Banking on grounding of solar and wind farms

One more item to add to the list of technical items to consider when interconnecting solar and wind farms is grounding. The grounding issue often appears when (a) integrating solar power inverters or wind turbines to existing distribution circuits, or (b) designing collector systems for solar and windpower farms.

Most medium to large scale solar inverters and wind turbines are supplied or sold as three-phase, delta or wye-ungrounded systems. This means their electrical systems have no path to ground of their own. And therein lies the concern.

When interconnected with grounded or four-wire systems, the inverter or turbine (generalized, for purposes of this article as distributed generation, or “DG“) poses a potential for very high ground fault overvoltages from the phenomenon known as neutral shift.

The way that neutral shift comes about can be described in the following sequence of events:
  1. One phase of a three-phase system is shorted to ground, as may occur if a conductor fails and comes into contact with a grounding path such as a tree limb.
  2. The fault is detected by the system side circuit breaker, which then opens, islanding the portion of the distribution or collector circuit.
  3. The DG remains energized on the islanded section for some duration.

This is illustrated in the figure below.

The resulting voltage on the unfaulted phases of the islanded section is 1.73 times the nominal voltage.

If capacitance is present, as in the case of underground cables, the voltage may be even higher.

The overvoltage, depending on the duration that it is present, may damage insulation, surge arresters, and any customer load connected to the island. The longer the overvoltage remains, the higher the likelihood of damage and failure to equipment. In addition, there are safety issues for both utility workers and the public.

The overvoltage is cleared when all the DG on the island shuts down.

The phenomenon manifests itself differently in existing distribution circuits and in farm collector systems, so we will tackle each separately.

Existing Distribution Circuits

On a distribution feeder, the DG may stay energized on an islanded circuit for a few milliseconds up to 2 seconds. (IEEE Std-1547 requires that the DG cease to energize within 2 sec.)

The magnitude of overvoltage is reduced by the presence of customer loads on the islanded circuit. In fact, if there is enough customer load, the overvoltage may remain below the limit set by industry standards of 1.25 to 1.35 times the nominal voltage, and thus be acceptable. Generally, a load-to-generation MW or kW ratio, measured during light load conditions, of about 3 sufficiently minimizes potential overvoltage from ground faults.

Solar and Wind Farm Collector Systems

On a collector system, the overvoltage condition may remain even longer if the DG is equipped with fault ride-through capability. If the collector system uses underground cables, the capacitance of the cables may increase the magnitude of overvoltages. Even if the collector system does not use underground cables but has overhead cables instead, an increase in voltage may still occur if the current on the cables leads to capacitive operation.

Countermeasures

The basic solution to the issue is to provide a path to ground for the ungrounded island. This is generally accomplished by providing a grounding bank.

A grounding bank is a three-phase transformer connected wye-grounded/delta which is used to provide a ground source for the primary neutral wire.

Typical criteria for specifying grounding banks are:

  1. During a ground fault, the transient overvoltage should be limited to about 1.25 PU — 1.35 PU, where PU means per unit or 100% of nominal value.
  2. The ground current in the grounding transformer should be within ANSI transformer withstand limit.
  3. In order to avoid loss of sensitivity, ground fault current from the grounding transformer should be approximately 10% or less than the current from the distribution or collector circuit.

The grounding bank is applied to each distribution or collector circuit that is protected by a breaker or switching device. Hence for a wind farm with three collector circuits, each protected by a circuit breaker, three grounding banks will be needed. The transformer does not carry any normal load, so its size need only be sufficient to handle the short-term current of a line-to-ground event.

An alternative to the two-winding transformer for the grounding bank is the zig-zag transformer.

If adding a transformer is not possible for space or cost reasons, another type of countermeasure is to use distribution circuit or collector breakers that have the crowbar feature. This is essentially a low resistance short circuit applied to the circuit side as the breaker opens.

Another method for addressing ground fault overvoltage is a transfer trip scheme that trips out all DG on the islanded circuit once the ground fault conditions are present.

Conclusions

Manufacturers give as a reason for providing ungrounded DG, the cost and reliability of their products. However, as consequence of that savings, the risk of transient overvoltages need to be addressed when interconnecting DG with existing distribution circuits or as part of a farm collector system. The countermeasures may take the form of additional equipment such as grounding transformers or crowbars, or operating measures such as transfer trip schemes.

Essentially, there are added costs associated with addressing the grounding risk.