High voltage testing guide

1.0 Insulation Resistance Test IR Test• For all electrical equipment the key indicator to High Voltage Equipment Testing are: 1.. Test for circuit breakerIR Test of vacuum CB • In this o

HIGH VOLTAGE

TESTS

• Need for periodic testing to guarantee integrity

and reliability of HV system

• Need for diagnostic test for comparative

measurement and determination of fault serenity

• Need to simulate fault conditions for

verification of system components stability

• The high voltage (e.g 6.6 kV) installation

covers : the generation, main supply

• cables, switchgear, transformers, electric

propulsion (if fitted) and a few large motors e.g

for side-thrusters and air conditioning

compressors

Condition of HV insulation is governed by factors such as:

• Temperature, humidity, surface condition and operating voltage level

when testing and maintaining HV insulation.

• Before applying an IR test to HV equipment its power supply must be switched off, isolated,

HV Test type

2) -Insulation Resistance (IR) test

3) -Polarity Index (PI) Test

4) -Infrared Imaging test

5) - Circuit breaker test

6) Partial discharge test

7) BIL (Basic Insulation level) test

8) Life test

1.0 Insulation Resistance Test (IR) Test

• For all electrical equipment the key indicator to High Voltage

Equipment Testing are:

1 its safety and

2 general condition of its insulation resistance (IR) - The IR must be

tested periodically between phases and between phases and earth

• HV equipment that is well designed and maintained, operated within its power and temperature ratings should have a useful insulation life

of 20 years

• An IR test is applied with a high d.c voltage which applies a

reasonable stress to the dielectric material (insulation)

• For 6.6 kV rated equipment, a periodical 5000 V d.c insulation

resistance (megger) test is recommended

• The minimum IR value is usually recommended as (kV + 1) MO

where V is the equipment voltage rating e.g 7.6 MO would be an

acceptable IR value for a 6.6 kV machine

Permanent magnet

Permanent Magnet Rotor

reconnected before the IR tester is disconnected]

• A 5000 V dc megger tester now be applied between phases and earth, and between phases, and the values are recorded

• The megger test should be applied for 1 minute

• The recommended minimum value is (KV rating of the machine + 1) MΩ

IR Test Procedure

• For machines with healthy insulation, an IR test result may indicate

a value up to 100 times greater than the recommended minimum

• First the reading is checked for 1 minute and for a better test it is checked for 10 minutes

• The correct procedure is to connect the IR tester to the circuit under test with the safety earth connection ON

• The safety earth may be applied through a switch connection at the supply circuit breaker or by a temporary earth connection local to the test point

• This is to ensure that the operator never touches a unearthed

conductor

• With the IR tester now connected, the safety earth is disconnected (using an insulated extension tool for the temporary earth)

• Now the IR test is applied and recorded

• The safety earth is now reconnected before the IR tester is

disconnected

• This safety routine must be applied for each separate IR test

IR Test Procedure

• Large currents flowing through machine windings, cables, bus-bars and main circuit breaker contacts will cause a temperature rise due to I2R resistive heating

• Where overheating is suspected, e.g at a bolted bus-bar joint in the main switchboard, the local continuity resistance may be measured and checked against the manufacturers recommendations or compared with similar equipment that is known to be satisfactory

• A normal ohmmeter is not suitable- as it will only drive a few mA

through the test circuit

• A special low resistance tester or micro-ohmmeter must be used

which drives a calibrated current (usually I=10 A) through the circuit while measuring the volt-drop (V) across the circuit

• The meter calculates R from V/I and displays the test result For a

healthy bus-bar joint a continuity of a few megerOhm) would be

expected

2.0 Polarity Index test (PI) Test

• A more involved IR test (the polarization index or P.I.)

is used when the insulation value may be suspect or

recorded during an annual survey

• The P.I value is the ratio of the IR result after 0 minutes

of testing to the value recorded after 1 minute of testing

• To apply a P.I test over a ten minute period requires a

special IR tester that has a motor-driven generator or an

electronic converter powered from a local 220 V a.c

supply

• Experience shows that using polarity index method give

far more reliable figure on the condition of insulation

3.0 Infrared Imaging Tester

• Normally the safe testing of HV equipment requires that it is

disconnected from its power supply

• Unfortunately, it is very difficult, impossible and unsafe to closely observe the on-load operation of internal components within HV enclosures

• This is partly resolved by temperature measurement with an

recording infra-red camera

• Electric Propulsion and High Voltage Practice Infrared image

testing distance

• The camera is used to scan an area and the recorded infra-red image

is then processed by a computer program to display hot-spots and a thermal profile across the equipment

• To examine internal components, e.g busbar joints, a camera

recording can be made immediately after the equipment has been switched off and isolated in accordance with an EPTW safety

procedure

Infrared Imaging Tester

• Alternatively, some essential equipment, e.g a main

switchboard, can be monitored on-line using specially

fitted and approved enclosure windows suitable for

infra-red testing

• These windows arc small apertures with a permanently

fixed steel' mesh through which the camera can view the

internal temperature from a safe position

• An outer steel plate fixed over the window mesh

maintains the overall enclosure performance during

normal operation

Infrared imaging tester

• A conventional photograph of the equipment is taken

simultaneously to match the infra-red image and both

are used as part of a test report

• Such testing is usually performed by a specialist

contractor who will prepare the test report and propose

recommendation / repair advice to the ship operator

• Fig 8.31 (unfortunately not in colour like the original)

gives typical results from an infra-red camera test on a

bus-bar connection

Test for circuit breaker

IR Test of vacuum CB

• In this on-line test, the camera recorded hot-spot temperatures

and the report recommended that this copper connection is

checked for tightness as High Voltage Equipment Testing it is

running very hot compared to that on the neighboring

copper-work

• To test the insulating integrity of an HV vacuum-type circuit

breaker requires a special high voltage impulse test - The tester

produces a short duration voltage pulse, of typically 10 kV for a 6.6 kV circuit, which is connected across the open breaker

contacts

• Any weakness in the insulating strength of the vacuum in the

interrupter chamber will be detected as a current flow and the

tester will display the condition as a pass

Test for circuit breaker

IR Test of vacuum sf6

• Gas (SF6) HV circuit breakers rely on the quality and pressure of

the gas acting as the insulation between the contacts.

• A falling gas pressure can be arranged to initiate an alarm from

pressure switches fitted to each switching chamber.

• Normal gas pressures are typically 500 kPa or 5 bar.

• Overall circuit protection of HV equipment is supervised by

co-ordinated protective relays -These must be periodically tested to

confirm their level settings (for current, voltage, frequency etc.)

and their tripping times

• This requires the injection of calibrated values of current and

voltage into the protective relays which is usually performed by ^

specialist contractor during a main ship survey while in dry-dock

• Partial discharges are small electrical discharges that takes place

in a gas filled void or on the dielectric of a solid or liquid insulation system

• The discharges are basically small arcs that only partially bridge the gap between phase to ground and phase to phase insulation

• Partial discharge serves to provide an early warning of an imminent equipment failure

• The ultimate failure is the result of the heating effect caused by the discharges

• This leads to deeper pits and finally puncture the insulator

• Oil impregnated paper deteriorates very rapidly

• Some epoxy resin insulators are moderately resistant

• Porcelain, ceramics and glasses are practically immune to partial discharges

ACCEPTABLE LIMIT OF PARTIAL DISCHARGES

• To except zero discharges is not practical and it

is generally acceptable to accept a maximum

limit for partial discharges

• Experience has shown that a discharge of less

than 10 pico coulombs at 0.75% of line to

ground voltage is acceptable.

• [Ref: Page 252, High Voltage Circuit Breakers by Ruben D Garzon]

Partial Discharge and Dielectric Strength

• The term ‘Dielectric Strength’ is used to describe the capacity of

an insulating material to withstand electrical stresses It is not a constant value

• The dielectric strength of a material is considerably influenced

by numerous parameters- temperature, form and frequency of voltage, field distribution, size of the stressed volume, duration

of stress, etc

• If the dielectric strength of a cable insulation specified under definite conditions is exceeded, discharge processes always occur, and these can be divided into two categories; partial discharges and complete break down

• Dielectric Strength: The Potential gradient necessary to cause

breakdown of an insulating medium is termed its dielectric strength and is usually expressed in MVs/meter of thickness.

Dielectric Strength of different insulation materials

If thickness of the insulation material is 1 mm

Ref: Page 119, Hughes Electrical Techonology

SF6 = About twice of Air

PARTIAL DISCHARGE IN INSULATION OF THE HIGH VOLTAGE CABLES.

• The occurrence of partial discharge (PD) within a dielectric -means that either the electric field or the dielectric strength or both are distributed in a highly inhomogeneous manner

• As a result, discharges occur in the void above a definite voltage that can be measured externally and can lead to a gradual erosion of

• The figure below shows an example: a gas-filled cavity in the dielectric

that disturbs both the field pattern and the distribution of the dielectric

strength.

Field Strength in void increased (doubled)

Fig: 1

Simplified relationship between Electric stress and relative permittivity or dielectric constant

In such an arrangement of this type, the electrical field strength E of

neighboring individual components behaves as inversely proportional to

the relative dielectric constants εr.

That is if relative permittivity is less Electrical Stress goes high (V/m).

Ref: Page 40, Cable Systems for High and Extra-High Voltage by E Peschke, R von

Olshausen.

Fig: 2

Partial discharge test on polymer-insulated cable

• The best way to explain the processes

taking place here is by using the simplified

equivalent circuit diagram comprising three

capacitances representing the void itself,

the dielectric connected in series with it

and the intact dielectric connected in

parallel with them both.

• Parallel to the void the equivalent circuit

diagram provides a spark gap which breaks

down when a specific voltage Uz, assumed

to be constant, is exceeded, and thus has

the effect of discharging the capacitor This

results in repeated voltage collapses at

capacitor C1 as indicated in the

Display unit

Test on Cables for partial discharge

Test voltage Voltage at void without discharge

Voltage at void with discharges

C1

C2 C3

Transfor

mer

Display unit

Insulation under test

On-site partial discharge monitoring

• It is available for the accessories,

particularly joints, and only where

they are equipped with sensors.

• Inductive Coupler: Pulses from

the joint that is being monitored

pass through Rogowiski coils in

opposite directions and, in

summation, produces a signal

with almost twice the amplitude

because the winding in the coil in

in opposite directions.

• For the same reason, pulses that

originate from the right or left of

the joint and are consequently

passing through the coils in the

same direction are largely

cancelled out during summation.

Signal addition

Rogwisky Coil

-Joint

5.0 Basic Impulse Insulation Level (BIL) TEST

• Insulation can withstand very high voltage, if it is applied for a very brief period

• If a 60 Hz sinusoidal voltage between the insulation and ground

is applied and if the voltage is slowly increased, a point will be reached where break down occurs

• On the other hand if we apply a dc impulse voltage for a extremely short period, it takes much higher voltage before insulation breaks down

• Same happens with other insulators, bushing, etc

• In the interest of standardization, and to enable a comparison between the impulse withstand capability of insulators, the insulators are tested by a defined impulse wave as follows

by a lightning stroke of 20 kA.

• Calculate the voltage across each insulator string under normal conditions

• Describe the sequence of events during and after the lightening stroke

• Under normal condition:

• Line to neutral voltage= 69/√3 = 40kV

• The insulator is therefore at the same potential to the

ground

• When lightening strikes:

• Voltage across the insulator and the ground resistance

suddenly jumps to 20kA X 20 = 400kV

• Therefore insulator burns immediately causing short

circuit in all three phases

6.0 Life Test

• A factory test to determine expected life.

Time to break down