The effective solar grounding conundrum

Utilities are increasingly requiring effective PV-plant grounding to limit risk of temporary overvoltage, but their regulations don’t necessarily translate to the solar inverter market.

By Emily Hwang, Senior Applications Engineer, Yaskawa – Solectria Solar

With the increase of PV generation on the grid, utilities are growing more concerned about the risk of temporary overvoltage (TOV) possibilities on their lines. Significant TOV can cause great damage to distribution systems and connected equipment—on both sides of the meter. Some utilities, such as PG&E, already employ overvoltage protection for PV systems. Others that have not—NGRID, XCEL, HECO and PacifiCorp—are increasingly turning to effective grounding as their TOV solution. Effective grounding uses impedance grounding, via the use of grounding banks or reactors, to limit the fault current while allowing a limited and safer amount of overvoltage.

The figure shows the before (left) and after (right) effects of a ground fault on the phase voltages (VA, VB, VC) and line-to-line voltages (VAB, VBC, VCA) for ungrounded, solidly grounded and effectively grounded systems. In this example, a line to ground fault occurs on phase A. For ungrounded systems, there will be no fault current; however, single phase loads may be damaged due to overvoltage. In solidly grounded systems, there will be high fault currents that could damage equipment and some fault protection relays may be desensitized. Finally, with effectively grounded systems, the impedance limits overvoltage to <1.39 p.u., eliminating harmful overvoltage and high fault currents. Effective grounding also helps protection relays to recognize faults.

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In the utilities’ attempt to protect their distribution lines from TOV, some require PV plants to abide by the same effective grounding requirements as conventional generator plants.  The difference between conventional generators and PV inverters is important to note­­­ since IEEE 142 (the Green Book) defines “effective grounding” as the ratios between the zero sequence reactance (X0) and the zero sequence resistance (R0), with the positive sequence reactance (X1) as follows:

0 < X0/X1 <= 3,   0 < R0/X1 <= 1

These definitions for the sizing of a PV systems’ effective grounding solution do not take into consideration the lack of an industry-standard definition for the output impedance of an inverter. Impedance is the AC equivalent to DC resistance, but it contains both a magnitude component (resistance) and a phase component (reactance).  For conventional generators, otherwise known as synchronous generators, measured physical winding impedance from the rotor and stator windings is used to calculate for X1.  Because the inverter has little to no rotational inertia and no winding impedance there exists no one-to-one industry accepted value equivalent for X1 in PV inverters. Some inverter companies use rated voltage and the measured maximum output current during a fault condition to derive X1 (Vrated/Irated). Other inverter companies use the output filter impedance.

This lack of consistency generates confusion for utilities hoping to use the IEEE definition for effective grounding. To avoid this, some utilities, such as HydroOne and Green Mountain Power, acknowledge the difference between synchronous generators and inverters, and define zero sequence reactance without the use of the positive sequence reactance, as seen in the definition below.

X0 = 0.6 ± 10% p.u.

Another point of contention stems from a fundamental difference between conventional generators and grid-connected PV inverters. Conventional generators can be regarded as voltage sources; inverters, on the other hand, are considered current sources where the plant’s terminal voltage is dependent on the grid. This difference is important because it raises the question of whether a PV inverter could generate significant TOV. ­­The National Renewable Energy Laboratory (NREL) is currently performing comprehensive testing and research into inverter load rejection overvoltage and inverter ground fault overvoltage testing. The hope is with more data the PV industry will be able to prove the need for a standard method to protect utility lines from TOV, or disprove the need for effective grounding on PV systems entirely.

Until then, it is the installation company’s responsibility to know whether their utility calls for effective grounding. While these requirements are more common for larger systems, they’re not unheard of on smaller projects. Before starting to budget for a PV project, be sure to understand the local utility’s effective grounding requirements for the specific project size and location/substation. Ignoring this possible requirement can lead to expensive retrofits, change orders and delays. For most utilities, these requirements can be found in the interconnection policy.

For more information, Yaskawa – Solectria Solar offers an effective grounding design tool on its website to help calculate the size of the grounding bank or reactor. You can also learn more in its white paper Effective Grounding For PV Plants.

References:

Hong, Soonwook; Yoo, Il Do; Bruno J. M., Terry; Zuercher-Martinson, Michael. Solectria Renewables. Effective Grounding for PV Plants.

IEEE Std 142-2007, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems.

Advanced Energy. Neutral Connections and Effective Grounding.

Nelson, A.; Hoke, A; Chakraborty; Chebahtah, J; Wang, T; Zimmerly, B. National Renewable Energy Laboratory and SolarCity Corporation. Inverter Load Rejection Over-Voltage Testing (SolarCity CRADA Task1a Final Report). February 2015.

Hoke, A; Nelson, A; Chakraborty, S; Chebahtah, J; Wang, T; McCarthy, M. National Renewable Energy Laboratory and SolarCity Corporation. Inverter Ground Fault Overvoltage Testing. August 2015.