Protection, Automation and Control with DER - Testing Procedures for Distributed Energy Resources

Author: Thomas Schossig, OMICRON electronics GmbH, Austria

 

The usage of (DER) is challenging for the entire grid and especially for the protection functions of the power system. Just to mention some of them are variable and increased fault levels, bidirectional power flow, false tripping of feeders,  blinding of protection, increase or decrease of fault levels with connection and unwanted islanding. (See below).
One of the major challenges is, that the network is protected effectively against Loss of Mains (LoM or anti-islanding).

Loss of Mains Protection
During this fast development the electric power supply the operation of the grid, the equipment used as well as the philosophy of protection changed. The conventional structure of the grid was based on vertical, unidirectional power flow, from big centralized generation plants to customer demand locations. The grid, as well as the generation plants; have been controlled by the utilities. As long as the number of local small generation units was small, it was philosophy during fault occurrences in the grid, to immediately disconnect all decentralized generation units. The reason was to concentrate control of the grid to the utility in charge, to be able to clear the fault reliable and, after wide-area faults, be able to reconstruct normal operation of the power system as reliable as possible and to prevent islanding of partial sub-networks.

In distribution systems where there is a significant penetration of DER there are many protection issues to be considered. Providing adequate LoM protection is a primary concern.  LoM is the situation in which a section of the distribution system is isolated from the remainder of the power system, but continues to be energized by one or more DER units, and so effectively forming an islanded subsystem.
The purpose of the LoM relay is to disconnect the generator when the connection to the main system is lost, as there are many potential hazards associated with safety, power quality and system integrity during unintentional islanded operation.

These risks include: line worker safety; public safety; unearthed or incorrectly earthed system operation; inadequately controlled voltage and frequency; low system fault levels and potential non-operation of short circuit protection systems; unsynchronised reclosing of the interface between the islanded system and the main system, with potentially grave consequences in terms of system damage and widespread instability. Because of these hazards, it is very important to quickly and accurately detect and react to islanding situations.

Several anti-islanding techniques have been developed. Techniques based on measurements taken at the DER terminals are most commonly used, including Rate-Of-Change-Of-Frequency (RoCoF), voltage vector shift/ vector surge (VS), under/overvoltage and under/over frequency.
In e.g. the UK RoCoF is widely used, in the rest of Europe vector shift is the main choice.
Vector shift is based on a sudden voltage angle change at the coupling point between the generating unit and the grid (Figure1).

 

The stable reference is the angle of the voltage Up, which is stable in the first instance due to the rotating masses of the rotor. During faults in the grid there is sudden change in current flowing through the generator. This makes a sudden change to be measured as difference of voltage drop over the generator impedances and can be calculated accordingly. Figure 3 shows the phasors in such a case. The voltage drop is shown in Figure 2.

Unfortunately, it is difficult to achieve the correct balance between required sensitivity and stability under all operational conditions when applying such LoM detection methods.  There are two main challenges for LoM: one is associated with sensitivity, that is ensuring that the protection always operates for an LoM event; the second is associated with stability, that is ensuring that the protection never operates for any transient condition that is not a true LoM event - the most challenging situation is usually where there is a short-circuit in the vicinity of the LoM relay.

In “Testing the Performance of Loss of Mains Protection for Networks Incorporating Distributed Generation”-IPTS, 2011, a project is described, involving simulation and secondary injection tests to establish the behaviour, under various scenarios, of LoM protection relays that are typically applied to the protection of DER. The scenarios were developed to test both the sensitivity and stability of LoM protection relays using the most commonly applied algorithms mentioned already. As results and findings in the above mentioned publication, the following interesting ones have been published:

  • The setting of RoCoF is always a balancing exercise between sensitivity and stability
  • VS as well as RoCoF are not always sensitive during true LoM events when the load is close to generator outputs
  • Different frequency measurements and RoCoF calculation methods will result in different behavior. Different responses from relays of different vendors with same settings were observed
  • Different generation technologies have a significant bearing on the ease of providing LoM protection

These results show the importance of simulation and testing.

Power. Flexible. Easergy.
Protecting your electrical assets? today and tomorrow