Testing and Maintaining High Voltage Systems on Ships

1
To Be A World Class Maritime Academy
HV Course March 2007
HIGH VOLTAGE
TESTS

2
Objective
• 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

3
Introduction
• 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.

4
Condition of HV insulation is governed by factors such as:
• Temperature, humidity, surface condition and
operating voltage level.
• Guide by the manufacturers recommendations
when testing and maintaining HV insulation.
• Before applying an IR test to HV equipment its
power supply must be switched off, isolated,
confirmed dead by an approved live-line tester
and then earthed for complete safety in
accordance with the current EPTW regulations.

5
HV Test type
• HV Test on board ship
1) -Insulation Resistance (IR) test
2) -Polarity Index (PI) Test
3) -Infrared Imaging test
4) - Circuit breaker test
5) Partial discharge test
6) BIL (Basic Insulation level) test
7) Life test

6
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.

7
Insulation tester
Motor
α
0
Control coil
Deflecting
coil
Permanent
magnet
Permanent
Magnet Rotor

8
Procedure of IR testing
• Isolate power, test and prove the conductors are dead by a proven live-line
tester.
• The circuit is earthed
• Issue EPTW.
• It is to be ensured that the operator never touches an unearthed
conductor.
• With the megger reader is connected between conductor and earth,
the safety earth is disconnected. [The safety earth must be
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.

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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.

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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.

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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.

12
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.

13
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.

14
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.

15
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.

16
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

17
4. PARTIAL DISCHARGES
• 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

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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]

19
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.

20
Dielectric Strength of different insulation materials
If thickness of the insulation material is 1 mm.
Air- 4.46 MV/m
Mica- 61 MV/m
Glass (density)- 28.5 MV/m
Ebonite- 50 MV/m
Paraffin-waxed paper- 40-60 MV/m
Transformer oil- 200 MV/m
Ceramics- 50 MV/m
Ref: Page 119, Hughes Electrical Techonology
SF6 = About twice of Air

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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.
Referring figure 1:
• Gases usually have a considerably lower dielectric strength than
solid, liquid or impregnated dielectrics.
• Field strength inside the void (εr=1) exceeds the stress in
surrounding dielectric by a factor of nearly εri, the permittivity of
the insulation material.
• 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 surrounding material.

22
CONTINUED
• 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

23
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

24
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
oscillogram.
C1
C2
C3
C1
C2
C3
Display
unit

25
Test on Cables for partial discharge
Test voltage
Voltage at void without
discharge
Voltage at void with
discharges
C1
C2
C3
Transform
er
Display
unit
Insulation under
test

26
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

27
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.

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CONTINUED
0%
100%
50%
Time
1.2μs 50μs
1.2 X 50 μs BIL Puse
Peak voltage

29
CONTINUED
• The BIL of a device is usually several times higher than its normal ac
operating voltage.
• For example, the standards require that 69 kV distribution transformer
must have a BIL of 350 kV.
• The peak voltage at which a safety begins to conduct must always be
lower than the BIL of the apparatus it is intended to protect.
• A 3 phase, 69 kV transmission line having a BIL of 300 kV is
supported on steel towers and protected by a circuit breaker. The
ground resistance at each tower is 20Ω whereas the neutral of the
transmission line is solidly grounded at the transformer just ahead of
the circuit breaker. During an electric storm, one of the towers is hit
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.

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CONTINUED
• 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.

31
6.0 Life Test
• A factory test to determine expected life.
Time to break down
Field
strength,
E
Impregnated paper
Polymer

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