by Alex Apostolov, Editor-in-Chief

The Interdependence between Protection and DERs

The world of electric power systems protection has been pretty well established from the protection and control point of view. Even with the wide acceptance of microprocessor based multifunctional protection IEDs and communications between them, the principles of protection and the methods to calculate their settings have not changed dramatically.

This is because the electric power system itself did not change that much - large synchronous machines in power plants connected to the transmission grid and radial distribution systems connected through substations to the transmission lines.  

We know how to deal with this, because we have been doing it for more than a century. We know how to calculate the fault current contribution of synchronous generators and how to analyze their dynamic behavior during wide area disturbances. We know how to coordinate the transformer protection with the distribution feeder protection and the fuses and reclosers - it is so simple on a radial feeder. However, the world has changed and we need to learn and adapt in order to handle the new challenges. There are several things that we need to take into consideration:

The requirements for the fault clearing time in the past were defined by the requirements to maintain system stability. That is why we had to implement fast fault clearing at the bulk and transmission levels of the electric power system.

At the distribution level we could live with fault clearing times in the range of several seconds, even when we started being concerned with the quality of power. With the large number of Distributed Energy Resources (DERs) being connected to the electric power system and their specific capabilities to withstand voltage drops over a period of time defined by the ride through characteristic, it becomes clear that we need to improve the protection systems at all levels in order to reduce the maximum fault clearing time. This will allow us to keep in service the distributed generators for the duration of the fault - something that will help maintaining the security of the system after the clearing of the fault.

At the transmission level of the system the reduction of the fault clearing time can be achieved using communications based line protection schemes.

At the distribution level things are much more complicated, because we need to change the protection principles used. This is due to the conversion of the radial distribution system supplied from a single source - the power transformers in the substation - to a system with multiple resources that can be connected at any location on each of the distribution feeders. The time coordinated non-directional overcurrent protection of the past needs to be replaced with directional overcurrent protection and communications based protection schemes in order to bring down the fault clearing times. At the same time the fact that many renewable DERs are affected by the weather and the light makes their availability probabilistic in nature.

Last, but not least, we may have DERs of different types, sizes and groupings installed as individual units or as farms. The non-deterministic behavior of the DERs has a direct impact on the requirements for the wide spread use of adaptive protection which will help to minimize the fault clearing time.

From everything above it is clear that there is significant interdependence between the protection of the transmission and distribution system and the DERs - we need to improve the protection performance in order to keep the DERs in service, while at the same time we need to know which DERs are in service and what type they are to be able to optimize the protection performance. 

"Interdependence is a fact, it's not an opinion."

Peter Coyote


Relion advanced protection & control.
BeijingSifang June 2016