Assessment of Short Circuit Power and Protection Systems for Future Low Inertia Power Systems

With the proliferated renewable energy integration and the gradual retirement of conventional generation units, power systems are experiencing significant changes in regard to the generation mix. The conventional power plants comprise large synchronous generators that naturally provide instantaneous short circuit power.

The high short circuit current is of crucial importance to activate the protective relays. In contrast, renewable-based generations typically interface with the grids through power converters, whose short circuit contribution can be significantly different from that of synchronous generators both in the quality and the quantity.

This Ph.D. work, as part of the Danish project “Synchronous Condenser Application in Low Inertia System (SCAPP) ”, concerns the assessment of short circuit power and protection systems for future low inertia power systems. Voltage source converters are widely utilized in the renewable energy integration.

To characterize the short circuit power for future low inertia power systems, a comparison on the short circuit response between synchronous sources and voltage source converters was carried out considering the converter peak current limit and the reactive power support capability. The short circuit response of voltage source converters is significantly different from that of the synchronous sources, especially during grid unbalanced faults.

For voltage source converters, the conventional synchronous reference frame control only in positive-sequence can result in distorted voltage and current outputs, and uncontrollable converter peak current. This indicates that it is necessary for voltage source converters to have current control loops in both positive- and negative-sequence.

Base on the instantaneous power theory, the dual-sequence current control strategies of voltage source converters under grid unbalanced faults were reviewed aiming at the short circuit power provision. They were classified into two groups, namely power-characteristic-oriented control strategy, and voltage-support-oriented control strategy, in terms of the properties that are being directly controlled.

The review also covered the inner current controllers and the design of the converter peak current limit. Through various simulations and discussions, the short circuit power from voltage source converters can be characterized by the different combinations of the positive and negative-sequence powers.

In addition, the dual-sequence current controls were extended to microgrids as a case study to examine the short circuit power in a system with 100% penetration of renewable energy. To investigate the challenges brought by low inertia power systems to protection systems, a hardware-in-the-loop test platform was created integrating power system real-time simulations and protective relays into a closed loop.

The platform is able to automate the relay testing under different scenarios through the bi-directional communication between Real Time Digital Simulator and MATLAB. On the transmission level, the performances of distance relays installed at a voltage source converter substation and protecting the neighboring line were explored through hardware-in-the-loop tests.

It was revealed that the measuring error caused by the fault resistance will be enlarged in low inertia power systems. The occurrence of underreach and overreach problems become unpredictable due to the low short circuit current level and the non-conventional short circuit power characteristics. Synchronous condensers, as synchronous machines in principle, have the advantage of short circuit contribution.

An investigation on the combined effect of voltage source converters and synchronous condensers was carried out considering the diverse dual-sequence current controls. It focused on the aspects of the combined short circuit current, the voltage at the point of common coupling, the DC-side voltage and the system frequency response during faults.

In particular, it was verified through hardware-in-the-loop tests under various scenarios that the associated short circuit current interaction could jeopardize the reliability of distance relays. The investigation was concluded with suggestions on selecting the dual-sequence current control strategies when incorporating synchronous condensers.

In order to analytically perform the fault analysis, a static fault analysis method was developed and verified taking the short circuit contribution from voltage source converters and their dualsequence current controls into account. The method was used to help explain a phenomenon, which has not been revealed in the literature, that the system may not have a stable response under unbalanced faults due to the negative-sequence reactive power injection.

The developed fault analysis method helps to understand the impact of the short circuit contribution from voltage source converters on low inertia power systems. With the proposed method, the optimal sizes and locations of synchronous condensers for the western Danish power system of a future scenario were investigated through optimizations.

The allocation results indicated that there is a need of more synchronous condensers for future low inertia power systems to maintain the system short circuit ratios.