Effective Grounding for PV Plants

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Effective Grounding for PV Plants Il Do Yoo, Soonwook Hong, Michael Zuercher-Martinson th June 6 , 2012 I. INTRODUCTION

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ITH the onset of high penetration of photovoltaics (PV), more utility companies are starting to look at PV plants the same way they would look at other major generators. The operational aspects and influence of a PV plant on the distribution network can be compared to other conventional generators as follows:

• A PV plant is comprised of inverters utilizing power semiconductor switches and microprocessors. Abnormal operation can be detected instantaneously by the control processor and the system can be protected with response times that are much faster than those of conventional generators. Many protective relay functions are directly built into the PV inverter. • A PV inverter does not have rotating parts and hence no mechanical inertia. During a grid fault condition, the inverter short circuit current is equivalent to or less than its rated current and the inverter can disable its operation within one cycle.

network. The purpose of a grounding reactor is also discussed for the reliable operation of overcurrent relay protection. Throughout this document, the voltage rise in the distribution line due to the line capacitance is ignored to simplify the analysis. II. DISTRIBUTION LINE FAULTS AND GROUNDING During normal operation, the star point voltage in a three phase system is close to zero, regardless of whether the neutral point is tied to the earth ground or not. When a three phase system with an ungrounded neutral experiences a fault condition, three phase voltages and currents may no longer be balanced; the electrical virtual neutral voltage can become significantly higher than zero and cause overvoltage in the phase voltages. A single line-to-ground fault is the most common fault type in the distribution network and can generate severe overvoltage conditions, and hence is used to analyze the overvoltage generation as shown in Figure 1.

Due to these inherent characteristics, PV inverters can effectively meet the IEEE 1547 utility interface requirements and the required breaking capacity of overcurrent protection devices is driven only by the impedance of the distribution network and the inverter’s integrated transformer in a single phase-to-ground fault scenario. When a PV plant is installed at the point of common coupling (PCC) of a private network or inside the utility territory, the plant shall meet the IEEE 1547 standard and the interface requirements of the local utility company. Some utility companies require PV inverters to have AC side grounding in order to assure compatibility with their grounding scheme, generally referred to as effective grounding. There are also cases in which utility companies do not require a specific grounding of the PV inverters, which raises the question: When is effective grounding needed? This article explains how grounding in the distribution network works, why utilities ask for effective grounding and elaborates on different fault protection and PV plant grounding schemes. The fault current paths with different transformer configurations are analyzed within the sequence

a) Circuit Configuration

b) Vector Diagram Figure 1. Single Line-to-Ground Fault on an Ungrounded System 1|P a g e

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Before the fault condition, three phase voltages are well balanced and the transformer star point voltage is near zero (VN = VG) as shown in the left side of Figure 1. b). When a single phase to ground fault is applied to phase A, phase A becomes grounded and the voltage vector diagram is shifted down as shown in the right side of Figure 1. b). As a consequence, phase B and phase C voltages are increased from line-to-neutral to line-to-line. This overvoltage may damage distribution assets and customer loads. Figure 2.a) shows a similar vector diagram on an ideal grounded system with the same single line to grounded fault. The transformer neutral is solidly tied to the ground so that the neutral potential does not change with a fault applied. In consequence, the phase A voltage vector falls down to the neutral point and the new voltage vector is established as shown at the right side of Figure 2.b). The phase B and C voltage magnitudes remain at their nominal values in contrast to the ungrounded case and no overvoltage is generated.

grounding scheme to the DERs for the case of an islanded operating condition in combination with a single fault condition within the island and during field switching of the substation feeder breaker. If the DER is designed without a neutral terminal and cannot provide effective grounding, the utility can recommend installing a grounding transformer as an alternative. III. OVERCURRENT PROTECTION ON THE DISTRIBUTION LINE The power distribution system is protected from faults by fuses, reclosers and circuit breakers that are coordinated with protective relays. Fuses are self-destructive protection devices that can interrupt the flow of excessive current when they are overloaded. Reclosers are self-contained circuit breakers that are rated to interrupt the available fault current and designed to reduce the outage duration during temporary faults. Protective relays sense abnormal or fault conditions, such as an overcurrent, under- or overvoltage, and send a signal to trip a circuit breaker to protect the system by isolating it from the faulted section. Figure 3 shows a typical overcurrent protection based scheme in a distribution line. In the feeders and transformers, the 50/51 relays detect overcurrents on all phases, and the 50N/51N relays monitor the current mismatch of all phases.

a) Circuit Configuration

b) Vector Diagram Figure 2. Single Line-to-Ground Fault on a Grounded System In North American power distribution, the four-wire multi-grounded neutral system is predominant, which is less susceptible to a line-to-ground fault induced overvoltage scenario as described above. Utility companies can require manufacturers to follow the same grounding scheme when a new Distributed Energy Resource (DER) is introduced to their network. The purpose of the requirement is to extend the

Figure 3. Typical Distribution Line Protection Scheme

IV. EFFECTIVE GROUNDING According to the IEEE 142 standard, a conventional circuit is considered to be effectively grounded when the power system constants are X0/X1 < 3 and R0/X1 < 1, where X0 and R0 are the zero sequence reactance and resistance, and X1 is the positive sequence reactance. By limiting these constants to within these boundaries, any overvoltage caused by a neutral shift is safely limited to 125% of the nominal voltage, and the ground fault current can be kept between 60% to 100% of the 2|P a g e

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available three-phase fault current. Typically, effective grounding is accomplished by directly connecting the neutral point of the wye side of power transformers to the station ground. Solid grounding is almost always used at the 480VAC voltage level, therefore commercial PV plants will most likely to be installed in an effectively grounded network. Figure 4. shows a case in which a single line-to-ground fault is applied to a system with two different medium voltage transformers (Tr2 and Tr4) connected to two identical PV inverters with internal isolation transformers (Tr3 and Tr5). This arrangement is used to illustrate how different medium voltage transformer winding configurations affect the flow of fault current through the inverters and the effectively grounded plant.

Figure 5. Zero Sequence Circuit During a Fault Condition (impedance in p.u., 1MVA base) Figure 6. shows the case of a single line-to-ground fault applied to a system with two identical wye-to-wye medium voltage transformers connected to two PV inverters with different grounding schemes. This case illustrates the influence of effective grounding of the inverter’s internal transformer on the flow of fault current.

(a) With a Grounded Neutral

Figure 4. Four-Wire Multi-Grounded System Figure 5. shows the zero sequence equivalent circuit during a fault using typical distribution line and 500kW PV plant parameters. The investigation of the zero sequence impedance network reveals that the majority of the fault current will flow into the higher voltage network as its associated impedances (X10 and XT0) are lower than that of the PV plant circuits downstream (X20 and X30) and will trigger the upstream overcurrent protection relay. Nevertheless, about 20% of the fault current flows into the PV Inverter 1 through the grounded network. The overcurrent protection device inside the PV inverter will detect the fault current and disconnect the inverter from the fault if it is sufficiently higher than the normal inverter operating current. If the fault current to the PV inverter (through X20 and X30) is high enough to interfere with the reliable operation of the protective relay, it can be reduced by adding a grounding reactor at the ground wye terminals of Tr2 or Tr3. In the meantime, no fault current can flow into PV inverter 2 because the delta connection in Tr4 does not provide a fault current path in the zero sequence equivalent circuit.

(b) With an Ungrounded Star Point Figure 6. Inverter Transformer Configurations Comparison As shown in Figure 7.a), the fault current can flow through Tr3 (X30) which is effectively grounded; whereas in Figure 7.b), Tr5 (X50) will not provide a current path in the zero sequence equivalent circuit. In both cases, the system does not experience an overvoltage as the medium voltage transformers already provide effective grounding regardless of the inverter transformer configurations.

a) With a Grounded Neutral

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b) With an Ungrounded Neutral Figure 7. Zero Sequence Circuit During an Islanded Condition (impedance in p.u., 1MVA base) In some cases, the utility may want to cover the case in which the inverters might run in an islanded condition or during breaker field switching operation with other customer loads on the same circuit. In this special case the utility side effective grounding system is no longer present and the inverters may be required to provide the effective grounding. Only PV Inverter 1 in Figure 6.a) provides effective grounding when the CB is open. PV Inverter 2 in Figure 6.b) does not provide effective grounding and may have a potential to generate overvoltage on assets within the island if the anti-islanding detection was turned off or failed. For new construction sites, a customer-owned, dedicated grounded wye-to-delta medium voltage transformer is preferred (see Figure 8.). The absence of a path for the fault current from the medium voltage side to the PV plant side will automatically satisfy most utility requirements for effective grounding. This transformer configuration is the most economical one for dedicated PV plants because of the copper savings on the eliminated neutral wire and the shortened protection scheme review time.

• Most of the existing 480VAC systems in the United States use a neutral conductor to accommodate the single phase loads and to prevent over-voltages at the load during an unbalanced fault condition. If a DER without neutral grounding is installed in the same network and operated in an islanded condition together with customer loads, it is capable of generating an overvoltage in a single line-to-ground fault scenario. For three phase four-wire systems, the PV inverter shall provide the effective grounding by means of tying the transformer neutral to the utility ground. • If the utility requires effective grounding, the neutral point of the Solectria inverter’s internal transformer can be solidly tied to ground. The products are designed to handle the higher short circuit currents and additional voltage harmonic heating effect that can result when a neutral wire connection to the inverter is made. Inverter user manuals include information on the landing terminal and the wire gauge that can be used. • In some cases, the utility company may request to install a grounding reactor between the neutral point and ground to provide for safe current relay operation on the grid side by reducing the fault current flow through the inverter. Such an impedance grounding configuration is permitted and will not adversely affect the inverter operation. • If PV inverters are installed at a private network without grounding (three wire system), the inverters provide the same functionality without compromising safety or normal operation of the inverter. • Finally, when a dedicated wye-to-delta medium voltage transformer can be used, a three wire connection to the PV inverters is the most economical solution.

Figure 8. System Diagram with Dedicated Wye-to-Delta Transformer

V. SUMMARY

REFERENCES 1. IEEE Std 142-2007, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems. 2. IEEE Std 1547-2003, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems.

Solectria Renewables’ large commercial inverters are available with a fourth power terminal for neutral connection where required to satisfy effective grounding requirements. The following guidelines can be used to determine whether a neutral connection is necessary or not: 4|P a g e