Neutral Connections and Effective Grounding

1 Introduction Neutral Connections and Effective Grounding are not recommended to mitigate temporary overvoltage when using listed photovoltaic inverters. Millions of dollars are being wasted because power companies are attempting to mitigate temporary overvoltage (TOV) from photovoltaic inverters using techniques designed for synchronous generators. This paper lays out fundamental differences between the two power generation technologies and associated differences in line-to-ground voltage during faults. It explains why IEEE 142 Effective Grounding requirements do not work in PV inverter systems and proposes a sound, cost- Effective way to ground PV modeling distribution-connected photovoltaic power systems, focusing on TOV during line-to-ground faults on both the distribution line and the low-voltage customer system, this paper examines how various configurations of distribution transformers and Grounding of the inverter isolation transformer affect TOV.

2 Laboratory tests validate the Over-VoltageTemporary overvoltage (TOV) poses a serious hazard to equipment connected to a power grid. TOV can occur during a ground fault such as a tree limb falling on a power line. In distribution and transmission lines fed by synchronous generator power sources, utilities traditionally have mitigated this danger using a technique called IEEE 142 Effective Grounding . In their efforts to maintain safe and reliable power systems, utilities have attempted to apply the same standard to photovoltaic inverter-based generation. However, it is impossible to make a PV system comply with IEEE 142 as currently written. The attempt to do so diminishes the effectiveness of utilities protective systems and wastes power and different causes of TOV must be delineated in order to evaluate the effectiveness of applying those standards in PV Connections and Effective GroundingCONTENTS Introduction Page 1 Temporary Over-Voltage Page 1 TOV Mechanisms Page 2 Important Mechanism of TOV in PV Systems Page 2 IEEE 142 Effective Grounding Page 3 Differences Between Generators and Inverters Page 3 Neutral Connection Generators vs Inverters Page 3 Why Most Inverters Do Not Have a Solid Neutral Connection Page 4 Current Source vs Voltage Source Generation Page 4 Switching High Generation Into

3 Light Load Page 5 Overmodulation or Current Control Saturation Page 6 Conclusion Page 62 TOV MechanismsThe mechanisms that cause TOV can be divided into six categories: i 1 Ground potential rise 2 Derived Neutral shift 3 Inductive coupling of fault currents 4 High generation to load ratio 5 Interruption of inductive currents 6 Over-modulation / saturation of current controlsGround potential rise occurs when large currents flow into Grounding electrodes. The resistance between the Grounding electrode and remote earth results in a voltage rise between the local ground reference and other more distant ground references. Derived Neutral shift occurs when one phase of a distribution line is faulted to ground. If the substation breaker opens in response to the fault, the distribution lines lose their ground reference and the phase conductors float with respect to ground.

4 If the distribution line is being backfed by a synchronous generator, the phase-to-phase voltage is maintained even when one of the phases is at zero potential to ground. For the unfaulted phases, that means the phase-to-ground voltage (and therefore the phase-to- Neutral voltage) can be the same value as the phase-to-phase voltage. In other words, the Neutral point can shift so that the phase-to- Neutral voltages on the unfaulted phases are equal to the phase-to-phase voltage. When derived Neutral shift occurs, devices connected phase-to- Neutral or phase-to-ground can be subjected to as much as times their rated voltage. The condition can persist until the fault clears, the distributed generation source trips off line, or protective devices separate the generation source from the fault.

5 The Effective Grounding standards in IEEE 142 are primarily designed to mitigate this TOV coupling of fault currents in faulted phases or the Neutral and unfaulted phases can induce voltages in the unfaulted phases. Because fault currents supplied by central generation can be so large, inductive coupling is the dominant TOV mechanism during a line-to-ground fault while the substation breaker remains closed. Once the substation breaker opens, TOV is dominated by other generation to load ratio, also known as load rejection, occurs when a significant portion of the load becomes separated from the generation source. If a switch, breaker or line-sectionalizing device opens, and the remaining connected load is lower than the output of the distributed generation system, a voltage rise can can occur during load rejection when the inverter current control loop saturates, or goes to a maximum duty cycle, leaving the (typically IGBT) switches on for long periods.

6 Under these conditions, the output voltage can approach the open-circuit voltage of the DC source times the transformer (if present) winding ratio. This can be a very high voltage, on the order of to 3 times the nominal peak instantaneous AC output voltage. If present, this condition would persist until the inverter trips off due to phase overvoltage. However, most inverters have protections against this possibility. Important Mechanisms of TOV in PV SystemsWork by Michael Ropp at Northern Plains Power Technologies showed that the dominant TOV inducing mechanisms in distributed generation systems are mechanisms 2 (derived Neutral shift) and 4 (high generation to load ratio). Because it is relevant to the topic of TOV, current control saturation will also be discussed.

7 What remains to be seen is whether or not synchronous generator Effective Grounding techniques mitigate TOV when applied to PV PaperFigure 1 - Illustration of derived Neutral 142 Effective GroundingIEEE 142 (the Green Book ) is a well-established standard that describes how to ground industrial and commercial power systems. This standard provides the following definition for providing Effective Grounding of generators: The zero sequence reactance of the Grounding source must be positive and greater than three times the positive sequence reactance of the generator. In addition, the zero sequence resistance of the Grounding source must be positive and greater than the source reactance of the is clear from the text of the standard that it was intended to be applied to rotating machine generators.

8 Significant problems arise when attempting to apply IEEE 142 Effective Grounding when the generation source is an Between Generators and InvertersThe physical characteristics of inverters are very different from those of generators. Generators have large reactance because they are constructed from massive coiled conductors with magnetic cores. The typical X/R ratio for a generator is on the order of 30 to 50. For this reason, the restive portion of a typical generator impedance is ignored because it is so small when compared to the reactance. By contrast, inverters have essentially no reactance. Only the relatively small choke inductors and the isolation transformer leakage inductance contribute any positive reactance to the circuit some basic assumptions are made based on their short circuit characteristics, it can reasonably be approximated that a typical PV inverter has an X/R ratio of to It would therefore be reasonable to ignore the reactive portion of inverter impedance.

9 The reactance of a typical PV inverter is essentially on their short circuit current characteristics, the resistive portion of inverter impedance would then have to be quite high to explain their relatively low output fault current (in the range of 1 - 2pu). Typical resistive impedance values are in the range of - we attempt to strictly satisfy the Effective Grounding equations from IEEE 142 using inverter reactance values, the equations become mathematically impossible to solve: The equations would have to be modified to have any kind of logical application to PV systems: The IEEE 142 Effective Grounding standard cannot be applied to PV inverters in a straightforward fashion. This raises the question of whether Effective Grounding is even applicable to Connection Generators vs Inverters Synchronous generators are grounded by making a solid (low-impedance) connection between the generator Neutral and ground.

10 However, no inverter with a solid Neutral connection that has been fully tested and listed to standard UL 1741 in this configuration - is offered for sale in North America. This begs the question, why do inverters not have solid Neutral Connections ?White PaperFigure 2 - Typical PV inverter equivalent circuit. Why Most Inverters Do Not Have a Solid Neutral ConnectionPhotovoltaic inverters are designed and intended to operate as balanced, 3 phase current sources. Therefore, a Neutral conductor is not necessary for the export of power. Since the Neutral conductor is not actually necessary, most inverters do not even have terminals for a Neutral inverters which measure voltage phase-to- Neutral do not have solid connection between the isolation transformer Neutral output and the Neutral terminal.