Adaptive Droop Control Shunt Active Filter and Series AC Capacitor Filter for Power Quality Improvement in Power System

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29 International Journal for Modern Trends in Science and Technology, Volume 3, Special Issue 4, July 2017

Adaptive Droop Control Shunt Active Filter and Series AC Capacitor Filter for Power Quality Improvement in Power System

G.Anil Kumar¹| M.V.Raghavendra Reddy²

1PG Scholar, Department of EEE, Godavari Institute of Engineering and Technology, Rajahmundry, Andhra Pradesh, India.

2Asssitant Professor, Department of EEE, Godavari Institute of Engineering and Technology, Rajahmundry, Andhra Pradesh, India.

To Cite this Article

G.Anil Kumar and M.V.Raghavendra Reddy, “Adaptive Droop Control Shunt Active Filter and Series AC Capacitor Filter for Power Quality Improvement in Power System”, International Journal for Modern Trends in Science and Technology, Vol. 03, Special Issue 04, July 2017, pp. 29-35.

Most of the pollution issues created in power systems are due to the non-linear characteristics and fast switching of power electronic equipment. Power quality issues are becoming stronger because sensitive equipment will be more sensitive for market competition reasons, equipment will continue polluting the system more and more due to cost increase caused by the built-in compensation and sometimes for the lack of enforced regulations. Efficiency and cost are considered today almost at the same level. Active power filters have been developed over the years to solve these problems to improve power quality. Among which shunt active power filter is used to eliminate and load current harmonics and reactive power compensation.

In this paper the study is carried out on both PI controller based and Artificial Neural Network (ANN) controlled, three-phase shunt active power filter to compensate harmonics and reactive power by nonlinear load to improve power quality is implemented for three-phase three wire systems.

Keywords— Active Power Filter, Harmonic, Instantaneous P Q Theory, Balanced, Unbalanced Loads, DC Link Voltage Controller, Voltage Source Inverter (VSC)

I. INTRODUCTION

Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Seventy five percent of total global energy demand is supplied by the burning of fossil fuels. But increasing air pollution, global warming concerns, diminishing fossil fuels and their increasing cost have made it necessary to look towards renewable sources as a future energy solution. Since the past decade, there has been an enormous interest in many countries on renewable energy for power generation. The market liberalization and

government’s incentives have further accelerated the renewable energy sector growth.

Renewable Energy Source (RES) integrated at distribution level is termed as Distributed Generation (DG). The utility is concerned due to the high penetration level of intermittent RES in distribution systems as it may pose a threat to network in terms of stability, voltage regulation and Power-Quality (PQ) issues. Therefore, the DG systems are required to comply with strict technical and regulatory frameworks to ensure safe, reliable and efficient operation of overall network. With the advancement in power electronics and digital control technology, the DG systems can now be actively controlled to enhance ABSTRACT

Available online at: http://www.ijmtst.com/ncceeses2017.html

Special Issue from 2^nd National Conference on Computing, Electrical, Electronics and Sustainable Energy Systems, 6^th – 7^th July 2017, Rajahmundry, India

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30 International Journal for Modern Trends in Science and Technology, Volume 3, Special Issue 4, July 2017 the system operation with improved PQ at Point of

Common Coupling (PCC). However, the extensive use of power electronics based equipment and non-linear loads at PCC generate harmonic currents, which may deteriorate the quality of power. Generally, current controlled voltage source inverters are used to interface the intermittent RES in distributed system. Recently, a few control strategies for grid connected inverters incorporating PQ solution have been proposed. In an inverter operates as active inductor at a certain frequency to absorb the harmonic current. But the exact calculation of network inductance in real-time is difficult and may deteriorate the control performance. A similar approach in which a shunt active filter acts as active conductance to damp out the harmonics in distribution network is proposed. A control strategy for renewable interfacing inverter based on theory is proposed. In this strategy both load and inverter current sensing is required to compensate the load current harmonics.

The non-linear load current harmonics may result in voltage harmonics and can create a serious PQ problem in the power system network [3]. Active Power Filter (APF) is extensively used to compensate the load current harmonics and load unbalance at distribution level. This results in an additional hardware cost. Another solution is to incorporate the features of APF in the, conventional inverter interfacing renewable with the grid, without any additional hardware cost. Here, the main idea is the maximum utilization of inverter rating which is most of the time underutilized due to intermittent nature of RES. The grid-interfacing inverter can effectively be utilized to perform functions as transfer of active power harvested from the renewable resources (wind, solar, etc.), load reactive power demand support, current harmonics compensation at PCC , current unbalance and neutral current compensation in case of 3-phase 4-wire system. Moreover, with adequate control of grid-interfacing inverter, all the four objectives can be accomplished either individually or simultaneously. The PQ constraints at the PCC can therefore be strictly maintained within the utility standards without additional hardware cost.

Figure 1: Basic Principal of Shunt Current Compensation in Active Filter

The control strategy for a shunt active power filter generates the reference current, that must be provided by the power filter to compensate reactive power and harmonic currents demanded by the load. This involves a set of currents in the phase domain, which will be tracked generating the switching signals applied to the electronic converter by means of the appropriate closed-loop switching control technique such as hysteresis or deadbeat control. Several methods including instantaneous real and reactive power theory have been proposed for extracting the harmonic content [2-5]. With the development of power electronic technology, low voltage and high current switching power supply has been applied widely in production and life. But harmonic pollution becomes severe increasingly in switching power supply. Generally, there are two methods to eliminate harmonics, which are mainly passive power filter and active power filter. The former is relatively low cost, but filtering effect is far from desirability. By contrast, the latter can suppress the harmonics instantly and compensate reactive power, and it becomes an effective approach to inhibit harmonics. Thus, this paper employs active power filter to suppress harmonic current of low voltage and high current switching power supply

II. DESIGN OF APF Principle of APF

A APF, which is schematically depicted in Figure 2, consists of a two-level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of three-phase

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31 International Journal for Modern Trends in Science and Technology, Volume 3, Special Issue 4, July 2017 ac output voltages. These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the APF output voltages allows effective control of active and reactive power exchanges between the APF and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power.

Figure 2: Schematic Diagram of a APF

The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to

three quite distinct purposes:

 Voltage regulation and compensation of reactive power

 Correction of power factor

 Elimination of current harmonics.

Here, such device is employed to provide continuous voltage regulation using an indirectly controlled converter.

As shown in Figure.-1 the shunt injected current Ish corrects the voltage sag by adjusting the voltage drop across the system

impedance Zth. The value of Ish can be controlled by adjusting the output voltage of the converter.

The shunt injected current Ish can be written as, Ish = IL – IS = IL – ( Vth – VL ) / Zth (1)

Ish /_η = IL /_- θ (2)

It may be mentioned that the effectiveness of the APF in correcting voltage sag depends on the value of Zth or

fault level of the load bus. When the shunt injected current Ish is kept in quadrature with VL, the desired voltage correction

can be achieved without injecting any active power into the system. On the other hand, when the value of Ish is minimized, the same voltage correction can

be achieved with minimum apparent power injection into the system.

Principle of APF Applied to Drive

Figure 3: Schematic Diagram of a APF

An active power filter, APF, typically consists of a three phase pulse width modulation (PWM) voltage source inverter [5]. When this equipment is connected in series to the ac source impedance it is possible to improve the compensation characteristics of the passive filters in parallel connection [6], [7]. This topology is shown in Figure 3, where the active filter is represented by a controlled source, where is the voltage that the inverter should generate to achieve the objective of the proposed control algorithm.

Instantaeous Power Theory

The control scheme of the shunt active power filter must calculate the current reference signals from each phase of the inverter using instantaneous real-power compensator. The block diagram as shown in Figure.4, that control scheme generates the reference current required to compensate the load current harmonics and reactive power. The PI controller is tried to maintain the dc-bus voltage across the capacitor constant of the cascaded inverter. This instantaneous real- power compensator with PI-controller is used to extracts reference value of current to be compensated.

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Figure 4: Reference Current Generator Using Instantaneous Real-Power Theory

The reference currents (isa*, isb * and isc *)are compared with actual source current (is)isa , isb and isc that facilitates generating cascaded multilevel inverter switching signals using the proposed triangular-sampling current modulator.

The small amount of real-power is adjusted by changing the amplitude of fundamental component of reference currents and the objective of this algorithm is to compensate all undesirable components. When the power system voltages are balanced and sinusoidal, it leads to constant power at the dc bus capacitor and balanced sinusoidal currents at AC mains simultaneously.

III. PROPOSED INSTANTANEOUS PROPOSED CONCEPT

The proposed instantaneous real-power (p) theory derives from the conventional p-q theory or instantaneous power theory concept and uses simple algebraic calculations. It operates in steady-state or transient as well as for generic voltage and current power systems that allowing to control the active power filters in real-time. The active filter should supply the oscillating portion of the instantaneous active current of the load and hence makes source current sinusoidal

Figure 5: α-β Coordinates Transformation

The p-q theory performs a Clarke transformation of a stationary system of coordinates a b c to an

orthogonal reference system of coordinates α, β . In a b c coordinates axes are fixed on the same plane, apart from each other by 120o that as shown in Figure 5. The instantaneous space vectors voltage and current Va , ia are set on the a-axis, Vb , ib are on the b axis, and Vc , ic are on the c axis. These space vectors are easily transformed into α, β coordinates. The instantaneous source voltages vsa, vsb, vsc are transformed into the α, β coordinate’s voltage vby Clarke transformation as follows:

Similarly, the instantaneous source current isa,

isb, isc also transformed into

thecoordinate’s current iiby Clarke transformation that is given as;

Where α and β axes are the orthogonal coordinates.

They Vα, iα are on the α-axis, and Vβ, iβ are on the β-axis.

Real-Power (p) Calculation

The orthogonal coordinates of voltage and current Vα, iα are on the α-axis and Vβ, iβ are on theβ-axis.

Let the instantaneous real-power calculated from the α-axis and β- axis of the current and voltage respectively. These are given by the conventional definition of real-power as :

This instantaneous real-power pac is passed to first order Butterworth design based 50 Hz low pass filter (LPF) for eliminating the higher order components; it allows the fundamental component only. These LPF indicates ac components of the real-power losses and it’s denoted as pac The DC power loss is calculated from the comparison of the dc-bus capacitor voltage of the cascaded inverter and desired reference voltage. The proportional and integral gains (PI Controller) are determining the dynamic response and settling time of the dc-bus capacitor voltage. The DC component power losses can be written as

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33 International Journal for Modern Trends in Science and Technology, Volume 3, Special Issue 4, July 2017 The instantaneous real-power ( p) is calculated

from the AC component of the real-power loss pac and the DC power loss pDC(Loss) ) ; it can be defined as follows;

The instantaneous current on the α,β coordinates of Icα and icβ are divided into two kinds of instantaneous current components; first is real-power losses and second is reactive power losses, but this proposed controller computes only the real-power losses. So the α,β coordinate currents Icα and icβ are calculated from the v,vvoltages with instantaneous real power p only and the reactive power q is assumed to be zero.

This approach reduces the calculations and shows better performance than the conventional methods. The coordinate currents can be calculated as

From this equation, we can calculate the orthogonal coordinate’s active-power current. The

axis of the instantaneous active current is written as:

Similarly, the -axis of the instantaneous active current is

Written as:

Let the instantaneous powers p(t) in the -axis and the - axis is represented as pand prespectively. They are given by the definition of real-power as follows:

IV. SIMULATION RESULTS

Fig: 6 Simulation Diagram

Fig: 7 Source Voltage and Current

Fig:8 Power Factor of the supply

Fig:9 Active Power and Reactive Power from the supply

Fig:10 DC Link Voltage

Discrete, Ts = 5e-005 s.

powergui

dc

A B C

+

-

A B C

a b c

A B C

a b c

A B C

a b c

A B C a b c A

B C a b c

G A1 B1 C1

A2 B2 C2

Subsystem

Step1

MEASUREMENT S

A B C Grid Connected Converter 1 Converter 1 Controller

c12 c12 c12

+

- A B

C ACT IVE FILT ER

ACT IVE FILT ER

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Fig:11 THD Analysis of for current before filters (36.99%)

Fig: 12 THD Analysis of for current with filters (17.62%)

Fig:13 Proposed Simulation circuit with fyzzy logic

Fig:14 Load Current, Source Current and Filter Current

Fig:15 Proposed Circuit Power Factor

Fig:16 THD analysis for Source Current (0.78%)

Fig: 17 THD analysis for Load Current (9.41%)

V. CONCLUSION

This paper presents a novel method to improve the power quality at point of common coupling for a 3-phase 4-wire DG system using PI controller for grid interfacing inverter. The grid interfacing inverter is effectively utilized for power conditioning. This approach eliminates the additional power conditioning equipment to improve power quality at PCC. The grid-interfacing inverter with the proposed approach can be utilized to inject real power generation from RES to the grid, and operate as a shunt Active Power Filter (APF). The current unbalance, current harmonics and load reactive power, due to unbalanced and non-linear load connected to the PCC, are compensated effectively such that the grid side currents are always maintained as balanced and sinusoidal at unity power factor. Moreover, the load neutral current is prevented from flowing into the grid side by compensating it locally from the fourth leg of inverter. When the power generated from RES is more than the total load power demand, the grid-interfacing inverter with the proposed control approach not only fulfills the total load active and reactive power demand (with harmonic compensation) but also delivers the excess generated sinusoidal active power to the grid at unity power factor.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-500 0 500

Selected signal: 42 cycles. FFT window (in red): 2 cycles

Time (s)

0 200 400 600 800 1000

0 5 10 15 20 25

Frequency (Hz) Fundamental (60Hz) = 151 , THD= 36.99%

Mag (% of Fundamental)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-500 0 500

Time (s)

0 200 400 600 800 1000

0 5 10 15 20 25

Frequency (Hz) Fundamental (60Hz) = 146.5 , THD= 17.62%

Discrete, Ts = 5e-005 s.

powergui

A B C + - Universal Bridge1 A B C + - Universal Bridge

T imer1 com A B C a b c Three-Phase Breaker1

A B C a b c Three-Phase V-I Measurement2 A B C a b c Three-Phase V-I Measurement1 A

B C a b c Three-Phase V-I Measurement

A B C

a b c Three-Phase Transformer (Two Windings)

A B C

Subsystem1 A

B C

Subsystem

Series RLC Branch3

Series RLC Branch2 Series RLC Branch10

Series RLC Branch1

Series RLC Branch

Scope4 Scope

Iabc From9 Vabc From8 Irefsaf

From7 Iabcsaf From6 Iabc From3 Iabcsaf From2 -T - From1

Add

V I

power f actor Active & Reactive Power AC Voltage Source2

AC Voltage Source1 AC Voltage Source

0 0.2 0.4 0.6 0.8 1

-200 0 200

Time (s)

0 200 400 600 800 1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Frequency (Hz) Fundamental (50Hz) = 333.3 , THD= 0.78%

0 0.2 0.4 0.6 0.8 1

-400 -200 0 200 400

Time (s)

0 200 400 600 800 1000

0 2 4 6 8 10

Frequency (Hz) Fundamental (50Hz) = 500 , THD= 9.41%

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