Observer-Based Quadrature Signal Generator for UPQC in a Single-Phase Distribution System

- This paper presents a control algorithm for generation of reference voltage and current based on Observer-based QSG (Quadrature Signal Generator) for UPQC (Unified Power Quality Conditioner) in a Single-Phase Distribution System. The proposed UPQC is a combination of DSTATCOM and DVR used for load and source compensation such as voltage sag and swell, voltage and current harmonics etc. The proposed control algorithm is capable of generating reference signals from the distorted voltages and currents. In this paper, an improved OQSG-based PLL (Phase-Locked Loop) is utilized over OQSG to enable and operate with increased bandwidths that will improve the dynamic response, tracking accuracy and faster detection of reference signal under all varying load and grid conditions. The control algorithm is tested and evaluated using MATLAB / Simulink.


I. INTRODUCTION
oday's interest is towards the renewable energy sources to produce electrical power. In this regard, single-phase photovoltaic (PV) power generation is increasing widely for residential applications. But, these are creating severe power quality (PQ) issues to the distribution system while connecting to the grid. Power electronic loads such as rectifiers, arc furnaces and fluorescent lamps etc are causing PQ problems such as harmonics, flickering, and excessive reactive power demand and sudden switching of loads at the point of common coupling (PCC) are also causing voltage related problems [1][2][3][4]. 92% of interruptions are due to voltage sags and to be compensated within 3 cycles. Voltage swells are less frequent; but, these swells are producing electromagnetic stress in power transformers due to flux saturation [5]. Voltage flicker is generally caused by the lightning intensity. It is described as voltage fluctuation by IEEE . Normally the voltage ranges in between 0.95 and 1.05p.u. Nearly 48% of power quality problems are due to voltage sags/swells and 22% are due to harmonics [6].
A UPQC is a combination of DVR and DSTATCOM and provides single solution for both voltage and current related problems such as harmonics, sag, swell and power factor improvement. It consists of two voltage source converters (VSC's) connected back to back with a common DC link capacitor at the DC bus.
Single-phase power generation is increasing widely based on PV system interfaced with a grid-tied inverter. Therefore, study of UPQC for single-phase applications has a prominent role to avoid both voltage and current related problems.
Different UPQC topologies are explained in the literature Multi variable regulator (MVR), Neural Network etc. [7]- [11]. Control algorithms based on single-phase UPQC are investigated in [12]- [13]. Control algorithms based on park transformation [14]- [15] and PQ theory (Instantaneous Active and Reactive Power) [16] are most popular and widely used techniques. In [17], three control algorithms such as UVT (Unit Voltage Template), DQ Theory and theory based on Fourier analysis are compared for single-phase applications.
In general, UPQC based compensation mainly depends on reference voltage and current generation that should be injected into the system by controlling of VSI. The performance of the UPQC depends on the tracking accuracy, speed and control method that was employed.
In this paper, Improved Observer Based Quadrature Signal Generator based control algorithm for UPQC in a single-phase distribution system is implemented for Voltage T sag/swell compensation and load harmonic compensation. A single-phase OQSG is used for better estimation and accuracy of sinusoidal signal with wide range of frequencies.
However, this implementation for a single-phase UPQC is reported less in the literature. Simulations are carried out using MATLAB / Simulink, simpower systems block set.

II. PROPOSED CONFIGURATION AND CONTROL ALGORITHM
The UPQC shown in Fig. 1 consists of two VSC's comprising of 4 IGBT switches in each converter feeding Linear/Nonlinear loads. The UPQC is a combination of Series and Shunt Active Filters connected in between the source and load. A ripple filter is connected to reduce any high frequency noises that are produced by the UPQC. The proposed controller for UPQC is capable of maintaining the total harmonic distortion (THD %) of voltage and current within the IEEE 519-1992 limits and load voltage is maintained at 1 p.u. even under sag and swell conditions. The proposed control algorithm has peak amplitude extraction for both voltage and current, unit template generation using OQSG-PLL and reference load voltage generation and reference source current generation and PWM generator to generate gating pulses for both VSC's.

A. OQSG-PLL
In Fig. 2, the observer-based QSG is outlined, which represents a basic QSG observer proposed in [29] with DC offset compensation. In the figure, Vi represents the QSG input signal, Vo the output that represents the direct component of the Vi signal, V  the output signal that represents the quadrature component of the input signal, ωn the input signal angular frequency, and K1, K2 and K3 the observer parameters.
Based on the observer QSG outlined in Fig. 2, the following transfer functions (1) and (2) are obtained.
By analyzing the transfer functions (1) and (2), the following conclusions can be made: • By using parameters K1, K2 and K3, the observer pole values can be set to any desired value. • For s = jωn the OQSG outputs are V  (jωn) = V g (jωn), and V  (jωn) = Vi(jωn)/j, which means that V  represents the direct and V  the quadrature input signal Vi component. • The problem related to transfer functions (3) and (4) is that their zeros zV  = − k3ωn/k2 and zV  = k2ωn/k3 cannot be tuned independently to the observer poles. This may in some cases result in GV  (jω) and GV  (jω) frequency responses in which the higher frequency inputs signal components are significantly amplified.
The outputs from the orthogonal QSG are fed to the transformation block to convert into dq frame. Park transformation is used to convert αβ to dq.
The transformation output V d is used to eliminate high frequency noises by passing it through a Loop Filter (LF) and the estimated phase angle ˆ is generated by adding fundamental frequency (W). The bandwidth response is tuned for various gains k1, k2, k3 and implemented as shown in Fig.  3. Fig.3 shows the closed loop block diagram of OQSG-PLL. Single-phase OQSG-PLL is used for source voltage (V s ) and load current (I L ) to generate in-phase and quadrature templates. The output phase angles (Sin (θ), Cos (θ)) that are generated from the OQSG-PLL are considered as unit templates. In OQSG-PLL, in-phase and quadrature signals are generated and supplied it to park's transformation to convert into d-q signals. Either d or q are fed to loop filter based on the requirement (d in this case) to generate error in angular frequency and then added to the nominal frequency to obtain actual angular frequency and then supplied to voltage controlled oscillator to produce phase angle (θ PLL ). This phase angle converted to sine angle acts as unit template (U PLL ).

C. Reference Source Current Generation
The peak amplitude of active component of current is estimated as shown in Fig. 1. Then I P is passed through lowpass filter to eliminate ripples produced from the OQSG and to produce a reference magnitude of current (I LP ). Then it is added to I LOSS component. The measured voltage (V dc ) across DC capacitor is compared with reference DC bus voltage (V dc * ). The error of the signal at n th sampling instant is given by: V d (n) = V * dc (n) -V dc (n) (4) The voltage error V d (n) is then supplied to Proportional-Integral controller to regulate the DC bus voltage of Shunt APF to produce I LOSS component. At n th sampling instant, the output of the PI controller is as: I LOSS (n)=I Loss (n-1)+k p {V dcer (n)-V dcer (n-1)}+k i V dcer (n) Where, K p and K i are proportional gain and integral gains of the PI controller. V dcer (n) and V dcer (n-1) are DC bus voltage errors in n th and (n-1) th instant and I Loss (n) and I Loss (n-1) are the amplitudes of active component of currents at the fundamental reference current in n th and (n-1) th instant.
The average magnitude of current (I LP ) and the output of the PI controller (I Loss ) are summed up and multiplied with the unit templates that are generated by the grid voltage to generate reference source current. The estimated reference source currents (i s * ) is compared with the sensed source current at the point of common coupling (i s ) and supplied to a PWM generator to generate gating pulses for Shunt APF as shown in Fig. 1.

D. Reference Load Voltage Generation
The peak value amplitude of reference load voltage is generated by using unit template (U PLL ) generated by OQSG as shown in Fig.1. The magnitude of load voltage is multiplied with the U PLL to obtain estimated load voltage and then subtracted with the actual load voltage measured across the load and then supplied to the PWM generator to generate gating pulses to the Series APF.

III. SIMULATION RESULTS AND DISCUSSION
In this section, OQSG was proposed in the control algorithm for reference voltage generation and it is tested and evaluated using MATLAB/Simulink on a single-phase distribution system. Fixed time step of 10µs with ode1 (Euler) solver is chosen for simulation. Source Voltage (V S ), Source Currents (I S ), Load Currents (I L ), Load Voltage (V L ) and DC link Voltage (V dc ) are observed. Some of the test cases are chosen in the simulation.

Steady State Condition under Non-Linear Load.
A diode bridge rectifier load is applied to test the performance of UPQC at time t=0s to 2s. The source current is maintained sinusoidal even under non-linear load condition.

30 % of Voltage Sag is introduced in source voltage.
A voltage sag is introduced into the supply system and still the load voltage is maintained constant as shown in Fig. 7 from time t=0.5s to 0.7s. 3. 30 % of Voltage Swell is introduced in source voltage. A voltage swell is introduced into the supply system and the load voltage is maintained constant as shown in Fig.  8 from time t=1s to 1.2s. 4. 5 th and 7 th harmonics are injected into source voltage. 5 th and 7 th harmonics are injected into the source voltage at time t=1.5s to 1.7s. At this condition, the UPQC is performing well and the load voltage is maintained constant and pure sinusoidal. 5. Sudden increase in Non-Linear load.
There is a sudden increase in Non-Linear load to test the dynamic performance of the system from time t=2s to 2.2s. All these cases are tested and evaluated using MATLAB / Simulink and the results found satisfactory. Below table shows the total harmonic distortion (THD %) with the above mentioned test cases. In all the cases both Shunt and Series APF are working together and satisfying the IEEE limits.

IV. CONCLUSION
In this paper, a simple and effective control algorithm based on Observer QSG for single-phase UPQC has been analyzed, presented and validated using MATLAB/Simulink. This theory is adopted to work in sinusoidal and non-sinusoidal grid voltages such as sag, swell and harmonics and load conditions such as harmonics and increase in load. The source current is maintained within IEEE 519-1992 limits. The control algorithm is very promising and easy to implement than the conventional controllers proposed in the literature because of its simple structure and accuracy.