ON THE INSULATION CO-ORDINATIONSTUDIES USING (EMTP)

M. Aburaida (AMIEE)Sceco-east, Secti,

Dammam, P. O. Box 5190Dammam 31422, Saudi Arabia

ABSTRACT:The reliability of supply provided by an electric power system, as judged by the frequency and duration of

supply interruptions to its customers, depends to a great extent on the surge performance of the system. Althoughthere are many other causes of interruptions, breakdown of insulation is one of the most frequent. In reality, theinsulation has to withstand a variety of overvoltages with a large range of shapes, magnitudes and duration. Thesevarious parameters of overvoltages affect the ability of insulation to withstand them. So proper insulation levelsare required for equipment as well as proper overvoltage protection.

The objective of this paper is to explain the different methodologies of insulation co-ordination studiesand show how the EMTP can give an easy and reliable approach to insulation co-ordination problem. This willinclude modeling of the different elements of the power system and simulating their behavior under different typesof surges (lightning, switching, fault clearing, etc..). The insulation level of each equipment will be examinedwith reference to BIL as well as the selection, specification and location of arresters. Practical cases will beexplained.

1. INTRODUCTIONInsulation co-ordination refers to the art of selectingappropriate insulation levels for equipment (e.g.transmission lines, transformers, generators..etc..)and the corresponding overvoltage protection system(arresters). For transmission lines, this means thedesign of the conductor-to-conductor and towerclearances, the selection and specification ofinsulator strings, the placement of ground (and insome cases shield and counterpoise) conductors andthe specification of towers and tower footingresistance. For equipment such as transformers thismeans the specification of BIL (basic lightningimpulse insulation level) and. Both conventional andstatistical ways of specifying these insulation levelsare recognized in the standards. For equipment suchas breakers, additional specification parametersinclude the TRV (transient recovery voltage)characteristics [1].

Selection of insulation levels for equipmentand selection and location of overvoltage protectionequipment are interrelated: the insulation level of apiece of equipment or line depends on the type ofovervoltage protective equipment installed. Recentpractice has been to attempt to reduce insulationlevels by more liberal use of arresters, particularlyzinc oxide arresters, and also the limiting ofovervoltages by means of closing resistors.

A fundamental distinction must be made inthe design process between self-restoring and non-self-restoring insulation. With solid insulation, suchas in many cables systems and transformers, once itfail it must be replaced: this is non-self-restoringinsulation. Open-air insulator strings, on the other

hand, will usually fail by arcing around the string.Once the initial arc is extinguished, the insulationcapability is restored. Statistical specification of aBIL is not appropriate to a non-self-restoringinsulation, but is quite appropriate to a self-restoringinsulation.

It is important to understand the meaning ofBIL and BSL specifications. These specifications areintended to measure the ability of equipment towithstand representative overvoltages, intended tocapture the general ability of the device with respectto a general kind of overvoltage condition. The BILis specified with reference to a specific waveform:1.2 microseconds rise time, 50 microseconds time todecay to half value. The BSL waveform has a 250microseconds to peak, 2500 microseconds to decayto half value. The implication (not necessarilycorrect) is that typical lightning and switchingwaveforms have the same general characteristics andshape. While the expected characteristics of theovervoltage resulting from switching can usually bedetermined with good accuracy using the EMTPtype programs (and consequently the designer canverify to what extent the standard test waveform isapplicable), actual lightning waveforms can differconsiderably from the assumed ones [2].

Insulation coordination also means theselection, specification and location of equipmentintended to reduce overvoltages, such as lightningarresters and to some extent breakers, pre-insertionresistors and other such items. There are fiveparameters of interest in the selection of a lightningarrester:

[1] The lightning current discharge capability andits corresponding voltage. [2] The switching current discharge andcorresponding voltage. [3] The energy dissipation characteristics of thearrester, or temporary overvoltage capability. [4] The arrester rated voltage. Generally themaximum temporary overvoltage expected duringoperation before the arrester has a significant effecton the system. [5] The continuous operating voltage. Thisdetermines the steady-state losses in the arrester.Arrester location is always an important subject. Ingeneral, arresters should be located as close to theequipment as practical, with as short leads aspossible. However, there are many reasons whysometimes arresters must be located at somedistance from the protected equipment. For example,in gas Insulated Substations (GIS) arresters are oftenlocated outside the substation for economic reasons,but encapsulated arresters may also be required.Thus, we see that insulation coordination is, as hasbeen said of many other aspects of engineeringdesign, as much as an art as it is an exact discipline,where the designer attempts to weight and considermany diverse factors and come up with a singleanswer. Insulation coordination methodologies arelikely to vary with different companies andindividuals.

2. INSULATION COORDINATIONMETHODOLOGIES

Methods for insulation coordination can be classifiedinto four general categories: [1] Rule of thumb methods. [2] Precalculated deterministic studies. [3] Deterministic methods and semi-statisticalmethods. [4] Statistical methods.Rule of thumb methods and precalculated studies arepractical ways of designing insulation levels formost standard substations with simpleconfigurations. Designs based on those methods are,generally, too conservative. For deterministic designmost of the parameters needed for the design are tobe selected before hand. This means that a lot ofdetails are required for this method. For statisticaldesign the values of those parameters are allowed tovary randomly within a known distribution.2.1 STEPS FOR AN INSULATIONCOORDINATION STUDYThe insulation coordination study can be done fromthree different perspectives: [1] To select and size arresters for an existingsystem.

[2] To specify the BIL for new equipment added toan existing system. [3] To select the insulation level for a new line.The first step in any insulation coordination study isto determine which of these categories of studies isto be performed. Once the desired study has beenclassified, three slightly different methodologies canbe adopted.(1st) If the study is to select, size and locate

arreters, then(a) Select an initial arrester operating

voltage and rated voltage based onthe system voltage and expectedtemporary overvoltage.

(b) Define all important serviceconfigurations (equipment in and outof service possibilities).

(c) Locate the arrester at the entrance ofthe substation and verify that for allservice configurations and fordifferent locations of lightningstrokes that all substation equipmentwill be adequately protected.

(d) Identify all probable switchingoperations and load rejections thatmay lead to arrester operation.

(e) If the results are not within thespecified limits, select newparameters for the arrester and checkagain.

(2nd) If the case is to select BIL for a newequipment,(a) Proceed as before with the exception

of step (i )(b) Using an appropriate computer

simulation program verify that thaaddition of the new equipment willnot affect the existing ones for alltested cases. Then using the resultsof this simulation the BIL of the newequipment can be determined.

(c) Otherwise try to relocate the existingarresters or add new ones andperform the simulation once more.

(3rd) When the objective is to select the insulationlevel for a new line, then(a) Define all important service

configurations.(b) Run simulations for all possible

lightning and switching transientconditions.

(c) Determine the overvoltages producedby all those simulations.

(d) Using the worst conditions, specifythe insulation level for the new line.

3. ELECTROMAGNETIC TRANSIENTPROGRAM

The Electromagnetic Transient Program (EMTP) is acomputer program for simulating theElectromagnetic, Electromechanical and controlsystem transients or transient Analysis of ControlSystems (TACS) on multiphase electric powersystems. It was first developed as a digital computercounterpart to the Analog Transient NetworkAnalyzer (TNA). Many other capabilities have beenadded to the EMTP over the years and the programhas become as a culture in the electric utilityindustry. The actual EMTP is the result of acooperative development effort among many users[2].3.1 EMTP APPLICATIONSStudies involving use of EMTP can be put into twogeneral categories. One in design which includesinsulation coordination, equipment ratings,protective device specification, control systemdesign, etc. The other is solving operating problemssuch as unexplained outages or system failures. Apartial list of typical EMTP studies follows: Switching surges, Lightning Surges, InsulationCoordination, Shaft Torsional Stresses, HighVoltage DC (HVDC), Static VAR Compensation(SVC), Transient Stability Studies, Motor Starting.

This is only a partial list. One of theEMTP major advantages is its flexibility inmodeling; an experienced user can apply theprogram to a wide variety of studies.

3.2 USING EMTP FOR INSULATIONCOORDINATION STUDIESThe EMTP provides the ability to perform insulationdesign. The questions that faces the designer whenusing the EMTP are: [1] What runs should be performed [2] What outputs are required? [3] What models to be used? [4] How are the results to be interpreted?The systems in the following cases are modeled andsimulated using the EMTP program [3].

4. CASES UNDER STUDY4.1 CASE STUDY (1):The system used in this study consists of atransmission line and a transformer as shown in Fig(1). The rated voltage of the system is 20kV. Thetransient is a lightning strike of 20kA and durationof 20us.The objective of the study is to select an arresterwith an appropriate operating voltage. Theprocedure followed is as outlined above.

Fig (1) Transmission line model usedin case study 1

4.2 CASE STUDY (2)In this case the effect of using a piece of cable on theon the BIL of a connected transformer isinvestigated. The system under study is shown in Fig2. The BIL for the transformer is 900kV. Thetransient is a lightning stroke of 20kA for 20us [4].

5. RESULTS AND DISCUSSIONFigures 3 and 4 show the resulting overvoltage incase study (1) with and without using the surgearrester.

A B C D L i n e

345 k V

20 K A / 20 u S

C a b l e L i n e

FIG. ( 12 : Circuit used to investigate cable effect on system overvoltage

20 kV

Zno

Fig (3) Voltage on transformer terminals during faultWithout the use of a surge arrester

As seen from the figures, the arrester reduced the overvoltage from above 6MV to less than 200kVwhich is well below the BIL of the transformer.Figures 5 and 6 show the resulting overvoltage incase study (2) in the two conditions with withoutusing a piece of cable to connect the terminatingtransformer.

Fig (4) Voltage at the transformer terminals duringfault withThe use of a surge arrester

lightnng>NODE-C(Type 1)

0.0 0.2 0.4 0.6 0.8 1.0 0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

Time (mS)

Voltage (V)

Max:Min:Avg:Abs:RMS:CF :FF :

6.97504e+06 0 341547 6.97504e+06 1.53819e+06 4.53459 4.50359

Fig 5 Overvoltage on transformer terminalsWithout the use of cable

lightnng>NODE-D(Type 1)

0.0 0.2 0.4 0.6 0.8 1.0 0

200000

400000

600000

800000

1000000

1200000

1400000

Time (mS)

Voltage (V)

Max:Min:Avg:Abs:RMS:CF :FF :

1.22919e+06 0 301605 1.22919e+06 461541 2.66322 1.53028

Fig. 6 Overvoltage on transformer terminalsUsing a piece of cable

As seen from the figures, the piece of cablereduced the overvoltage on the transformerterminals from about 7 MV to only 1.2 MV,which can easily be handled by any type ofsurge arrester.

6. CONCLUSIONA comperhensive explanation of thedifferent methodologies of insulation co-ordination studies is presented. Two caseswere simulated one with a lightning arresterand the other using only a piece of cable tostudy its effect on the determination on thetransformer insulation level. It is shown thatthe EMTP allows the overvoltage transientsand hence the required insulation level to beanalyzed quickly and accurately.

7. ACKNOWLEDGMENTThe authors appreciate the full computingand publishing support of KFUPM.

8. REFERENCES:(1) Electromagnetic Transient Program (EMTP), Workbook Volume 3, 1992. (2) Electromagnetic Transient Program (EMTP) , RevisedRule Book Version 2.0 , Volume 1; Main Program , June1989 .(3) A. Greenwood, Electrical Transients in PowerSystems . John Willey & Sons , 1991(4) P.P. Barker, Voltage Quadrupling on a UD cable .IEEE Trans. PWRD vol. 5, No. 1 Jan. 1990 . (5) W. Diesendorf, Insulation co-ordination in high-voltage electric power systems. Butterworth 1974.

6204>NODE-B(Type 1)

0.00 0.05 0.100. 15-15 00 00

-100000

-5 0 0 00

0

5 000 0

10000 0

1500 00

200000

Time (mS)

Voltage (V)

Max:Mi n:Avg:Abs:RMS:

C F :F F :

1 9 0 2 0 1 -107260 4 9 1 9 9 190201 6 4 3 0 2 . 9 2.95789 1. 307

litsurg>NODE-B(Type 1)

0.00 0 .050.10 0.15-2000000

0

2000000

4000 000

600000 0

8000000

Time (mS)

Voltage (V)

Max:Min:Avg:Abs:RMS:CF :

FF :

7.26675e+06 -1.26494e+06 3.23169e+06 7.26 67 5e +06 4.16582e+06 1.7443 8 1.28 9 0 5

9. BIOGRAPHY M. Aburaida was born in Sudan. He received hisB. Sc. in Electrical Engineering from University ofKhartoum (Sudan) in 1980. He worked for National

Electricity Corporation in Sudan and now withSCECO-EAST in Dammam, Saudi Arabia. He is

currently working for an M. Sc. In ElectricalEngineering in KFUPM in Dhahran. His present

research interest includes power system modeling,transients and EMTP applications.

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