Utilizing UPQC-Based PAC-SRF Techniques to Mitigate Power Quality Issues under Non-Linear and Unbalanced Loads

The distribution system is becoming more complicated due to a fast expansion in capacity and the addition of modern loads. This generates new power quality challenges inside the distribution system. Keeping up with PQ criteria is getting more difficult, posing a challenge for several researchers [1]. "Electric PQ means keeping the almost sinusoidal waveform of voltages and currents on the distribution bus at the rated magnitude and frequency". As the power system develops, the number of nonlinear loads like transformers, electric furnaces, and power electronic equipment are growing quickly [2]. System PQ distortion is caused by nonlinear loads linked to the point of common coupling (PCC), where this is harmful to the power grid, more costly electrical equipment will be damaged by voltage distortion, and system losses will rise [3]. The UPQC is one of the suitable solutions for mitigating the PQ issues on the source and load sides [4]. The UPQC consists of a shunt and a series APFs connected by a common DC-link. This device can mitigate the PQ issues, including voltage imbalance, voltage harmonics, voltage sag/swell, flickers in voltage, current harmonics, reactive current, current unbalance, etc. [5].

The voltage compensating on a system can be controlled using one of four control approaches: UPQC-P, UPQC-Q, UPQC-VA_min, and UPQC-S [6]. The UPQC-Q among these methods can only compensate for voltage sag while it cannot compensate for voltage swell. The series APF injects voltage in quadrature with the source current. So, a series inverter doesn't require any active power to compensate for the voltage drop [7]. The UPQC-P employs active power to provide voltage sag compensation, which, unlike the other three methods, needs a minimum amount of series injection voltage. In UPQC-VA_min, to achieve optimum series APF volt-ampere (VA) loading, the series APF injects a voltage at the optimal angle at the voltage sag/swell situation [8]. Finally, the UPQC series inverter is controlled to execute concurrent voltage sag/swell compensation and load reactive power sharing with the shunt inverter using the maximum VA capability of series and shunt APFs. In this case, the UPQC series inverter gives both active and reactive power, so it is named UPQC-S [9].

The first practical UPQC was created by Fujita and Akagi [10], who used a shunt APF and a series APF connected through a DC connection. Since then, this device has been the focus of much study, resulting in several topologies, models, and control schemes of this device. In Jindal et al. [11] suggested a novel connection for a unified power quality conditioner (UPQC) to enhance the PQ of two feeds in a distribution system. Because the UPQC is linked among two various feeds, this link is known as an interline UPQC (IUPQC). Feeder-1 provides an unbalanced and nonlinear load (L-1), whereas Feeder-2 provides a sensitive load (L-2). This IUPQC was designed to protect the sensitive load from upstream disturbances and regulate the voltage at the Feeder-1 terminals. The IUPQC has been tested under several scenarios, including a fault on one feeder, voltage drops on both feeders, and a change in the load. The IUPQC could manage systems with imbalanced and distorted loads.

In a study conducted by Khadkikar [8], the author examined the challenges of controlling, operating, and rating UPQCs in voltage sag and swell scenarios. The Power Angle Controller (PAC) of the UPQC concept was employed as a controller to ensure an effective power angle between the resultant load voltage and the source voltage. There are two proposed control strategies: fixed and variable PAC techniques. It has been shown that these strategies may impact the total kVA loadings of shunt and series APFs. Series and Shunt APFs share different amounts of active and reactive power dependent on the mechanisms utilized (fixed or variable) without changing the system's overall power balance. For voltage swell compensations, the constant power angle technique may be preferable, while for voltage sag compensations, the variable power angle method may be superior.

Integration of UPQC with the microgrid and the utility to mitigate PQ-related difficulties was developed by Samal and Hota [12]. This system includes photovoltaics, a fuel cell, wind turbines, batteries, and various loads (coupled to the bus). According to the results, the distribution PQ issues can be solved by UPQCs connected to both the utility and the microgrid. Also, the microgrid could switch to stand-alone mode while reliably powering critical loads in the event of a significant failure or power disruption in the main grid.

In a study performed in 2018, Lakshmi and Ganguly [13] devised a multi-topical planning approach aimed at the best location of an open UPQC-O. This strategy aimed to enhance the hosting capacity of PV systems and minimize energy loss in distribution networks. The series and shunt converters of the UPQC-O model are not connected to a common DC link. To find out where the UPQC-O inverters should be placed and the optimal PV generating capacity in each bus, the Pareto-dominance-based technique was employed to optimize the objective functions simultaneously. The operational constraints in this planning problem are the maximum PV generating capacity in each bus, the maximum line current flow, and the percentage of voltage-sag-mitigated demand. As a method for achieving a solution, the multi-objective particle swarm optimization approach was used. According to the findings, installing PV in distribution systems minimizes energy loss.

In 2018, Campanhol et al. [14] presented comprehensive research on the stability analysis, size, and power flow of a multifunctional 3-phase DG system comprised of a one-stage Photovoltaic linked to the UPQC. Whenever the Distributed generation system is placed between the grid and general loads or an AC microgrid, the UPQC acts as a bidirectional interface. This study stands out as a key method for effectively developing inverters, considering the nonlinear characteristics of the load and, the PV array's maximum power production and the effect of various current disturbances in the grid voltages. Additionally, two methods for reducing the UPQC inverters' power rating were shown and addressed. The analysis of system stability showed that the system stability was ensured by the voltage and current controllers that have been employed. Experiment findings were provided, taking into account various static circumstances of operation for the PV-UPQC system in addition to simulation results, which demonstrated the efficacy of overcoming the parallel converter's over-power rating.

Mansor et al. [15] investigated the effectiveness of the UPQC in mitigating PQ problems and harmonics caused by non-linear loads connecting to the grid. Battery Energy Storage System (BESS) and Photovoltaic System (PV) backed up the UPQC, where the PV system supplies the load with real power. Since the PV-BESS system provided a constant supply, the DC voltage was stable. As a result, the algorithm used to regulate the DC link voltage may be simplified. The shunt and series APF compensator successfully implemented the STF-UVG technique for synchronization phases to create reference voltage and current. The suggested method has shown that the source current harmonics match IEEE-519.

A hybrid UPQC model that includes distribution generation (HCUPQC-DG) was proposed by Wang et al. [16] in 2021. The HCUPQC-DG has a pair of DC ports located at its DC link. The low-voltage DC port is directly connected to the DG, while the front-end DC-DC converter establishes a connection between the high-voltage DC port and the DG. Most active power may flow directly from the DG to the AC load or grid through the direct power flow path. Thus, the power capacity of the front-end dc-dc converter was reduced significantly, with the conversion was greatly improved. In the HCUPQC-DG architecture that has been proposed, the voltage of the DG wasn't constrained by the voltage of the UPQC's common DC bus, so it could be low and vary greatly. Since some of the DG's active power was transmitted via the direct power flow channel, the front-end DC-DC converter's power capacity was reduced.

In 2021, Abdalaal and Ho [17] developed a new control approach for transformerless UPQC (TL-UPQC) so that distribution networks could take in and send out reactive power from and to the grid. One converter regulated both the voltage on the load side and the voltage on the input grid. With no additional sensor circuits, the improved control strategy operated based on local data gathered by the TL-UPQC. With the help of the suggested enhanced control strategy, the TL-UPQC can improve the voltage profile of the input grid and eliminate the components of harmonic caused by nonlinear load. It was also able to compensate for voltage fluctuations across sensitive loads quickly. The practical results confirmed the system's ability to regulate the voltage at the PCC and simultaneously powering linear and nonlinear sensitive loads.

Krishna et al. [18] introduced a UPQC controller based on Fractional Order Fuzzy Logic (FOFL) to deal with PQ issues in (2022). Four different controllers, the FLC, Adaptive FLC, the Fractional Order Proportional Integral (FOPI), and the Fractional Order FLC, were used to explain the operation of the UPQC. Compared to UPQCs using FOPI controllers, UPQCs using FOFL controllers are significantly effective at addressing power quality issues.

A passive sliding mode control (PSMC) method was created by Jiang and Zhang [19] for a modular multilevel converter (MMC-UPQC) system experiencing grid imbalance. Unbalanced power grids were represented using a mathematical model structurally comparable to MMC-UPQC. A positive and negative sequence separation technique was used to split the detected quantity without PLL. The suggested controller could enhance the control of system parameters, response speed, and compensation performance.

This work is based on a steady-state analysis of UPQC under different operating conditions. Section 2 of this study describes the UPQC configuration. Section 3 of the UPQC's Proposed Control explains this. While section 4 highlights the MATLAB/Simulink simulation outcomes, and Section 5 concludes the study.

Combining shunt and series inverters, this section presents the proposed UPQC control technique. To compensate for voltage-based PQ problems, the UPQC's Series APF acts as a voltage source, injecting the necessary voltage into the supply voltage. In contrast, shunt APF acts as a current source, injecting the suitable current into the system to compensate for load-side power quality problems. The DC-link voltage of UPQC is also controlled by a shunt APF [21]. P-q theory and SRF theory are two of the most often utilized techniques for reference signal generation that are based on the time domain. Methods based on p-q or SRF theory are simpler when compared to more sophisticated control approaches used of adaptive filters like the adaptive notch filter, etc. [22]. PAC is a method that successfully distributes reactive power between the shunt and series APF without hindering with UPQC's primary function, relieving some of the load on the shunt converters [23]. The following is an in-depth description of the control of each of the APFs. The power angle control (PAC) technique was used to control the series APF, and synchronous reference frame (SRF) theory was used to control the shunt APF.

3.1 Control of the series APF

In the PAC approach, the series APF injects the required series voltage to compensate for problems with the source voltage's power quality while supplying a portion of the reactive power to the load. The phase difference between the load voltage and the supply voltage results from injecting series voltage at a certain angle with the source current to supply a portion of the reactive power to the load. To generate reference signals, the power angle (δ) with respect to the source voltage/current must be determined [24]. Using the suggested PAC approach, control of a series APF can be broken down into two parts: power angle estimate and creating pulses for switching.

3.1.1 Power angle estimate

Figure 2 illustrates the phasor diagram for UPQC using the PAC technique. In this diagram, V_S represents the nominal supply voltage with its RMS value equal to (k). In normal conditions, the load voltage (V_L) and source voltage (V_s) form an angle (δ). A series APF injects a series voltage (V_Sr), the phasor difference between the source and load voltages [21]. The operations of UPQC-P and UPQC-Q are shown at angles of δ=0 and φ_sr=90°, respectively, whereas UPQC-S is operated at any angle 0<δ<90° [25].

In the fixed PAC method, when sag occurs, the supply voltage's magnitude RMS is lowered to k^′. The magnitude of the series voltage (V ^′_Sr) injected during sag will be lower than that of the V_Sr injected under health operation. The condition is reversed when there is a system swell. The series voltage (V ^′′_Sr), as seen in Figure 2, is the greatest in all three circumstances under swell. As a result, the rating of the series APF should be chosen based on the maximum swell situation if the fixed power angle approach is utilized. The series APF rating will be underused in normal operation due to the low incidence of swells, which is a disadvantage of this method [26].

2.png

Figure 2. Phasor diagram of UPQC based on the PAC method

In the variable PAC method, when sag or swell happens, the value of δ is changed. If there is sufficient demand of reactive power from load, series APF works at its total rating during normal operation. When sag or swell occurs, these power angles are changed to maintain the series inverter's maximum capacity [26].

Under sag conditions, the following equations can be obtained from Figure 2 [24].

$x=k \cos \delta$               (1)

$y=k \sin \delta$               (2)

$\left|V_{s r}{ }^{\prime}\right|=\sqrt{y^2+\left(k^{\prime}-x\right)^2}$               (3)

$\left|V_{s r}^{\prime}\right|=\sqrt{k^2+k^{\prime 2}-2 k k^{\prime} \cos \delta}$               (4)

where, (f_S) is assumed to be the fraction of voltage sag and (f_Sr) to be the ratio of the series voltage (V ^′_Sr) to the rated source voltage k, further simplifications result in the following formulations [24]:

$f_S=k^{\prime} / k$               (5)

$f_{S r}=\left|V_{s r}^{\prime}\right| / k$               (6)

$f_{S r}=\sqrt{1+f_S^2-2 f_S \cos \delta}$               (7)

$\delta=\cos ^{-1}\left[\frac{1+f_S^2-f_{s r}^2}{2 f_s}\right]$               (8)

If series APF provides a rated voltage (V_Sr,max), then $\delta$ will be at its maximum value at a particular supply voltage. The following can be concluded from Eq. (8) [21]:

$\delta_{\text {max }}=\cos ^{-1}\left[\frac{1+f_S^2-f_{s r-\max }^2}{2 f_S}\right]$               (9)

It is important to note that $\delta_{\max }$ varies with the source voltage and is not a constant because f_S is present in Eq. (9). Eq. (10) is used to calculate the actual value of $\delta_A$ in the case of normal UPQC from the reactive power (Q_Sr) supplied by series APF and the load active power (P_L) [27].

$\delta_A=\sin ^{-1}\left(\frac{Q_{s r}}{P_L}\right)$               (10)

where, the following equation can be used to calculate the maximum reactive power handled by a series APF under normal supply voltage situations [26].

$Q_{s r-\max }=P_{L} \sin \delta_{\max }$               (11)

3.1.2 Creating pulses for switching

The next step for series APF after the power angle has been determined is to generate reference load voltages leading to supply voltages by power angle. Figure 3 depicts the control block diagram of the series converter controller. The unit vector template generation (UVTG) approach is used to generate the reference signals for the series inverter. A phased locked loop (PLL) has been used to produce (ωt), which corresponds to phase A's fundamental component [28]. When the produced (ωt) is combined with the estimated $\delta$ computed by the power angle estimator block, Eq. (12) is used to generate three-phase balanced unit vectors [24].

$\left[\begin{array}{l}V s a \\ V s b \\ V s c\end{array}\right]=\left[\begin{array}{c}\sin (\omega t+\delta) \\ \sin (\omega t+\delta-2 \pi / 3) \\ \sin (\omega t+\delta+2 \pi / 3)\end{array}\right]$               (12)

3.png

Figure 3. Series converter controller employing PAC technique

The unit vectors are then multiplied by the desired magnitude of the load voltages (V_Lm) to produce reference load voltages (V ^*_Labc). Series inverter PWM controllers compare actual load (V_Labc) and reference load (V*_Labc) voltages to create switching signals for the IGBT switches.

3.2 Shunt APF control

This research presents a shunt active power filter (APF) that can be utilized to compensate for the current harmonics and reactive power of a nonlinear load. The shunt APF was controlled by the SRF method, as shown in Figure 4 [20]. The proposed SRF-based shunt APF reference supply current signal is generated from source voltages, load currents, and DC link voltages. The abc-dq0 transformation (Park's transformation) was used to get the basic component of load current, as well as the ($\omega t$) signal required for this conversion has been generated by supplying the source voltages to PLL [29].

A low pass filter has been used to get the load current average component ($I_{L d}^{-}$) from the d-axis component. A PI controller calculates the current required (I_{d loss}) for maintaining the dc-link voltage constant by comparing the dc-link voltage (V_dc) to its reference value (V ^*_dc). As shown in Eq. (13), the source's current fundamental reference component $I_{s d}^*$ is found by adding the ($I_{L d}^{-}$) and the loss current (I_{d loss}) [30].

$I_{s d}^*=I_{L d}^{-}+I_{d \ l o s s}$               (13)

To compensate for imbalance, harmonics, and reactive power in the source current, the zero-and negative-sequence components of the reference source current ($I_{s 0}^*$ and $I_{s q}^*$) in the zero- and q-axes are set to zero. Inverse Park's transform has been used to generate the current reference signals $I_{s a b c}^*$, where the 3-phase balanced reference source currents ($I_{s a b c}^*$) are obtained from ($I_{s d}^*$) in Eq. (14) [20].

$\left[\begin{array}{c}I_{s a}^* \\ I_{s b}^* \\ I_{s c}^*\end{array}\right]= T^{-1}\left[\begin{array}{c}I_{s d}^* \\ 0 \\ 0\end{array}\right]$               (14)

In order to create the inverter's IGBT switching signals, a hysteresis band current controller compares the generated reference source currents ($I_{s a b c}^*$) with the actual source currents ($I_{s a b c}$), that compensates for all problems with current, such as reactive power, current harmonics, zero-and negative sequence components, dc-link voltage regulator, and load-current imbalance.

4.png

Figure 4. Shunt converter controller using SRF method

The specification of harmonic voltage and current restrictions for supply voltage may be found in IEEE Standard 519-1992 [31].