Effective Grounding for PV Inverters: What You Need to Know
While only a handful of utilities nationwide currently require effective grounding for three- phase commercial photovoltaic (PV) installations, that number is growing. As more PV projects come online, more utilities (ie. NGRID, HECO, XCEL, PEPCO, BGE, etc.) are looking for methods to mitigate temporary overvoltage (TOV) from PV inverters. Yaskawa – Solectria Solar is an expert on this topic since we have worked with the first utilities to help customers size their grounding banks. We’ve created an Effective Grounding Design Tool to help calculate the impedance of grounding devices, but more on that later.
Protection requirements: Not one size fits all
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. This can lead to oversized protection requirements given to PV installers. 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
Pacificorp (operating as Pacific Power & Rocky Mountain Power) states that in addition to IEEE requirements, the X/R ratio of the grounding bank used for effective grounding shall be greater than or equal to four. These definitions for the sizing of a PV systems’ effective grounding solution do not take into consideration that there lacks an industry standard definition for the output impedance of an inverter. For conventional generators, otherwise known as synchronous generators, measured physical winding impedance is used for X1. Since the PV inverter has little to no rotational inertia and no winding impedance, 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.
“These definitions for the sizing of a PV systems’ effective grounding solution do not take into consideration that there lacks an industry standard definition for the output impedance of an inverter.”
This lack of consistency generates confusion for utilities hoping to use the IEEE definition for effective grounding. In order 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; while 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. Hopefully, with more data the PV industry will be able to prove the need and standardize on the best method to protect utility lines from TOV, or disprove the need for effective grounding on PV systems entirely.
Measure twice, design once
Before starting to budget for a PV project, be sure to understand whether the local utility has an effective grounding requirement and if so, be sure to understand what is required 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 example, Pacific Power’s policy states that for projects with inverters incapable of stand-alone operation, rated over 10kW, and listed to UL1741, the installation may require effective grounding depending on installation location and local generation penetration. It also states that effective grounding is required for all three-phase distributed energy resource facilities with the potential to carry more than 10% of the electric power system’s minimum load. In conclusion, one of the most important things to do in regards to effective grounding is to be clear on your utilities’ requirements before solidifying your project budget.
The Effective Grounding Design Tool from Yaskawa - Solectria Solar is useful in calculating the impedance of grounding devices - namely grounding transformer banks or neutral grounding reactors, commonly employed in effective grounding for PV plants and in estimating the neutral current with the given impedance. This tool can be used for the following calculations:
- Impedance of the neutral grounding reactor or grounding transformer bank
- Voltage Imbalance Contribution to ground current at POI
- Grid contribution to a ground fault
- Inverter contribution to a ground fault
See more of our Effective Grounding tool here: http://solectria.com/support/effective-grounding-design-tool/
See our Effective Solar Grounding Conundrum article in Solar Power World here: http://www.solarpowerworldonline.com/2016/04/effective-solar-grounding-conundrum/
See our Effective Grounding: Oregon's New Frontier paper written for the Oregon solar market here: https://solectria.com/site/assets/files/1990/effec...