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In this paper, nine switches UPQC topology with different control techniques have been depicted and compared their performances to accomplish clear inspection of the best control techniques. As conventional UPQC having twelve switches of six legs enhances the switching losses, circuit complication, costly etc. Hence nine switches UPQC topology is intended to reduce the switching losses, complexity, size and cost. This proposed topology has only three legs with three switches in each leg making total number of nine switches. The three control techniques such as Unit vector template, PQ theory and Synchronous reference frame theory have been employed in the nine switch UPQC topology. The simulation has been carried out separately for each control technique in MATLAB/SIMPOWER environment and results have been verified for higher efficiency of nine switch UPQC topology. The comparative THD of source currents and load voltages under these control techniques have been analysed. The obtained THD values are below 5% as per IEEE519 standard.
Active and Reactive Power (PQ), Synchronous Reference Frame (SRF), Total Harmonic Distortion (THD), Unified Power Quality Conditioner (UPQC), Unit Vector Template (UVT)
Nowadays, power quality improvements become an important factor for the smooth transfer of power in a distribution system [13]. Harmonics in voltage and in current, voltage dip and voltage swell, interruptions, transient over voltage, voltage variations, voltage steps and voltage fluctuations, high reactive power and distortion of voltage and current waveforms are some of the major issues of power quality decrement [39]. These power quality problems are caused due to nonlinearity behavior of equipment. Nonlinear devices, IT and office equipment, arching devices, load switching, large motor starting, interconnection of power system, lightning strikes and environmental changes are the major sources of power quality issues [3, 5, 812]. In industrial scenario, three phase power converters, arching devices and saturable devices are the major source of harmonics in current and voltage. In order to compensate these power quality issues, custom power devices (CPDs) have been employed in distribution lines [2, 3]. Many CPDs have been introduced like DSTATCOM as a shunt compensation, DVR as a series compensation and then UPQC, comprising both shunt and series compensation [13, 14]. The shunt and series inverter of conventional UPQC have 12 switches [15]. The 10switch based UPQC have been designed and developed in the studies [1618]. With the increased number of switches, the switching losses and complexity of the circuit will be more which requires complex control technique. Hence 9s UPQC has been proposed with nine switches for switching loss reduction and to create simplicity of the circuit [1922].
The practical application of 9s UPQC topology is in a typical steel manufacturing industry employing induction furnace, arcing devices and saturable devices. More over 9s UPQC can also be adopted in grid integration of solar PV system. The main work of this paper is to alleviate power quality matters in the distribution network by the proposed 9s UPQC topology. Different control schemes have been introduced for generation of gate signals for the inverter switches [3, 13, 2328]. These switches are controlled rectifiers controlled by gate signals which are generated by hysteresis or SPWM generator. In this paper, SRF, UVT and PQ theory have been employed in the nineswitch UPQC and compared their performances in terms of THD (Total harmonic distortion) of source current and load voltage.
2.1 Existing UPQC model
The present UPQC equipped with traditional components; DVR and DSTATCOM which can be combined to form UPQC [13, 14]. In DVR and DSTATCOM, inverter is used, in which six switches are used for series and shunt compensation. The basic structure of UPQC is given in Figure 1 and consists of Shunt compensator, Series compensator, Linear transformer, DC link and Nonlinear load.
Figure 1. Basic UPQC structure
2.2 Proposed UPQC model
The proposed model consists of both shunt and series compensator but the number of switches get reduced from 12 to 9. The reduction in number of switches not only reduces switching losses but also reduces the complexity of the UPQC system. The new model is termed as 9S UPQC which utmost will be helpful in getting economical benefits to the distribution companies [19]. The basic structure of proposed UPQC model is given in Figure 2 and can be analyzed by the following points:
a) Dual shunt and series compensator: This compensator consists of total nine switches which can be used as per requirement for the compensation. Upper three switches can be used in series compensation and lower three switches can be used in shunt compensation and the middle switches can be used in both compensations.
b) Linear transformer: This transformer works on the same principle as in traditional UPQC. Similar ratings can be used in both 9S and normal UPQC.
c) DC link: Similar inductor or capacitor can be used as an energy storing device for maintaining the voltage and current waveform sinusoidal.
Figure 2. 9S UPQC
Basically control schemes are being used for controlling the switches of inverters used in UPQC [13]. Three different control techniques have been analyzed and compared their performances to examine the best among these.
3.1 Unit vector template (UVT)
Unit vector template generates the reference signals of current and voltage for shunt and series inverters respectively. For series inverter, the three phase source voltage is applied to the phase locked loop to achieve the sine angle. From these sine angles, three unit vectors (Ua, Ub and Uc) are obtained as:
$\left[ \begin{matrix} {{U}_{a}} \\ {{U}_{b}} \\ {{U}_{c}} \\ \end{matrix} \right]=\left[ \begin{matrix} 1 & 0 \\ {}^{1}/{}_{2} & {}^{\sqrt{3}}/{}_{2} \\ {}^{1}/{}_{2} & {}^{\sqrt{3}}/{}_{2} \\ \end{matrix} \right]\left[ \begin{matrix} \sin \theta \\ \cos \theta \\ \end{matrix} \right]$ (1)
The three unit vectors are multiplied by the voltage magnitude V_{LM} to get threephase reference PCC voltages and these reference PCC voltages are compared with the actual PCC voltages.
$\left[ \begin{matrix} V_{la}^{*} \\ V_{lb}^{*} \\ V_{lc}^{*} \\ \end{matrix} \right]={{V}_{LM}} \left[ \begin{matrix} {{U}_{a}} \\ {{U}_{b}} \\ {{U}_{c}} \\ \end{matrix} \right]$ (2)
At last hysteresis controller play important role by converting these compared signals to the gate pulses which is fed to the series inverters for voltage control to the main UPQC circuit. For shunt inverters, voltage error is computed from the comparison of average and the reference value of the dc bus voltage. This error is fed to PI controller (dc voltage controller). The output of PI controller (i.e. reference amplitude (I*_{Sp}) of the supply currents) is multiplied with the three unit vectors (Ua, Ub and Uc) to get three phase reference supply currents.
$\left[ \begin{matrix} I_{Sa}^{*} \\ I_{Sb}^{*} \\ I_{Sc}^{*} \\ \end{matrix} \right]=I_{Sp}^{*} \left[ \begin{matrix} {{U}_{a}} \\ {{U}_{b}} \\ {{U}_{c}} \\ \end{matrix} \right]$ (3)
The computed threephase supply reference currents are compared with the actual threephase supply currents and then feed to hysteresis controller to generate the switching signals for the inverter switches. Figure 3 indicates UVT control scheme for series and shunt inverter.
3.2 PQ theory
In this theory Clarke’s and Inverse Clarke’s transformation is used for generating reference signals for gate pulses. In Clarke’s transformation abc rotating reference frame is transformed to αβ0 static reference frame and vice versa in inverse Clarke’s transformation.
$\left[ \begin{matrix} {{V}_{\alpha }} \\ {{V}_{\beta }} \\ \end{matrix} \right]=\sqrt{\frac{2}{3}} \left[ \begin{matrix} 1 & {}^{1}/{}_{2} & {}^{1}/{}_{2} \\ 0 & {}^{\sqrt{3}}/{}_{2} & {}^{\sqrt{3}}/{}_{2} \\ \end{matrix} \right]\left[ \begin{matrix} {{V}_{Sa}} \\ {{V}_{Sb}} \\ {{V}_{Sc}} \\ \end{matrix} \right]$ (4)
$\left[ \begin{matrix} {{I}_{\alpha }} \\ {{I}_{\beta }} \\ \end{matrix} \right]=\sqrt{\frac{2}{3}} \left[ \begin{matrix} 1 & {}^{1}/{}_{2} & {}^{1}/{}_{2} \\ 0 & {}^{\sqrt{3}}/{}_{2} & {}^{\sqrt{3}}/{}_{2} \\ \end{matrix} \right]\left[ \begin{matrix} {{I}_{La}} \\ {{I}_{Lb}} \\ {{I}_{Lc}} \\ \end{matrix} \right]$ (5)
Active and reactive power is calculated from the αβ component of voltages and currents.
${{P}_{L}}=({{V}_{\alpha }}*{{I}_{\alpha }})+({{V}_{\beta }}*{{I}_{\beta }}) \\ {{Q}_{L}}=({{V}_{\beta }}*{{I}_{\alpha }})({{V}_{\alpha }}*{{I}_{\beta }})$ (6)
To achieve DC component of active and reactive power, calculated Active and reactive power is processed through LPF. The loss component of active power P_{loss} is calculated from compared DC voltage which is obtained from the PI controller. VAR reactive power i.e. Q_{vr} is computed from the compared PCC voltage which is obtained from AC voltage controller. The p* is calculated from P_{loss} and $\overline{P_{L}}$ and q* is calculated from Qvr and $\overline{Q_{L}}$. The reference current in αβ frame is calculated from the reference value of active and reactive power i.e. p* and q*.
${{P}_{L}}=\overline{{{P}_{L}}}+\overline{\overline{{{P}_{L}}}}, {{Q}_{L}}=\overline{{{Q}_{L}}}+\overline{\overline{{{Q}_{L}}}}$ (7)
${{p}^{*}}=\overline{{{P}_{L}}}+{{P}_{LOSS}}, {{q}^{*}}=\overline{{{Q}_{L}}}{{Q}_{vr}}$ (8)
From the inverse clarke’s transformation, three phase reference current in abc frame is obtained.
$\left[ \begin{matrix} I_{Sa}^{*} \\ I_{Sb}^{*} \\ I_{Sc}^{*} \\ \end{matrix} \right]=\sqrt{{}^{2}/{}_{3}}\left[ \begin{matrix} 1 & 0 \\ {}^{1}/{}_{2} & {}^{\sqrt{3}}/{}_{2} \\ {}^{1}/{}_{2} & {}^{\sqrt{3}}/{}_{2} \\ \end{matrix} \right]{{\left[ \begin{matrix} {{V}_{\alpha }} & {{V}_{\beta }} \\ {{V}_{\beta }} & {{V}_{\alpha }} \\ \end{matrix} \right]}^{1}}\left[ \begin{matrix} {{p}^{*}} \\ {{q}^{*}} \\ \end{matrix} \right]$ (9)
This reference three phase current is compared with the actual three phase current and fed to the hysteresis controller to produce gate pulses for thyristors. Figure 4 indicates block diagram representation of PQ Theory for generating gate pulses.
3.3 Synchronous reference frame
In this technique, Phase locked loop (PLL) is synchronized with the PCC voltage and generates phase angle for the Park’s and Inverse Park’s transformation [25, 26]. In Park’s transformation, three phase source current is converted into d component of source current and then fed to Low pass filter.
$\left[ \begin{matrix} 0 \\ {{I}_{sD}} \\ 0 \\ \end{matrix} \right]=\sqrt{\frac{2}{3}}\left[ \begin{matrix} \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ \sin \theta & \sin (\theta \frac{2\pi }{3}) & \sin (\theta +\frac{2\pi }{3}) \\ \cos \theta & \cos (\theta \frac{2\pi }{3}) & \cos (\theta +\frac{2\pi }{3}) \\ \end{matrix} \right]\left[ \begin{matrix} {{I}_{Sa}} \\ {{I}_{Sb}} \\ {{I}_{Sc}} \\ \end{matrix} \right]$ (10)
The Low pass filter is used for extraction of DC component of current and voltage i.e. I_{sDdc} and V_{sDdc}. The compared signal of DC capacitance voltage and DC reference voltage is passed through PI controller to generate current loss i.e. I_{loss}. The current loss i.e. I_{loss }is to be added with the DC component of I_{sD} to obtain reference current I*_{D}.
$I_{D}^{*}={{I}_{Loss}}+{{I}_{sDdc}}$ (11)
Figure 5. Synchronous reference frame
At last, I_{D}*and V_{sDdc} are transformed to abc component by Inverse Park’s transformation and further sent to hysteresis controller. In hysteresis controller, the reference signals are compared with the corresponding actual signals and generate gate pulses for the thyristors. Figure 5 indicates block diagram of Synchronous reference frame technique.
$\left[ \begin{matrix} I_{Sa}^{*} \\ I_{Sb}^{*} \\ I_{Sc}^{*} \\ \end{matrix} \right]=\sqrt{\frac{2}{3}}{{\left[ \begin{matrix} \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ \sin \theta & \sin (\theta \frac{2\pi }{3}) & \sin (\theta +\frac{2\pi }{3}) \\ \cos \theta & \cos (\theta \frac{2\pi }{3}) & \cos (\theta +\frac{2\pi }{3}) \\ \end{matrix} \right]}^{1}}\left[ \begin{matrix} 0 \\ I_{D}^{*} \\ 0 \\ \end{matrix} \right]$ (12)
$\left[ \begin{matrix} 0 \\ {{V}_{sD}} \\ 0 \\ \end{matrix} \right]=\sqrt{\frac{2}{3}}\left[ \begin{matrix} \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ \sin \theta & \sin (\theta \frac{2\pi }{3}) & \sin (\theta +\frac{2\pi }{3}) \\ \cos \theta & \cos (\theta \frac{2\pi }{3}) & \cos (\theta +\frac{2\pi }{3}) \\ \end{matrix} \right]\left[ \begin{matrix} {{V}_{Sa}} \\ {{V}_{Sb}} \\ {{V}_{Sc}} \\ \end{matrix} \right]$ (13)
$\left[ \begin{matrix} V_{La}^{*} \\ V_{Lb}^{*} \\ V_{Lc}^{*} \\ \end{matrix} \right]=\sqrt{\frac{2}{3}}{{\left[ \begin{matrix} \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ \sin \theta & \sin (\theta \frac{2\pi }{3}) & \sin (\theta +\frac{2\pi }{3}) \\ \cos \theta & \cos (\theta \frac{2\pi }{3}) & \cos (\theta +\frac{2\pi }{3}) \\ \end{matrix} \right]}^{1}}\left[ \begin{matrix} 0 \\ {{V}_{sDdc}} \\ 0 \\ \end{matrix} \right]$ (14)
4.1 Series part calculations
4.1.1 Linear transformer
The rating of this transformer can be assigned on the basis of voltage coming from source:
${{V}_{inj}}=X*{{V}_{S}}$ (15)
VA rating of transformer= $3 * V_{i n j} * I_{i n j}$ (sag condition)
4.1.2 Inductors on series side
Secondary side current can be determined by load real power and this current can be helpful to find series inductance.
${{I}_{\sec }}=\frac{{{P}_{L}}}{3*(1+X)*{{V}_{S}}}$ (16)
${{L}_{se}}=\frac{\sqrt{3}}{2}*\frac{M*K*{{V}_{dc}}}{{{f}_{s}}*6*a*{{I}_{\sec }}}$ (17)
4.2 Shunt part calculations
4.2.1 Inductor on shunt side
It can be determined from DC voltage, ripple current and switching frequency.
${{L}_{f}}=\frac{\sqrt{3}*M*{{V}_{dc}}}{12*a*{{f}_{s}}*{{I}_{rc}}}$ (18)
4.2.2 Switch rating of voltage and current
Dynamic conditions can be calculated by following equation;
${{V}_{r}}={{V}_{dc}}+{{V}_{w}}$ (19)
${{I}_{r}}=1.25*({{I}_{rc}}+{{I}_{pk}})$ (20)
4.3 Switching mechanism
There are nine thyristors in 9S UPQC so in order to work upper six thyristors as series compensator and lower six thyristors as shunt compensators; XOR gate has been used [19]. XOR gate confirms that which thyristors must be working in particular situations. Following scheme has been used for this mechanism:
$G7=\overline{G1.G4}, G8=\overline{G2.G5}, G9=\overline{G3.G6}$
4.4 DC voltage control
For controlling the current of the inverter V_{dc} means DC voltage across capacitor should be always greater than the line to line voltage (peak) of distribution lines. The DC voltage is selected in such a way that it reduces the ripples as much as possible. It can be calculated as:
${{V}_{DC}}=\frac{2\sqrt{2}*{{V}_{L}}}{\sqrt{3}*m}$ (21)
DC Capacitor value of 9S UPQC can be calculated from the following equation:
${{C}_{DC}}=\frac{{{K}_{1}}*6*V*I*a*t}{V_{DC}^{2}V_{DC1}^{2}}$ (22)
The nineswitch UPQC topology with the UVT, PQ and SRF control techniques have been modeled and executed in MATLAB/ simulink environment using sim power tool boxes. The simulation works have been performed by considering 0.8 sec simulation time in ode5 solver.
Simulation results in nineswitch UPQC of Figure 6, Figure 7 & Figure 8 show the load voltages of each phase a, b and c under after compensation and before compensation condition for SRF, PQ and UVT theory respectively. The load voltages are compensated by series voltages, injected by DVR circuit of nineswitch UPQC. Previously load voltages are harmonically distorted due to nonlinear loads but after compensation, load voltages become sinusoidal.
(a)
(b)
Figure 6. Three phase load voltages (a) after and (b) before compensation in SRF
(a)
(b)
Figure 7. Three phase load voltages (a) after and (b) before compensation in PQ theory
(a)
(b)
Figure 8. Three phase load voltages (a) after and (b) before compensation in UVT
(a)
(b)
Figure 9. Three phase source currents (a) after and (b) before compensation in SRF
(a)
(b)
Figure 10. Three phase source currents (a) after and (b) before compensation in PQ theory
(a)
(b)
Figure 11. Three phase source currents (a) after and (b) before compensation in UVT
Now the simulation results in nineswitch UPQC topology have been observed by considering source currents parameter. Figure 9, Figure 10 & Figure 11 show the source currents of three phase a, b and c on after compensation and before compensation in SRF, PQ and UVT theory respectively. The source currents are compensated by shunt current injected by DSTATCOM circuit of nine switch UPQC. Previously, source currents are harmonically distorted due to nonlinear loads but after compensation, THD of source current get reduced and become sinusoidal.
From the simulation results of nine switch UPQC, THD analysis of load voltages have been observed. Figure 12, Figure 13 & Figure 14 depict THD spectrum of load voltages of phase a, b & c after compensation which are 1.04%, 1.03% and 1.17% respectively in SRF control technique, 1.11%, 2.06% and 2.23% respectively in PQ Theory & 3.88%, 3.53% and 3.57% respectively in UVT control technique where the deducted THD values are within the IEEE 519 harmonic Standard limits [29].
Now the nine switch UPQC results have been explained in terms of source currents THD. Figure 15, Figure 16 & Figure 17 show THD spectrum of Source currents of phase a, b & c for the case of after compensation. In SRF technique, Source currents THD is reduced to 2.55% for phase a, 1.88% for phase b &0.89% for phase c, In PQ Theory, Source currents THD is reduced to 2.39% for phase a, 2.47% for phase b & 3.06% for phase c and In UVT Theory, Source currents THD is reduced to 4.75% for phase a, 4.32% for phase b & 4.59% for phase c. In all control techniques, the deducted THD value of Source currents is less than 5% which successfully fulfills criteria of IEEE 519 standard limits.
Table 1 represents comparative THD analysis of three phase load voltages and source currents for the case of after compensation and before compensation in the nineswitch UPQC topology under SRF, PQ and UVT techniques. From the comparative THD analysis of Table 1, it has been found that the lowest THD value of voltages and currents are achieved by SRF theory. Figure 18 and Figure 19 depict pictorial THD analysis of three phase load voltages and source currents for the case of after compensation and before compensation under SRF, PQ and UVT techniques.
(a) Load Voltage a
(b) Load Voltage b
(c) Load Voltage c
Figure 12. %THD in SRF
(a) Load Voltage a
(b) Load Voltage b
(c) Load Voltage c
Figure 13. %THD in PQ theory
(a) Load Voltage a
(b) Load Voltage b
(c) Load Voltage c
Figure 14. % THD in UVT
(a) Source Current a
(b) Source Current b
(c) Source Current c
Figure 15. %THD in SRF
(a) Source Current a
(b) Source Current b
(c) Source Current c
Figure 16. %THD in PQ theory
(a) Source Current a
(b) Source Current b
(c) Source Current c
Figure 17. % THD in UVT
Figure 18. Pictorial representation of THD (After compensation)
Figure 19. Pictorial representation of THD (Before compensation)
Table 1. THD analysis of load voltage and source current
THD AFTER COMPENSATION (%) 



SRF Theory 
PQ Theory 
UVT 
Three Phase Load voltages 
Va 
1.04 
1.11 
3.88 
Vb 
1.03 
2.06 
3.53 

Vc 
1.17 
2.23 
3.57 

Three Phase Source currents 
Ia 
2.55 
2.39 
4.75 
Ib 
1.88 
2.47 
4.32 

Ic 
0.89 
3.06 
4.59 

THD BEFORE COMPENSATION (%) 



SRF Theory 
PQ Theory 
UVT 
Three Phase Load voltages 
Va 
16.72 
39.80 
39.80 
Vb 
26.89 
42.29 
42.29 

Vc 
20.37 
80.74 
86.23 

Three Phase Source currents 
Ia 
18.29 
25.54 
25.54 
Ib 
24.85 
23.42 
23.42 

Ic 
17.31 
85.14 
91.50 
Now a days, there are many custom power devices such as DSTATCOM, DVR used in power distribution system and UPQC is most efficient among all due to its dual scheme simultaneous compensation in the source current and the load voltage. UPQC also have different topologies having different number of switches. This paper presents the comparative analysis of SRF, PQ and UVT based nineswitch UPQC topology. From the comparative analysis, it has been found that SRF based nineswitch UPQC topology is much efficient in reducing THD in load voltages and source currents to large extent as compared to PQ and UVT control techniques. Moreover, due to decrease in switches from 12 to 9, nineswitch UPQC topology have less switching losses which reduce upto 33%. It has been also concluded that the THD in load voltages and source currents of all the three control schemes satisfy the IEEE 519 harmonic standards limits.
CPD 
Custom Power Device 
DC 
Direct Current 
DSTATCOM 
Distribution STATCOM 
DVR 
Dynamic Voltage Restorer 
IEEE 
Institute of Electrical and Electronics Engineers 
PCC 
Point of Common Coupling 
PI 
Proportional Integral 
PLL 
PhaseLocked Loop 
PQ 
Power Quality 
SRF 
Synchronous Reference Frame 
THD 
Total Harmonic distortion 
UPQC 
Unified Power Quality Conditioner 
VSI 
Voltage Source Inverter 
VA 
Volt Ampere 
Table. Parameters used in simulations and mathematical modeling
Elements 
Parameters 
Values 
AC Source 
Voltage Va, Vb, Vc 
110 V 
Frequency f 
50 Hz 

Resistance 
0.01 Ώ 

Inductance 
10 mH 

Linear transformer 
Nominal Power 
2 KVA 
Frequency 
5 Hz 

Series Line Impedance 
Resistance (Rse) 
1 Ώ 
Inductance (Lse) 
5 mH 

Shunt line Impedance 
Resistance (Rsh) 
1 Ώ 
Inductance (Lsh) 
3 mH 

DC voltage Capacitance 
Capacitance (Cdc) 
5000μF 
Modulation Index 
m 
1 
Turns Ratio 
k 
3 
Voltage Variation 
X 
30% 
Overloading Factor 
a 
1.2 
DC dynamic voltage 
V_{w} 
10% 
Ripple current 
Irc 
1520% 
Non Linear loads 
Three phase diode rectifier 
 
Nominal load Rn 
10 Ώ 

Nominal load Ln 
5mH 
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