**1. Introduction**

The number of small grid-connected photovoltaic and wind power plants is constantly growing. These plants supply energy to low-voltage lines of the utility. The quantity of energy supplied by these power plants depends on natural conditions, which often change. On the other hand, the electrical grid loads in low-voltage lines are not just three-phase but single-phase as well, and many of them use reactive power. This leads to the fact that one of the main present-day problems of the electrical grid is compensation of the reactive power in the low-voltage grid [1–18] and since the low-voltage three-phase lines are often loaded asymmetrically [14,19–21], the reactive power, which has to be compensated, is di fferent in di fferent phases.

At present, most parts of reactive power compensation systems in low-voltage lines of the utility are based on the electro-mechanical commutation technology of capacitor banks, which are split into steps connected in parallel to the utility grid [22,23]. However, capacitor banks have fixed discrete reactive power capacity, i.e., the reactive power produced by the capacitor bank cannot be changed smoothly. Therefore, it is impossible to fully compensate reactive power of the utility grid using capacitor banks [24].

Smooth reactive power compensation can be achieved by employing static synchronous compensators (STATCOMs) based on a voltage source inverter [24–30]. STATCOM devices are capable of compensating for both capacitance and inductance reactive power. The inverter of a STATCOM device produces PWM voltage to the utility grid employing a low pass filter. Capacitance reactive power is supplied if the magnitude of the voltage provided by the inverter is higher than the voltage magnitude of the utility grid. In cases when the magnitude of the inverter voltage is lower than the magnitude of the utility grid voltage, inductive reactive power is consumed. The STATCOM compensator based on inverter has a fast response time and is capable of full reactive power compensation. The main disadvantages are its high price [19,20,31] and that STATCOM devices can provide just symmetric compensation of reactive power in all three phases of the grid [32].

Smooth reactive power compensation can be performed by employing the static VAR compensator (SVC), a shunt-connected variable reactance, which either generates or consumes reactive power. The static VAR compensator is a power electronic device based on thyristor-switched capacitors (TSCs) for discrete control of generated reactive power and thyristor-controlled reactors (TCRs) for smooth control of consumed reactive power [24–27]. A TCR consists of a reactor and a bidirectional thyristor connected in series. Consumed inductive reactive power is controlled by variation of the thyristor firing angle α, where α = 90 corresponds to full reactive power and α = 180 corresponds to zero reactive power [24–27,33–37]. The first step in this approach is to overcompensate the utility grid using the TSC (to make the utility-grid load slightly capacitive) and after that, by consuming the required amountofinductivereactivepowerbytheTCR,thetotalcompensationofreactivepowerisachieved.

Despite the fact that research in the field of SVCs has been carried out for many years, the topic is still relevant. This fact is evidenced by many new publications devoted to the theory and application of SVCs, e.g., [4,8,12,38–43]. One of the directions of recent research works in this field is the expansion of SVC application areas [8]. A new area of expansion could be the development and application of the SVC for smooth asymmetric compensation of reactive power in low-voltage grids as a cheaper alternative to the inverter-based STATCOM compensator. The novelty of such work can be proved by the following facts:


The novelty of this work is that the proposed TCR compensator is capable of compensating reactive power in a three-phase low-voltage grid utility asymmetrically and that the proper operation of the proposed compensator is proved experimentally, using developed experimental reactors and the test bench of the compensator.

### **2. The Topology and Operation of the TCR Compensator**

The block diagram of the experimental test bench for the investigation of the developed TCR compensator for smooth asymmetric compensation of reactive power for a low-voltage utility grid is presented in Figure 1. It consists of a three-phase power supply (|*U*| = 230 V, *f* = 50 Hz); commutation switches SW1–SW3; zero crossing circuits for each phase; thyristor switches T1–T3 for each phase; control block. The test bench was designed for the experimental investigation of the TCR compensator operation with the three-phase single-cored reactor and with separate reactors for every phase. Therefore, the single-cored three-phase Y-connected reactor (L1) with the middle point connected to neutral, three-phase Y-connected separate phase reactors (L2–L4) with the middle point connected to neutral and switches SW2 and SW3 for commutation of reactors were included

into the structure of the reactive power compensator (Figure 1). The power quality analyzer and oscilloscope were used for the measurement of the reactive and active power and waveforms of utility-grid voltage and current. The investigation was performed in low-voltage lines of the utility for symmetric and asymmetric phase load reactive power compensation. The Δ connection of coils as well as Y-connection with an unconnected midpoint were not used because they are not suitable for asymmetric compensation of the reactive power.

**Figure 1.** Block diagram of the thyristor-controlled reactor (TCR) compensator experimental test bench.
