effects of voltage sag due to starting of induction motor-ed2
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Effects of Voltage sag due to starting of induction motor
Introduction
Voltage sags are short duration reduction in rms voltage, cause by short circuits,
overloads and starting of large motors. The excite in voltage sags is mainly due to
the problem it cause on several types of equipment for example the adjustable
speed drive, process control equipment and computers are notorious for their
sensitivity. Of course a voltage sags is not as damaging to industry as a (long or
short) interruption. But as there are far more voltage sags than interruptions the total
damage due to sags is still larger. Short interruptions and most long interruptions
originate in the local distribution network. Most of the current interest in voltage sags
due to short circuit faults. These voltage sags are the ones which cause the majority
of equipment trips. But also the starting of induction motors leads to voltage sags.
Voltage sags due to induction motor starting last longer than those due to short
circuits, the typical duration are seconds to ten of seconds.
Problem statement
Electronics and power electronics equipment has especially become much more
sensitive than it is in over past 10 or 20 years ago. Not only has equipment become
more sensitive, companies also have become more sensitive to loss of production
time due to their reduced profit margins. On the domestic market electricity is more
considered a basic right which should simply always be present. The consequence is
that an interruption of the supply will much more than before lead to complaints,
even if there are no damages or cost related to it.
Large load changes or motor starts can also cause voltage sags. An induction motor
will draw six to ten times its full load current while starting. This lagging current then
causes a voltage drop across the impedance of the system. Should the current
magnitude be large relative to the system available fault current, the resulting voltage
sag may be significant. Figure 1 illustrates example the effect of a large motor being
started.
Objectives
The objectives of this project as follows :
a. To model and simulate distribution system consisting of induction motor for
voltage sag study during starting of an induction motor
b. To validate the results from simulation with measurements on actual
distribution panel.
c. To propose and test a solution to voltage sag problem using simulation and
actual setup on the distribution panel.
The scope of project
The project covers the following scopes:
a. Voltage sag study on 3phase 400V 50Hz 300kVA supply system
b. The system consists of 120 kW resistive load connected to two 10 hp
induction motors and lift simulator PLC controlled connected to controller
(CPU).
Figure 1
Literature review
Alexis Polycarpou et. al. [1] developed the technique to predict the depth and
duration of a voltage sag sensed on the various buses of an interconnected power
system due to large induction motor load switching on one of the system buses by
proposed methodology consists of three steps:
Step 1 - Calculation of ZPCC (Impedance at point of common coupling)
Step 2- Obtaining Power waveforms
Step 3- Instantaneous transient Load flow calculation
These method done through mathematically described and validate through PSCAD
simulation and MathCAD mathematical programming. However, the technique only
proven by mathematically and simulation and also usually on common industry did
not practice using interconnected system, they prefer on using radial system which is
more practical.
J.C. Gomez, et. al. [2] developed the technique of transform the information of
starting current/time characteristics to voltage sag depth/time characteristics that are
directly comparable with the sensitive equipment susceptibility curves (CBEMA and
other curves). The method also allows them to consider motor repetitive starts and
different starting cycles. They found that From the tests and calculations proposed
method makes it easy to evaluate the consequences of the changes (in the motor
starting commutation time and initial voltage) on the SE dropout and also allows
consideration of repetitive starts as well as different starting cycles. However, the
technique have disadvantages such as if the specific energy below the minimum
steady-state value reaches the SE characteristic stored energy, the equipment will
drop out. It is also did not show on what percent of voltage sags should effect with
sensitive equipment.
J.D. Li , et. al. [3] developed the technique of determining the switching device
current rating and the energy storage device capacity of the restorer. They found that
if the DVR is installed upstream of the motor, the large starting current will also flow
through the DVR and might damage the restorer. the large starting current will also
require more energy to be drawn from the upstream system. In order to guarantee
the restorer would offer a good supply quality for its downstream loads, a method to
estimate the peak current and required energy for the motor starting process is
presented However, the technique have disadvantages such as in term of cost,
maybe this technique only suitable for large industry. And also the injection
transformer have its own impedance, the design should consider the impedance.
Background on Voltage Sag
The main load having influence on the voltage sags during and after a sag is formed
by induction and synchronous motor as they have the largest currents during and
after a short circuit fault. These voltage sags are the main cause of equipment failure
and malfunction , and one of the main reasons of power quality to become an issue
during the last decade. Another important cause of voltage sags, is one which has
actually been of much more concern to designers of industrial power systems in the
past, is the starting of large induction motors. During a start up an induction motor
takes a larger current than normal, it is typically five to six times larger than nominal
current rating. This current remains high until the motor reaches its nominal speed
typically between several seconds and one minute. The drop in voltage usually
depends strongly on the system parameters. For example from the system below
shown in Figure 2, where ZS is the source impedance and ZM of the motor
impedance during start up.
Figure 2
The voltage experienced by a load fed from the same bus as the motor is found from
the voltage divider equation as above:
௦ = ெ
ௌ + ெ
Then ZS and ZM can be obtain by this formula,
ௌ = మ
ௌೄೠ
ெ = మ
ఉௌ
Then rewritten would get,
௦ = ௦௨
௦௨ + ௧ߚ
The voltage at the motor terminal during motor starting can be estimated using this
calculation,it may not be as the same with the actual simulation and experiment.
Methodology of the project
The proposed methodology consists of following steps
Step 1 : develop a model based on the project using matlab simulation to get initial
results
Step 2 : set up experimental work to capture voltage sags readings and observe the
effects on the other loads
Step 3 : select an appropriate method to solve the voltage problem and observe the
performance
Step 4 : make comparison between simulation and experimental work after the
implementation of method
Step 5 : the results then compiled and analysed
In this project a two 10Hp motor connected with 120kW resistive load and lift
simulator PLC controlled connected to controller (CPU) with incoming supply of
400V 50Hz 300kVA. This line up will be use to demonstrate a voltage sags cause by
starting of induction motor. This test is set to be running with a certain period of time
to enable the data collection. Scope meter 190B (fluke) will help in the process of
data collection. Below a block diagram and picture set up of experiment work that
propose earlier.
Block diagram
Before doing the above mentioned experiment, simulation on the same model will be
done using matlab to get the initial results. The improvement on this process then
will be improve by adding a method of mitigation that is suitable for this model.
Resistive load Inductive load
Capasitive load
Main switchboard
RLC load simulator
Induction motor
Lift simulator
Expected result
In this result, it will discuss the probability success of this project. So at the final of
the project, the expected result has been targeted as followed;
1. The modelled design should display output wave required
2. Set up of experiment work should give out readings of voltage sags.
3. Propose and test a solution to voltage sag problem using simulation and
actual setup on the distribution panel should correct the problem.
Appendix
Project title
Develop model
Run in matlab
Obtain o/p
Experimental work
Obtain o/p
Run in matlab
Develop method solve voltage sag
Experimental work
Obtain o/p
Obtain o/p
comparison
Compile and
analyze
End
YES
YES
YES
YES
NO NO
NO
NO
Project flow chart
No Task Weeks
W1 W2 W3 W4 W5 W6 W7 W8 W11
W12
W13
W14
W15
1 Project Title Confirmation
2 Discuss about project
3 Define the problem statement, objective
and scope
4 Research on configuration of the project
system
5 Literature review and previous works
6 Study the theory of the project
7 Study for the methodology
8 Construct the flow chart and system
design
9 Determine experimental set up
10 Discussion on expected result
11 Conclusion for PSM1 report
12 Seminar paper preparation
13 Submit seminar for checking by
supervisor
14 Correction of seminar paper
15 Completer seminar paper
16 Seminar PSM1
17 Submit PSM1 report
Task Weeks
W1 W2 W3 W4 W5 W6 W7 W8 W11
W12
W13
W14
W15
Arrangement of project
Discuss on strategy to start project
Define the task should be work on
Research on configuration of the project
system
Literature review and previous works
Simulate model get result
Set up experimental work
Experimental work result
Implement method solving voltage sags
Obtain result from method
Result compile and anallize
PSM2 report and seminar preparation
Submit report and seminar for checking
by supervisor
Correction
Complete seminar paper
Seminar PSM2
Submit PSM2 report
Reference
[1] Alexis Polycarpou, Hassan Nouri, Validation of a Proposed Voltage Sag
Prediction Methodology for Interconnected Systems during Motor Starting, : Control,
Automation, Communication and Energy Conservation, 2009. INCACEC 2009. 2009
International Conference on ; 4-6 June 2009, page 1 - 4
[2] J.C. Gomez, M.M. Morcos, Voltage Sag Effects on Sensitive Equipment Due to
Starting Cycles of Induction Motor, Power Engineering Review, IEEE; Aug. 2002,
Volume : 22 , Issue:8 , page: 51 - 52
[3] J.D. Li, S.S. Choi , D.M. Vilathgamuwa, Design of a Series Compensator for
Enhancing Power Quality during the Starting of an Induction Motor, Power
Engineering Conference, 2005. IPEC 2005. The 7th International; Nov. 29 2005-Dec.
2 2005, page 1 - 516