Effects of Voltage Sag Due to Starting of Induction Motor-ed2

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