Modelling And Analysis Of D Statcom

Published Date: 02 Nov 2017

K.Mahammad Rafi1 and prof. j.Amarnath2

1Department of Electrical and Electronics Engineering, Muffakham Jah College of Engineering and Technology, Hyderabad, Andhra Pradesh-500155. India.

[email protected]

2Department of Electrical and Electronics Engineering, JNTUH,Kukatpally, Hyderabad Andhra Pradesh-500072. India. [email protected]

ABSTRACT:

Power quality problems are rising in importance particularly for highly integrated plants that are sensitive to distortions or voltage dips, Almost all PQ problems originate in distribution networks. In most countries, there exist regulations which place limits on the distortion and unbalance that a customer can inject to a distribution system. These regulations may require the installation of compensators (filters) on customer premises. This Paper specifically examines the use of a power electronic shunt compensator to correct the current drawn from a utility to closely approximate balanced sinusoidal waveforms, without adversely affecting the voltage at the point of common coupling.

A distribution static compensator (DSTATCOM) is a voltage source converter (VSC)-based power electronic device. Usually, this device is supported by short-term energy stored in a dc capacitor. The DSTATCOM filters load current such that it meets the specifications for utility connection. This paper is simulated under MATLAB 7.8.0(R2009a) environment using SIMULINK. Simulation results demonstrate the performance of the DSTATCOM.

Index Terms: DSTATCOM, PLL, PCC, power quality (PQ),Reactive power control, FACTS, voltage source converter (VSC).

1. INTRODUCTION

1.1 FACTS CONCEPT

FACTS (Flexible AC transmission system) are one aspect of the power electronics revolution that is taking place in all areas of electric energy. A variety of powerful semiconductor devices not only offer the advantage of high speed and reliability of switching but, more importantly, the opportunity offered by a variety of innovative circuit concepts based on these power devices enhance the value of electric energy.

D-STATCOM system is composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network. It is generally power electronics based system. The opportunities of STATCOM controllers to control the interrelated parameters that govern the operation of transmission systems including shunt impedance, voltage, phase angle, and the damping of oscillations at various frequencies.

Figure- 1.Basic circuit diagram of D- STATCOM connected to AC Bus bar

1.2 BASIC TYPES OF FACTS CONTROLLERS

In general, FACTS controllers can be divided into the following categories:

Series Controller, Shunt Controller, Combined series-series controller, Combined series shunt controller, Various Types of Shunt Connected Controllers are: Static synchronous compensator (STATCOM), Static var compensators(SVC),Thyristor controlled reactor (TCR), Thyristor switched reactor (TSR), Thyristor switched capacitor (TSC).

2.0 PROBLEM FORMATION:

The analysis can be done by combining in different steps that are shown below and explained in detail in consequence pages.

1. Instantaneous reactive power theory (PQ Theory), 2. Voltage Source converter (VSC), 3.Phased Lock Loop (PLL), 4.PID Controller.

2.1Generating Reference Currents using Instantanious PQ Theory:

Hirofumi akagi and his coworkers have described an instantaneous method of generating reference currents for shunt compensator; this method is applicable to three phase four wire system. The main purpose of the park’s transformation is to transfer parameters in ABC/RYB co-ordinates to new co-ordinates called d,q,o/, where d-stands by direct-stands for quadrature, and o-stands by zero. The advantages of this transformation technique is that, under balance three-phase system, three-phase A.C parameters in ABC/RYB-co-ordinates can be represented by two-phase D.C parameters in dqo consequently, the classical control technique, which is valid only in D.C space, can be simply applied in the AC environment.

A co-ordinate in the ABC/RYB can be represented by a vector, whose direction points from the origin to its co-ordinate the output current/voltage of the STATCOM, for example, can be represented as following 3x1 matrix.

iabc (t) =, or vabc (t) =--(1)

where ia(t)/va(t), ib(t)/vb(t)and ic(t)/vc(t),are phase R, phase B and phase Y output currents/voltages of a STATCOM respectively.

Figure- 2.The rotating d-q-o coordinates with respect to the coordinates

Three phase voltages from a-b-c frame to d-q-o/ frame and vice versa using the following power invariant transformation.

=….(2)

=…..(3)

We can also use the same transform matrix for transforming currents .the instantaneous three-phase power is then given by …(4)

Where p is the total instantaneous real power in the three phase wires and is the instantaneous power in the zero-sequence network.let us define the following variable

q= ………...(5)

We thus see the quantity q given in equation (5)is the reactive power observedby a circuit when both voltages and currents contain only the fundamental frequency.

=……………….(6)

This is equivalent to writing

……………(7)

The instantaneous powers in α-axis and β-axis be denoted respectively by and we can write from equation (6)&(7)

……….(8)

We now define the following quantities

α-axis instantaneous active power:

α-axis instantaneous reactive power:

β-axis instantaneous active power:

β-axis instantaneous reactive power:

let us now expand these expressions

adding above two expressions we get similarley adding the reactive power components we get now we can conclude the following

-The sum of and is equal to the instantaneous real power. Therefore they are referred to as instantaneous active powers.

- The sum of andis equal to the instantaneous real power. Therefore they are referred to as instantaneous active powers.

Figure-3.Simulation model of a generating d,q,o/, of voltage and current blocks.

Figure-3.out put wave form of a dqo/, of voltage.

3.0 Voltage Source Converter(VSC):

The below matlab model shows the three phase VSC with PWM pulses.

Figure- 4.the mat lab model of a VSC with PWM

Out put pulses of three phase VSC to control the D-STATCOM.

4.0 PHASE-LOCKED LOOP (PLL)

Phase-locked loop (PLL) is a feedback loop which locks two waveforms with same frequency but shifted in phase. The fundamental use of this loop is in comparing frequencies of two waveforms and then adjusting the frequency of the waveform in the loop to equal the input waveform frequency. A block diagram of the PLL is shown in Figure-5. The heart of the PLL is a phase comparator which along with a voltage controlled oscillator (VCO), a filter and an amplifier forms the loop. If the two frequencies are different the output of the phase comparator varies and changes the input to the VCO to make its output frequency equal to the input waveform frequency.

Figure-5: Block Diagram of Basic Phase-Locked Loop

3.3.3 PLL structures

PLL structures can be broadly classified into following three Categories.

1) Zero crossing detection (ZCD) based PLL 2) Stationary reference frame based PLL

3) Synchronously rotating reference frame (SRF) based PLL

ZCD based PLL is the simplest among all but its performance becomes poorer if frequency variation or line notching is present. Stationary reference frame based PLL structure is not capable of accurate phase tracking during unbalanced voltage condition. The well known SRF PLL works well under most abnormal grid conditions.

3.3.4 Basic operation of three phase SRF PLL

Operation of a three phase SRF based PLL can be schematically shown in the Figure-15.

Figure-15: Basic structure of SRF PLL

To obtain the phase information the three phase voltage signals ( Va, Vb and Vc) are transferred into stationary two phase system (V and V). Where,

Va = Vm cos(wt) -------------- (1) Vb = Vm cos(wt −2π/3) -------------- (2)

Vc = Vm cos(wt −4π/3) -------------- (3)

Now phase angle (θ) can be obtained by either synchronizing the voltage space vector (V, Figure-16) along q axis or along d axis of synchronously rotating reference frame. Let us assume that the voltage space vector is to be aligned with q axis (Figure-16). Then position of d axis (θ) is related to wt by

θ = wt –π/2 ------------ (4)

Figure-16: Reference Frames

In the PLL structure (Figure-15) θ gets estimated as θ′ by integrating estimated frequency (w′) which is the summation of output of PI controller (Kp(1 + ssτ) and the feed forward frequency (wff ). The voltage vectors in synchronously rotating reference frame can be found out using θ′ from following equations.

Vd = − (3/2) Vm sin(θ − θ′) ------------ (5)

Vq = (3/2) Vm cos( θ− θ′) ------------ (6)

The controller gains are designed such that Vd follows reference value which will result in estimated frequency (w′) to lock to system frequency (w) and estimated phase angle (θ′) to be equal to the phase angle θ. Now if θ ≈ θ′ then space vector of voltage gets aligned to q axis. The voltage vector along synchronously rotating d-axis (eqn. 5) can be expressed as follows.

Vd = −(3/2) Vm (θ − θ′) ------------ (7)

So, overall system in Figure-15 can be simplified to that shown in Figure-17.

Figure-17: Simplified block diagram of SRF PLL

3.3.5 Control of DSTATCOM with PLL:

The DSTATCOM controller is shown in Figure-18.We can see that the PLL plays important role in the controller. The performance of the whole DSTATCOM is decided by the PLL which produces the synchronous phase angle to control the PWM modulator. At the same time the proposed PLL can provide the voltage magnitude of the fundamental components which is used for the AC voltage controller. The outputs of PLL are the key factors to decide the quality of current and AC voltage controllers. PLL is also used to generate unit sine and cosine signals synchronized to system frequency from utility voltage required for abc to dq transformation. Also, from Figure-18, the phase angle is used for the PWM modulation. Consequently, the PLL decides the DSTATCOM performance.

Figure-18: DSTATCOM system controller with proposed PLL

RESULTS:

OUT PUT PHASE VOLTAGE WITH FAULT WITHOUT D-STATCOM :

OUT PUT LINE VOLTAGE WITH FAULT WITHOUT D-STATCOM:

OUT PUT LINE VOLTAGE UNDER FAULT WITH D-STATCOM:

D-STATCOM IS SIMULATED USING THE SYSTEM PARAMETERS GIVEN BELOW

The system voltage:415V+/-10%(peak) ,sinusoidal and may contain harmonics and exhibits sag, swell, Terminal bus voltage :415V(peak),sinusoidal, balanced, Feeder resistance(Rs): 1.57Ω,Feeder reactance(Ls) :40mH.DC Capacitor : 5000 µF,Reference value of total DC capacitor voltage:1200V, DC link Resistor:6000Ω,Balanced load: R=50Ω, L=200mH, Unbalanced load: Ra=500Ω La=2000mH,Rb=750Ω, Lb=2250mH&.Rc=250 Ω Lc=1750mH.

KP, KI VALUES FOR CURRENT AND VOLTAGE MEASUREMENT LOOPS

For Id ,Iq ,Vd, &Vq the Kp values are 40,8,7and 8 respectively,For Id ,Iq ,Vd, &Vq the Ki values are 150,4,55and 4respectively.

CONCLUSIONS:

In this paper Voltage Oriented Control algorithm is applied to operate a DSTATCOM to regulate the voltage of the terminal bus at a nominal value. A closed loop control scheme, consisting of an outer dc capacitor voltage loop and an inner load angle control loop, is proposed. The control scheme maintains the power balance at the Point of common coupling (PCC) to regulate the dc capacitor voltages. It has been shown that the DSTATCOM is able to regulate the PCC voltage against disturbances either in the load or in the source side. Transient Characteristics with VOC Strategy Rise Time (tr) 0.002 sec, Fall Time (tf ) 0.065 sec.