Assessment of HVDC Frequency Control Methods in the Nordic Test System

The Frequency Containment Reserve (FCR) is one of the balancing actions to keep the frequency within acceptable limits. The objective of the FCR (also known as primary control) is to stabilize the system frequency within a short time interval after a disturbance. Related to that, maximum steady-state frequency deviation and maximum Instantaneous Frequency Deviation (IFD) are defined. With higher integration of renewable energy sources, power systems will reduce its impact on pollution, but face much more often with low inertia scenarios. With low inertia values, the system decreases its inherent property to react to large power disturbances. In these cases, IFD is profoundly affected, and there is a need for fast and cost-effective solutions.
High Voltage Direct Current (HVDC) links with appropriate control strategies may be a potential solution for the challenge mentioned above. According to current system requirements, HVDC links must be capable of providing frequency support. Several studies analyzed the impact of FCR action via HVDC systems with various control methods. However, the question is which of these methods has the best properties in terms of reliability, robustness, and cost-effectiveness.
This work investigates and applies two control methods for HVDC frequency support in the Nordic test system, where these actions are referred to as Emergency Power Control (EPC). The first EPC method is currently used in the Nordic Power System (NPS), and it is based on ramp power injections and frequency triggering activations. The second one is a frequency droop based EPC, and this work proposes it as a new method for future EPC operation. This work assesses the comparison between the two EPC methods for two different disturbances and with the same EPC power capacities. The main objective of the EPC is to meet the frequency requirements and avoid any negative interactions. The Nordic test system has been designed and tuned by authors to capture the frequency dynamical response of the NPS. Furthermore, an equivalent single machine model with low inertia is used to study the performance of the frequency droop based EPC.