IEC TR 60919-3-2021 Performance of high-voltage direct current (HVDC) systems with line- commutated converters – Part 3: Dynamic conditions

IEC TR 60919-3-2021 Performance of high-voltage direct current (HVDC) systems with line- commutated converters – Part 3: Dynamic conditions. 4 AC system power flow and frequency control 4.1 General Active power control of an HVDC system can be used to control the power flow and/or frequency in related a.c. systems in order to improve the performance of a.c. systems in steady-state operation and under disturbance. In this clause, the HVDC active power operation modes, which are used to improve the a.c. system performance for the following purposes, will be covered: – HVDC power control to minimize the total power system losses under steady-state operation; – HVDC power control for prevention of a.c. line overload under a disturbance as well as steady state; – coordinated HVDC power control with an a.c. system generator governor control; – HVDC power control for suppression of an a.c. system frequency deviation under a disturbance as well as steady state. HVDC active and/or reactive power modes used to improve a.c. system dynamic and transient stability or improve a.c. voltage control is discussed in Clauses 5 and 6. 4.2 Power flow control 4.2.1 Steady-state power control requirements The power of an HVDC system is sometimes controlled to minimize overall power system losses, to prevent a.c. line overloading, and to coordinate with the governor control of a.c. system generators. Such power control requirements differ from time to time according to the role of HVDC systems in the overall power system. When an HVDC system is used to transmit power from remote generating stations, the HVDC transmission power control is coordinated with the governor control of the power station generators. In this case, the generator voltage, frequency or the rotor speed may be used as a reference to the HVDC power control system. When two a.c. power systems are connected by an HVDC link, the HVDC power is controlled to a pre-determined pattern under normal circumstances, but an additional function can be incorporated to this HVDC power control so that the frequency of either or both a.c. power systems is controlled. When one of the a.c. systems is an isolated system, such as one supplying a separate island, frequency control of this isolated a.c. system may have to be realized by the HVDC system. The a.c. system frequency control by an HVDC system is discussed in 4.3. When two a.c. systems are interconnected by more than one d.c. link...