An enhanced controller for shunt active filter in soft starting of induction motor

1. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
149
AN ENHANCED CONTROLLER FOR SHUNT ACTIVE
FILTER IN SOFT STARTING OF INDUCTION MOTOR
DIVIYA RADHAKRISHNAN1
, RESHMA P S2
1
P G Scholar, Department of EEE Sree Narayana Gurukulam College of Engineering Kolenchery, India
2
Assisstant Professor, Department of EEE Sree Narayana Gurukulam College of Engineering, Kolenchery, India
ABSTRACT
A shunt active filter controlled by Icosɸ algorithm is designed which is capable of reducing THD of source
current and also enables power factor improvement of the system. Here ac voltage controller fed induction motor is the
non linear load which draws reactive power from the source and imposes harmonics into the source. Simulation of three
phase and single phase shunt active filter controlled by I cosɸ algorithm with non linear load is performed. THD of
source current can be reduced below 4%.
Keywords: Ac voltage controller, Icosɸ algorithm, shunt active filter, total harmonic distortion
1. INTRODUCTION
With development in the field of electrical and electronics power electronics equipments are widely used today.
The main drawback of these equipments is the non linear behavior in system. Non linear loads will reduce the power
factor and also impose harmonics to the system. The solution for these problems ends up with proper filtering
equipments. Active filters [1] [2] are using power electronic switches and they are most commonly used now a day. The
main aim of filter is to supply the reactive power drawn by the non linear load. The active filters are nothing but inverter
circuits in which the switching can be controlled by any control algorithm to give compensation currents.
The shunt active filter is kept parallel with line which is mainly used for current harmonic elimination. Fig 1
shows the schematic diagram of three phase shunt active filter installed in a system with ac voltage controller fed
induction motor as the non linear load. IRPT algorithm[3], synchronous detection algorithm[4],dc bus voltage
algorithm[5] are some of the control algorithms for controlling shunt active filter. But these algorithms are not suited
under distorted and non ideal voltage conditions. Icosɸ algorithm[6] gives better results under these system conditions.
So a shunt active filter controlled by Icosɸ algorithm can reduce THD of source current and improves the power factor of
the system.
2. AC VOLTAGE CONTROLLER FOR SOFT STARTING OF INDUCTION MOTOR
Ac voltage controller fed induction motor is a non linear load, which will draw reactive power from the source.
Ac voltage controller can be used for the soft starting purpose of the induction motor. Initially when the motor is starting
stator current is very high which will damage the winding. At the time of starting the firing angle of the controller is kept
to a high value. Stator current can be reduced considerably by keeping the firing angle to a high value. After sometime
when the motor attains its speed the firing angle is gradually reduced. Starting current reduction improves the lifetime of
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
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2. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
150
motor and the losses can be reduced. Speed control is also achieved. The shunt active filter in the system is controlled by
Icosɸ algorithm which will give better results in distorted and unbalance voltage conditions. It can reduce the harmonics
below 4%.
Fig. 1 Proposed system with SAF
3. PROPOSED ICOSɸ ALGORITHM
Here the shunt active filter generates a reference filter or compensation current which is injected to the system to
provide compensation for harmonic and reactive portion of the load current. So source current need to supply only active
part of the load current. In this algorithm the desired source current is the product of Icosɸ and a unit amplitude sine
wave in phase with the mains voltage.
Consider R phase,
Source voltage = Vm sinwt (1)
Since the load is an RL load current lags voltage by an angle ɸ. Also as the load is non linear, the load current consists of
harmonic and dc current also.
Load current, i = Im sin(wt-ɸ) +Ih + Idc (2)
Fundamental load current If = Im sin(wt-ɸ) (3)
With a phase delay of 90 degree
i= Im sin(wt-ɸ-90) (4)
i= Im sin(wt-ɸ)cos90- Im cos(wt-ɸ) sin90 (5)
i= -Im cos(wt-ɸ) (6)
From fig 2 at first crossing of phase voltage wt = 180
i= -Imcos(180-ɸ)
i=-Im(cos180cosɸ+sin180sinɸ) (7)
i= Imcosɸ (8)
At second zero crossing of the voltage, wt =360
i= -Imcos(360-ɸ) (9)

3. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
151
i=-Im(cos360cosɸ+sin360sinɸ) (10)
i= -Imcosɸ
(11)
Since at every zero crossing of the phase voltage the load current is sampled and holded as Icosɸ magnitude,
until the next zero crossing, it is called Icosɸ algorithm.
Fig. 2 Description of Icosɸ algorithm
4. CONTROL CIRCUIT
Fig. 3 Control circuit of the algorithm

4. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
152
The load current is sensed and is given to the 2nd
order low pass filter to get a phase shift of 90 degree lagging.
This is given to the sample circuit of a sample and hold circuit. A zero crossing detector is used to detect the negative
going zero crossing of the phase voltage. The output pulse from zero crossing detector goes as the hold input to the
sample and hold circuit. This is then multiplied with unit amplitude sine wave in phase with source voltage which is the
desired source current. The reference filter current is obtained by taking the difference between the desired source current
and the load current. This reference filter current is compared with the actual filter current with a comparator to obtain
the switching pulses for the shunt active filter.
5. SIMULATION RESULTS
Fig 4 shows the simulink model of three phase ac voltage controller for soft starting of induction motor ie open
loop circuit.
Fig. 4 Simulink model of three phase ac voltage controller for soft starting of induction motor
Firing pulses are controlled by a clock timer where the initial firing angle is set to 110 degree and after 10 time
units of simulation time it is switched to 20 degrees. A separate control subsystem called firing control is kept for that
purpose. At the time of starting the firing angle is set as 110 degree. After some time the clock signal controls a selector
switch to change the firing angle from 110 degree to 20 degree. Considerable reduction in stator current can be observed
by this operation. The current drawn by this load is non linear and rich in harmonics. Shunt active filter can do better in
this case to reduce harmonics and to improve the power factor.
Fig. 5 Stator current of phase A of motor

5. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
153
Initially the stator current is low and gradually it increases and reaches the rated value. After switching the firing
angle from 110 degree to 20 degree the current will gradually increases. The stress of stator windings can be considerably
reduced. Initially at starting the current range was 20 Amperes and after the motor attains its speed the current level
switches to 40 Amperes.
Fig. 6 THD window of source current in open loop
In the figure 5 it is clear that for a particular time period the stator current is kept in the order of 20 amperes and
after some simulation time it is switched to 40 amperes. Thus the starting current reduction is achieved successfully.
The THD window of open loop control is shown in figure 6. Source current THD is measured. This much
harmonic level is not affordable for a precise load case. The THD level should be kept below 5%. For this we are using a
shunt active filter.
The THD level of source current is 18.95% which is very high and it is not affordable. For the reduction of THD
of source current we are using a shunt active filter in closed loop operation , thus power factor can also be improved.
Fig.7 Simulink model of closed loop model with controller
The switching pulse generation is given in separate sub system. Icosɸ controller is simulated separately in 3
phases and given in the subsystem shown as Icosɸ controller. The output of the controller gives the reference filter
current for three phases. The reference filter current is compared with the actual filter current and switching pulses are
generated. These switching pulses are given to VSI and it is then controlled in closed loop manner once the actual filter
current is obtained. By controlling the filter current properly the reactive power compensation can be done in a successful
manner. Considerable THD reduction can be obtained once the system is connected in closed loop, also the source
current becomes sinusoidal and power factor is improved.

6. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
154
Fig. 8 THD window of source current with controller
It is found that after controlling the system with shunt active filter the THD level is reduced to 3.98 %. Thus
power factor improvement is achieved.
Fig. 9 Compensated source current
Simulation of single phase shunt active filter with single phase thyristor converter as non linear load is also
done. It is shown in fig 10. Considerable reduction in the THD of source current is obtained.
Fig. 10 simulink model with single phase SAF

7. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
155
Fig. 11 THD window of source current with single phase SAF
6. CONCLUSION
A Shunt active filter controlled by Icosɸ algorithm is designed which reduces the THD of source current
imposed by the non linear load effectively. It also makes the source current sinusoidal and improves the power factor of
the system. Simulation of both three phase and single phase system with shunt active filter is done. THD of source
current is reduced below 5%. The main advantage is that Icos ɸ algorithm is suited under distorted and non ideal voltage
conditions and it has less response time. But the limitation is that the source current is not perfectly sinusoidal.
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