Protection performance assessment with Grid Forming Converter acting as a reference generator...

Session Title: Protection during black-start/network restoration*

Protection performance assessment with Grid Forming Converter acting as a  reference generator during black-start.

Objectives and context: 

The objectives of the studies presented in this paper aim to assess the operation of the existing protective schemes (i.e. over-current protection) within UK networks at various voltage levels, primarily based on discriminating between load and fault current magnitudes and secondarily considering the reduced fault levels during black-start emanating from distributed energy resources.

The ultimate goal of this research is to provide observations and findings to assist the design of future black start schemes within dis

Methods / approach: 

The assessment of the performance of the protection schemes, has been conducted using a verified model of the SP Distribution and SP Transmission network associated with Chaperlcross 132/33kV grid supply point (GSP), located in south west Scotland. This incorporates voltage levels from LV (415 V) to 400kV, and considers two different generating technologies used as main reference generator, i.e. conventional Synchronous Generator (SG), and Grid-Forming Converter (GFC) driven by a battery storage energy system (the network and resource have been modelled and validated in PowerFactory/DigSILENT software). Reference generator is the initial generating unit used to energise the system and must be capable of i) creating its own independent voltage source ii) regulating the voltage and frequency in restoration iii) ensuring the demand restoration iv) facilitating the restoration of the wider network where it is possible. The fault response of the SG and GFC  examined with respect to fault current magnitude and duration. It shall be noted that the deployment of a SG is examined for benchmarking purposes.

In particular, a systematic fault level analysis has been conducted to investigate to what extent an equivalent scale GFC (i.e. same MVA size) could deliver the same benefits as a SG, in terms of power systems protection performance. 

The case studies considered transmission and distribution-level (i.e. 11kV, 33kV, 132kV and 400kV), fault level calculations and the assessment of the existing protection performance under balanced/unbalanced faults considering a second group of revised relay settings which are utilized during black start conditions.

Outcomes & conclusions.

For the case of the SG, the operation of the overcurrent protection, with the second group of protection settings, considered reliable. For the case of the GFC, its fast-acting voltage response has been examined under selected faults as a useful feature for sustaining relatively high currents. The simulation RMS studies revealed that at distribution voltage levels (i.e. 415V and 11kV) the response from the GFC, was adequate to ensure the correct operation of the over-current relays. At higher voltage levels (i.e. 33 kV, 132kV and 400kV) the resulting fault currents exceeded the GFC’s current limitation threshold and forced the GFC to operate as a constant current source, resulting in reduced fault levels. Therefore, the fault infeed was not adequate to trigger existing over-current protection (even the second group of revised relay settings), at these voltage levels. To address this problem at 33kV (and potentially at 132 kV), simulation-based studies will be presented, considering voltage-assisted protection schemes as a potential protection solution.