Unified power quality conditioner based on a three-level NPC inverter using fuzzy control techniques for all voltage disturbances compensation

Abstract

This paper presents a novel and efficient control scheme for unified power quality conditioner (UPQC) based on three-level neutral point clamped (NPC) inverter using fuzzy logic techniques. The proposed UPQC is capable of mitigating source current harmonics and compensate all voltage disturbances such as voltage sags, swells, unbalances and harmonics. It is designed by the integration of series and shunt active filters (AFs) sharing a common DC bus capacitor. The DC voltage is maintained constant using proportional integral voltage controller. The synchronous reference frame (SRF) theory is used to get the reference signals for shunt active power filters (APFs) and the power reactive theory (p-q theory) for series APFs. The shunt and series APF reference signals derived from the control algorithm and sensed signals are injected in two controllers to generate switching signals. To improve the UPQC capability, fuzzy logic techniques are introduced to control the series APF. The performances of the proposed UPQC system are evaluated in terms of power factor correction, mitigation of voltage or current harmonics and all other voltage disturbances compensation using Matlab-Simulink software and SimPowerSystem toolbox. The simulation results illustrate the performance of the proposed UPQC at the common connection point of the nonlinear load to improve the power energy quality.

Keywords

three-level neutral point clamped (NPC) inverter / unified power quality conditioner (UPQC) / current harmonics mitigation / fuzzy logic controller / voltage disturbance compensation / shunt active filter / series active filter / power quality energy improvement

Introduction

There has been a continuous rise of nonlinear loads over the years due to intensive use of power electronic control in industry. The utility supplying these nonlinear loads has to supply large vars. Moreover, the harmonics generated by the nonlinear loads pollute the utility. The basic requirements for the compensation process involve precise control with fast dynamic response and online elimination of load harmonics. In the past, these identified power quality problems were mitigated by using switched capacitor and thyristor controlled inductor [1,2] coupled with conventional passive filters [1,2]. But limitations, such as fixed compensation, resonance with source impedance and the difficulty in tuning time dependence of filter parameters, have ignited the need for active power filters (APFs) [3,4]. The two types of APFs are shunt and series APF. The shunt APFs are used to mitigate current harmonics and reactive power compensation. The series APFs are used to compensate voltage related problems, such as voltage harmonics, sags, swells, unbalances, flicker, and etc.

The unified power quality conditioner (UPQC) is one of the best solutions to compensate both current- and voltage-related problems simultaneously [5,6]. It is the integration of shunt and series APFs through a common DC link capacitor. UPQC has been widely studied to eliminate or mitigate the disturbances propagated from the source side and the other loads interconnected [7,8]. In the normal operation of UPQC, the shunt APF control circuit calculates the harmonic currents compensation and generates the inverter pulses to power circuits (shunt APF). The series APF compensates the harmonics and the all other voltage disturbances. The arrangement of series and shunt filters are interchangeable. In general, when a UPQC is used in a power distribution system, the series filter is installed ahead of the shunt filter.

There are many control strategies reported in the literature to control the UPQC for power quality improvements, the most common of which are the instantaneous active and reactive power theory (the p-q theory) proposed by Akagi [9], symmetric component transformation, synchronous reference frame (SRF) theory, and unit template technique (UTT) etc. In some works, only the SRF strategy is adopted for controlling shunt and series APF [5]. The major drawback of this strategy is the two phase locked loop (PLL) used to achieve synchronization with the distorted supply voltage. Other techniques reported in literature produce poor results under distorted and/or unbalanced input/utility voltages, and they involve many calculations. In this paper, a simple control scheme is proposed based on PQ method for series APF control and SRF method for the shunt APF to achieve effective compensation for source current harmonics, reactive power compensation and voltage harmonic mitigation even under distorted and/or unbalanced input/utility voltages.

The controller is the core of any APF operation and has been the subject of numerous researches in recent years [10,11]. The conventional control scheme to generate pulses, based on hysteresis technique control, presents several drawbacks such as uneven switching frequency that causes acoustic noise and difficulty in designing input filters in case of SAPF. To improve the APF performance, the tendency has been to use intelligent control techniques, particularly fuzzy logic controllers. The use of fuzzy logic controllers in power electronics applications has generated considerable interest [12,13]. Their advantages are robustness, ability to control nonlinear systems and needing no mathematical model, etc. For these reasons, in this paper, a novel control scheme of UPQC is proposed based on fuzzy controller for the series APF.

This paper presents a novel UPQC system based on a three-level neutral point clamped (NPC) inverter using fuzzy the control technique. The series active filter (AF) is controlled to maintain voltage load to the reference level and to eliminate supply voltage sag/swell, harmonics and unbalance from the load terminal voltage. The shunt AF is controlled to mitigate the supply current harmonics. The DC bus voltage is maintained constant by the shunt AF. The performances of the proposed UPQC system are verified through simulations for transient and steady-state conditions using Matlab-Simulink software and SimPowerSystem toolbox.

Unified power quality conditioner

Figure 1 shows the proposed three-phase three-wire UPQC connected to the power system feeding a nonlinear load. It consists of two three-level NPC inverters, one for the shunt and the other for a series AF. The DC link of both AFs is connected to a 3000 μF common DC capacitor. The series filter is connected between the supply and load terminals using three single phase transformers with turns ratios of 1:1. In addition to injecting the voltage, these transformers are used to filter the switching ripple of the series AF. A small capacity rated C_sf filter [14] is used with inductance to eliminate the high switching ripple content in the series AF injected voltage. The three-level inverters are designed with insulated gate bipolar transistors (IGBTs). The three leg shunt AF is connected ahead of a series filter using a small capacity rated inductive filter. The control algorithm of UPQC is based on the SRF detection method for the shunt AF and instantaneous reactive power theory for the series AF [15].

Since its introduction in 1981 [16], the three-level NPC voltage source inverter has attracted popular great deal of attention. Apart from its application in high-capacity AC motor drive, other interesting applications of this topology include high voltage direct current (HVDC) transmission, static synchronous compensator (STATCOM), APFs, pulse width modulation (PWM) rectifier, as well as renewable energy interfacing applications. Although the three-level NPC topology provides significant advantages over the conventional two-level one in high-power applications. In power quality applications, the three-level topology has been used in static var compensators (SVCs) [3], unified power-flow controller (UPFC) [17], etc., due to its high speed and wide range of reactive power. On the other hand, the application of the NPC voltage source converters to UPQCs is being limited by the unbalance DC link voltages due to the inherent transient operating condition. The advantages of these structures are near sinusoidal current waveforms due to reduced unwanted harmonics in the voltage PWM waveforms, each power valve taking half the DC link voltage and the first set of unwanted harmonics being at twice the switching frequency.

Figure 2 demonstrates the power circuit of the three-level NPC inverter based on the six main switches (T₁₁, T₂₁, T₃₁, T₁₄, T₂₄, T₃₄) of the traditional two-level inverter, with six auxiliary switches (T₁₂, T₁₃, T₂₂, T₂₃, T₃₂, T₃₃) and two neutral clamped diodes added on each bridge arm. The diodes are used to create the connection with the point of reference to obtain midpoint voltages. This structure allows the switches to endure larger DC voltage input on the premise that the switches will not raise their withstand voltage. For this structure, three output voltage levels can be obtained, namely, U_d/2, 0, and –U_d/2 corresponding to three switching states A, 0, and B. As a result, 27 states of switching output exist in the three-phase three-level inverter [18-20].

Control strategies

The control strategy is basically the way to generate reference signals for both shunt and series APFs of UPQC. The compensation effectiveness of the UPQC depends on its ability to follow with a minimum error and time delay the reference signals to compensate the distortions, unbalanced voltages or currents or any other undesirable condition [21]. The conventional techniques reported in literature produce poor results under distorted and/or unbalanced input/utility voltages, and they involve many calculations. The proposed control scheme is a simple scheme to achieve effective compensation for source current harmonics, reactive power compensation and voltage harmonic mitigation even under distorted and/or unbalanced input/utility voltages.

Shunt APF

The shunt APF control strategy adopted here to compensate harmonic currents is based on the SRF detection method. The principle of this technique is described below [22]. The three-phase load currents i_La, i_Lb and i_Lc are transformed from three phase (abc) reference frame to two phase’s (α-β) stationary reference frame currents i_α and i_β using

(1)\left[\begin{array}{l}{i}_{\alpha }\\ i_{\beta }\end{array}\right]=\sqrt{{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}}\left[\begin{array}{lll}\mathrm{1}& -{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}& {\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}\\ \mathrm{0}& {\displaystyle \frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}& -{\displaystyle \frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}\end{array}\right]\left[\begin{array}{l}{i}_{\mathrm{L}a}\\ i_{\mathrm{L}b}\\ i_{\mathrm{L}c}\end{array}\right].

Using a PLL, cos(θ_est) and sin(θ_est) can be generated from the phase voltage source u_sa, u_sb and u_sc. The i_α and i_β currents expression in (d-q) reference frame are given by

(2)\left[\begin{array}{l}{i}_d\\ i_q\end{array}\right]=\left[\begin{array}{ll}\sin ({\theta }_{\mathrm{e}\mathrm{s}\mathrm{t}})& -\cos ({\theta }_{\mathrm{e}\mathrm{s}\mathrm{t}})\\ \cos ({\theta }_{\mathrm{e}\mathrm{s}\mathrm{t}})& \sin ({\theta }_{\mathrm{e}\mathrm{s}\mathrm{t}})\end{array}\right]\left[\begin{array}{l}{i}_{\alpha }\\ i_{\beta }\end{array}\right].

The current i_d is transformed to DC and harmonic components using a low pass filter

(3)\left[\begin{array}{l}{i}_d\\ i_q\end{array}\right]=\left[\begin{array}{lll}{\overline{i}}_d& +& {\tilde{i}}_d\\ & i_q& \end{array}\right].

The expression of the reference current i_α-ref and i_β-ref are given by

(4)\left[\begin{array}{l}{i}_{\alpha -\mathrm{r}\mathrm{e}\mathrm{f}}\\ i_{\beta -\mathrm{r}\mathrm{e}\mathrm{f}}\end{array}\right]={\left[\begin{array}{ll}\sin ({\theta }_{est})& -\cos ({\theta }_{est})\\ \cos ({\theta }_{est})& \sin ({\theta }_{est})\end{array}\right]}^{-\mathrm{1}}\left[\begin{array}{l}{i}_d\\ i_q\end{array}\right],

(5)\left[\begin{array}{l}{i}_{\alpha -ref}\\ i_{\beta -\mathrm{r}\mathrm{e}\mathrm{f}}\end{array}\right]=\left[\begin{array}{ll}\sin ({\theta }_{est})& \cos ({\theta }_{est})\\ -\cos ({\theta }_{est})& \sin ({\theta }_{est})\end{array}\right]\left[\begin{array}{lll}{\overline{i}}_d& +& {\tilde{i}}_d\\ & i_q& \end{array}\right].

The correspondent reference currents in the (abc) frame are given by

(6)\left[\begin{array}{l}{i}_{a-ref}\\ i_{b-ref}\\ i_{c-ref}\end{array}\right]=\sqrt{{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}}\left[\begin{array}{ll}\mathrm{1}& \mathrm{0}\\ -{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}& {\displaystyle \frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}\\ {\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}& -{\displaystyle \frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}\end{array}\right]\left[\begin{array}{l}{i}_{\alpha -ref}\\ i_{\beta -\mathrm{r}\mathrm{e}\mathrm{f}}\end{array}\right].

Finally, the compensation currents i_ca, i_cb and i_mL are given by

i_{ca}=i_{a-\mathrm{r}\mathrm{e}\mathrm{f}}-i_{\mathrm{L}a},

(7)i_{cb}=i_{b-\mathrm{r}\mathrm{e}\mathrm{f}}-i_{\mathrm{L}b},

i_{cc}=i_{c-\mathrm{r}\mathrm{e}\mathrm{f}}-i_{\mathrm{L}c}.

To compensate the inverter losses and regulate the DC link voltage U_dc, a proportional integral voltage controller is used. The control loop consists of the comparison of the measured voltage (U_dc1 + U_dc2) with the reference voltage U_dc-ref. The loop generates corresponding current I_c,los as given by

(8)I_{c,\mathrm{l}\mathrm{o}\mathrm{s}}=K_{\mathrm{p}}\mathrm{\Delta }{U}_{\mathrm{d}\mathrm{c}}+K_i{\displaystyle \int \mathrm{\Delta }{U}_{\mathrm{d}\mathrm{c}}}\mathrm{d}t.

Figure 3 shows the shunt AF strategy control.

Series APF

The control strategy used to extract the reference voltages of series APF is based on the p-q theory [20,23,24]. The three-phase voltage source in the grid is assumed to be symmetric and distorted

(9)\left[\begin{array}{l}{U}_a\\ U_b\\ U_c\end{array}\right]=\left[\begin{array}{l}{\displaystyle \sum _{n=\mathrm{1}}^{\infty }\sqrt{\mathrm{2}}{U}_n\sin (n\omega t+{\theta }_n)}\\ {\displaystyle \sum _{n=\mathrm{1}}^{\infty }\sqrt{\mathrm{2}}{U}_n\sin \left[\left(n\omega t-{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)+{\theta }_n\right]}\\ {\displaystyle \sum _{n=\mathrm{1}}^{\infty }\sqrt{\mathrm{2}}{U}_n\sin \left[\left(n\omega t+{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)+{\theta }_n\right]}\end{array}\right],

where U_n and θ_n are respectively the rms voltage and initial phase angle, n is the harmonic order. When n = 1, it means three-phase fundamental voltage source

(10)\left[\begin{array}{l}{U}_a\\ U_b\\ U_c\end{array}\right]=\left[\begin{array}{l}\sqrt{\mathrm{2}}{U}_{\mathrm{1}}\sin (\omega t+{\theta }_{\mathrm{1}})\\ \sqrt{\mathrm{2}}{U}_{\mathrm{1}}\sin \left[\left(\omega t-{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)+{\theta }_{\mathrm{1}}\right]\\ \sqrt{\mathrm{2}}{U}_{\mathrm{1}}\sin \left[\left(\omega t+{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)+{\theta }_{\mathrm{1}}\right]\end{array}\right].

Equation (10) is transformed into (α-β) reference frame

(11)\left[\begin{array}{l}{U}_{\alpha }\\ U_{\beta }\end{array}\right]=C_{\mathrm{32}}\left[\begin{array}{l}{U}_a\\ U_b\\ U_c\end{array}\right]=\sqrt{\mathrm{3}}\left[\begin{array}{l}{\displaystyle \sum _{n=\mathrm{1}}^{\infty }{U}_n\sin (n\omega t+{\theta }_n)}\\ {\displaystyle \sum _{n=\mathrm{1}}^{\infty }\mp U_n\sin (n\omega t+{\theta }_n)}\end{array}\right],

(12)C_{\mathrm{32}}=\sqrt{{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}}\left[\begin{array}{lll}\mathrm{1}& -\mathrm{1}/\mathrm{2}& -\mathrm{1}/\mathrm{2}\\ \mathrm{0}& \sqrt{\mathrm{3}}/\mathrm{2}& -\sqrt{\mathrm{3}}/\mathrm{2}\end{array}\right].

The three-phase positive fundamental current template is constructed as

(13)\left[\begin{array}{l}{i}_a\\ i_b\\ i_c\end{array}\right]=\sqrt{{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}}\left[\begin{array}{l}\sin (\omega t)\\ \sin \left(\omega t-{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)\\ \sin \left(\omega t+{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)\end{array}\right].

Equation (13) is transformed to (α-β) reference frame

(14)\left[\begin{array}{l}{i}_{\alpha }\\ i_{\beta }\end{array}\right]=C_{\mathrm{32}}\left[\begin{array}{l}{i}_a\\ i_b\\ i_c\end{array}\right]=\left[\begin{array}{l}\sin (\omega t)\\ -\cos (\omega t)\end{array}\right].

According to the instantaneous reactive power theory [14], then

(15)\left[\begin{array}{l}p\\ q\end{array}\right]=\left[\begin{array}{ll}{u}_{\alpha }& u_{\beta }\\ u_{\beta }& -u_{\alpha }\end{array}\right]\left[\begin{array}{l}{i}_{\alpha }\\ i_{\beta }\end{array}\right],

where DC and AC components are included

(16)\left[\begin{array}{l}p\\ q\end{array}\right]=\left[\begin{array}{l}\overline{p}+\tilde{p}\\ \overline{q}+\tilde{q}\end{array}\right],

where p and q are passed through low pass filter (LPF), and DC component are obtained by

(17)\left[\begin{array}{l}\overline{p}\\ \overline{q}\end{array}\right]=\sqrt{\mathrm{3}}\left[\begin{array}{l}{U}_{\mathrm{1}}\cos ({\theta }_{\mathrm{1}})\\ U_{\mathrm{1}}\sin ({\theta }_{\mathrm{1}})\end{array}\right].

According to Ref. [15], transformation is made

(18)\left[\begin{array}{l}p\\ q\end{array}\right]=\left[\begin{array}{ll}{u}_{\alpha }& u_{\beta }\\ u_{\beta }& -u_{\alpha }\end{array}\right]\left[\begin{array}{l}{i}_{\alpha }\\ i_{\beta }\end{array}\right]=\left[\begin{array}{ll}{i}_{\alpha }& i_{\beta }\\ -i_{\beta }& i_{\alpha }\end{array}\right]\left[\begin{array}{l}{u}_{\alpha }\\ u_{\beta }\end{array}\right].

The DC components of p and q

(19)\left[\begin{array}{l}\overline{p}\\ \overline{q}\end{array}\right]=\left[\begin{array}{ll}{u}_{\alpha f}& u_{\beta f}\\ u_{\beta f}& -u_{\alpha f}\end{array}\right]\left[\begin{array}{l}{i}_{\alpha }\\ i_{\beta }\end{array}\right]=\left[\begin{array}{ll}{i}_{\alpha }& i_{\beta }\\ -i_{\beta }& i_{\alpha }\end{array}\right]\left[\begin{array}{l}{u}_{\alpha f}\\ u_{\beta f}\end{array}\right].

The fundamental voltages in (α-β) reference frame are

(20)\left[\begin{array}{l}{u}_{\alpha f}\\ u_{\beta f}\end{array}\right]={\left[\begin{array}{ll}{i}_{\alpha }& i_{\beta }\\ -i_{\beta }& i_{\alpha }\end{array}\right]}^{-\mathrm{1}}\left[\begin{array}{l}\overline{p}\\ \overline{q}\end{array}\right]=\left[\begin{array}{ll}{i}_{\alpha }& -i_{\beta }\\ i_{\beta }& i_{\alpha }\end{array}\right]\left[\begin{array}{l}\overline{p}\\ \overline{q}\end{array}\right].

The three-phase fundamental voltages are given by

(21)\left[\begin{array}{l}{U}_{af}\\ U_{bf}\\ U_{cf}\end{array}\right]=C_{\mathrm{23}}\left[\begin{array}{l}{u}_{\alpha f}\\ u_{\beta f}\end{array}\right]=\sqrt{\mathrm{2}}{U}_{\mathrm{1}}\left[\begin{array}{l}\sin (\omega t+{\theta }_{\mathrm{1}})\\ \sin \left(\omega t+{\theta }_{\mathrm{1}}-{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)\\ \sin \left(\omega t+{\theta }_{\mathrm{1}}+{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)\end{array}\right],

where

(22)c_{\mathrm{23}}=\left[\begin{array}{ll}\mathrm{1}& \mathrm{0}\\ -\mathrm{1}/\mathrm{2}& {\displaystyle \frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}\\ -\mathrm{1}/\mathrm{2}& {\displaystyle \frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}\end{array}\right].

The block diagram of the series APF control is shown in Fig. 4.

UPQC control

Shunt APF controller

The main component of any AF is the current controller. The conventional scheme uses hysteresis controller, which is very complicated and requires many calculations for its implementation [25]. To replace the conventional controller, a new control scheme to use the three-level NPC inverter is proposed, as given in Fig. 5. The difference between the injected current and the reference current determines the error current signal (e). This input is compared with two triangular-carrying identical waves shifted from one to the other by a half-period of chopping and generating of switching pulses. The control of the inverter is summarized in the following two steps:

Determination of the intermediate signals V_i1 and V_i2:

If error E_c ≥ carrying 1, then V_i1 = 1;

If error E_c < carrying 1, then V_i1 = 0;

If error E_c ≥ carrying 2, then V_i2 = 0;

If error E_c < carrying 2, then V_i2 = –1.

Determination of control signals of the switches T_ij (i = 1, 2, 3; j = 1, 2, 3, 4):

If (V_i1 + V_i2) = 1, then T_i2 = 1, T_i1 = 1, T_i3 = 0, T_i4 = 0;

If (V_i1 + V_i2) = 0, then T_i2 = 0, T_i1 = 1, T_i3 = 1, T_i4 = 0;

If (V_i1 + V_i2) = –1, then T_i2 = 0, T_i1 = 0, T_i3 = 1, T_i4 = 1.

Series APF controller

Fuzzy logic controllers (FLCs) have been an interesting and good alternative in more power electronics application. Their advantages are robustness, non-requirement of a mathematical model, and acceptance of nonlinearity [26]. To benefit from these advantages, a fuzzy logic voltage controller is proposed for use in the three-level NPC series APF. The controller is designed to improve the compensation capability of APF by adjusting the voltage error using fuzzy rules. Fuzzy logic control is the evaluation of a set of simple linguistic rules to determine the control action. The desired inverter switching signals of the three-level series active filter are determined according to the error between the compensation voltages and reference voltages. In this case, the fuzzy logic voltage controller has two inputs, error “e” and change of error “de”, and one output “s” [24,25,27]. To convert the inputs into linguistic variable, three fuzzy sets are used: N (Negative), ZE (Zero), and P (Positive). Figure 6 shows the membership functions used in fuzzification.

The fuzzy controller for every phase is characterized by three fuzzy sets for each input; five fuzzy sets for output; Gaussian membership function for the input and triangle membership function for the output; implication using the “min” operator, Mamdani fuzzy inference mechanism based on fuzzy implication; and defuzzification using the “centroid” method.

1) If error is Negative and error rate is Negative, then output is Big Negative.

2) If error is Zero and error rate is Negative, then output is Positive.

3) If error is Positive and error rate is Negative, then output is Big Positive.

4) If error is Negative and error rate is Zero, then output in Big Negative.

5) If error is Zero and error rate is Zero, then output is Zero.

6) If error is Positive and error rate is Zero, then output is Big Positive.

7) If error is Negative and error rate is Positive, then output is Big Negative.

8) If error is Zero and error rate is Positive, then output is Negative.

9) If error is Positive and error rate is Positive, then output is Big Positive.

The errors for each phase are discretized by the zero order hold blocks. The error rate is derivative of the error and is obtained by the use of unit delay block. The saturation block imposes the upper and lower bounds on a signal. When the input signal is within the range specified by the lower limit and upper limit parameters, the input signal passes through unchanged. When the input signal is outside these bounds, the signal is clipped to the upper or lower bound. The output of the saturation blocks are the input to fuzzy logic controllers. The outputs of these fuzzy logic controllers are used in the generation of pulse switching signals of the three-level inverter. The switching signals are generated by comparing a two-carrier signal with the output of the fuzzy logic controller. The Simulink model of the fuzzy logic switching signal generation is depicted in Fig. 7.

Simulation results and discussion

Figure 8 displays the block diagram of the proposed UPQC. The simulation is performed using Matlab-Simulink software and SimPowerSystem toolbox. The performances of UPQC are evaluated in terms of voltage and current harmonics mitigation, sags, swells and voltage unbalances compensation. The parameters of the proposed UPQC are V_s = 220 V, frequency f_s = 50 Hz, resistor R_s = 0.1 mΩ, inductance L_s = 0.0002 mH, resistor R_l = 48.6 Ω, inductance L_l = 40 mH, C_dc = 3000 μF, resistor R_c = 0.27 mΩ, and L_c = 0.8 mH.

Performances of UPQC for current and voltage harmonics compensation

To visualize the shunt APF and series APF performance individually, both APFs are put into operation at different instants. At time t₁ = 0.05 s, shunt APF is put in operation first for compensating current harmonics. Before time t₁ = 0.05 s the source current is highly distorted (THD_i = 26.58%). After this instant, it becomes sinusoidal (THD_i = 5.76%) and in phase with utility voltage. The obtained results for current harmonics compensation are illustrated in Fig. 9.

At time t₂ = 0.1 s, the series APF starts compensating voltage harmonics immediately by injecting out of phase harmonic voltage, making the load voltage at load distortion free. The voltage injected by series APF and the DC voltage are presented in Fig. 9(e) and Fig. 9(f) respectively. In this case, the load voltage THD_v (%) is improved from 46.93% to 3.67%. The harmonic spectrum of the source current and the load voltage before and after compensation are shown in Fig. 10.

Performances of UPQC for voltage sags compensation

To analyze the performance of UPQC during voltage sag conditions, the voltage source is assumed sinusoidal and does not contain any harmonics. The simulation results are shown in Fig. 11. There are three instants: t₁, t₂ and t₃. At time t₁ = 0.05 s, the shunt APF is put into operation. A sag (25%) is introduced into the system at time t₂ = 0.1 s, and the series APF is put into operation instantly. This sag lasted till t₃ = 0.22 s, where the system is again at normal working condition. During the voltage sag, the series APF provides the required voltage by injecting in phase compensating voltage (25%) equal to the difference between the reference load voltage and the real load voltage. During the sag voltage disturbance, it is observed that the source current is increased in Fig. 11(d), while the UPQC maintains the load voltage at desired constant voltage in Fig. 11(g), and the DC voltage is maintained constant in Fig. 11(f).

Performances of UPQC for voltage swells compensation

A swell (35%) is introduced into the system between t₁ = 0.1 s and t₂ = 0.22 s, as shown in Fig. 12. Under this condition, the series APF injects an out-of-phase compensation voltage (35%) in the line through series transformers, equal to the difference between the reference load voltages.

The UPQC controller acts in such a way that source delivers the reduced current, as shown in Fig. 12 (e). In other words, the extra power due to the voltage swell condition is fed back to the source by taking reduced fundamental source current. The shunt APF maintains the DC link voltage at almost a constant level. It increases slightly due to the swell on the system, as shown in Fig. 12 (f). The load voltage profile in Fig. 12 (g) indicates that the UPQC is effectively maintaining the load bus voltage at the desired constant level.

Performances of UPQC for voltage unbalance compensation

An unbalance is introduced into the system between t₁ = 0.1 s and t₂ = 0.22 s, as shown in Fig. 13. In this case, the three-phase voltage sources do not contain harmonic components; their expressions are given by

u_{sa}=\mathrm{311}\sin (\omega t)+\mathrm{31}\sin (\omega t),

(24)u_{sb}=\mathrm{311}\sin \left(\omega t+{\displaystyle \frac{\mathrm{4}\pi }{\mathrm{3}}}\right)+\mathrm{31}\sin \left(\omega t+{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right),

u_{sc}=\mathrm{311}\sin \left(\omega t+{\displaystyle \frac{\mathrm{2}\pi }{\mathrm{3}}}\right)+\mathrm{31}\sin \left(\omega t+{\displaystyle \frac{\mathrm{4}\pi }{\mathrm{3}}}\right).

Performances of UPQC for all voltage disturbances compensation

The performance of the proposed UPQC is also tested under all voltage disturbances simultaneously. The simulation results are exhibited in Fig. 14. The voltage sags (25%) is introduced voluntary between t₁ = 0.06 s and t₂ = 0.12 s. After that, a voltage swells (35%) is introduced between t₂ = 0.12 and t₃ = 0.18 s. The voltage harmonics is introduced between t₃ = 0.18 s and t₄ = 0.24 s. The unbalances is introduced between t₄ = 0.24 s and t₅ = 0.3 s. The system is again at normal working condition. This illustrates that the proposed UPQC is capable of mitigating all voltage disturbances and does not show any significant effect of disturbance type present in the utility voltages on its compensation capability.

Dynamic performances of UPQC for sudden change of load

To evaluate the performance of the proposed UPQC under transient condition, the load on the system is changed suddenly. The simulation results under this condition are shown in Fig. . Before t₁ = 0.05 s, the shunt and series APFs are not working, and the source current is highly distorted. After t₁ = 0.05 s the shunt AF is only on operation (the source current after compensation is nearly sinusoidal and in phase with the source voltage). The source voltage disturbances such as sag, swell, unbalance and harmonic voltages introduced between t₂ = 0.16 s and t₃ = 0.4 s, are effectively compensated using the proposed UPQC system. When the sudden load current disturbance is introduced voluntarily between t₄ = 0.25 s and t₅ = 0.35 s, the UPQC controller acts immediately without any delay, and the shunt APF injects a current equal to the sum of harmonic. In all the dynamic condition, the DC voltage is maintained constant and equal to the reference value U_dc-ref = 800 V using the proportional integral voltage controller. It is observed that the DC voltage passes through a transitional period of 0.02 s before stabilization and reaches its reference with moderate peak voltage approximately equal to 5 V. Before the shunt AF application, the source current is distorted with poor power factor, but after compensation the source current shown in Fig. 15(f) is sinusoidal and in phase with the source voltage for all voltage disturbances. The effectiveness of UPQC in reducing the supply current and load voltage harmonics for all disturbance conditions is proved.

The proposed control scheme for UPQC has been validated through simulation results using Matlab-Simulink software and SimPowerSystem toolbox. Through visualization (Figs. 9, 11, 12, 13, 14 and 15), it can be concluded that the operation of the proposed unified power quality energy based on three-level NPC inverters is successful. Before the application of shunt APF, the source current is equal to nonlinear load current, highly distorted and rich in harmonic. After compensation, the THDi is considerably reduced from 26.58% to 5.76% (without a passive filters) and the load voltage is instantly improved using the proposed UPQC for separate or simultaneously voltage disturbances such us sags, swells, unbalances and harmonics. The DC voltage is maintained at a constant value which is equal to the reference value U_dc-ref = 800 V by using PI voltage controller. Figure 14 (c) illustrates the dynamic response of the control loop. It is observed that the DC voltage passes through a transitional period of 0.02 s before stabilization and reaches its reference U_dc-ref = 800 V with moderate peak voltage approximately equal to 5 V when a step change in load current is introduced between t₁ = 0.25 s and t₂ = 0.35 s. The effectiveness of the proposed UPQC has been demonstrated in maintaining the three-phase load voltages balanced and sinusoidal with sinusoidal source current. Moreover the proposed system does not show any significant effect of disturbance present in the utility voltages on its compensation capability, and the load voltage under all voltage is maintained constant, balanced and sinusoidal.

Conclusions

To enhance the power quality by reducing the source current harmonics and improve the voltage delivered to sensible and critical loads, a novel UPQC configuration system based on three-level NPC inverter topology using fuzzy control techniques has been proposed in this paper. The adopted control strategy is based on the SRF detection method for the shunt AF and the instantaneous power method for the series AF. The UPQC model is developed and validated using Matlab-Simulink software and SimPowerSystem toolbox. The control algorithm of UPQC has been observed to be satisfactory for various power quality improvements like voltage harmonics mitigation, current harmonic mitigation, voltage sag, swell and unbalance compensation. The source current THD_i (%) is improved from 26.58% to 5.76%, while the load voltage THD_v (%) is improved from 46.93% to 3.67%. The proposed UPQC acts immediately (fast dynamic responses) with a small delay in the operation and has been found satisfactory under transient conditions.

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