Power Quality Enhancement Using Power Balance Theory Based DSTATCOM

The DSTATCOM (Distributed Static Compensator) is used for current harmonic mitigation, Power Factor Correction (PFC), reactive power compensation, load balancing and neutral current compensation in the Power Distribution System (PDS). In this paper, the power balance theory based DSTATCOM is used for power quality enhancement like current harmonic mitigation, power factor correction (PFC), reactive power compensation, load balancing and neural current compensation and load balancing. A nonisolated star/delta transformer is to reduce dc-link voltage (vdc) of Voltage Source Converter (VSC) and neutral current compensation. The reference source currents can be extracted quickly by using proposed power balance theory. The proposed power balance theory based DSTATCOM is modeled and simulated using MATLAB/SIMULINK under PFC and ZVR (Zero Voltage Regulation) operations.


Introduction
The power distribution system network is highly polluted due to the presence of power electronic and lagging power factor loads.Because of this, the power quality of supply delivered to the residential, commercial and industrial loads is also much affected [1], [2].These power electronic based converters create pollu-tion to PDS and also reduce utilization of equipment to their rated value [3].The major power quality problems are voltage/current harmonics, voltage flicker, transients, sag/swell, load unbalancing, high reactive power, excessive neutral current and poor voltage regulation, etc [4].A group of power electronic based Custom Power Devices (CPD) such as DSTATCOM, DVR and UPQC are used for mitigation of power quality problems at the three-phase AC source and as well as at load [5].The power quality at power distribution system is governed by many standards such as IEEE519, IEEE1159, IEEE241, and IEC61642, etc [6].
DSTATCOM is a shunt connected custom power device, which mitigates PQ problems such as power factor correction at the source, current harmonics, reactive power compensation, load unbalancing, etc.The classification of various topologies and control algorithms are reported in the literature [7], [8].The design, selection of DC bus capacitor, interfacing AC inductor and IGBTs of DSTATCOM are discussed [9].The primary requirement of any CPD is a quick detection of distortion with high accuracy, a quick response of controller and a generation of reference signals [10], [11].For a generation of reference signals, many control algorithms are reported in the literature such as sinusoidtracking algorithm [12], SRF theory based control algorithm [13], parallel neural network based algorithm [14] and nonlinear control algorithm [15].In this paper, power balance theory based DSTATCOM for power factor correction and zero voltage regulation mode operations of DSTATCOM for mitigation of current related PQ problems such as excessive reactive power component, currents harmonics, power factor correction and unbalanced loads have been presented [16].A three-phase star/delta transformer is connected at the Point of Common Coupling (PCC) to compensate neutral current and also reduces DC link voltage of capacitor [17], [18].
The features of proposed DSTATCOM are: • Power Factor Correction (PFC) operation: harmonics compensation, unity power factor maintenance at source and load balancing.
• Zero Voltage Regulation (ZVR) operation: harmonics compensation, unity power factor maintenance at the source, reactive power compensation, and load balancing.
• Control strategy is robust, simple and efficient.
• Star/delta transformer is used to reduce the DClink voltage of VSC and for neutral current compensation.
The performance of power balance theory based DSTATCOM is modeled and simulated using MATLAB/SIMULINK with linear/non-linear R-L loads under PFC and ZVR operations.

Design and Characteristics of DSTATCOM
The schematic diagram of power balance theory based DSTATCOM with star/delta transformer in a distri- Three-phase four-wire Non-linear loads Computation of Active and Reactive Powers  

In-Phase Component of Reference Source Currents
The in-phase unit vectors for reference source currents are estimated as: where v t is the amplitude of the terminal voltage at PCC.
The instantaneous active load power (p L ) are estimated as: This instantaneous active load power (p L ) consists of AC and DC components.An LPF (Low Pass Filter) is used to extract DC component of load power (P Ldc ).
The magnitude of the fundamental active power component of load current can be estimated as: The DC bus voltage error of VSC (v erdc(m) ) at m th sample instant is given by: where v * dc(m) is the reference DC bus voltage, and v dc(m) is the sensed DC bus voltage of VSC at m th sample instant.The output of the DC PI controller is to control DC bus voltage of the VSC at mth sample instant, which is given by: where i dloss(m) is current loss component of DC bus voltage controller.The k dp and k di are the proportional and integral gains of the DC bus PI controller.This loss component current (i dloss ) is added to the magnitude of the fundamental active power component of load current (i Ld ): where i * sd is considered as the magnitude of in-phase reference source current of the VSC.The in-phase component of reference source currents (i * sad , i * nsbd , i * scd ) are given below:

Quadrature Component of Reference Source Currents
The quadrature vector for reference source currents are estimated as: The instantaneous reactive load power (q L ) are estimated as: This instantaneous reactive load power (q L ) consists of AC and DC components.An LPF is used to extract DC component of load power (q Ldc ).The magnitude of the fundamental reactive power component of load current can be estimated as: The terminal voltage error at PCC (v ert(m) ) at m th sample instant is given by: where v * t(m) is the reference terminal voltage at PCC and v t(m) is the sensed terminal voltage at PCC at m th sample instant.The output of the AC bus PI controller is to control terminal voltage at m th sample instant, which is given as: where i qloss(m) is current loss component of AC bus voltage controller.The k qp and k qi are the proportional and integral gains of the AC bus PI controller.This loss component current (i qloss ) is added to the magnitude of the fundamental reactive power component of load current (i Lq ): where i * sq is considered as the magnitude of quadrature reference source current.The quadrature phase component of reference source currents (i * saq , i * sbq , i * scq ) are given below: i * saq = i * sq w aq , i * sbq = i * sq w bq , i * scq = i * sq w cq . (17)

Generation of Reference Source Currents for VSC
The generations of reference source currents for threeleg VSC are summation of in-phase and quadrature phase component of reference source current:

Star/Delta Transformer Connection
The three-phase star/delta transformer is used for neural current compensation and also reduces DC link voltage of VSC.The star/delta transformer connection is depicted in Fig. 2. The primary winding voltage per phase is: Hence, star/delta transformer rated voltage is selected as 240 V.The secondary voltage of delta winding is also selected as 240 V.The rating of three single-phase transformers is 3 kVA, 240/240 V.

Current Controlled PWM Controller
The three-phase source current errors (i sae , i sbe , i sce ) are the difference between reference source currents (i * sa , i * sb , i * sc ) and source currents (i sa , i sb , i sc ).These source current errors are compared with triangular wave frequency (f s ) of 10 kHz for generating of switching signals for three-leg VSC.

Results and Discussion
The proposed power balance theory based DSTATCOM is modeled and simulated by using MATLAB/SIMULINK.The performance of the power balance theory for DSTATCOM is studied for PFC and ZVR modes of operation under linear/nonlinear R-L loads and results are discussed in the following sections.

Performance of Power Balance Theory Based DSTATCOM Under PFC Mode with Linear Loads
The performance of power balance theory based DSTATCOM under PFC mode with linear loads is shown in Fig. 3.The proposed DSTATCOM parameters such as source voltage (v sabc ), source current (i sabc ), load phase currents (i La , i Lb , i Lc ), compensating current (i cabc ), load neutral current (i Ln ), source neutral current (i sn ), compensating neutral current (i cn ), DC bus voltage (v dc ), its corresponding reference DC bus voltage (v * dc ), terminal voltage (v t ) and its corresponding reference terminal voltage (v * t ) are shown in Fig. 3.At t = 0.75 second, suddenly one phase of the three-phase load was disconnected and after five cycles, it was reapplied again.With different variations in the three-phase linear load, it was observed that the source voltage (v sabc ) and source current (i sabc ) are inphase.These variations in three-phase linear load, the performance characteristics of DSTATCOM show that the source voltages (v sabc ) and source currents (i sabc ) are balanced and harmonic free.The star/delta transformer is used to compensate the load neutral current under unbalanced load condition, during this period source neutral current (i sn ) is maintained at zero.The performance parameters shown in Fig. 3 depict DC bus voltage (v dc ) and terminal voltage (v t ), which are maintained at 400 V and 320 V respectively.

4.2.
Performance of Power Balance Theory Based DSTATCOM Under PFC Mode with Non-Linear Loads The performance of power balance theory based DSTATCOM under PFC mode with non-linear loads is shown in Fig. 4. The proposed DSTATCOM parameters such as source voltage (v sabc ), source current (i sabc ), load phase currents (i La , i Lb , i Lc ), compensating current (i cabc ), load neutral current (i Ln ), source neutral current (i sn ), compensating neutral current (i cn ), DC bus voltage (v dc ), its corresponding reference DC bus voltage (v * dc ), terminal voltage (v t ) and its corresponding reference terminal voltage (v * t ) are shown in Fig. 4. At t = 0.75 second, suddenly one phase of the three-phase load is disconnected and after five cycles again it was reapplied.At different variations in the three-phase non-linear load, it was observed that the source voltage (v sabc ) and source current (i sabc ) are in-phase.These variations in threephase non-linear load, the performance characteristics of DSTATCOM show that the source voltages (v sabc ) and source currents (i sabc ) are balanced and harmonic free.The star/delta transformer is used to compensate the load neutral current under unbalanced load condition.During this period source neutral current (i sn ) is maintained at zero.The performance parameters shown in Fig. 4

4.3.
Performance of Power Balance Theory Based DSTATCOM Under ZVR Mode with Linear Loads The performance of power balance theory based DSTATCOM under ZVR mode with linear loads is shown in Fig. 5.The proposed DSTATCOM parameters such as source voltage (v sabc ), source current (i sabc ), load phase currents (i La , i Lb , i Lc ), compensating current (i cabc ), load neutral current (i Ln ), source neutral current (i sn ), compensating neutral current (i cn ), DC bus voltage (v dc ), its corresponding reference DC bus voltage (v * dc ), terminal voltage (v t ) and its corresponding reference terminal voltage (v * t ) are shown in Fig. 5.At t = 0.75 second, suddenly one phase of the three-phase load is disconnected and after five cycles again it was reapplied.At different variations in the three-phase linear load, it was observed that the source voltage (v sabc ) and source current (i sabc ) are in-phase.These variations in three-phase linear load, the performance characteristics of DSTATCOM show that the source voltages (v sabc ) and source currents (i sabc ) are balanced and harmonic free.The star/delta transformer is used to compensate load neutral current under unbalanced load condition -during this period source neutral current (i sn ) is maintained at zero.The performance parameters shown in Fig. 5 depict DC bus voltage (v dc ) and terminal voltage (v t ), which are maintained at 400 V and 339 V respectively.

4.4.
Performance   five cycles again it was reapplied.At different variations in the three-phase non-linear load, it was observed that the source voltage (v sabc ) and source current (i sabc ) are in-phase.These variations in threephase non-linear load, the performance characteristics of DSTATCOM show that the source voltages (v sabc ) and source currents (i sabc ) are balanced and harmonic free.The star/delta transformer is used to compensate load neutral current under unbalanced load condition, during this period source neutral current (i sn ) is maintained at zero.The performance parameters shown in Fig. 6 depict DC bus voltage (v dc ) and terminal voltage (v t ), which are maintained at 400 V and 339 V respectively.The source voltage (v s ) and source current (i s ) THD are 4.51 % and 3.47 % whereas load current (i L ) THD is 46.46 % for three-phase non-linear R-L loads with harmonic spectra as depicted in Fig. 7.

Conclusion
The dynamic performance of power balanced theory based DSTATCOM shows satisfactory results under the application of linear/non-linear R-L loads.The advantage of using star/delta transformer is to mitigate neutral load current and also to reduce the DC link voltage at 400 V.The performance characteristics show that power balance theory based DSTATCOM compensates power quality problems like mitigation of harmonics, power factor correction at the source, load balancing, and reactive power compensation.The power balance theory based DSTATCOM is simple, efficient, reliable and robust as observed from the satisfactory results.During PFC and ZVR operations of DSTATCOM, it maintained the terminal voltage (v t ) and DC bus voltage (v dc ) at 339 V and 400 V respectively.The proposed DSTATCOM with non-linear loads reduces the THD of the source voltage (v s ) and source current (i s ) to 4.51 % and 3.47 % respectively, which is well within IEEE519, IEEE1159, IEEE241 and IEC61642 standards.

Figure 1
Figure 1 shows the proposed power balance theory based DSTATCOM in the three-phase distribution system.The main advantage of power balance theory is the fast detection of distortion with high accuracy and quick response extraction of reference source currents.The star/delta transformer is connected at PCC to compensate neural current and to reduce DC link voltage.The basic equations of power balance theory for generation of switching signals for VSC are given below.
(a) source voltage (b) source current (c) load current
VoltageSource Converter), a DC bus capacitor (C dc ) and three single-phase AC inductors (L f ).The VSC consists of six IGBT switches with anti-parallel diodes, and AC inductor (L f ) is used to mitigate ripples in the compensated currents.The three-phase supply voltages (v sa , v sb , v sc ) with source impedance Z s (R s and L s ) and three-phase loads may be linear/nonlinear R-L loads.A Ripple filter consists of resistance (R f ) and capacitance (C f ) connected across PCC (Point of Common Coupling) to compensate high frequency voltage harmonics.The proposed DSTATCOM injects compensating currents (i ca , i cb , i cc ) to cancel the reactive power component of load currents and harmonics of the non-linear loads.
La , i Lb , i Lc ), compensating current (i cabc ), load neutral current (i Ln ), source neutral current (i sn ), compensating neutral current (i cn ), DC bus voltage (v dc ), its corresponding reference DC bus voltage (v * dc ), terminal voltage (v t ) and its corresponding reference terminal voltage (v * t ) are shown in Fig.6.At t = 0.75 second, suddenly one phase of the three-phase load is disconnected and after c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING