Adaptive Control of HVDC Links for Frequency Stability Enhancement in Low-Inertia Systems

Abstract

Decarbonization of power systems has put Renewable Energy Sources (RES) at the forefront when it comes to electric power generation. The increasing shares of converter-connected renewable generation cause a decrease of the rotational inertia of the Electric Power System (EPS), and consequently deteriorate the system capability to withstand large load-generation imbalances. Low-inertia systems are subjected to fast and large frequency changes in case of in-feed loss, where the traditional primary frequency control is not sufficient to preserve the frequency stability and to maintain the frequency above the critical value. One possible solution to this rising problem is seen in Fast Frequency Response (FFR) provided by the High-Voltage Direct-Current (HVDC)-based systems. This paper presents the adaptive FFR control of HVDC-based systems for frequency stability enhancement in the low-inertia system. The EPS is considered as a “black box” and the HVDC response is determined only using the locally measured frequency change. Sliding Mode Control (SMC) of the Modular Multilevel Converter (MMC) was developed and demonstrated to provide faster and more appropriate frequency response compared to the PI controller. The described adaptive HVDC control considers the size of disturbance and the inertia of the power system, and it is verified by simulations on the IEEE 39 bus test system implemented in MATLAB/Simulink for different system configurations and different sizes of disturbance.

A VSG-Based Coordinated DC Voltage Control for VSC-HVDC to Participate in Frequency Regulation

“…The DC voltage is decreased to release energy for an inertial response, and the voltage deviation is detected by offshore wind farms, which will adjust their active power output to recovery the DC voltage. Similar work can be found in [13][14][15]. Recently, the VSG (virtual synchronous generator) method, which could mimic the operating mechanism of synchronous generators, is attracting more and more concerns in the control of voltage source converters [16,17].…”

“…One is that smaller deviation takes less time for adjustment, which accelerates the change of transmitted power; the other one is that bigger deviation may jeopardize the DC capacitor and even threaten the stability of the DC side of the VSC-HVDC system. The first constraint in (14) guarantees that the transmitted power from renewable energy generators to gird is equal to P* calculated in (3). The last two constraints are the safety range of DC voltages.…”

“…To solve (14) with general methods may be tedious. Actually, the optimization goal and constraints have a clear geometric meaning if ( 14) is expressed in a graph, as shown in Figure 4.…”

“…The red dot represents the initial state, the physics meaning of ( 14) is to find a candidate point on the blue line in the green area whose distance from the red dot is shortest. Obviously, when the line connecting the red dot and the candidate point is perpendicular to the tangent line of the candidate point, the candidate point is just the solution of (14).…”

Adaptive Droop Control for Compromise Between DC Voltage and Frequency in Multi-Terminal HVDC

“…However, limiting the maximum allowed support power does not ensure a frequency nadir within the frequency standard since the inertia constants, droop gains, dead bands, and ramp rates of generators in that grid substantially affect the frequency nadir [23]. In [24][25][26], droop gain is adaptively changed by the margin to frequency limit, which protects the supporting grid's frequency from violating the frequency standard. However, the condition of DC voltage is neglected in these papers, which may pose instability to the system.…”