Improving the Protection of Distribution Systems with DERs

Author: Alexander Apostolov, USA

Effects of Short Circuit Faults  

The reduction of the impact of short circuit faults on distributed energy resources (DER) can be achieved in several different ways. Like any other problem that has to be solved, we need first to understand the nature of the problem and its effect on the wind generators.

The most common short circuit faults in the system - single-phase to ground faults are characterized by the fact that they introduce a voltage sag in the faulted phase, and at the same time they result in a voltage swell in the two healthy phases.
This is clearly seen in Figure 2 that shows the recorded waveform of voltages and currents for a single-phase to ground fault.
The first characteristic of a voltage sag - the depth - is a function of the type of fault, fault location and the system configuration. This is something that we cannot control, but we have to study in order to be able to predict or estimate the effects of different faults on the wind generators

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The second characteristic of the voltage sag - the duration - is the parameter that we can control by properly applying the advanced features of state-of-the-art multifunctional protection relays. Monitoring the changes of the power system configuration and adapting the relay to these changing conditions can further improve the performance of the relays and reduce the effect of short circuit faults on the DERs.

Using high speed peer-to-peer communications to implement protection schemes will also lead to a significant reduction in the fault clearing times that can be handled by the wind turbine generators.
In order to define the capabilities of wind turbine generators to withstand the effects of short circuit fault conditions many countries have developed and implemented national codes that require them to be able to remain in operation for voltage drops to a certain percentage of the nominal value for a specified period of time. Such requirements are known as Fault Ride Through (FRT) characteristic.

Considering the example of a FRT characteristic of a wind generator shown in Figure 1, it is clear that we need to revisit the distribution protection principles implemented in the last one hundred years and look into more advanced distribution protection schemes that will make it possible to reduce the fault clearing time in order to help the wind generators to remain in service for the duration of the fault. This is not that critical if the generator is connected to a faulted feeder, but is especially important for the wind generators connected to a part of the distribution system that remains healthy after the clearing of the fault.

Distribution Feeder Protection
Typical distribution feeder protection is based on non-directional phase and ground overcurrent relays set to protect the line for three-phase, phase-to-phase or phase-to-ground faults.
On distribution feeders it is common that an instantaneous relay is used to operate for close-in faults and a time overcurrent relay with inverse characteristic provides protection for most faults on the line. The time overcurrent relay has to coordinate with any fuses used to protect distribution transformers connected to the feeder. The coordination requirements for high current faults result in significant increase in the operating times for faults further down the feeder, with the longest times for faults at the remote end.

In order to reduce the number of electromechanical or solid state relays, backup protection for bus faults or breaker failure has been traditionally provided by the transformer protection relays. Considering the fact that they also have to coordinate with the feeder relays, it is obvious that the operating times for bus faults or feeder faults with breaker failure will not meet the requirements defined in the FRT characteristics. (Figure 3).
One of the first required changes to the protection of distribution feeders that have DERs providing fault current contribution is that the overcurrent protection will need to have directional supervision in order to avoid undesired operation for faults on adjacent feeders.
State-of-the-art multifunctional protection relays have many features that allow significant improvements in the performance of the relays under different short circuit fault conditions.

Definite Time versus Inverse Time Overcurrent
Modern distribution feeder protection relays have multiple phase and ground overcurrent elements that can be used to reduce the operating time of the relay for different fault conditions. Figure 3 shows the inverse time characteristic of a phase overcurrent relay with the operating points for different fault locations on the protected feeder. The characteristic is coordinated with a downstream fuse.

Let?s start with organization in protection testing