chapter 4 enhancement of power quality using dvr...

CHAPTER 4

ENHANCEMENT OF POWER QUALITY USING DVR

4.1. INTRODUCTION

At present, modern industrial devices are typically based on

power electronic devices such as programmable logic controllers and

adjustable speed drives. The electronic devices are very responsive to

disturbances and become less tolerant to power quality problems such

as voltage sags, swells and harmonics. Voltage dips are considered to

be one of the most severe disturbances to the industrial equipments

[130].

DVRs are a class of custom power devices for providing

consistent distribution power quality. They use a series of voltage

boost technology using solid state switches for compensating voltage

sags/swells [115]. The DVR applications are mostly for sensitive loads

that may be considerably affected by fluctuations in system voltage.

Among the power quality problems viz. sags, swells, harmonics

etc.., voltage sags are the most severe disturbances. Overcome these

problems from the concept of custom power devices. DVR is the most

efficient and effective modern custom power device used in power

distribution networks [94]. It is recently proposed series connected

solid state device that injects voltage into the system in order to

control the load side voltage. It is usually installed in a distribution

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system between the utility and the critical load feeder at the point of

common coupling.

DVR has been modelled with PI Controller and Fuzzy Logic

Controllers to enhance the performance of the system. The main

objective of the above model is to improve the power quality of the

system during different types of faults like three phase fault, single

line to ground fault and double line to ground fault points are to be

discussed in this chapter.

4.2. BASIC CONFIGURATION OF DVR

The common configuration of the DVR consists of the following

necessary units.

1 An Injection/ Booster transformer

2 A Harmonic filter

3 Storage Devices

4 A Voltage Source Converter

5 DC charging circuit

6 A Control and Protection system

4.2.1. Injection/ Booster Transformer

The Injection / Booster transformer is a particularly designed

transformer that attempts to limit the coupling of noise and transient

energy from the primary side to the secondary side [26, 16]. Its major

tasks are given below.

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Fig. 4.1 Schematic Diagram of DVR.

1. It connects the DVR to the distribution network using the HV-

winding of the transformer and couples the injected

compensating voltages generated by the voltage source

converters to the incoming supply voltage.

2. In addition, the Injection / Booster transformer serves the

function of isolating the load from the system (VSC and control

mechanism).

4.2.2. Voltage Source Converter

A VSC is a power electronic system consists of a storage device

and switching equipment, which can produce a sinusoidal voltage at

any necessary frequency, magnitude and phase angle. In the DVR

application, the VSC is used to momentarily replace the supply voltage

or to produce the part of the supply voltage which is missing [109].

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The purpose of storage devices is to supply the necessary energy

to the VSC using a DC Link for the generation of injected voltages. The

different kinds of energy storage devices are superconductive magnetic

energy storage (SMES) [55, 64], batteries and capacitance [17].

4.2.3. DC Charging Circuit

The dc charging circuit has two main tasks.

1 The first task is to charge the energy source after a sag

compensation event.

2 The second task is to maintain DC Link voltage at the nominal

DC Link voltage.

4.2.4. Equations related to DVR

Fig. 4.2 Equivalent Circuit Diagram of DVR.

The load impedance Zth depends on the fault level of the load

bus. When the system voltage (Vth) drops, the DVR injects a series

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voltage VDVR through the injection transformer so that the desired load

voltage magnitude VL can be maintained. The series injected voltage of

the DVR can be written as

THLTHLDVR VIZVV (4.1)

Where

VL: The desired load voltage magnitude

ZTH: The load impedance.

IL: The load current

VTH: The system voltage during fault condition

The load current IL is given by,

L

LL

LV

jQPI

(4.2)

When VL is considered as a reference equation can be rewritten

as,

THTHLDVR VZVV 0 (4.3)

,, are angles of VDVR, ZTH ,VTH respectively and is Load

power angle

The complex power injection of the DVR can be written as,

*

LDVRDVR IVS (4.4)

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It requires the injection of only reactive power and the DVR itself

is capable of generating the reactive power.

4.3. CONTROL STRATEGIES

There are various control methods proposed for DVR, such as

the Voltage Space Vector PWM suggested by Changjiang Zhan et al in

[23 & 24]. Teke K. Bayindir et al [115] presented a Fuzzy Logic

Control. These studies show that the transient response of the FLC is

better than that of PI Controller. A new approach to estimate

symmetrical components for controlling the DVR was suggested by

Mostafa I. Marei et al [85].

H. Ezoji et al [59] recommended the use of DVR based on

hysteresis voltage control. Hysteresis voltage control based on

unipolar pulse width modulation has been used to control DVR. The

hysteresis voltage control in terms of quick controllability and easy

implementation hysteresis band voltage control has the highest rate

among other control methods.

4.3.1. PI Controller

The endeavor of the control method is to maintain constant

voltage magnitude at the point where a sensitive load is connected,

under system disturbances. The control system only evaluates the

root mean square (RMS) voltages at the load terminals, i.e., no reactive

power measurements are required. The VSC switching approach is

based on a sinusoidal PWM technique which proposes simplicity and

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good response. Since custom power is a comparatively low-power

application, PWM methods offer a more flexible alternative than the

Fundamental Frequency Switching (FFS) methods. Besides, high

switching frequencies can be used to get better on the efficiency of the

converter, without incurring considerable switching losses.

The controller input is an error signal acquired from the

reference voltage and the RMS value at the terminal voltage measured.

Such error is processed by a PI Controller, the output angle is δ,

which is provided to the PWM signal generator. An error signal is

obtained by comparing the reference voltage with the RMS voltage

measured at the load point [115]. The PI Controller process the error

signal, then produces the required angle to drive the error to zero, i.e.,

the load RMS voltage is brought back to the reference voltage.

Fig. 4.3 PI Controller.

4.3.2. Hysteresis Voltage Control

Hysteresis voltage control is used to get better the load voltage

and find out switching signals for inverters gates. A necessity of the

hysteresis voltage control is based on an error signal between an

injection voltage (Vinj) and a reference voltage of DVR (Vref) which

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produces appropriate control signals [59]. There is a Hysteresis Band

(HB) above and under the reference voltage. When the difference

between the reference and inverter voltage reaches to the upper/ lower

limit, the voltage is forced to decrease/increase as shown in Fig.4.4.

Fig. 4.4 Hysterisis Band Voltage Control.

4.3.3. Fuzzy Logic Controller

The fuzzy logic controller for the proposed DVR has two real

time inputs measured at every sampling time, named error and error

rate and one output named actuating-signal for each of the phases.

The input signals are fuzzified and represented in fuzzy set notations

by membership functions. The defined ‘if … then …’ rules produce the

linguistic variables and these variables are defuzzified into control

signals for comparison with a carrier signal to generate PWM inverter

gating pulses.

ErrA = VpllA - VSA (4.5)

)1()( nErrnErrErr AAA (4.6)

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Fuzzy logic control involves three steps: fuzzification, decision-

making and defuzzification. Fuzzification transforms the non-fuzzy

(numeric) input variable measurements into the fuzzy set (linguistic)

variable that is a clearly defined boundary.

Fig.4.5 Membership function Input variable “Error”

Fig. 4.6 Membership function Input variable “Error rate”

Fig.4.7 Membership function for output

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In the proposed controller, the error and error rate are defined

by linguistic variables such as large negative (LN), medium negative

(MN), small negative (SN), small (S), small positive (SP), medium

positive (MP) and large positive (LP) characterized by memberships.

The memberships are curves that define how each point in the input

space is mapped to a membership value between 0 and 1. The

membership functions belonging to the other phases are identical.

Membership functions for the inputs are shown in Fig.4.5 and Fig.4.6.

The membership function of output variable is shown in Fig.4.7.

Table 4.1 Decision table for fuzzy logic control

There are 49 rules to carry out optimum control action and each

rule expresses an operating condition in the system as shown in Table

4.1. The person’s experience and knowledge about the system

behavior help to define the rules. The correct combinations of these

rules improve the system performance. All rules are evaluated in

parallel and the order of the rules is not important. The decision table

for fuzzy logic control rules is shown in Table 4.1

The outputs of FLC process are the control signals that are used

in generation of PWM inverter switching signals by comparing a

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carrier signal. The generation of switching signals for single phase of

the system is shown in Fig. 4.13 where the configuration will be the

same for other phases.

4.4. SIMULATION OF DVR WITH PI CONTROLLER AND

FUZZY LOGIC CONTROLLER

Computer simulation has become an indispensable part of the

power electronics design process. DVR is a power electronics device

which consists of voltage source converter as an important element.

The overall design process can be shortened through the use of

computer simulations, since it is usually easier to study the influence

of a parameter on the system behavior in simulation. Main system

simulation model without DVR has been created in Matlab/Simulink

as shown in Fig 4.8.

Fig. 4.8 Main System without DVR.

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The test system employed to carry out the simulations for DVR

with PI Controller and Fuzzy Logic Controller is given in Fig 4.9 and

Fig 4.10. Such system is composed by a 13 kV, 50 Hz generation

system, feeding two transmission lines through a 3-winding

transformer connected in Y/Δ/Δ, 13/115/115 kV. Such transmission

lines feed two distribution networks through two transformers

connected in Δ/Y, 15/11 kV.

To verify the working of a DVR employed to avoid voltage sags

during short-circuit, via a resistance of 0.4 Ω. Such fault is applied for

400msec. The capacity of the dc storage device is 5 kV.

Fig. 4.9 PI Controller subsystem.

The test system of without DVR during faults like three phase

fault, single line to ground fault and double line fault is analyzed.

Observed load voltage problems like sag, swell and interruption. Then

to overcome these problems DVR is connected for the test system in

addition to the controllers like PI Controller with parameters KP = 0.5,

Ki = 50 & sample time 50µsec and Fuzzy Logic Controller for the

enhancement of the performance and THD improvement.

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Fig. 4.10 Main diagram of DVR with Fuzzy Logic Controller.

Fig. 4.11 DVR subsystem.

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Fig. 4.12 Fuzzy Logic Controller subsystem.

Fig. 4.13 Phase Modulation (for control angle ) Sub system.

4.4.1. Simulation Results for Voltage Sag during Three Phase

Fault

The first test system contains no DVR and a three-phase short-

circuit fault is applied, via a fault resistance of 0.4Ω, during the period

500 to 900ms. The load voltage is 35% with respect to the reference

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voltage. Fig. 4.14 shows the RMS load voltage sag at the RL load (R =

10Ω and L = 10-6H) for the case when the system operates with no

DVR.

The second test system is carried out using the same scenario

as above but now DVR with PI Controller and DVR with Fuzzy Logic

Controller are in operation. The total simulation period is 1400 ms.

When the DVR is in operation the voltage sag is mitigated almost

completely and the rms voltage of the sensitive load is maintained at

98% while DVR with FLC as shown in Fig. 4.15 with the capacity of

the dc storage device is 5 kV.

Fig. 4.14 Voltage VRMS at the load without DVR.

The PWM control scheme controls the magnitude and the phase

of the injected voltages, restoring the RMS voltage very effectively. The

sag mitigation is performed with a smooth, stable and rapid DVR

response. Two transient undershoots are observed when the DVR

comes in and out of operation. It should be noted that in the DVR, the

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dc voltage is supplied by a dc capacitor. Several simulations were

carried out to assess the performance of the DVR as a function of

short circuit proximity and the compensation capability of DVR

depends on the capacity of the energy storage device. Fig. 4.16 and

Fig. 4.17 show that the compensating capability of DVR depends on

the capacity of the DC energy storage.

Fig. 4.15 Voltage VRMS at the load with 5KV Energy Storage.


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4.4.2. Simulation Results for Voltage Sag during Single Line

to Ground Fault

The first test system contains no DVR and a single line to

ground fault is applied and the corresponding waveform as shown in

Fig. 4.18, via a fault resistance of 0.2 Ω, during the period 500 to 900

ms. The voltage sag at the load voltage is 30% with respect to the

reference voltage.




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When the DVR is in operation the voltage sag is mitigated

almost completely and the RMS voltage of the sensitive load is

maintained at 98%, as shown in Fig. 4.19.

Fig. 4.19 Voltage VRMS at the load with 6.5KV Energy Storage.

The DVR response is smooth, stable and rapid to mitigate sag

during single line to ground fault. Two transient undershoots are

observed when the DVR comes in and out of operation. The

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compensation capability of DVR depends on the capacity of the energy

storage device. Fig. 4.19, Fig. 4.20, Fig. 4.21 and Fig. 4.22 shows DC

energy storage of 6.5KV, 6KV, 4KV and 8KV respectively.



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4.4.3. Simulation Result of Voltage Sag during Double Line

Fault

The first test system contains no DVR and a double line fault is

applied via a fault resistance of 0.1 Ω, during the period 500 to 900

ms. The voltage sag at the load voltage is 65% with respect to the

reference voltage. Fig. 4.23 shows the RMS voltage at the load point

for the case when the system operates with no DVR.




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Fig. 4.24 Voltage VRMS at the load with DVR Energy Storage of

6.68KV.

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Fig. 4.24, Fig. 4.25, Fig. 4.26 and Fig. 4.27 shows that the

compensating capability of DVR depends on the capacity of the DC

energy storage of 6.68KV, 6.2KV, 5.6KV and 8KV respectively.



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Fig 4.27 Voltage VRMS at the load with 8KV Energy Storage

4.4.4. Simulation Result of Voltage Interruption during

Three- Phase Fault

The first test system contains no DVR and three phase fault is

applied via a fault resistance of 0.001 Ω, during the period 500 to 900

ms. The voltage at the sensitive load is 0% with respect to the

reference voltage. Fig. 4.28 shows the RMS voltage at the load point

for the case when the system operates with no DVR.




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Fig. 4.29 Voltage VRMS at the load with DVR Energy Storage of 4.9KV.

The sag mitigation is performed with a smooth, stable and rapid

DVR response, two transient undershoots are observed when the DVR

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comes in and out of operation. The compensation capability of DVR

depends on the capacity of the energy storage device. Fig. 4.29 and

Fig. 4.30 show that the compensating capability of DVR depends on

the capacity of the DC Energy Storage.



Fig. 4.32 shows that if the energy storage is more than the

required it will give swell at the load.

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4.4.5. Simulation Result of Voltage Swell

The first test system contains no DVR and capacitive load C =

28.4µF is applied during the period 500 to 900 ms. The voltage swell

at the load point is 25% with respect to the reference voltage is shown

in Fig. 4.33.


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as above but now with the DVR in operation. The total simulation

period is 1400 ms.

When the DVR is in operation the voltage swell is mitigated

almost completely and the RMS voltage at the sensitive load is

maintained normal, as shown in Fig. 4.34. Two transient undershoots

are observed when the DVR comes in and out of operation.

Fig. 4.34 Voltage VRMS at the load with DVR Energy Storage of 6.8KV.

4.5. SUMMARY

DVR is a recently proposed series connected solid state device

that injects voltage into the system in order to regulate the load side

voltage. It is normally installed in a distribution system between the

supply and the critical load feeder at the point of common coupling.

Other than voltage sags and swells compensation, DVR can also

added other features, those are line voltage harmonics compensation,

reduction of transients in voltage and fault current limitations and

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then analyze the dynamic and steady-state performance of DVR. The

results of voltage and current waveforms of DVR using PI Controller

and Fuzzy Logic Controller with voltage sag during three phase fault,

voltage sag during single line to ground fault, voltage sag during

double line fault, voltage interruption during three phase fault and

voltage swell points are analyzed. DVR with Fuzzy Logic Controller

performs better among DVR with PI Controller and Fuzzy Logic

Controller.

chapter 4 enhancement of power quality using dvr...

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