Design Parameters of HV Testing Station

8/8/2019 Design Parameters of HV Testing Station

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International Journal of Power and Energy Systems, Vol. 23, No. 3, 2003

DESIGN PARAMETERS OF HV TESTING

STATIONAT QATRANA SUBSTATION

H. Obeid

Abstract

This article presents the procedure implemented to calculate

the design parameters of High-Voltage Pollution Testing Center(HVPTC) at Qatrana substation. These parameters were correctlydefined and verified based on manufacturers data in order to

construct a testing facility for the investigation of contaminatedinsulators. A short-circuit current was evaluated to comply with the

existing regulations in this field.

Key Words

High voltage, testing, pollution, leakage current, Jordan

1. Introduction

The High-Voltage Pollution Testing Center (HVPTC) wasdesigned to test various types of insulator strings for high-voltage (HV) transmission lines under natural pollution

conditions. The objectives of these investigations are:

1. to determine the severity of site conditions, whichrepresents the desert in terms of type and degree ofpollution;

2. to maintain the proper insulation level of HV trans-mission lines (types and number of insulators perstring);

3. to accumulate information about the pollution of insu-lators, which will be used in the future to calculate theprobability of flashover and the risk of failure of HVtransmission line insulation.

2. Power Supply

The HVPTC is located at Qatrana substation (south ofAmman) and is connected to 33 kV busbar at substation by350 m power cable through 33/0.4 kV; 100 kVA, deltastarconnected, transformer.

The measuring and control equipment is connectedto low-voltage (LV) side of the power transformer (PT).

Department of Electrical and Computer Engineering, Applied

Sciences University, Amman, Jordan; e-mail: [email protected]

(paper no. 203-3249)

The applied voltage to the primary winding of the voltageregulator (VR) is phase-to-phase voltage (380 V).

The voltage applied to the terminals of control systemand instrumentation is single-phase 220 V. In this regardthere are two possibilities of maintaining that voltage:

1. From the LV side of the PT (Fig. 1).

Figure 1. Power supply from LV side.

In this case, the control system and measuring equip-ment may be affected when high leakage current appears onthe surface of the polluted insulators before flashover. Toavoid that, a shielding transformer seems to be necessaryto separate the control system and measuring equipmentfrom the test circuit.

2. The other possibility is to feed the control desk andmeasuring equipment from the HV side of PT throughpole-mounted transformer 33/0.4 kV (Fig. 2).

Figure 2. Power supply from HV side.

In this case, two phases of the LV side of PT are loadedand the other one will be open circuit.

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The technical data of the PT is as follows:

Connection: deltastarApparent power: S= 100 kVAImpedance voltage: UZ% = 5.5%Active component of impedance voltage: UR% =

0.081% (estimated value)Reactive component of impedance voltage: UX% =

5.4994%Single-phase short-circuit level at 33 kV busbar at

Qatrana substation is: Ssc =87MVA

3. Characteristics and Parameters

of HV Equipment

The most important HV equipment in the testing cir-cuit is the HV transformer and the VR. The parametersof HV testing transformer (TT) are primary voltage upto 0.380 kV, secondary voltage up to 350 kV, apparentpower STT = 700 kVA, impedance voltage UZ% = 14.75%,active component of impedance voltage UR% = 1.02%, andreactive component of impedance voltage UX% = 14.71%.

The primary winding of TT consists of two windings,and this construction permits the connection of these twowindings in series or in parallel. In each case we obtaindifferent transformation ratio and values of HV side ofthe transformer. For series connection (Fig. 3) the ratedtransformation ratio n will be equal to 460. The value ofthe HV side is 175 kV.

In the case of parallel connection of primary windings(Fig. 4), the nominal transformation ratio is equal to 921and the value of voltage at HV side of TT will be 350 kV,and this is the maximum voltage that might be obtainedfrom the TT.

In some cases it is required to change the value of theapplied voltage to the terminals of primary windings of HVTT, and this can be done by means of VR. In case of series

Figure 3. Series connection of primary winding.

Figure 4. Parallel connection of primary winding.

connection of primary windings of HV HT VHV = 101.2 kVand 202.2 kV in the case of parallel connection.

3.1 Voltage Regulator

The primary winding of the VR is connected to line voltageand the voltage at the secondary winding may be changedin the range of 00.38 kV; this winding is connected to theterminals of primary windings of HV TT. The power of

VR is equal to 700 kVA.The most important characteristics of VR are the

variation of its power and voltage impedance during theprocess of regulation.

Table 1 shows the values of power and voltageimpedance of VR when the voltage is changed from 100%to 13%.

It is necessary to point out here that the variationof impedance of VR will affect the value of short-circuitcurrent of the testing circuit.

Table 2 and Fig. 5 show the value of short-circuitcurrent of the regulator with the variation of voltage at theterminals of secondary winding of the VR.

Table 2 and Fig. 5 show that the process of voltageregulation is accepted in the range of 10045%. As thevoltage decreases, the short-circuit current increases andwill reach its maximum value when the voltage at theterminal of the secondary winding is equal to 54% of itsnominal value, and this is very important in pollution tests

When the VR is operated with HV TT, the variation ofshort-circuit current at HV side of TT during the processof voltage regulation is shown in Table 3.

4. Short-Circuit Current Calculation

In HV testing techniques, the short-circuit current mustbe calculated for any testing circuit. As an example, whenthe testing object is short circuited due to flashover, theshort-circuit current will cause a voltage drop, which willaffect the actual value of the HV that has been applied tothe test equipment.

In pollution testing, when the surface of insulator isclean (for natural pollution this is the case at the beginningof the test), the value of leakage current will be very small(about a few milliamperes), and therefore the associatedvoltage drop is negligible. It has been found that the surfaceimpedance of an individual unit, as the moisture condition

progresses, is reduced and changed from capacitive at thebeginning of the test to resistive when the sallinationactivity appeared. This means that the value of leakagecurrent will be of high amplitude before flashover, andwill be raised very much during flashover. Therefore, thetesting voltage will be distorted due to voltage drop inthese conditions.

4.1 Testing Circuit

The testing circuit for the main case is depicted in Fig. 6In this case the value of voltage at the terminals of HV

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Table 1Values of Voltage Impedance during Regulation

Voltage Voltage Apparent Impedance Active ReactiveV (V) Regulation V (%) Power* S (kVA) Voltage* (Z%) Component* (R) Component* (X)

380 100 700 5.7 6.9 7.2

205 54 378 5.1 3.2 3.4

141 37 259 10.1 4.5 4.7

87 23 161 20.0 5.6 5.8

49 13 91 43.0 6.8 7.0

These are measured values during testing of VR.

Table 2Values of Short-Circuit Current during

Regulation

Voltage Regulation Short-Circuit CurrentV (%) Isc (%)

100 100

90 101

80 101

70 103

60 109

54 113

50 111

45 100

40 65

30 40

20 22

Note: 100% of voltage corresponds to 380 V and 100%of short-circuit current corresponds to 32317.6 A.

Figure 5. Short-circuit current as a function of voltage atthe terminal of VR.

winding of TT is equal to 350 kV, and the applied voltageto the primary winding of VR is phase-to-phase voltage(380 V) and is taken from two phases of 33/0.38 kV PT.The primary windings of TT are connected in parallel,

Table 3Effect of Voltage Regulation on Short-Circuit

Current of HV Side of TT

Voltage Regulation* Short-Circuit Current**V (%) Isc (%)

100 100

90 80

70 77

65 72.5

54 62

37 40

23 23

100% of voltage corresponds to 380 V and 100% ofshort-circuit current corresponds to 32317.6A. Isc is the short-circuit current at the HV winding ofTT when it is connected in series with VR in testing

circuit.

Figure 6. Testing circuit.

and the applied voltage is equal to 380 V. In this case thetesting object is subjected to 350 kV at the HV side of TT

4.2 Short-Circuit Calculation

The equivalent circuit for short-circuit current calculationis shown in Fig. 7.

The reactance of the source in ohms referred to 33 kVside is equal to 12.52 ; the resistance and reactance oPT in ohms referred to 380 V side are equal to 0.0012 and

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Figure 7. Equivalent circuit of the testing circuit.

0.008 , respectively. The resistance and reactance of VRat full voltage are equal to 6,900 and 7,200 , respectively.

All parameters of the circuit are referred to 33 kV sideand expressed in per units (pu).

The total resistance and reactance of the short-circuited circuit are equal, respectively, to 0.0554 and0.2349pu, and the R/X ratio is equal to 0.2358 pu. Theshort-circuit current is equal to 4.14 pu, which is equivalentto 8.28 A. The reactance in ohms per phase is equal to42.3 k.

The most important condition for pollution tests is themaximum permissible voltage drop caused by the leakagecurrent pulse occurring during the whole test. In order tohave consistent and comparable results on all insulators,the test circuit must fulfil the condition that the voltagedrop during the whole test does not exceed 5% [1]. Thenthe voltage drop for our test circuit may be calculated bythe following formula [2]:

V% =

1 +

I

Isc

R/X

1 + (R2/X2)

1

I2

I2sc

1

1 + (R2/X2)

The voltage drop in our test circuit is 3.5%, which isconsistent with the above-mentioned requirements.

5. IEC Recommendations for Pollution Test

At the present time there are two IEC recommendationsregarding pollution test:

1. HV test technique, Part 2: test procedure publication60-2, 1973

2. Artificial pollution tests on high-voltage insulators tobe used on AC systems, publication 507, 1975

The first recommendation defines the requirements forthe transformer test circuit as:

The voltage in the test circuit should be stable enoughto be practically unaffected by varying leakage cur-rents. Partial discharges, or pre-discharges in the testobject, should not reduce the test voltage to such anextent and for such a time that the measured disrup-tive discharge voltage of the test object is affected [3].

This is usually achieved if, simultaneously, (1) the totalcapacitance of the test object and any additional capacitoris not less than about 1000 pF; and (2) the steady-state

current delivered by the transformer, if the test object isshort circuited at the test voltage, is not less than 1 A rms

Expectations for requirement on steady-state short-circuit current are:

For dry tests on small samples of solid insulationinsulating liquids, or combinations of the two, a short-circuit current of the order of 1 A rms may suffice;

For tests under artificial pollution, the required short-circuit current depends on the ratio of series resistanceRs to the steady-state reactance Xs of the voltagesource, including the generator or supply network atthe test frequency (Table 4).

Table 4Effect of Source Impedance

on Short-Circuit Current

Rs/Xs 0.1 0.10.2 0.20.3

Isc (A rms) 6 8 12

In IEC publication 507 [4] the requirement for testvoltage is defined as:

Throughout the test, the insulator shall be contin-uously energized at the specified test voltage whichgenerally is the highest line to earth voltage of the sys-tem in which the insulator is to be used. The frequencyshall lie between 48 Hz and 62 Hz.

The minimum short-circuit current of the testingplant at the test voltage depends on the ratio resistance/reactance (R/X) of the test voltage source. Table 5 givesthe limits of the short-circuit current of the testing plant

(Isc) that are recommended for different values ofR/X.

Table 5Minimum Short-Circuit Current

as a Function ofR/XRatio

R/X Minimum

Salt Fog Solid Layer Short Circuit

Method MethodCurrent (A)

0.05 0.05 5

0.050.15 0.050.15 7

0.150.5 0.150.3 10

0.51.5 0.30.5 15

Various HV laboratories all over the world have dif-ferent voltage source characteristics. Table 6 shows thevoltage source characteristics of some testing facilities [5].

It should be noted here that the above-mentionedrecommendations, which were in effect in the 1980s, arenot sufficient. It is more accurate to relate the creepagedistance (mm/kV) to short-circuit current of the testingcircuit. Table 7 shows this relationship.

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Table 6Voltage Source Characteristics of Some Testing Facilities

Laboratory Voltage Source Characteristics

Nominal Nominal Short- R/XVoltage Circuit Current

(kV) (A)

Bonneville Power 290 18 0.36

administrationCeraver 250 18 0.5

ENEL 350 50 0.5

Institute de recherche 200 43 0.2dHydro Quebec

Lapp Insulator 500 23

NGK Insulator 200 11 0.1

Ohio Brass Co. 250 16

Ontario Hydro 350 23 0.51

Project UHV 150 22 1.78

University of Southern 50 4 California

Table 7Relationship betweenLeakage Distance to

Short-Circuit Current

Leakage Path Short-Circuit(mm/kV) Current (A)

16 6

20 9

25 1215

31 1821

6. Earthing System

Earthing, fencing, and shielding are very important factorsin HV facilities and are intended to prevent risk to persons,installations, and apparatus. The main function of earthingsystems is to provide safety for the working personnelduring ground faults and to protect equipment; therefore,step and touch voltages must be kept at acceptable limitsduring ground faults.

For HV substation, standards indicated that the earth-ing resistance must be less than 0.5. Some standardsindicate that step and touch voltages must be less than250 V for clearing time of ground faults less than or equalto 0.5 s. Therefore, step and touch voltages should be keptless than 125V for clearing time of ground faults in therange of 0.51.0 s.

Figure 8. Direct connection of two earthing systems.

6.1 Ground Resistivity at Qatrana Substation

Ground resistivity at Qatrana substation (at the site ofHV PTC) was measured during May. Four measurementswere performed at various points, and the average value ofresistivity was found to be equal to 30 m.

The earthing system of HVPTC will consist of twoparts:

Direct connection to earthing system of 132/33 kV

substation Earthing mesh at HVPTC

To avoid the rise of potential in HVPTC due to faultsin 132/33 kV system, a direct and reliable connection musbe maintained between the two earthing systems through(1) the sheath of the power cable and (2) two strips ofcopper, the cross-sectional area of which must be not lessthan 95 mm2 each (Fig. 8).

6.2 Earthing Mesh

At HVPTC earth mesh consists of horizontally laid copperstrips as shown in Fig. 8, the cross-sectional area is 95 mm2

and the contact area between the equipment and the earthmesh must not be less than 200mm2. The strips mustbe laid at 800 mm below the surface of the ground. Theearth resistance of this mesh was calculated and is equal to0.37 .

6.3 Control of Voltage Gradients

outside the Fence

The HVPTC will be protected by means of a metallicfence. In this case, the potential outside the metallic fence

during ground faults should be controlled. The best way ofreducing the value of this potential is to put a horizontacopper strip 1 m outside the fence at 1.5 m below theground level.

In HVPTC all tests will be carried out at power fre-quency voltage, and it is not necessary to take precautionsand protection against impulse voltage.

7. Measuring the Leakage Current

There are several methods, in pollution tests, for measuringsite severity. One of these methods is the leakage current

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Design Parameters of HV Testing Station