International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
POWER QUALITY DISTURBANCE ON PERFORMANCE OF
VECTOR CONTROLLED VARIABLE FREQUENCY INDUCTION
MOTOR
A. N. Malleswara Rao1, K. Ramesh Reddy2, B. V. Sanker Ram3
1
Research Scholar, JNT University Hyderabad, Hyderabad, India
G.Narayanamma Institute of Science and Technology, Hyderabad, India
3
JNTU College of Engineering, JNTUH, Hyderabad, India
2
ABSTRACT
Sensitive equipment and non-linear loads are now more common in both the industrial/commercial sectors and
the domestic environment. Because of this a heightened awareness of power quality is developing among
electricity users. Therefore, power quality is an issue that is becoming increasingly important to electricity
consumers at all levels of usage. Continuous variation of single-phase loads on the power system network leads
to voltage variation and unbalance, most importantly; the three-phase voltages tend to become asymmetrical.
Application of asymmetrical voltages to induction motor driven systems severely affects its working
performance. Simulation of an Induction Motor under various voltage sag conditions using Matlab/Simulink is
presented in this paper. Variation of input current, speed and output torque for vector controlled variable
frequency induction motor-drive is investigated. Simulation results show that the variation of speed and current
in motor-drive system basically depends on the size of the dc link capacitor. It is shown that the most reduction
of dc-link voltage happens during voltage sag. It is also observed that as the power quality become poor, the
motor speed decreases, causing significant rise in power input to meet the rated load demand.
KEYWORDS: Power quality disturbance, Sag, Vector Control Induction Drive
I.
INTRODUCTION
Electric power quality (PQ) has captured much attention from utility companies as well as their
customers. The major reason for growing concerns are the continued proliferation of sensitive
equipment and the increasing applications of power electronics devices which results in power supply
degradation [1]. PQ has recently acquired intensified interest due to wide- spread use of
microprocessor based devices and controllers in large number of complicated industrial process [2].
The proper diagnosis of PQ problems requires a high level of engineering ability. The increased
requirements on supervision, control and performance in modern power systems make power quality
monitoring a common practice for utilities [3].
In general, the main PQ issue can be identified as, voltage variation, voltage imbalance, voltage
fluctuations, low frequency, transients, interruptions, harmonic distortions, etc. The consequences of
one or more of the above non-ideal conditions may cause thermal effects, life expectancy reduction,
dielectric strength and mis-operation of different equipment. Furthermore, the PQ can have direct
economic impact on technical as well as financial aspects by means of increase in power consumption
and in electric bill [4]. PQ problems affecting Induction Motor performance are harmonics, voltage
unbalance, voltage sags, interruption etc. Voltage sags are mainly caused by faults on transmission or
distribution systems, and it is normally assumed that they have a rectangular shape [5]. This
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International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
assumption is based on neglecting a change in the fault impedance during the fault progress.
However, this assumption does not hold in case of the presence of induction motors and longer
duration faults since the shape of voltage sags in such cases gets deformed due to the motors’ dynamic
responses [6]. When voltage sags appear at the terminals of an induction motor, the torque and speed
of the motor will decrease to levels lower than their nominal values. When voltage sags are over,
induction motor attempts to re-accelerate, resulting in drawing an excessive amount of current from
the power supply.
In this paper first, various types of voltage sag are simulated in Matlab / Simulink environment.
Thereafter, performance of an (Vector Controlled Variable Frequency Induction Motor)VCVF IMdrive system is simulated and the results are analyzed in order to identify the parameters affecting the
drive-motor performance.
II.
TYPES OF SAGS
Due to different kinds of faults in power systems, different types of voltage sag can be produced.
Different types of transformer connections in power grid have a significant role in determination of
voltage sag type [7]. Voltage sag are divided in to seven groups as type A, B, C, D, E, F and G as
shown in Table I. In this table "h" indicates the sag magnitude. Type A is symmetrical and the other
types are known as unsymmetrical voltage sag.
There are different power quality problems that can affect the induction motor behaviors such as
voltage sag (affecting torque, power and speed), harmonics (causing losses and affecting torque),
voltage unbalance (causing losses), short interruptions (causing mechanical shock), impulse surges
(affecting isolation), overvoltage (reducing expected life time), and under voltage (causing
overheating and low speed) . There are several power quality issues which until today were normally
not included in motor protection studies. However, they should be taken into consideration due to
their increasing influence. Other actual power quality problems have been considered for many years
now, such as voltage imbalance, under voltages, and interruptions [8].
This type of problems is intensified today because power requirements of sensitive equipment, and
voltage– frequency pollution have increased drastically during recent years. The actual trend is
anticipated to be maintained in the near future. Principally, voltage amplitude variations cause the
present power quality problems. Voltage sags are the origin of voltage amplitude reduction together
with phase-angle shift and waveform distortion and result in having different effects on sensitive
equipment. Voltage sags, voltage swells, overvoltages, and undervoltages are considered such as
amplitude variations [8].
New power quality requirements have a great effect on motor protection, due to the increasingly
popular fast reconnection to the same source or to an alternative source. The characteristics of both
the motor and supply system load at the reconnection time instant are critical for the motor behavior.
Harmless voltage sags can be the origin of great load loss (load drop) due to the protection device
sensitivity
TABLE-I : Types of Sags
150
Type A
Type B
Va = hV
1
1
Vb = − hV − jhV 3
2
2
1
1
Vb = − hV + jhV 3
2
2
Va = hV
1
1
Vb = − V − jV 3
2
2
1
1
Vb = − V + jV 3
2
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International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
Type C
Type D
Va = V
1
1
Vb = − V − jhV 3
2
2
1
1
Vb = − V + jhV 3
2
2
Va = hV
1
1
Vb = − hV − jV 3
2
2
1
1
Vb = − hV + jV 3
2
2
Type E
Type F
Va = V
Va = hV
1
1
1
Vb = − jV 3 − hV − jhV 3
3
2
6
1
1
1
Vc = + jV 3 − hV + jhV 3
3
2
6
1
1
Vb = − hV − jhV 3
2
2
1
1
Vb = − hV + jhV 3
2
2
Type G
2 h
Va = ( + )V
3 3
1
1
Vb = − (2 + h)V − hVj 3
6
2
1
1
Vb = − ( 2 + h)V + hVj 3
6
2
Where 0 ≤ h < 1
(h= sag magnitude)
2.1 Symmetrical Faults
The voltage during the fault at the point-of-common coupling (pcc) between the load and the fault can
be calculated from the voltage-divider model shown in Figure 1.
Figure 1. Voltage divider model for voltage sags due to faults.
For three-phase faults, the following expression holds:
V=
Z F+
E
Z F + + Z s+
----(1)
where ZS+ and ZF+ are the positive-sequence impedance of source at the pcc and impedance
between the pcc and faulty point including the fault impedance itself. Through this relation it can be
concluded that the current through the faulted feeder is the main cause for the voltage drop [8].
2.2 Non-Symmetrical Faults
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International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
For non-symmetrical faults the expressions are similar but slightly more complicated. This leads to
resulting characterization of unbalanced dips due to non-symmetrical faults. For two-phase-to-ground
and phase-to-phase faults the characteristic voltage is found from (2); for single-phase faults also the
zero-sequence quantities affect the result:
1
Z F + + (Z F 0 + Z S 0 )
2
V=
E
1
Z F 1 + Z S1 + ( Z F 0 + Z S 0 )
2
----(2)
where ZS0 and ZF0 are the zero-sequence source impedance at the pcc and the zero-sequence
impedance between the fault and the pcc, respectively [9]. For two-phase-to-ground faults it can also
be obtained from:
V=
Z F + + 2( Z F 0 + Z S 0 )
E -------(3)
Z F 1 + Z S 1 + 2( Z F 0 + Z S 0 )
The main assumptions behind these equations are that the positive-sequence and negative-sequence
impedances are equal and that all impedances are constant and time independent. They lead to a
“rectangular dip” with a sharp drop in rms voltage, a constant rms voltage during the fault, and a
sharp recovery. Under the assumption of constant impedance, all load impedances can be included in
the source voltage and impedance equivalent, and the voltages at the motor terminals are equal to the
voltages at the PCC.
III.
BEHAVIOUR OF AN INDUCTION MOTOR SUPPLIED WITH NONSINUSOIDAL VOLTAGE
When induction motors are connected to a distorted supply voltage, their losses increase. These losses
can be classified into four groups:
1) Losses in the stator and rotor conductors, known as copper losses or Joule Effect losses.
2) Losses in the terminal sections, due to harmonic dispersion flows.
3) Losses in the iron core, including hysterics and Foucault effects; these increase with the order
of the harmonic involved and can reach significant values when feeding motors with skewed
rotors with wave forms which contain high frequency harmonics[7,8,9].
4) Losses in the air gap. The pulsing harmonic torques is produced by the interaction of the
flows in the air gap with those of the rotor harmonic currents, causing an increase in the
energy consumed.
These increased losses reduce the motor’s life. Further information on each of the groups is given
below. The effect of the copper losses intensifies in the presence of high frequency harmonics, which
augment the skin effect, reducing the conductors’ effective section and so increasing their physical
resistance [10].
3.1 Induction Motor Behaviour
The study can be done experimentally or analytically, by using dynamic load models mainly designed
for stability analysis, but they are rather complicated, requiring precise system data and high level
software [11-13]. Therefore, in this investigation, the study is adopted as a preliminary step. When a
temporary interruption or voltage sag takes place, with time duration between 3 seconds and 1 minute,
the whole production process will be disrupted. Keeping the motor running is useless because most of
the sensitive equipment will drop out. The induction motor should be disconnected, and the restart
process should begin at the supply recovery, taking into account the reduction and control of the hot
load pickup phenomenon.
Keeping the motor connected to the supply during voltage sags and short interruptions, rather than
disconnecting and restarting it, is advantageous from the system’s stability point of view. It is
necessary to avoid the electromagnetic contactor drop out during transients. This scheme improves the
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International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
system ride-through ability due to the reduction of the reacceleration inrush [14]. Such problems
result in the initial reduction of the motor speed, keeping for a while a higher voltage supplied by its
internal, or back electromotive force (emf). The voltage reduction is governed by the stored energy
dissipation through the available closed circuits, which are the internal rotor circuit (including the
magnetizing inductance) and the external circuit composed of the load (paralleled by the faulted path
in case of fault-originated voltage sags.) The whole circuit time-constant determines the trend which
the decaying voltage will follow until the final voltage magnitude is reached or the event is ended.
When the transient ends, the motor speed increases demanding more energy from the supply until the
steady state speed is reached. The load torque in this case shows very different characteristics as
compared to normal start up conditions, due to several reasons such as the motor generated voltage
that might be out of phase, heavily loaded machinery, and a rigorous hot-load pickup [15].
As mentioned above, the single line-to-ground fault is the most probable type of fault, and through a
∆Y transformer is transferred as a two-phase voltage sag, in which case normal and extremely deep
voltage sags should be considered as a case of transient unbalanced supply. The effect of voltage
unbalance is the decrease of the developed torque and increase of the copper loss due to the negativesequence currents. The thermal effect of the short duration considered can be neglected. Besides,
three-phase voltage events represent the worst stability condition. Therefore, only balanced
phenomena were experimentally studied here, leaving the unbalanced behavior for future
investigation [16],[17].
IV.
CASE STUDY AND SIMULATION RESULTS
This paper also investigates the impact of power quality on sensitive devices. At this stage, the focus
is on the operation characteristics of a Vector Controlled Variable Frequency Induction Motor Drive
(as shown in Fig. 2) in the presence of sag events. The motor under consideration is a 50 HP, 460V
and 60 Hz asynchronous machine. A DC voltage of 780V average is obtained at the DC link from the
diode bridge rectifier which takes a nominal 3-phase (star connected) input of 580V rms. line-to-line.
Voltage sags are normally described by magnitude variation and duration. In addition to these
quantities, sags are also characterized by unbalance, non sinusoidal wave shapes, and phase angle
shifts.
Fig 2 . Vector controlled Variable Frequency Induction Motor Drive
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ISSN: 2231-1963
Fig 3: Wave forms of 3 phase currents and Vdc during LG Fault
Fig 4: waveforms of Vabc ,Iabc, Speed and Torque during LG fault
Fig 5 : Wave forms of 3 phase currents and Vdc during LLG Fault
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International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
Fig 6 : waveforms of Vabc ,Iabc, Speed and Torque during LLG Fault
Fig 7: Wave forms of 3 phase currents and Vdc during 3 phase Fault
Fig 8: waveforms of Vabc ,Iabc, Speed and Torque during 3phase fault
Fig. 3-8 illustrate disturbance inputs, the fall in DC link voltage and change in rotor speed for Case C
corresponding to the sag event that occurs at time t= 3 seconds when Phase A and Phase B experience
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International Journal of Advances in Engineering & Technology, Nov 2011.
©IJAET
ISSN: 2231-1963
a line to ground fault. The fall in DC link voltage, and the rotor speed are observed for the period of
the event. When normal supply resumes, the DC link voltage stabilises at 780 Volts and the rotor
speed at 120 radians per second. There might be different kinds of short circuit faults on the network
resulting in voltage sags such as single phase-to-ground, phase-to-phase, 2 phase-to-ground and 3
phase-to-ground faults. Studying the speed variation waveform of the induction motor due to the
different voltage sags caused by such faults at a specific place in the network as shown in Figure 5, it
is proved that single phase-to-ground fault causes the least variation in speed profile but a 3 phase-toground fault the highest variations. Also, the ability of the drive to ride-through a voltage sag event is
dependent upon the energy storage capacity of the DC link capacitor, the speed and inertia of the load,
the power consumed by the load, and the trip point settings of the drive. The control system of the
drive has a great impact on the behaviour of the drive during sag and after recovery. The trip point
settings can be adjusted to greatly improve many nuisance trips resulting from minor sags which may
not affect the speed of the motor. Table II shows three cases of inputs “A” to “C” supplied as
unbalanced sags to the above system, and the corresponding outputs observed.
TABLE II: SIMULATION RESULTS
INPUT CASE
LG
Sag magnitude : Phase A
0.1
(p.u.)
Phase B
1
Phase C
1
Start time of sag (sec)
4
Duration of sag (sec)
1
Phase angle shift: Phase A
0
(radians)
Phase B
-1.047
Phase C
1.047
Load torque (N-m)
50
Start time of load (sec)
0
Duration of load (sec)
4
Reference rotor speed (rad/s)
120
OBSERVATIONS
Nominal DC link Voltage (V)
780
DC link Voltage during event (V)
450
Change in DC link Voltage (%)
42.3
Rotor speed during event (rad/s)
120
Change in rotor speed (%)
0
V.
LLG
1
0.1
0.1
4
1
0
0
0
50
0
4
120
3φ
Fault
0.1
0.1
0.1
4
1
0
0
0
50
0
4
120
780
370
52.6
93
22.5
780
250
68
25
79.7
CONCLUSIONS
Voltage sags and short time interruptions are a main power quality problem for the induction motors
utilized in the industrial networks. Such problems can also lead to the unbalanced voltages of the
network. Their result is the effect on torque, power and speed characteristics of the motor and the
increase in the losses. In this paper, the short interruption and voltage sag effects on the motor
behaviour were studied where through the simulations done with MAT LAB, the different behaviours
of induction motors due to voltage sags from different origins and other related problems were
investigated. In addition the amount of effect of different sources of the faults leading to voltage sag
and imbalanced voltage sag were observed. Behaviour of a Vector controlled Variable Frequency
Induction Motor Drive in the presence of sag events has been simulated as our initial investigation of
impact of power quality on sensitive equipment.
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[16] A.N.Malleswara Rao, Dr.K.Ramesh Reddy and Dr. B.V.Sanker Ram”A new approach to diagnosis of power
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[17]A.N. Malleswara Rao, Dr. K. Ramesh Reddy and Dr. B.V. Sanker Ram” Effects of Harmonics in an Electrical
System” International Journal of Advances in Science and Technology (IJAET), Vol. No. 3, Issue No. 2, 25 – 30
AUTHORS
A. N. Malleswara Rao received B.E. in Electrical and Electronics Engineering from Andhra
University, Visakhapatnam, India in 1999, and M.Tech in Electrical Engineering from JNT
University, Hyderabad, India. He is Ph.D student at Department of Electrical Engineering, JNT
University, Hyderabad, India. His research and study interests include power quality and power
electronics.
K. Ramesh Reddy received B.Tech. in Electrical and Electronics Engineering from Nagarjuna
University, Nagarjuna Nagar, India in 1985, M.Tech in Electrical engineering from National
Institute of Technology(Formerly Regional Engineering College), Warangal, India in 1989, and
Ph.D from SV University, Tirupathi, India in 2004. Presently he is Head of the department and
Dean of PG studies in the Department of Electrical & Electronics Engineering, G.Narayanamma
Institute of Technology & Science (For Women), Hyderabad, India. Prof. Ramesh Reddy is an
author of 16 journal and conference papers, and author of two text books. His research and study interests
include power quality, Harmonics in power systems and multi-Phase Systems.
B. V. Sanker Ram received B.E. in Electrical Engineering from Osmania University,
Hyderabad, India in 1982, M.Tech in Power Systems from Osmania University, Hyderabad,
India in 1984, and Ph.D from JNT University, Hyderabad, India in 2003. Presently he is
professor in Electrical & Electronics Engineering, JNT University, Hyderabad, India. Prof.
Sanker Ram is an author of about 25 journal and conference papers. His research and study
interests include power quality, control systems and FACTS.
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