Load Influence on Voltage Dip Characteristics

I. Rendroyoko R.E. Morrison Peter K.C. Wong*

Department of Electrical & Computer ScienceMonash University, PO BOX 35, CLAYTON, VICTORIA 3800

Phone: 61-03-99053465, Fax: 61-03-99053454Ignatius.rendroyoko@eng.monash.edu.au

* United Energy Ltd., Moorabbin, VIC 3189

Abstract

Voltage dips are short duration reductions in rms voltage. They are mainly caused by short circuits.Voltage dips are one of the most troublesome power quality problems due to the interference theycause to several types of equipment. An important issue concerning voltage dip problems is theinfluence of loads on voltage dip characteristics. This paper explains in part the load influence onvoltage dip characteristics. Measurements of voltage dip, which are collected from a distributionsystem under study, are presented to quantify the analysis. The voltage dip measurements show thatthe remaining rms voltage present during the dip for busbars closer to the load. The measurementsalso show that the load influence the voltage dip shape.

1. Introduction

Voltage dips have become a major concern in powerquality in the past decade. The cost of economicalloses and inconveniences caused by voltage dips havetriggered some studies and research activities. Manyexperts have tried to characterise voltage dips [1,2,3].The existing standard on voltage dip characterises thevoltage dips in terms of magnitude and duration. Thecharacterisation of the standard is based on theassumption that faults will cause rectangular voltagedips. It is also assumed that the voltage drops to acertain low value immediately and when the fault iscleared, the voltage recovers back to normalimmediately.

The assumption of rectangular voltage dips, however,is not correct in a realistic system, which largelyconsists of rotating machines or motors. When a faultoccurs, all the rotating machines in the system slowdown and after the fault is cleared, the motors willaccelerate to the normal condition. Duringacceleration, motor will draw high current from thesystem and thus prolong the voltage dip.

This paper will discuss load behaviour on a systemduring and after a voltage dip. For the purpose of thispaper, one sub-system in south-eastern Victoria wasselected. This sub-system has significant differencesin load characteristics between the summer and winterseason. In the summer season, there is an increase ofenergy consumption, which is mostly due to theoperation of air conditioners. Therefore, part of theload consists of electrical rotating machines.

2. System Components

The distribution system under study is presented infig. 1. A main 66kV bus bar supplies the 22kVdistribution system trough 2 66/22kV 30-MVAtransformers and the sub-system is supplied at 415 Vfrom a 22/0.415kV 400-kVA transformer.

Figure 1One-line diagram of the sub-system under study

The transmission and distribution systems supply

electric power to a south-east area of Melbourne, it isowned by United Energy Ltd. (UE). The sub-systemsupplies mostly commercial customers, and a fewresidential and light industrial customers.

The circuit parameters of the distribution systemunder study are shown on table 1.

Table 1Sub-system's Circuit Parameters

For monitoring and measurement purposes, UE hasinstalled power quality monitoring equipment at the22kV bus and 415V bus. Thus, every fault occurringin the system will be recorded.

3. Summer and winter loads

The subsystem under study has a specificcharacteristic in load trend. To some extend, theamount of load is difference between the summer andwinter seasons. Usually, in summer, the system hasmore loads than in winter. The load variation betweensummer and winter is shown in fig. 2.

Figure 2Summer and Winter Max Load Variation

Since, the subsystem consists of residential,commercial and light industrial customers, thedifference of load between summer and winter in thissubsystem could be due to air conditioners. Insummer, the sub-system will have more rotatingmachines.

4. Post Fault Voltage Recovery

Faults in the distribution system might cause voltagedips. The location of fault, type of fault, fault clearingtime and the electrical system configuration will alsoaffect the voltage dip [3].

A Voltage dip is normally characterised by amagnitude and duration, however, another researcheralso mentioned phase angle jump and post fault dip asa further important characteristics [4]. A voltage dipoccurring in a system that has resistive loads, willhave rectangular shaped dips. When the fault occurs,the voltage directly reduces to a particular value, andwhen the fault is cleared, the voltage recovers back toits original level immediately [1].

This does not happen when parts of the load consist ofrotating machines such as induction motors or airconditioner motors.

One of the results of the voltage dip recording isshown in figure 3. Figure 3 shows 415V bus voltagedue to a fault of 200ms on a 22kV distribution system.The fault which has occurred on the 22kV system is asingle phase to ground fault. However, it is seen in thelow voltage side as a two phase to ground faultbecause of transformer connection. The voltage dipwas recorded at 20:51:34 PM, on 10 December 2000.

Figure 3Voltage dips in the subsystem

When the fault occurred, the bus voltage decreased toa certain point and then decaying to a lower rapidlyvoltage level during the short circuit period. After thefault is cleared, the voltage did not directly recover toits level before fault. The voltage need a longer time torecover back and this could be caused by airconditioning motor loads.

Circuit Voltage Postv Seq-% on 100MVA Zero Seq-% on 100MVA

From To No kV Type R1 X1 B1 R0 X0 B0

Source NW 66kV 66 Generator/Source 2.05% 6.99% 2.11% 17.95%

NW 66kV NW 22kV 1 66/22 Trfr - 20/30MVA 1.98% 51.36% 7.92% 20.54%

NW 66kV NW 22kV 2 66/22 Trfr - 20/30MVA 1.93% 50.96% 7.72% 20.38%

NW 22kV WHORSE-SVLE 1 22 O/H - 19/3.25AAC 0.19 0.33 1.75 0.34 1.59 0.67

22kV/433V 400kVA 1 22/0.433 Trfr - 400KVA 4.00% 4.00%

Cap Banks NW 22kV 1 22 4.7 Mvar

Cap Banks NW 22kV 2 22 6.0 Mvar

Sub-system Max LoadSummer 2000 and Winter 2001

0

50

100

150

200

250

300

350

6 7 8 9 10 11 12 13 14 15

Days

Load

kW June01 kVA June01 kVAR June01

kW Dec00 kVAR Dec00 kVA Dec00

Voltage Dip

200

210

220

230

240

250

260

0.05 0.15 0.25 0.35 0.45 0.55

Time (s)

Vo

ltag

e (V

)

V1 V2 V3

5. Simulation of Post Fault LoadBehaviour

In order to evaluate and analyse the rotating machine'sinfluence to the sub-system, a simulation model of thesub-system has been developed. The simulation usesthe power system blockset tools within the MATLABpackage. For simulation purposes, a SLGF and a3Ph-G fault are simulated at both 22kV and 415V.

In the model, the subsystem mimics the realsubsystem shown in figure 1. There are six feederssupplying electricity to consumers consisting ofresistive loads and rotating motor load models. The airconditioning motor load is 27% of the load in thesubsystem. The typical air conditioning motorparameters used here are given below:

Table 2Air conditioning motor parameters

5.1 Balanced Fault

During a 3Ph-G fault in the 22kV system, the 415Vbus bar voltage also drops in magnitude. Some of thesimulation results are shown below:

Figure 4Voltage dip for a 3Ph-G fault

Figure 5Load current during 3Ph-G fault

Figure 4 and 5 show the 415V-bus voltage and loadcurrent during a 3Ph-G fault in the 22kV system.

At the 3Ph-G fault, the bus voltage will be suppresseduntil the fault is cleared. The characteristics of the air-conditioning motor load affects the voltage drop andvoltage recovery. The subsystem voltage may swingfor a few cycles before returning to normal. These

swings lengthen the duration of the recoveryprocess [3].

5.2 Unbalanced Fault

Most of the faults on a medium voltage system aresingle-phase to ground faults [5]. Single-phase faultsoften result from lightning, wind, and tree-branchcontact or insulator failure. The behaviour of the sub-system under study during an unbalanced fault ismore complicated than during a three-phase to groundbalanced fault.

Figure 6Voltage dip for a 1Ph-G fault

Rating Rs Xs Rr Xr Xm J

1Ph Motor 1 Hp 1 4.14 0.9 4.14 69.11 0.146

3Ph Motor 2 Hp 0.6 2.98 0.5 2.98 41.73 1

Xm : magnetizing reactance

J : rotor inertia

Figure 7Load Current during 1Ph-G fault

The busbar voltage during a single phase to groundfault at 22kV line is shown in fig. 6. The 22/0.415kVtransformer connection makes the voltage dip seen asa 2Ph-G fault at the 415V busbar voltage.

Single phase to ground faults give less severeproblems to motor loads than 3Ph-G faults. However,voltage recovery after the fault is still affected.

Using symmetrical components, the characteristics ofthe voltage waveform can be clearly seen by dividethem into positive, negative and zero sequence voltagecomponents.

The result of this process is shown in figure 8 and 9below.

Figure 8Positive, negative and zero sequence voltage for the

single phase to ground fault shown in fig. 6

Figure 9Positive, negative and zero sequence current for the

single phase to ground fault shown in fig. 7

After fault initiation, the positive-sequence voltagedecreases while the negative-sequence voltageincreases. In fig. 9, the positive-sequence currentdrops after fault initiation and suddenly jumps toalmost three times than normal load current before itsslowly decays to a steady level during the fault. Thisphenomenon is caused by the air conditioning motorcharacteristics. When the fault occurs, the motors slowdown causing a decrease in positive-sequenceimpedance. This decrease in positive-sequenceimpedance is the cause of the increase in positive-sequence current and the decrease in positive-sequence voltage. The effect is probably due to speedreduction of the motor.

6. Conclusions

It has been reported that voltage dips which occur on asystem with no rotating machines result in arectangular profile dip. The voltage directly drops to aparticular level during fault. After the fault is cleared,the voltage returns to the level present before the faultoccurred.

A different phenomenon can be found in a systemwith rotating machine loads. When a fault occurs, thevoltage does not directly drop to its minimum levelbut it decays until reaching a steady condition beforethe fault is cleared. At that time, the voltage does notdirectly return, but recovers slowly until reaching itsoriginal level.

From these two results, it can be declared that the loadinfluences the voltage dip characteristization.

7. References

1. Math H.J Bollen, "Understanding Power QualityProblems: Voltage Sags and Interruptions", IEEEPress, New York, 2000.

2. Math H.J Bollen, The Influence of MotorReacceleration on Voltage sags, IEEE Trans. onInd. Applicat., Vol. 31, No. 4, July/August 1995.

3. J.C Das, The effects of momentary voltage dipson the operation of induction and synchronousmotors, IEEE Trans. Ind. Applicat., vol.26,pp.711-718, 1990.

4. Lidong Zhang; Math H.J. Bollen, Characteristicof Voltage Dips (Sags) in Power Systems, 8th

International Conference on Harmonics andQuality of Power ICHQP’98, 1998, pp. 555-560.

5. McGranaghan, Mark F., Mueller, David R.,Samotyj Marek J., Voltage Sags in IndustrialSystems, IEEE Trans. on Ind. Applicat., Vol. 29,No. 2, March/April 1993

6. Shaffer, John W., Air Conditioner Response toTransmission Faults, IEEE Transactions on PowerSystems, Vol.12, No.2, May 1997

Ignatius Rendroyoko, a student member of IEEE,was born in Indonesia in 1970. He graduated from theInstitute Technology of Bandung, Indonesia in 1994and served as an electrical engineer in PLN since1995. He is currently working towards a M.Eng.Sc.degree at Monash University, Australia.Professor RE Morrison was born in Stoke on Trent,United Kingdom in 1951. He received his BSc degreeand PhD degree from University of StaffordshireUniversity, UK in 1973 and 1981 respectively.Professor Morrison worked for ALSTOM (UK) from1973 to 1983 and at Staffordshire University from1983 to 1997. He joined Monash University, Australiain 1997.Peter KC Wong