Essay: SLs based on reactive (Q) compensation (SLQ) and impedance-type load

Due to growing penetration of asynchronous inverter-interfaced generation (e.g., wind, solar photovoltaic, etc.) the complexity of future power systems is expected to reduce extremely. This would cause undesirable large excursions and rates of change of the system frequencies (RoCoF) following a loss of infeed which would pressure the system security [1]. Besides, loss-of-infeeds more than the reserve capacity of the system could be more likely for example, due to a cable fault in a dc grid. These would make the control of primary frequency much more challenging than what it is now.

 

To control the problem, asynchronous generators like the wind farms and some selected categories of loads (demand) must be required to contribute in frequency control alongside the conventional synchronous plants using fast ramp-rates and natural response of the frequency-dependant loads. Various papers have been published on contribution of wind farms on frequency control [2][3]. On the demand/load end, the focus has primarily been on load scheduling [4] and grid frequency control [5] using on/off control of loads which has been referred to as “demand response” or “demand-side management (DSM)” [6][7]. Frequency control using changes in average power consumption of the loads can be achieved by switching those on/off with appropriate duty cycles [8], [9]. This is also predicated on that the potential candidate loads for DSM e.g., air conditioners, especially, the ones supplied through adjustable speed drives exhibit a constant power characteristics over a wide voltage range [10] which rules out any permissibility of continuous control.

For controlling voltage at different system nodes based on optimization of reactive power compensation various methods have been proposed [11][13]. The basic aim in all these methods is to control the system voltage within an acceptable limit instead of controlling the frequency by manipulation of load. With certain types of voltage-dependant loads e.g., electric heating, lighting (especially, LED lighting), small motors with no stalling problems (e.g., fans, ovens, dish washers, and dryers), it is possible, to exercise a continuous variation in active power consumed by regulating the supply voltage [14].

The concept of smart load (SL) using electric springs (ES) was proposed in [15] as a mean of utilization of both voltage and frequency in an unified framework. A SL consist of a voltage compensator (ES) connected in series between the supply/mains and a voltage-dependant load which can allow a wider variation in supply voltage. Such a load is referred to as NC load. The mains voltage can be regulated while allowing the voltage (and hence the power) across the NC load to be controlled by regulating the voltage injected by the compensator. In fact if the magnitude and phase angle (with respect to the current) of the voltage injected by the compensator is controlled, both the mains voltage and frequency can be simultaneously regulated. However, injection of voltage at any random phase angle other than 90 would require exchange of active power and additional storage element or a back-to-back converter arrangement.

In this paper, the contribution of the SLs with reactive compensation to primary frequency control is represented. In [16] the active and/or reactive power of the SL has been controlled essentially by controlling the active and/or reactive power of the series connected compensator accepting appropriate limits which is not necessarily the good strategy. Hence, an improved control principle is reported in this paper to directly change the active power consumption of the SL. The paper concentrates on SLs based on reactive (Q) compensation (SLQ) and impedance-type loads only. This suggest that the compensator in series with the impedance-type NC load can put in a voltage of controllable magnitude but only in quadrature with respect to the current. As there is only one control variable, the magnitude of the voltage, such as SLQ can be controlled either to control the mains voltage or the frequency, but not both at the same time.