IEC TR 60919-3:2021 pdf download – Performance of high-voltage direct current (HVDC) systems with line- commutated converters – Part 3: Dynamic conditions.
In some cases of a.c./d.c. system configurations described above, the HVDC power change control can be used to prevent overloading of one or more transmission lines in the power system. In certain special HVDC control schemes, such as the one designed to improve a.c. system performance by increasing the d.c. power during and after a disturbance, the steady-state d.c. transmission power may have to be set at a restricted value so that the d.c. power does not exceed the d.c. rated power, including overload capability, when the control is initiated. It is important to consider also the additional reactive power supply required both by the HVDC converters and the a.c. systems in such a situation. The following items a) to g) need to be considered in the specification of steady-state control requirements. Note that at the time of preparing the specification, the complete steady-state control requirements may not have been determined or designed, but allowance for possible future inputs is necessary. a) When a power flow control system is designed to have more than one function, including the a.c. system frequency control, the HVDC control system should be so designed that priorities are set between each control function. b) Under steady-state conditions, the control for prevention of a.c. line overloading is usually given higher priority over other power flow controls. The control for minimization of power system losses is implemented either by setting the d.c. power to a pattern which has been pre-determined by the power system data, or in response to an on-line computation which is conducted in the central load dispatching office. Usually, its control response is relatively slow, being several seconds or several minutes, even in the latter case.
c) In isolated systems or systems with a relatively large d.c. infeed, frequency is often maintained by the HVDC power. In such a case, HVDC frequency control could have a priority over system loss minimization, but may be limited by overload protection. d) The change in reactive power demand accompanying the power changes may result in frequent switching of reactive power equipment. In such a case, it is necessary to figure out particular a.c. voltage control measures such as reactive power control by converter units, or to set limits of the magnitude of HVDC power change. e) The need for special power order adjustment signals unique to the power system should be identified, studied, and specified. The signals cannot be permitted to cause d.c. current or power, or a.c. voltage to deviate beyond equipment and system ratings and limits. The priority of two or more input signals having simultaneous demand on d.c. link power should be established and coordinated. f) Bipolar d.c. links normally require that d.c. power and current be effectively shared between poles. For loss of one pole, an overload strategy for the remaining pole could be developed to minimize disruption to a.c. system power flow, voltage and frequency. g) Disruption of the telecommunication link between the sending and receiving system of the d.c. link should not cause disruption to the a.c. power system. A minimum specification requirement is that power transmission is maintained at the same power level which existed before the telecommunication failure. If additional functions such as frequency control are required during temporary outage of the telecommunication link, these should be specified. 4.2.2 Step change power requirement Under certain power system conditions, it may be required to change the HVDC power in steps in order to improve the performance of a.c. systems during and after power system disturbances. Under certain circumstances, the step change may involve d.c. power reversal.