**1. Introduction**

It is well known that distortions of supply voltage and load current cause power quality degradation, diminish the power factor of the supply system and may result in disruption of sensitive loads [1]. Shunt and series active power filters can alleviate these problems. Shunt filters are intended to compensate for load non-active current whereas series ones can improve supply voltage quality. The Unified Power Quality Conditioner (UPQC) can integrate advantages of both shunt and series active filters in order to achieve control over load voltage and source (line) current [2,3].

A wide review on UPQC configurations can be found in [3]. According to this classification the discussed UPQC can be classified as intended for a single-phase supply system that is based on a two-H-bridge converters employing the same DC-link capacitor. Depending on the point of injecting the compensating current with respect to the injecting transformer (Figure 1), the UPQC under study can be implemented as well in the UPQC-R (right-side shunt) as UPQC-L (left-side shunt) configuration. It is also classified as UPQC-P—It compensates source voltage sags using active power of supply sources. UPQCs can obtain reference signals on the base of frequency or time domain detection methods. Some researchers argue that "Harmonic current estimation is the key technology of power electronics systems to generate a harmonics reference current for harmonic control" and propose extensive and flexible solutions in this field [4]. Other researchers develop time-domain techniques [5,6]. Since the considered UPQC calculates references for load voltage and line current directly on the base of time variable quantities it can be classified as operating using time-domain signal analysis. In particular, this method refers to Fryze's concept of the load equivalent conductance [7].

**Figure 1.** UPQC power circuitry diagram and scheme of obtaining conductance signal on the base of DC-link capacitor voltage *vdc*, according to Equation (6). The S/H block is an sample-and-hold module that is synchronized with source voltage waveform using *sync* block.

Commonly used control methods of UPQC involves continuous measurement of source voltage and load voltage and current in order to calculate reference signals for UPQC action. Such control methods are known as direct control techniques. However, UPQC can be steered using a somewhat different scenario, which may be classified as the indirect control method [8–12]. A type of the indirect method, which has been dubbed the conductance signal control method, has been successfully implemented to control the shunt active power filter (SAPF) action [9]. However, there is no implementation of this technique for UPQC so far. Applying the conductance signal control method references required to control the UPQC operation are obtained on the base of measuring the voltage across the UPQC's DC-link capacitor. Since for this method the capacitor is employed as "a sensor" of load active power its voltage must not be controlled to be constant. On the contrary, the "freewheeling" capacitor voltage is measured and processed in order to obtain an equivalent (or hypothetical) conductance element that characterizes the consumption of the total active power taken from the source [8]. Since the load active power may vary in time, this conductance element should also be considered as time-dependent one. The ongoing information on conductance of this element may be referred to as the conductance signal. The first part of this paper shows that the conductance signal can be used to control the UPQC action.

As it turns out, if the conductance signal method is used some noteworthy additional functionalities of UPQC can be obtained. In particular, the UPQC can also be used to control the flow of energy between the supply source and the AC or DC passive or active elements of the network being connected with the particular UPQC. It can be said that in addition to perform the UPQC's conventional tasks, it can also serve as a local energy distribution center operating with high power factor. This center may serve as a spot improving grid's energetic efficiency. The second part of this paper describes these extra UPQC's functionalities.

There are many other technical problems related to UPQC extended operation in smart grids. From this perspective problems of UPQC real-time control [13–16] and their optimal sizing and siting are considered very important [13–19].

#### **2. Control of UPQC with the Use of Signal of Load Equivalent Conductance**

### *2.1. Basic Scheme of UPQC*

From the point of view of the studied control method the UPQC can be considered as composed of: (1) the shunt converter, which shapes source current *iS* to be active and of amplitude required to supply the load with the required active power and maintains the UPQC's DC-link capacitor voltage in the assumed range, and (2) of the series converter, which tracks the supply voltage *vS* and—If needed—injects suitable voltage corrections *vadd* (Figure 1). As the result load voltage *vL* is shaped to be sinusoidal and of nominal amplitude. In the vast majority of cases the control circuitry of the UPQC's shunt converter steers also the operation of the entire UPQC filter. The same rule is applied in this paper. A comprehensive review on the possible shunt converter control techniques can be found in [20,21].
