An adaptive controller for DSTATCOM for Power Quality Improvement in distribution system

An adaptive control technique has been implemented for DSTATCOM for power quality enhancement in three phase four wires(3P4W) distribution system. The reference source currents are extracted from active components of load current for each phase by using this technique. It has also been used for load balancing, harmonic suppression, power factor improvement and reactive power compensation in distribution system. The adaptive control technique has been tested at varying load in steady state and dynamic conditions. This technique is developed using MATLAB/Simulink and source side harmonics are mitigated under different load conditions which are acceptable in accordance with IEEE standard.


Introduction
With the introduction of power electronics-based load like UPS, TV, LED, variable speed derives etc. which introduce nonlinearity in distribution system and creates harmonics and DC offsets in the system [1]. Two type of power quality problems are arising in distribution system like current related power quality and voltage related power quality problems. For mitigation of these problems in distribution system the custom power devices are used. To mitigate the current related power quality problems the device DSTATCOM is used in distribution system which is a shunt connected CPD [2]. For enhancement of quality of supplied power by AC lines, by using reference current compensation, the various technique for DSTATCOM has been implemented [3,4]. It is so essential to learn the performance of DSTATCOM at different frequency [5]. In small scale generation as well as renewal energy resources the active shunt filters are used in distribution system [6][7][8]. The shunt compensating devices are used for above discussed application and their performance depends upon the control technique and procedure. Frequency and time domain control technique for DSTATCOM has been presented based on IRPT, SRF etc [9]. For control of shunt active filters, the ANN technique is used. It is further categorized as hidden layer, compressive learning, adaptive neural networkbased learning [10]. Performance of these technique depends upon the learning rate and operating conditions [11]. A least square adaptive control technique has been presented for calculation of in phase components of instantaneous power and regulation of DC link voltage of shunt connected active power device [12]. For harmonics elimination the commonly used adaptive control techniques like digital frequency control, time effective adaptive control and notch filter-based control has been presented in [13][14][15][16][17]. Three different adaptive control technique for DSTATCOM in distribution system has been implemented for power qualities issues mitigation [18]. For computation of essential load current without using the Voltage Controlled Oscillator (VCO) less PLL for three phases to two phases transformation has been implemented [19]. For VSC based DSTATCOM nonlinear adaptive second order Volterra filter (NLVF) has been executed for compensating current injection in distribution system for power quality enhancement [20]. Harmonics as well reactive power compensation using optimal step least square control technique for DSTATCOM for linear/nonlinear load has been presented in [21]. LMS-LMF based control technique for DSTATCOM has been presented for generation of reference supply currents [22]. Integration of Battery energy storage system with grid through DSTATCOM for supply /storage of active power to grid/from grid has been implement with SRFT control algorithm for PQ enhancement of grid system [23]. The NN with CGBP (Conjugate back propagation) using cos ∅ technique has been presented for various power problems mitigations [24]. The ANN (artificial Neural Network) technique based on HNN for control of DSTATCOM has been presented for different power related issues mitigations in distribution system [25].
In this paper the adaptive control technique for VSC based four legs DSTATCOM has been developed for regulation of DC link voltage, harmonics suppression of source current, load balancing and neutral current compensation in three phase four wire distribution system.

Proposed system and Control Method
The distribution system with nonlinear load and three phase four wire DSTATCOM is depicted in figure1. The Rs and Ls represent the source resistance and inductance respectively. For the ripple reduction from DSTATCOM output currents are passed through Lf coupling inductor which connected as shown in figure1.The harmonics of load currents are reduced by injected the DSTATCOM output current. Data for proposed system are presented in table 1.  Three phase linear load 4kVA,0.8 pf. Lagging.
Reference DC link voltage 400V

Proposed control method
Research Control technique developed in this paper is depicted in figgure2.The basic mathematical equations for the extraction of supply reference current are given below.

In phase templates calculation
The ( ) voltage of coupling point is computed as Where Vsa, Vsb and Vsc are voltages of phases a,b and c respectively at coupling point. The inphase templates of these voltage are computed by following formulations 2.1.2. Computation of total active component of load current from distorted load current The active current, several harmonic components and DC component are present in distorted load current. From active component of load current this distorted load current is subtracted for current error generation and this error current is multiplied with phase voltage of supply and pass through LPF (low pass filter) and integrated and multiplied with voltage ,which is in phase of this. By this manner a phase load current active component is computed. For estimation maximum value of active current, it is multiplied by a gain (k), and represent as . The remaining two-phase current and are computed using same procedure. Load current average value of (ILAP) is computed as Reference supply currents are subtracted from supply currents and error signal is given to hysteresis controller for generation of gating signal for VSC based DSTATCOM. This source neutral current ( ) is subtracted from reference neutral current ( * ) and error signal is given to hysteresis controller for generation of gating signal for fourth leg. * current is considered as zero.

Simulation and Results
An adaptive controller for four -legs VSC based DSTATCOM has been developed and simulated in MATLAB/simulink for distribution system power quality enhancement. The performance of DSTATCOM has been studied in time domain with linear/nonlinear loads for steady state (STC) and dynamic condition.  figure 5(a, b). The load current has THD 28.76% whereas source current has only 2.20 %THD which shows the DSTATCOM with adaptive control works efficiently for harmonics suppression for source current.

DSTATCOM performance under dynamic condition
The proposed controller is tested for linear as well nonlinear load variation to check the robustness of the controller under dynamic condition.

3.2.1.Linear load variation
DSTATCOM performance under dynamic state when the linear load (4kVA,0.8pf) and nonlinear load( 3P4W rectifier based non-linear load R=15Ω and L=100mH) is added at 0.6s to existing load is depicted in figure 6 in terms of PCC voltage( ),source current ( ),Load current( ) source neutral current( ) ,load neutral current( ),compensator neutral current( ), DC link voltage ( ) and power factor ( ) respectively. The source current is increased with increasing load at t=0.6s but remains balanced and sinusoidal with non-linear load and DC link voltage is also maintained constant at 400V. THD of source current ( ) and load current ( ) is depicted in figure   7(a, b). The load current has THD 14.02% whereas source current only 1.21%THD which shows the DSTATCOM with adaptive control works efficiently for harmonics suppression for source current. ( ) and load current ( ) is depicted in figure 9(a,b). The load current has THD 28.60% whereas source current only 2.25%THD which shows the DSTATCOM with adaptive control works efficiently for source harmonics suppression under this condition also. respectively. The source current is increased with increasing load at t=0.6s but is remains balanced and sinusoidal with non-linear load and DC link voltage is also maintained constant at 400V. THD of source current ( ) and load current ( ) is depicted in figure 13(a, b). The load current has THD 30.86% whereas source current only 4.67%THD which shows the DSTATCOM with adaptive control works efficiently for harmonics suppression under this condition also.

Conclusions
The adaptive controller for DSATCOM is developed for power quality enhancement in 3phase 4wire distribution system. This control technique is simulated for variable linear/nonlinear load under steady state and dynamic conditions. It has been observed from the simulated results that the proposed controller is capable for harmonic suppression of the supply side current, load balancing,