CHAPTER 3 INSULATION TEST OF POWER SYSTEM EQUIPMENT POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE DET310 CHAPTER 3 INSULATION TEST OF POWER SYSTEM EQUIPMENT INSUALTION TEST OF ELECTRICAL EQUIPMENT

-It is an object intended to support or separate electrical 3.0 INTRODUCTION WHAT IS INSULATION An Insulator is a material that resists the flow of electric current. -It is an object intended to support or separate electrical conductors without passing current through itself. -The term electrical insulation has the same meaning as the term dielectric.

3.0 Introduction (CONT-) QUIPMENT ANALOGUES: COMPARISON WITH WATER FLOW Pressure on water from a pump causes flow along the pipe (Fig. 1a). If the pipe were to spring a leak, you’d waste water and lose some water pressure.

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.0 Introduction (CONT-) OHM’S LAW R = V/I For a given resistance, the higher the voltage, the greater the current. Alternatively, the lower the resistance of the wire, more current that flows for the same voltage. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.0 Introduction (continue: ) NO INSULATION IS PERFECT Some current does flow along the insulation So what is “good” insulation? “Good” means a relatively high resistance to current flow. Measuring resistance can tell you how “good” the insulation is. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.1 Insulation Degradation Electrical Stress- Over voltages and under voltages, example: lightning surges, switching surges 2) Mechanical Stress- example cable been hit by mechanical forces, vibration of rotating machine 3) Thermal stress- Running machine in hot and cold condition- expansion and contraction of insulation INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.1 Insulation Degradation (Continue-) 4) Enviromental Contamination- moisture, humidity, chemical,heat INSUALTION TEST OF ELECTRICAL EQUIPMENT

3.2 Review of Dielectric Theory and Practice Dielectric circuit and testing involves several parameters: Dielectric - is term used to identify a medium such as insulation in which an electric field charge can be produced and maintained. The energy required to charge the electricity is recoverable, in whole or in part, when the charge is removed.

INSUALTION TEST OF ELECTRICAL EQUIPMENT b) Dielectric Constant- : Dielectric constant is known as specific capacitance, capacitive or permittivity c) Dielectric Absorption- Dielectric absorption is a phenomena that occurs in dielectrics whereby positive and negative charges are separated to respective polarities when a dc voltage is applied to the dielectric INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT d) Dielectric Loss – Is the time rate at which electric energy is transformed into heat in a dielectric when it is subjected to an electric field. Dielectric loss is associated with real component (watts) losses in a dielectric e) Dielectric Power factor – The dielectric power factor of a material is the ratio of the power dissipated in materials in watts (watt loss) to the effective volt-amperes when tested with sinusoidal (ac ) voltage. INSUALTION TEST OF ELECTRICAL EQUIPMENT

f) Dielectric loss factor or dielectric Loss Index – Is the product of its dielectric constant and dissipation factor. g) Dielectric strength – The dielectric strength of a material is the potential gradient (voltage ) at which breakdown ( electrical failure) occurs and is a function of the material’s thickness and its electrical properties.

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3 TEST ON INSULATION The properties of an insulation can be accessed by DC Test AC Test INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.1 DC VOLTAGE TEST ON INSULATION DC test can be modeled as Figure 2.0 below INSUALTION TEST OF ELECTRICAL EQUIPMENT 3 type of current will flows- a) Capacitive Charging Current (Ic), b) Dielectric Absorption Current (Ida), c) Resistive (Leakage) Current (Ir)

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.1.1 Capacitive Charging Current Charge the capacitance Current stop flowing in a few second 3.3.1.2 Resistive( leakage ) Current Good insulation will results a relative current flows and constant. - Cause by electrons which pass through the insulation INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.1.3 Dielectric Absorption Current (Ida) INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.1.3 Dielectric Absorption Current (Ida) The applied insulation voltage puts a stress on the molecules of the insulation. The positive side of the molecules are attracted to the negative conductor and the negative side of the molecules are attracted to the positive conductor. The result is an energy that is supplied to realign the molecules much like force will realign a network of rubber bands. Like Ic, Ida usually dies off fairly quickly as the molecules realign to their maximum extent. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT

3.3.2 Type of DC Testing Generally can be divided into Insulation Resistance Testing Hi-Pot Test

3.3.2.1 INSULATION RESISTANCE TEST can be accessed by - Short Time reading/Spot test - Time resistance Test - Polarization Index Test - Step Voltage Reading

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.2.2 SHORT TIME READING TEST Voltage is applied to Equipment for 60 secs/1 min An increase of resistance due to decreasing of absorption current and capacitive current INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT Insulation resistance over a period of time

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.2.3 STEP VOLTAGE TEST Involves Resistance testing at Various voltage setting Test duration in 60 secs/ 1 min Good insulation should withstand an increase over-voltages with insulation resistance remain the same Deteriorate/ damage insulation resulted in decrease insulation INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.2.4 Dielectric-absorption/ time-resistance test Compares the absorption current of good insulation with a bad insulation b) Test voltage is applied for 10 min with data recorded at every 10 sec for the first 1 min. Then data is recorded at I min interval c) A continuous increase in graphed resistance indicates good insulation d) A flat or downwards curve indicates deteriorate insulation INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT 3.3.2.5 Polarization Index The polarization index is the ratio of two time-resistance readings: one is taken after 1 minute and the other is taken after 10 minutes Polarization Index: R10/R1 b) A low polarization index usually indicates problems with the insulation. c) When time is constraint, a shortcut to the polarization index test is the dielectric absorption ratio (60/30) second test. INSUALTION TEST OF ELECTRICAL EQUIPMENT

3.3.2.6 Equipment for Insulation Resistance Testing This test may be conducted at applied voltages of 100–15,000 V. The instrument used is a megohmmeter, either hand cranked, motor driven, or electronic, which indicates the insulation resistance in megohms.

3.3.2.7 Insulation Resistance Test on Power System Equipment Power Transformer. - IR test a) Using 1 kV IR tester, connect the positive probe to Red phase and negative probe to ground. Measure the IR after 1 minute. Repeat the testing for Yellow Phase. b) Using 5 kV IR tester, connect the positive probe to Red Phase and negative probe to ground. Measure the IR after 1 minute. Repeat the testing for Yellow Phase. POWER TRANSFORMER

Polarization Index Test. a) Carry out IR test between Low Voltage to Earth with 1kV IR tester. Record the data. b) Carry out IR test between HV to Earth with 5kV IR tester c) Carry out IR test between HV to LV with 5kV IR tester Duration of test is 10 minutes Divide the value obtained with value for 1 min test Polarization Index = R10/R1 THE POWER TRANSFOREMR

IR test on power transformer (HV to Ground)

Circuit Breaker Insulation Resistance test is conducted to ensure that the circuit breaker bushing and conductor have sufficient insulation level to the ground. 5kV IR tester is used for the test.

Cables Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached. In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured.

DC test Voltages for IR test on Power Cables(Table 3.1)

Current Transformer The objective insulation resistance test current transformer (CT) is to ensure the CT windings have good insulation to ground. For testing, we use the 1kV, 5kV insulation tester. Use 1kV insulation tester to check the insulation between secondary winding and earth. Use 5 kV insulation testers to check the insulation between the primary windings and earth and between primary and secondary winding and earth

Primary winding to earth test of current transformer Secondary winding to earth test of current transformer

INSUALTION TEST OF ELECTRICAL EQUIPMENT DC Hi-Pot Testing Sometimes called a Dielectric Withstand test. Is used to verify the strength of the insulation between a product’s current-carrying components and its chassis or enclosure. test can also be performed to detect material and workmanship defects, most importantly small gap spacings between current-carrying conductors and earth ground. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT DC Hi-Pot Testing Principles The theory: if a voltage much higher than the product would normally see is applied across the insulation without a breakdown (which results in an excessive amount of leakage current flow), the product will be able to operate safely when run under nominal operating conditions. The Hipot tester is used to indicate whether or not a dielectric breakdown of the insulation has occurred by monitoring the leakage current resulting from the applied test voltage. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT . DC Hi-Pot Testing (cont)- Even under normal operating conditions, some leakage current will be present in any device under test (DUT), but at minute and safe levels. However, when the insulation breaks down or is damaged an excessive amount flows to the chassis. This can present a substantial shock hazard to anyone that comes into contact with the product. INSUALTION TEST OF ELECTRICAL EQUIPMENT

Preparation before Conducting DC Hi Pot Testing a) All equipment must be disconnected from DUT, i.e.,taps, motors, circuit breakers, surge arrestors, etc. This will preclude damage to such equipment and will prevent test interruptions due to flashovers and/or trip-outs resulting from excessive leakage current. b) Establish adequate clearance between the circuit test ends and any grounded object, and to other equipment not under test. c) Ground all circuit conductors not under test with all cable shields including nearby equipment.

DC Hi Pot Methodology Cables

The current observed during a DC pressure test will initially include the charging current. After full voltage is reached the charging current will decay and the remaining leakage current is used to indicate the condition of the cable. Table 3.2: DC Pressure Test for Cables

IEC Standards specify maintenance tests after installation should be 70% of permissible factory test DC voltages (15 minutes).   Example: For 11kV cable: After installation, at site: The test DC voltage is 70% x 22 kV = 15.4kV Example: For 6.0 kV:

Table 3.3: DC Pressure Test for Switchgear (IEC 60298) Nominal System Voltage (kV rms) Rated Maximum System Voltage (kV rms) Field Test DC Voltage (combined) after Erection (kV) DC Pressure Test Phase -Earth (Combined) DC Pressure Test Phase -Phase Voltage (Combined) 3.3 3.6 2964 5.134 6.6 7.2 5928 10.267 11 12 9880 17.112 15 17.5 13.47 23.33 22 24 19759 34.22 33 36 29639 51.336 Table 3.3: DC Pressure Test for Switchgear (IEC 60298)

AC VOLTAGE TEST ON INSULATION In the case of AC voltage application to an insulation, a large current is drawn which remains constant as the AC current alternately charges and discharges the insulation. The effect of dielectric absorption currents remains high because the dielectric field never becomes fully established due to the polarity of the current reversing each half cycle. When an AC voltage is applied to an insulation, the currents drawn by the insulation are due to capacitance charging, dielectric absorption, continuous leakage current, and corona

AC VOLTAGE TEST ON INSULATION Capacitance charging current: In the case of AC voltage, this current is constant and is a function of voltage, the dielectric constant of the insulating material, and the geometry of the insulation. Dielectric absorption current: When an electric field is set up across an insulation, the dipole molecules try to align with the field. Since the molecules in an AC field are continually reversing and never fully align, the energy required is a function of material, contamination, (such as water), and electrical frequency. It is not a function of time.

AC VOLTAGE TEST ON INSULATION Leakage current (conductivity): All insulation materials will conduct some current. If voltage is increased beyond a certain level, electrons will be knocked off of molecules causing current to pass through the insulation. This is a function of the material, contamination (especially water), and temperature. Excessive conductivity will generate heat causing the insulation to cascade into failure.

AC VOLTAGE TEST ON INSULATION Corona (ionization current): Corona is the process by which neutral molecules of air disassociate to form positively and negatively charged ions. This occurs due to overstressing of an air void in the insulation. Air voids in oil or solid insulations may be due to deterioration from heat or physical stress, poor manufacture, faulty installation, or improper operation. Corona breaks down the air into ozone which, in combination with water, forms nitrous acid. The ionized air bombards the surrounding insulation and causes heat. The combination of these conditions will result in deterioration of the insulation and carbon tracking. Corona losses increase exponentially as voltage increases.

Type of AC Insulation Tests. The AC voltage tests can be classified into the categories as listed below: 1. PF and DF 2. AC high potential tests 3. Very low frequency (VLF) 4. AC series resonant 5. Induced frequency 6. Partial discharge (PD) 7. Impulse tests

Type of AC Insulation Tests (cont:-) The AC tests may be classified as destructive and nondestructive tests. The PF and DF tests are considered nondestructive since the test voltages used in performing these tests do not exceed line-to-neutral voltages of the equipment being tested. The basic principle of the nondestructive testing is the detection of a change in the measurable characteristics of an insulation that can be associated with the effects of contaminants and destructive agents without overstressing the insulation.

Classifications of AC Testing. The AC high potential, VLF, and AC series resonant tests may be classified as destructive since the test voltages associated with these tests are higher than normal operating voltages which may overstress the insulation.

Testing with AC has two significant problems Since a large percentage of insulation current in both good and marginal insulation is capacitive, good insulation will have close to the same amount of AC current flow as marginal insulation. It is, therefore, not possible to evaluate the quality of insulation by simply measuring the magnitude of current flow. (Except for Tan Delta and PF Test) 2. The high amount of current flow drawn by the insulation requires a large test instrument to supply it. This causes AC test sets to be heavier and more difficult to transport than equivalent capability DC test sets.

AC High-Potential (Over-Potential Test) In this test a high voltage AC is applied to the insulation. Usually two or more times the rated insulation current will be used. The concept here is somewhat equivalent to overloading the dam with too much water. If the insulation is good, it will not fail during the test. Note that failure during an AC high potential test is usually very rapid and the insulation is completely destroyed. Because of this, the AC Over-potential test is classed as a go-no go type of test.

Methodology of AC Hi-Pot Testing Switchgear Circuit breaker in closed position. This test is to determine whether the insulation can withstand high voltage at rated frequency for certain period of time. With the circuit breaker in closed position, apply the high voltage to phase RED for 1 minute with the other two phases earthed. Measure and record the leakage current. Repeat for the other two phases. The test fails when there flashover or test set trips.

Methodology of AC Hi-Pot Testing Switchgear Circuit breaker in open position. This test is to check the insulating medium of the interrupter when high voltage is applied across it. With the circuit breaker in open position, earth the circuit side and apply high voltage on all three phase of the busbar side of the circuit breaker. Measure and record the leakage current after 1 minute. The test fails when there is flashover or test set trips.

Methodology of AC Hi-Pot Testing Switchgear Busbar Apply high voltage to phase RED of the busbar for 1 minute with the other two phases earthed. Measure and record the leakage current. Repeat for the other two phases. The test fails when there is flashover or test set trips. Remarks: IR test must be conducted before and after AC Hi-potential Test  

AC Hi –Pot Test for Switchgear

Nominal System Voltage (kV rms) Rated Max System Voltage (kV rms) AC test (one min at any frequency between 25 and 100Hz) Factory Test After Errection 0.415 0.44 2.0 2 0.6 0.66 3.3 3.6 9.5 8.6 6.6 7.2 17.0 15.2 11 12 27 24 15 17.5 36 32 22 52 46 33 76 68 Table 3.4 : AC Pressure Test Voltage Table Test Voltages for Hi-Pot Test on Switchgear (Based on BS 159) Remarks: In the absence of ac test, dc test also can be conducted (duration 15 minutes)

Nominal System Voltage (kV rms) Rated Max System Voltage (kV rms) DC Hi-Pot Test Voltages (kV) After Service 0.415 0.44 3 0.6 0.66 3.3 3.6 5 6.6 7.2 10.5 11 12 18 15 17.5 25 22 24 37.5 33 36 60 Table 3.5 : DC Hi-Pot Test for Switchgear (BS159)

Duration of Test Minute Percentage of one minute test according to Table above 1 100% 2 83.5% 3 75% 4 70% 5 66.6% 10 60% 15 57.7% Table 3.6: Percentage of AC Hi-Pot Pressure Test For longer duration of AC Hi-Pot Testing, use voltage as shown in Table above

Nominal System Voltage (kV rms) Rated Maximum System Voltage (kV rms) Field Test AC Voltage after Erection (kV rms) Field Test AC Voltages in Service (kV rms) IEC60694 IEC60298 3.3 3.6 10 Nil 6.6 7.2 20 11 12 28 22.4 12.1 15 17.5 38 30.4 16.5 22 24 50 40 24.2 33 36 70 56 36.3 Table 3.7: AC Pressure Test (Based on IEC 694 and IEC 298)