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CHAPTER 5

MITIGATION OF VOLTAGE SAG AND SWELL USING

DIRECT CONVERTERS WITH MINIMUM

SWITCH COUNT

5.1 INTRODUCTION

Though many DVR topologies have been proposed based on direct

converters, in the literature survey it is found that minimum number of

switches used in each phase for compensation is four. In order to reduce the

number of switches a new simplified topology with a series centre tapped

transformer at the output side of the direct converter is presented. The direct

converter is fabricated using only three bi-directional controlled switches. The

DVR can properly compensate long-duration, balanced and unbalanced

voltage sag and swell by taking power from the grid. The switches are driven

by Pulse Width Modulation (PWM) signals. The simulation and the hardware

results validate the idea that the proposed topology can mitigate sag and swell

effectively.

5.2 PROPOSED TOPOLOGY WITH MINIMUM SWITCH

COUNT

Each DVR consists of a direct converter with a LC filter and a

series centre tapped transformer with the turns-ratio of 1:1as shown in the

Figure 5.1.

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Figure 5.1 Proposed topology with minimum switches

Figure 5.2 Bidirectional switch topology

In this arrangement, the output voltage of the compensator can be

either in-phase with the grid voltage or out of phase with the grid voltage. In

the proposed topology, for the a-phase, when the switch Saa is put-on,

supplies an in-phase voltage to the series transformer. When the switch Saa is

put-on, the topology supplies the out-of-phase voltage to the series

transformer. The switch Sga is connected across the series transformer. When

the a-phase voltage is at the rated level, the switch Sga is kept closed, short-

circuiting the primary side of the series transformer. If the a-phase

experiences sag, in order to generate the compensating voltage in phase with

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the grid voltage, rnatively modulated. If it

has swell, then in order to generate the compensating voltage out of phase

with the grid voltage, the switches Saa and Sga are alternatively modulated.

The compensating voltage is added to the grid voltage through the

series transformer. Each DVR is structured by three bidirectional power

switches. The topology of the bidirectional switch used is shown in the

Figure 5.2. The switches are controlled by a simple PWM technique.

Considering Figure 1, the following equation can be obtained:

(5.1)

In (5.1), l, g and con subscripts are used for the load, grid, and compensating

quantities, respectively; the second subscript refers to the corresponding

phases, respectively. Assuming sinusoidal waveforms, and considering only

the a-phase, the voltages can be expressed as follows:

(5.2)

In the aforementioned equations , and are the peak

values of load, grid, and injected voltages, respectively. , is the phase angle

of the injected voltage and is defined as follows:

(5.3)

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5.3 CONTROL PROCEDURE

As mentioned in the first chapter, the voltage sag and swell is

identified using single phase d-q theory. In this section switching pulse

generation for the mitigation of sag and swell is explained in detail.

5.3.1 Voltage Sag Mitigation

In order to compensate the voltage sag, it is necessary to inject

voltage in-phase with the grid voltage. As the centre tapped series transformer

gives output voltages of different polarity, the voltage which is in-phase with

the grid voltage is chosen. This voltage is modulated using bi-directional

switches and added in-phase with the grid voltage through a series

transformer.

Figure 5.3 Block diagram of switching pulse generation

So if the voltage sag has occurred in a-phase, then the bidirectional

A

detailed block diagram for switching pulse generation to mitigate the voltage

sag is shown in Figure 5.3. Ugmax has been obtained from single phase d-q

transform as explained in the first chapter. Uref is the desired terminal voltage,

which is a user specified constant value set in the micro controller program.

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The difference between the reference voltage Uref and the peak value of the

grid voltage Ugmax gives the amount of voltage sag or swell in the grid. The

error signal is compared with the triangular carrier signal to generate the

switching pulses. The pwm pulses are given to the corresponding switches

through the logic gates as shown in the Figure 5.3.

5.3.2 Voltage Swell Mitigation

In order to compensate the voltage swell, it is necessary to inject

voltage out of phase with the grid voltage. So if there is voltage swell in phase

modulated to mitigate the swell. Switching pulse generation is same as that of

sag.

5.4 COMPENSATION RANGES OF THE DVR

In this section, the voltage sag and swell compensation ranges for

the proposed topology are calculated. It is assumed that the phase shift from

Vg to Vcon as well as the phase shift from Vcon to Vl are negligible. The

relationship between Vg and Vcon (the filtered output voltage of the converter)

can be expressed as

(5.4)

In (5.4

The above equation is valid because the transformation ratio of the series

transformer is 1:1.

5.4.1 Voltage Sag Compensation Range

According to Figure 1, for voltage sag condition:

(5.5)

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Considering (5.4), (5.5) can be rewritten as follows:

(5.6)

The voltage sag percentage is defined by

(5.7)

Considering (5.6) and (5.7), the voltage sag percentage can be simplified to

(5.8)

It is seen that the maximum value of sag that can be compensated is

50% for D=1.

5.4.2 Voltage Swell Compensation Range

According to Figure 1, for voltage swell condition:

(5.9)

Considering (5.4), (5.9) can be simplified to

(5.10)

The voltage swell percentage is defined as follows:

(5.11)

Considering (5.10) and (5.11), the voltage swell percentage is

simplified as follows:

(5.12)

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In this method the swell has been compensated by feeding the

voltage from the same phase. From (5.12) it is observed that for D = 0.5,

100% voltage swell can be compensated. As the value of D is increased the

topology can mitigate a greater amount of swell. It is not required to choose D

=1 to mitigate large swells since sufficient voltage is available in the phase

where swell is occurred.

5.5 SIMULATION RESULTS

The MATLAB/SIMULINK software has been used for simulation.

Three phase RL load (0.8 power factor lag, 240VA per phase,) were

connected to the lines. The desired terminal voltage has been set at 60 V rms

(1p.u), 50 Hz. The switching frequency of the converters is 8 kHz. The filter

is designed for a cut off frequency of 1000Hz with the value of inductance

1.732mH and the capacitance of 15uF according to the formula f = 1/

The ability of the DVR to mitigate balanced voltage sag of 50% in

all the phases is shown in Figure 5.4.

Mitigation of unbalanced voltage sag

Figure 5.5.

The compensation of balanced swell of 100% is illustrated in

Figure 5.6.

The ability of the DVR to mitigate unbalanced voltage swell of

100% in a-phase, 50% in b-phase, and 25% in c-phase can be observed in

Figure 5.7.

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Figure 5.4 Mitigation of balanced voltage sag (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR

Figure 5.5 Mitigation of unbalanced voltage sag (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR

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Figure 5.6 Mitigation of balanced voltage swell. (a) Grid voltage (b) load Voltage (c) compensation voltage

Figure 5.7 Mitigation of unbalanced voltage swell (a) Grid voltage (b) Load voltage (c) Compensation voltage produced by the DVR

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5.6 EXPERIMENTAL RESULTS

A three-phase DVR described in this chapter has been fabricated in

order to verify the design procedure. A photograph of the experimental

prototype is shown in the Figure 5.8. The PIC16F877A microcontroller has

been used to generate the switching pulses. The rating of the centre tapped

series transformer is 720VA, 120V with the transformation ratio of 1:1.

IRFP460 power MOSFET switches of rating 500V, 20A were used to

synthesize the direct converter. The hardware prototype has been designed for

a rated voltage of 60V.

Figure 5.8 Hardware prototype

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The ability of the DVR to mitigate balanced voltage sag of 50% in all the

phases is shown in Figure 5.9.

Figure 5.9 Balanced Sag Compensation (a) Grid voltage (b) Compensated load voltage (c) Compensating voltage produced by the DVR

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The compensation of unbalanced voltage sag of 50% in a-phase, 25% in b-

phase, and 10% in c-phase can be seen in Figure 5.10.

Figure 5.10 Unbalanced sag compensation (a) Grid voltage (b) Compensated load voltage (c) Compensating voltage produced by the DVR

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The compensation of voltage swell of 90% in a-phase, 100% in b-phase, and

115% in c-phase can be seen in Figure 5.11.

Figure 5.11 Swell compensation (a) Grid voltage (b) Compensated load voltage (c) Compensating voltage produced by the DVR

5.7 PERFORMANCE ANALYSIS

Babai et al (2010) proposed a topology based on direct converter,

with 5 switches per phase in which switches are controlled based on sampling

process and it needs computations throughout a cycle. Using the control

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algorithm explained, the compensation range of voltage sag is 0-33% and of

the range for the voltage swell is 0-100%.

Perez et al (2006) presented a topology for a single-phase DVR

based on a single phase matrix converter with 4 switches per phase and the

compensation range is 25% for voltage sags and 50% for swells. In this

topology, during compensation 3 switches are modulated. So generation of

switching pulses is complicated.

Wang & Venkataramanan (2009) designed a DVR based on an

indirect matrix converter for balanced voltage sags compensation of 60%.

This topology needs flywheel energy storage element and the capability of the

topology in voltage swell compensation has not been investigated.

In this work, a centre tapped series transformer is used only with

3 bi-directional switches such that the only 2 switches are modulated

during compensation. So the switching loss is less and switching pulse

generation is also easier. In this topology switches are controlled by ordinary

PWM. As a result, computation is avoided, control is simpler and 50% of

voltage sag and unlimited quantity of voltage swell is compensated as

mentioned in Table 5.1.

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Table 5.1 Comparison of various DVR topologies

S. No

Converter Topology Compensation

Range PWM

Technique Used

No of Switches

Sag Swell

1 Direct Converter topology by Babai et al (2010)

33% 100%

Pulse Width is computed during each switching period

5

2 Matrix Converter topology by perez et al (2006)

25% 50% Ordinary PWM

4

3

Indirect matrix Converter with Flywheel energy Storage by Wang & Venkataramanan (2009)

60% Nil Ordinary PWM

5

4

Proposed converter

50% Unlimit

ed Ordinary PWM

3

5.8 SUMMARY

A three-phase DVR based on direct converters has been presented

which do not require the dc link as in the conventional DVRs. The absence of

the dc link reduces the cost, weight, and volume of the DVR and also avoids

the maintenance of energy storage devices. The DVR in each of the three

phase lines is constructed using only three bidirectional switches. The control

of DVR is done by using a very simple PWM procedure. The DVR is able to

mitigate 50% of balanced and unbalanced voltage sag, unlimited quantity of

balanced and unbalanced voltage swell effectively. The topology presented

uses only three switches, with one centre tapped series transformer for each

phase for effective compensation with simple control logic.