Modern power systems are undergoing the transition from synchronous-generator-based ones towards converter-based ones. This transition tends to reduce the system strength and impair the system voltage and frequency stability. Therefore, synchronous condensers (SCs) are drawing an increasing attention world-widely in recent years, as SCs are able to enhance the system strength, provide dynamic voltage regulation, and contribute inertia to the system. However, less attention has been given to the protection systems for SCs in the literature, especially when a synchronous condenser is equipped at the point of common coupling (PCC) of a HVDC station. Firstly, an SC needs to be protected against over-speed in the rotor under the island scenario of the HVDC station and the SC. Secondly, the protection should not trip the SC undesirably when the SC is supposed to remain connected (within its physical limit) and support the gird operation subject to grid-side disturbances.
This paper investigates the possible undesired trip of an SC by reverser power and over-frequency protection under a certain grid configuration through hardware-in-the-loop (HiL) tests. The tests have identified that, when there are two transmission lines emanating from the substation and the VSC-HVDC station is operating under inverter mode, the trip of the line that exports active power from the substation may lead to an undesired trip of the SC due to the power swing. Losing the support from the SC during such power swing conditions can result in the instability of the grid. Therefore, it is of great importance to have suitable settings for the reverse power and over-frequency protection for the SC. Besides the physical limits of the SC, the following conditions should also be considered for the case study: (1) How much active power the HVDC is importing prior to the line trip; (2) How much active power the other transmission line is importing to the substation prior to the line trip; (3) How fast the HVDC is able to switch to weak grid mode after the line trip; (4) What is the active power reference for the HVDC under weak grid mode; (5) The strength of the grid that the HVDC is connected to after the line trip. The investigation in this paper points out a specific scenario that should draw more attentions for an improved application of synchronous condensers. It provides insights for grid operators and engineers regarding the planning of power dispatches and the settings of generator relays for synchronous condensers.
J. Jia, G. Yang, A. Nielsen, P. Roenne-Hansen, CIGRE Symposium Aalborg 2019, Session 11, paper 104, June - 7, 2019
KEYWORDS: Frequency, hardware-in-the-loop test, HVDC, low-inertia system, power swing, power system protection, reverse power, synchronous condenser, voltage source converter.