Low Voltage and High Voltage Cable Testing

 Low Voltage XLPE Distribution Cables:Insulation Resistance:

• Cables shall be tested for insulation resistance with an insulation tester (i.e. Megger) at 1000 Volts for 1 minute.

• The minimum insulation resistance to earth or between phases shall be 100 meg-ohms.

• The instrument used for this measurement shall have a minimum resolution of 10 meg-ohms on the 0 to 500 meg-ohm range.

• At the conclusion of LV insulation resistance testing, the neutrals must be connected to the earth stakes.

Phasing Test:

• The correct phasing of all LV circuits shall be checked at all positions where the LV cables are terminated into fuse bases and where any LV cable is run from point to point.

• This test shall be performed with an instrument designed for the purpose. Mains frequency voltage of 240 Volts is not acceptable for this test.

• The neutral conductor shall be connected to the earth stake for this test.

Continuity Test (resistance of bolted connections):

• For loop LV systems, a continuity test shall be carried out on each LV circuit to ensure that all bolted connections are complete and adequate. The test shall be carried out as follows:

• (1)     At the transformer firmly bond all 4 conductors together

•  (2)   Undertake a continuity test at every point where there is a service provision or open point. In a fused service pillar the bottom row of fuses bases must be the point at which the test is undertaken as that is the furthest extent of the network.

• The difference between the readings of each phase conductor and the neutral for each individual test shall not be greater than 10% of each other. Any difference greater than this may indicate a loose or dirty connection and will require further investigation.

• The instrument used for this measurement should have a resolution to the second decimal point in the 0 to 5 ohm range.

• A typical instrument would be the earth “Megger” type and taking into account the resistance values of the test leads.

Earth Resistance Test:

• In any overhead or underground network the earth resistance at any point along the length of a LV feeder is to have a maximum resistance of 10 ohms prior to connection to the existing network.

• In any overhead or underground network the overall resistance to earth Shall be less than 1 ohm prior to connection to the existing network.

 11 KV AND 33 KV XLPE Cables:Phasing Test

• The correct phasing of all HV circuits shall be checked at all positions where the HV cables have been terminated.

• This test shall be performed with an instrument designed for the purpose. 240 Volt mains frequency is not acceptable for the performance of this test. The test may be conducted on either the wire screens or the aluminum conductors.

• Where the test is performed on the wire screens, they shall be disconnected from earth.

 Outer Sheath Insulation Resistance (Screen wire test)

• The purpose of the test is to determine soundness of the outer polyethylene sheath against water ingress, mechanical damage and termite attack.

• Values below 0.5 meg-ohms (500 kΩ) can indicate sheath damage. Values between 1.0 and 10 meg-ohms may not indicate damage in a single location. Fault finding can often be very difficult. In new cables, values of greater than 100 mega ohms are required.

• The integrity of the outer sheath shall be checked after cables have been buried by an insulation tester (Megger) at 1000 Volts.

• The test shall be conducted for 1 minute between each wire screen and earth after the cable has been jointed and terminations installed.

• For cables after repairs, the resistance must not be less than 10 meg-ohms.

• Where HV cable circuits are cut and joined to new circuits, sheath testing must be carried out on the existing old circuit prior to joining to the new cable.

 HV test on XLPE cables already in service or previously energized

• Studies carried out on DC high voltage testing of XLPE cables now conclude that;

• DC testing above 5kV of field aged XLPE cables generally increases water tree growth and reduces service life.

• 5kV is not considered a “High Voltage DC Test”. The test voltages for tests on XLPE cables is now limited to 5kV after in service repairs and 10kV for new installations.

• A 5kV Megger is suitable for a 5kV test on cables after repairs.

• The changes to this section will also make it possible for a repaired cable to be tested by repair crews and made available for immediate return to service.

Application	Test Voltage	Criteria
After repairs – Sheath	1kV Megger 1 minute	10 meg-ohms min.
After repairs – Insulation	5kV Megger 1 minute	1000 meg-ohms min.
After repairs – Insulation	5kV DC 1 minute	5.0 μA (micro-amps) max.

 HV test on new XLPE cable:

•  Prior to the performance of this test, the screen wires must be connected to the permanent earth position.

• The cable shall be tested at the test voltage and the pass criteria shall be in accordance with the following table:

Application	Test Voltage	Criteria
New cables – Sheath	1kV Megger 1 minute	100 meg-ohms min.
New cables – Insulation	10kV DC 15 minute	1.0 μA (micro-amps) max
New cables – Insulation	10kV DC 15 minute	1000 meg-ohms min.

• If further repair works are undertaken, and they require additional joints to be installed, the complete HV testing procedure shall be repeated.

Alternative HV Test Requirement on Insulation for 11kV Cables

• Where it is not practical to conduct a high voltage test, the test requirements for insulation (core to screen wire) may be limited to testing for the condition of “safe to energize”. The following list of circumstances and conditions must be met as a minimum requirement:

• The cable circuit voltage shall be 11kV,

• The circuit outage duration shall be not more than 48 hours,

• The work shall involve extending or repairs to existing circuits,

• The insulation test shall be applied for 1 minute between each

• phase core and screen with a 1000 Volt minimum insulation tester (Megger),

• Typically the test result should be in the order of 1000 meg-ohms.

Paper Insulated Cables:Tests on LV Cables

• An insulation resistance test shall be conducted with a 1000 Volt megger. Test results as low as 10 meg-ohms on old cable circuits are common and therefore considered safe to energies.

Test on 11kV and 33kV Cables between Cores and Earth

• For three core belted cables, the test on any core shall be conducted between the core and lead sheath with the remaining two cores connected to earth.

• The test voltages and pass criteria shall be in accordance with the table below.

Application	Test Voltage	Criteria
11kV new cables	5kV Megger 1 minute	100 meg-ohms.
11kV after repairs	5kV Megger 1 minute	100 meg-ohms.
33kV – no TF’s connected	5kV Megger 1 minute	1000 meg-ohms.
33kV – with TF’s connected	5kV Megger 1 minute	15 meg-ohms.

66kV XLPE CABLESCore to Sheath Test after Repairs:

• After repairs have been carried out, the 66kV XLPE cable shall be energized at power frequency for 24 hours without load. DC testing is not permitted.

• The cable sheath link box/cross bonding system shall be put into its normal condition.

Outer Sheath Integrity Test:

• An insulation resistance test between the metallic sheath and earth shall be conducted. The anti-termite barrier must be connected to the metallic sheath and the insulation test performed to earth.

• The test voltage applied for 1 minute shall be 5kV DC applied with either a high voltage test set or insulation resistance tester (Megger).

• ETSA TECHNICAL STANDARD

         Except for New Cables, Testing at Voltage greater than 5.0KV is not permittedReferences:

Electrical Faults

Transient fault

A transient fault is a fault that is no longer present if power is disconnected for a short time and then restored; or an insulation fault which only temporarily affects a device's dielectric properties which are restored after a short time. Many faults in overhead power lines are transient in nature. When a fault occurs, equipment used for power system protection operate to isolate the area of the fault. A transient fault will then clear and the power-line can be returned to service. Typical examples of transient faults include:

• momentary tree contact
• bird or other animal contact
• lightning strike
• conductor clashing

Transmission and distribution systems use an automatic re-close function which is commonly used on overhead lines to attempt to restore power in the event of a transient fault. This functionality is not as common on underground systems as faults there are typically of a persistent nature. Transient faults may still cause damage both at the site of the original fault or elsewhere in the network as fault current is generated.

Persistent fault

A persistent fault does not disappear when power is disconnected. Faults in underground power cables are most often persistent due to mechanical damage to the cable, but are sometimes transient in nature due to lightning.

Symmetric fault

A symmetric or balanced fault affects each of the three phases equally. In transmission line faults, roughly 5% are symmetric. This is in contrast to an asymmetrical fault, where the three phases are not affected equally.

Asymmetric fault

An asymmetric or unbalanced fault does not affect each of the three phases equally. Common types of asymmetric faults, and their causes:

• line-to-line - a short circuit between lines, caused by ionization of air, or when lines come into physical contact, for example due to a broken insulator.
• line-to-ground - a short circuit between one line and ground, very often caused by physical contact, for example due to lightning or other storm damage
• double line-to-ground - two lines come into contact with the ground (and each other), also commonly due to storm damage.

Bolted fault

One extreme is where the fault has zero impedance, giving the maximum prospective short-circuit current. Notionally, all the conductors are considered connected to ground as if by a metallic conductor; this is called a "bolted fault". It would be unusual in a well-designed power system to have a metallic short circuit to ground but such faults can occur by mischance. In one type of transmission line protection, a "bolted fault" is deliberately introduced to speed up operation of protective devices.

Realistic faults

Realistically, the resistance in a fault can be from close to zero to fairly high. A large amount of power may be consumed in the fault, compared with the zero-impedance case where the power is zero. Also, arcs are highly non-linear, so a simple resistance is not a good model. All possible cases need to be considered for a good analysis.

Arcing fault

Where the system voltage is high enough, an electric arc may form between power system conductors and ground. Such an arc can have a relatively high impedance (compared to the normal operating levels of the system) and can be difficult to detect by simple overcurrent protection. For example, an arc of several hundred amperes on a circuit normally carrying a thousand amperes may not trip overcurrent circuit breakers but can do enormous damage to bus bars or cables before it becomes a complete short circuit. Utility, industrial, and commercial power systems have additional protection devices to detect relatively small but undesired currents escaping to ground. In residential wiring, electrical regulations may now require Arc-fault circuit interrupters on building wiring circuits, to detect small arcs before they cause damage or a fire.

source : https://en.wikipedia.org/wiki/Fault_(power_engineering)

Test Procedure For Bus Bar

1.      PURPOSE:                      The purpose of this procedure is to verify functions of   Metal Enclosed Bus Duct.

2.      APPLICABILITY:        This procedure applies to all types of Metal Enclosed Bus Duct.

3.      DEFINITIONS:              Non

4.      FORM:                             Site Test Report For Metal Enclosed Bus Duct.

5.      REFERENCES: 

All inspections and tests shall utilize the following references;

A.                Project design drawings and specifications

B.                 Manufacturer’s instruction manuals applicable to each particular apparatus.

6.      STANDARDS: 

The following codes and standards apply,

A.                National Electrical Testing Association: NETA

B.                 Institute of Electrical and Electronic Engineers: IEEE.

7.      EQUIPMENTS REQUIRED:

1)                  Megger.

2)                  DLRO.

3)                  Digital Multimeter.

4)                  Phase Sequence Meter.

5)                  Hi-Pot Test Set.

8.      PROCEDURE:

SAFETY PRECAUTIONS:

Always refer to instruction manuals of test equipment used for detailed safety instructions and precautions.

MECHANICAL CHECK AND VISUAL INSPECTION:

1.      Inspect for physical damage/defects.

2.      Check bus arrangement for conformance with approved drawings.

3.      Check tightness of all bolted connections (torque wrench method).

4.      Check that all enclosure grounding is securely connected.

5.      Inspect internal compartments for cleanliness (free from dust and moisture).

6.      Check for watertight seals at all joints including expanding interface points.

7.      Check bus conductor support insulators for cracked insulation chipped porcelain etc

8.      Check quality of paint work (inside and outside)

9.      Check that ventilation openings are not blocked and screened against ingress of insects and rain.

10.  Moisture drain holes available at bottom of enclosure.

11.  Check anti-condensation heaters mounted at the correct locations (Bottom)

      ELECTRICAL TESTS :

1.      Phasing check Ensure correct phase identification

2.      Continuity check

3.      Bus joint resistance (at 100 A DC)

4.      Insulation resistance test.

5.      Hi-pot test (to ground and between phases)

6.      Heaters and thermostats functional test Measure the heater currents.

9.       ACCEPTANCE  CRITERION :

Check the Test results against applicable standards and/or manufacturer’s instructions.

DC Voltage drop calculation

Surge arresters field testing

The Gas Insulated Switchgear (GIS)

·         The Gas Insulated Switchgear (GIS) shall be a complete unit comprised of main equipment, all control, monitoring, protection, measuring and auxiliary devices and systems including interconnections, all termination

·         Joints at interfaces, mechanical linkages, couplings, SF6 gas piping, cabling, wiring, grounding materials etc. required for proper and satisfactory operation.
·         The GIS shall be of compact and modular design with individual equipment modules connected together to form a complete assembly. However, combined disconnect switch and grounding switch, cable termination, PT, exit bus duct and CT (with cores located within gas compartment) shall be in separate gas tight compartment. If, CT is installed in breaker gas compartment; insulation class for CT shall be class F. of either single-phase enclosure type or three-phase enclosure type for voltage levels of 230kV and below. of single-phase enclosure type for voltage level of 380kV. Installed on suitable pads or supporting frames/structures with provision for leveling, including fasteners to foundation, which shall also be included in the supply. Support-structures shall be designed and fabricated to withstand the short circuit forces of the ground fault current.
·         GIS shall having expansion joints and flexible connections, where several enclosures are connected in the longitudinal direction, such as main bus. Expansion joints provided for installation alignment shall be locked in place when alignment is complete. Expansion joints for compensation of thermal expansion and erection tolerances shall have
·         the means to preserve mechanical integrity of the enclosure and the plug-in contacts for the conductor. Provided with galleries, stairways, movable platform or walkways for accessing the equipment above two meters for maintenance and operation.
·          All structures, movable platform, galleries, stairways and walkways shall have grounding provision and shall conform to the relevant Occupational Safety and Health Administration (OSHA) regulations.
·         For single-phase type GIS, creation of single gas zone by external piping connection between the phases is not acceptable. Also, external piping connection (with normally open valve) to create common gas zone (between same phase) in single-phase type GIS is not acceptable.
·         The GIS enclosure shall be safe to touch and fully ensure operational security and personnel safety under all normal and fault conditions with the maximum allowed induced voltage in the enclosure of 65 volts.
·         The switchgear and all its components and accessories shall be designed for minimum maintenance during service. The manufacturer shall state the minimum interval between minor inspections (which will be restricted to visual checking and adjustments of external parts only) and major inspection/overhaul, including refilling or replenishment of gas and cleaning of the contaminant or filter in the circuit breaker chamber(s).
·         Suitable openings shall be provided in the circuit breaker, disconnect and grounding switch modules for major inspection/adjustments.
·         All motors shall be in accordance with IEC 60034. The bearings and other such parts shall be permanently lubricated for the entire service life. Padlocks shall be made of stainless steel or brass.
The GIS interface points shall be carefully coordinated with other equipment such as overhead lines, cables, transformers, reactors, capacitor banks, etc., supplied by other manufacturers in order to ensure full compatibility. The design criteria for outdoor