than AC because a DC arc is harder to

extinguish than an AC. The absence of a zero crossing in DC makes it more difficult for the arc to

extinguish. DC voltages greater than 300V are bound have fire protection systems according to

UL1741.

3.1.2 Islanding

Islanding is a phenomenon where a portion of utility system that contains both the

load and generation source is isolated from the remainder of the utility system but

remains energized. The principal concern with Islanding is the safety of line

workers who come in contact with the line that is unexpectedly energized. Even

when these people are trained to test the safety of a line before working the

chances of accidents may remain high. The other concerns are damage to the

utility equipments from a DG system working outside of specifications, and

interface with automated distribution system protection function such as reclosing.

Therefore the phenomenon of islanding happens to be two ways- Intentional and

Unintentional.

Intentional Islanding can occur at customer level such as hospital uses its

emergency generators during a utility level. Potential safety concerns occur when

DG system that is not specifically and approved for intentional islanding fails to

detect the loss of utility power and continues to energize an isolated section of the

utility line. Grid Tied inverters certified with UL1741 standard are recommended for

this purpose.

In order to avoid unintentional islanding inverters use active methods such as

frequency shift, frequency instability, Power variation, Current Injection etc.

Sometimes over/under voltage and over/under frequency protection schemes gives

protection against unintentional islanding. IEEE 1547 standard specifies inverter to

automatically trip if the RMS voltage at the point of coupling is above 10% or 12%

below the nominal value or if the frequency is not between 1 Hz of the nominal

value. With static inverters there is range of loads which are called Non-Detection

Zone (NDZ) for which these methods of detection of islanding becomes difficult.


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Modern technologies with greater penetration want PV system to ride through such

voltage or frequency disturbances. But with incorporation such stringent conditions

the penetration of PV system would lead to grid instability.

3.1.3 Over/Under Voltage Protection

The inverter has to operate at higher voltage compared to the grid in order to inject

power into the grid. Considering the grid to be infinite and the level of penetration

of PV to be low, the increase in voltage is very less. However due to faulty

operation there is chance of increase in the terminal voltage at the point of coupling

to the grid. Overvoltage may reduce the life time of the equipment and under-

voltage may cause equipments to operate at lesser efficiency. International

standards1have posed limitation. The situation becomes worse with higher

penetration of PV into the system leading to unstable grid. If the power produced

by the PV at a point is higher than the power consumed at that point, the surplus

energy will flow into the grid. The reverse power flow causes the voltage to rise as

it goes to other end. However this not issue with urban grid with high impedance

network. In case of rural grid with low impedance and increase penetration of PV

could cause the voltage to rise beyond the limits. This issue is called Over-Voltage.

It is possible to minimize this effect by tap changing transformers but the lines near

to the transformers may face under-voltage. Over-Voltages and under-voltages

both have a negative impact on Generation side and distribution side. Over-voltage

1Refer appendix 1A for IEEE 1547 and IEC6127 standards on Voltage Constraints.


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may reduce the life time of the equipment and under-voltage may cause the

equipments to operate less efficiently.

There is a significant problem of voltage rise or fall when there is a large

penetration of PV in to the distributed system. The problem can be lessened andthe upper limit on penetration of PV can be subsequently increased by the

following ways.

Decrease in the utility impedance. Increase use of transformers, use of multiple

conductors and transformers with large de-rating factors results in lesser utility

impedance and would decrease the effect of voltage with PV penetration. But this can

be achieved with increase in cost and short circuit current which results in the redesign

of over current protection equipments.

Use of energy storage devices to store the excess power and deliver power whenthere is a shortfall would obviously reduce the impact of voltage rise or fall.

Using PV inverters to supply or absorb reactive power to the grid would be a cost-

effective solution for the voltage rise or fall. This in some cases results in under-

utilization of available energy from the PV system.

Energy Management systems (EMS) which are another technology coming into picture

esp. for micro grids where a central system manages loads shedding accordingly to

manage the problem of over/under voltage. Thus a careful disintegration of critical and

non critical loads are to be done and the EMS should be managed based on that.

3.1.4 Grounding

There is no standard specification on the grounding but it always makes sense to

protect your devices from lightning and shock. The grounding system is based on

the specifications made by the inverter manufacture and the panel manufacturer. It

is always good to have a surge arrester installed near the switchboard where the

inverter terminal wires are connected. This will prevent loss of costly equipments

due to lightning and surges.

3.1.5 Under/Over Frequency Protection

The disruption of balance between supply and demand leads to frequency

fluctuations. The effect becomes more noticeable with higher penetration of PV

system into the grid. There is a wide variety of impact on the quality of goods


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produced from industries due to frequency fluctuations. It also stated that the

resonance from frequency fluctuations may damage small generators and a chain

reaction may lead to outage. International standards2require inverter to get tripped

in the event of frequency disturbances occurring either in its output or the gridfrequency.

The rapid variation in the output power of the PV due to cloud transient has a

significant impact on the frequency of the system. This is because the variation is

directly linked with the ramp rate and response time of the other generating plant.

Battery storage systems are the best solutions for controlling the frequency

disturbance due to load mismatch. Excess energy generated can be stored during

normal operation and when there is an imbalance the stored power can be

injected

3

.

3.1.6 Manual Disconnect Switch

Grid Connected PV systems are mandated in some parts of world to provide a

lockable manual disconnect switch which the grid operator may use to ensure that

there is no unintentional islanding. This ensures the 100% safety of the operators

working on the line. But IEEE 929-2000 standards say that the utility systems may

relax the requirement of a manual disconnect if the inverter is non islanding one.

The cost of this switch when compared to cost of entire system is negligible butwhen they are compared with returns of the system they are relatively expensive.

This device needs to be placed near the point of connection to the grid at an

accessible height. In the UL 741 standards the provision of the utility disconnection

switch is made mandatory.

3.1.7 Short Circuit Capacity

Short circuit capacity is amount of current that flows out of the system when a short

circuit fault occurs in the network (Figure 2). If the short circuit current rating is high

than the breaker capacity then the breaker will not be able to isolate the fault and

the result would be the damage of equipments. PV inverters which are certified

according to international standards have generally lower magnitude of fault

2Refer appendix 1B for IEEE 1547 and IEC6127 standards on Frequency Constraints.

3Refer appendix 3A for Battery sizing based on PV plant size.


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current of the order 1-1.5 times the rated capacity. But there is a problem with

upstream protection devices when there is a large penetration of the PV systems

the fault current from the upstream would be low enough for the devices to act and

hence leads to very dangerous situation of continuous flow of fault current.

Figure [2]

3.2 QUALITY STANDARDS

3.2.1 Harmonics

Harmonics is generally caused by non linear loads and generally viewed as

pollution to the electrical network. The power electronic converters used with the

Distributed generation systems are also a source of harmonics. Due to the

capacitive coupling between the ground and the point of interconnection with the

grid, the converter injects a capacitive ground current due to its switching action

which can cause a high EMI. Square wave and Quasi square wave inverters are

known to be major sources of harmonics. But majority of the latest technology

inverters use Sinusoidal pulse width modulation techniques and digital active

harmonic filters to reduce the production of harmonics at the output. The effect of

harmonics not limited to be overloading of capacitors, resonance with the system

leading to overstressing, overheating of magnetic cores of transformers and other

rotating machines. If the disturbance happens in the 3rd5thand 7thharmonic then

there would a large magnitude of current flow in the neutral wire which may have


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been rated for less current carrying in which case will lead to burning of cables.

Harmonics create problems of interference with telephone lines, broadcasting

instruments. Electronic devices such a series reactor or static capacitors are

installed in the distribution station to reduce the level of harmonics.

IEEE 519 Section 11.54recommends that the voltage distortion limits, as a

percentage of nominal fundamental frequency voltage in the utility service, should

not exceed 5% of the total voltage harmonic distortion and 3% of any individual

harmonic. Spectrum analyzers are devices used for measuring the harmonics

produced by source. Standards require the measurement to be made with the

instruments which has good response up to 25thharmonic. The tests on the

inverter should be carried out under clear sky conditions. Certified inverters are

exempted from testing it on site.

3.2.2 Power Factor

Power factor is the ratio of the real power delivered to the apparent power. PV

inverters when operated at unity power factor impose the grid to supply all the

greater proposition of reactive power and hence reducing the power factor at the

distribution station. But it has an added advantage of reducing the transmission

losses since there is no or lesser real power transmission along the line. (Figure: 3)

The inverter should operate between 0.8 leading and 0.95 lagging for outputs

ranging from 20% to 100% of the rated VA. IEC 6127 requires the inverter system

to have a lagging power factor of 0.9 when the power is greater that of 50% of

rated power. The basic criterion for islanding is that the PV should supply the real

and reactive power of the load. The other reason to impose restriction on power

factor is to prevent islanding.

4Refer Appendix 2A for IEEE 519 on Harmonic Restrictions.


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Figure 3

3.2.3 Flicker

Increase or decrease in voltage for a short period of time is called flicker. There are

no standards defining flicker in numerical terms. The effect is based on the comfort

level of the user. It is considered objectionable when it causes the modulation of

lighting level sufficient to cause irritation to human eyes

The term flicker is very subjective and depends on the magnitude of voltage and

frequency. The lesser the frequency of flicker greater is its effect. A potential

source of flicker from PV systems is during startup and shutdown of the system.

The IEEE 1547-2003 allows 5% voltage fluctuations during the synchronization

part of the DG (Distributed Generation) system. There is a general concern that the

fluctuation in PV output would be a major cause for flicker in the output.

3.2.4 Voltage Imbalance

Voltage imbalance is a condition in which the amplitude of each phase voltage is

different in a three phase system or the phase difference is not exactly 120.

Voltage imbalance will generate current with twice the frequency and a backward

magnetic field in three-phase synchronous machines, and will have a negative

impact on generators, such as temperature rise of rotors, noise, and vibration. It

will also have an impact on induction machines and power electronic devices.

Generally for 33KV the voltage imbalance shall not exceed 3%. The phase current


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imbalance according to IEEE929-2000 should be less than 5% measured at 50%

and 100% of the rated current.

3.2.5 DC Injection

Transformer-less inverter inject DC current at the output of the AC terminal. IEEE

1547 insists that the injected DC current must be lesser 0.5% of the rated capacity

at the point of delivery. For a three phase inverter the injected DC current

measured between phase to phase must not exceed 0.5% or 5 milli-amperes,

whichever is greater. The limits is based on the harmonic standards and the limit of

the distribution transformers which can tolerate about 0.5% of the dc current

without its core getting saturated and the residual current devices (RCD) used forprotection (25 mA). When a number of inverters are connected to the grid in

parallel then their combined value should be less than 0.5% of the distribution

transformer used.

IEC 61727 insists that the dc injection shall not be greater than 1% of the rated

inverter current when the inverter is operating at the maximum power point.


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Appendix 1

A. Voltage Constraints for Grid Connected PV Inverters

The IEC 6127 guidelines values are to be satisfied when the inverter is running at 50% of therated power.

Voltage Condition

(% Nominal Voltage)

Maximum time to Disconnect

(IEC 6127)

V < 50% 0.1 Sec

50% V < 85% 2 Sec

85% V 110% Continuous Operation

110% V 135% 2 Sec

V > 135% 0.1 Sec

Voltage Condition

(% Nominal Voltage)

Maximum time to Disconnect

(IEEE 1547)

V < 50% 0.16 Sec ( 8 Cycles)

50% < V< 88% 2 Sec (100 Cycles)

110% < V < 120% 1 Sec ( 50 Cycles)

V > 120% 0.16 Sec (8 Cycles)


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Appendix 1

B. Frequency constraints for Grid Connected PV Inverters

Frequency Range

Maximum Clearance Time(IEEE1547)

F > 60.5 Hz0.16

F < 57.05

0.16

59.8 < F < 57.06

Adjustable ( 0.16 and 300)

559.3 if DER 30 kW.

6For DER > 30 kW


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Appendix 2

A. Harmonic Content Limitat ions on Grid Connected PV Inverters

IEEE 519 Sec 11.5

Total harmonic distortion : 5.0%

Maximum Distortion

Harmonic Number Even Harmonics Odd Harmonics

h < 11 1.0% 4.0%

10 < h < 17 0.5% 2.0%

18 < h 35 0.1 % 0.3%


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Appendix 3

A. Battery Sizing fo r Resolving Clou d Transient Issues.

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PRF - PV - Technical Note on Solar Grid Integeration Ver 1 0