Test On 110kV Power Cable After Installation - Part 2

General description of site test procedure

In previous part of this technical article first three procedures were explained. Now the rest will be explained in details:

1. Phase indication test (previous part)
2. DC conductor resistance measurement (previous part)
3. Capacitance test (previous part)
4. DC Sheath test on outher sheath
5. Insulation resistance measurement
6. Cross bonding check
7. Zero sequence and positive sequence impedance test (next part)
8. Earth resistance measurement at link boxes (next part)
9. Link box contact resistance measurement (next part)

4. DC sheath test on outher sheath

The test is applied when the cable sheath can be isolated from the earth to permit a voltage to be applied to the over-sheath to check the integrity of the covering.
This testing is generally applied at certain stages of cable system installation at specified parameter as follows:

1. When the cable is still on reel. The applied test voltage is 10 kV for 10 seconds, if a proper test lead is provided.
2. Once the cable are laid, dressed and tied together in trefoil configuration a test voltage of 10 kV for 30 seconds is applied.
3. Following backfilling sand beddind-2, a test voltage of 10 kV is applied for 1 minute on each cable. This is a formal testing with test records and signed by representatives of the responsible parties as witnesses.
4. Following completion of jointing activities between two cable sections in a joint bay and after backfilling of the joint bay, the jointed cable sections are then tested by applying 10 kV for 30 seconds.
5. Following the completion of cable system installation and prior to acceptance testing, as a pre-check testing a test voltage of 10 kV is applied for 1 minute.

Note – All above mentioned testing will be conducted in presence of project consultant.

References

• IEC 60840 – Power cables with extruded insulation and their accessories for rated voltages above 30 kV
• IEC 60229 – Electric cables // Tests on extruded oversheaths with a special protective function
• TES-P-104.08 – Bonding and grounding of insulated metallic sheath of power cable system

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Test On 110kV Power Cable After Installation - Part 1

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.

General description of site test procedure

Site test procedure covers all necessary electrical testing for the 110 kV cable and accessories to be carried out during and after installation of the cable system.

This procedure is in line with the requirements of the contract suitable for 110 kV, XLPE cables and accessories and the tests are in accordance with TCSP-104.08, IEC 229, IEC 540 and IEC 840.

110 kV, 115 kV and 132 kV XLPE Cables

(Standard Reference is IEC 60840 and relevant SEC Transmission Specifications 11-TMSS-02, Rev. 0 and TCS-P-104.02, TCS-P-104.03, TCS-P-104.06 and TCS-P-104.08)

1. Mechanical Check and visual Inspection

ITEM	Description	Remark
1	Inspection for physical damage or defects
2	Check tightness of all bolted connections (torque wrench method)
3	Check for proper cable bolted connections
4	Check cable bends to ensure that bending radius is equal to or greater than the minimum bending radius specified
5	Check for proper cable support, clamping, trays arrangements
6	Link box tightness check
7	Verify that shields are terminated as specified (through link box or directly grounded)
8	Verify the exact route length as per approved drawings from terminations to terminations
9	Check that all grounding points are securely connected to ground grid as specified
10	Check that phases are identified and color coded
11	Single core cable connected between power transformer and switchgear shall be single point earthed as switchgear side and at floating side SVL (sheath voltage limiter) should be installed
12	Check single point or both ends, via voltage limiter as per approved design
13	Inspection of label inside link boxes and water proofing
14	Check cable entry path trench as ducts are properly sealed
15	Check irregularities of outer jacket formed by non-uniform shield wire distribution
16	Check/inspect the transposition of cable phases
17	Check the cable outer jacket for any physical damage during and after installation
18	Check for the cross connection of cable metallic sheath in cross bonding system
19	Check the rubber seal in cable clamps to avoid any damage to cable outer jacket
20	Check the insulating shrouds are installed at the base of the cable terminations
21	For accessories (sealing terminations, instrument panels and link boxes) check the following:
	a. Name plates installed and data is correct
	b. Danger signs
	c. Bolt tightness check and paint work conditions

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5 key factors to the correct cable selection and application

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.

Cable selection can be based upon the following five key factors:

1. Cable installation
2. Cable construction
3. Cable operation (voltage and current)
4. Cable size
5. Shielding requirements

1. Cable installation

Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served.

A good understanding of local conditions, installation crews, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily! Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions.

Designs of conduit systems not only should minimize the number of conduit bends and distances between manholes but also should specify the pulling tensions.

The inspection personnel should ensure that installation crews do not exceed these values during installations. It is also important that correct bending radius be maintained in order to avoid unnecessary stress points. Once a correct installation is made, routine inspection, testing, and maintenance should be carried out on a regular basis to chart the gradual deterioration and upkeep of the cable system.

Cable systems are the arteries of the electric power distribution system and carry the energy required for the successful operation of a plant. Following is a brief discussion on cable installation and maintenance.

There are several types of cable systems available for carrying electrical energy in a given distribution system. The selection of a particular system may be influenced by local conditions, existing company policies, or past experience.

No set standards or established guidelines can be given for the selection of a particular system.

2. Cable construction

Selection and application of cable involves the type of cable construction needed for a particular installation. Cable construction involves conductors, cable arrangement, and insulation and finish covering.

2.1 Conductors

Conductor materials such as copper and aluminum should be given consideration with regard to workmanship, environmental conditions, and maintenance. The requirements for aluminum conductors with regard to these factors are more critical than for copper conductors.

Cable conductors should be selected based upon the class of stranding required for a particular installation.

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Corrosion Types Encountered With Power Cables

Introduction

There are numerous types of corrosion, but the ones that are discussed here are the ones that are most likely to be encountered with underground power cable facilities.
In this initial explanation, lead will be used as the referenced metal. Copper neutral wire corrosion is not discussed here.

Anodic Corrosion (Stray DC Currents)

Stray DC currents come from sources such as welding operations, flows between two other structures, and –in the days gone by — street railway systems.

Anodic corrosion is due to the transfer of direct current from the corroding facility to the surrounding medium, usually earth. At the point of corrosion, the voltage is always positive on the corroding facility.

In the example of lead sheath corrosion, the lead provides a low resistance path for the DC current to get back to its source. At some area remote from the point where the current enters the lead, but near the inception point of that stray current, the current leaves the lead sheath and is again picked up in the normal DC return path.

The point of entry of the stray current usually does not result in lead corrosion, but the point of exit is frequently a corrosion site.

 

Clean sided corroded pits are usually the result of anodic corrosion. The products of anodic corrosion such as oxides, chlorides, or sulfates of lead are camed away by the current flow. If any corrosion products are found, they are usually lead chloride or lead sulfate that was created by the positive sheath potential that attracts the chloride and sulfate ions in the earth to the lead.
In severe anodic cases, lead peroxide may be formed. Chlorides, sulfates, and carbonates of lead are white, while lead peroxide is chocolate brown.

Cathodic Corrosion

Cathodic corrosion is encountered less fiequently than anodic corrosion, especially with the elimination of most street railway systems.

This form of corrosion is usually the result of the presence of an alkali or alkali salt in the earth. If the potential of the metal exceeds -0.3 volts, cathodic corrosion may be expected in those areas.

In cathodic corrosion, the metal is not removed directly by the electric current, but it may be dissolved by the secondary action of the alkali that is produced by the current. Hydrogen ions are attracted to the metal, lose their charge, and are liberated as hydrogen gas.

This results in a decrease in the hydrogen ion concentration and the solution becomes alkaline. The final corrosion product formed by lead in cathodic conditions is usually lead monoxide and lead / sodium carbonate. The lead monoxide formed in this manner has a bright orange / red color and is an indication of cathodic corrosion of lead.


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Industrial control wiring and cabling guide

Wires and preparation for control wiring

Electrical equipment uses a wide variety of wire and cable types and it is up to us to be able to correctly identify and use the wires which have been specified. The wrong wire types will cause operational problems and could render the unit unsafe.

Such factors include:

• The insulation material;
• The size of the conductor;
• What it’s made of;
• Whether it’s solid or stranded and flexible.

These are all considerations which the designer has to take into account to suit the final application of the equipment.

A conductor is a material which will allow an electric current to flow easily. In the case of a wire connection, it needs to be a very good conductor. Good conductors include most metals. The most common conductor used in wire is copper, although you may come across others such as aluminium. An insulator on the other hand is a material which does not allow an electric current to flow. Rubber and most plastics are insulators.

Insulation materials

Wires and cables (conductors) are insulated and protected by a variety of materials (insulators) each one having its own particular properties. The type of material used will be determined by the designer who will take into account the environment in which a control panel or installation is expected to operate as well as the application of individual wires within the panel.
As part of the insulating function, a material may have to withstand without failing:

• Extremes of current or temperature;
• A corrosive or similarly harsh environment;
• Higher voltages than the rest of the circuit.

Because of these different properties and applications, it is essential that you check the wiring specification for the correct type to use.

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Fire Properties Of Cables

Standards relating to fire properties of cables

IEC, BS standards

This is an area of increasing public and legislative concern, and therefore of increasing interest to engineers. There have been major advances in the fire performance of cables in recent years, and table below lists some of the relevant standards.

Standard	Description
IEC 60331	Fire resisting characteristics of electric cables.
IEC 60332	Tests on electric cables under fire conditions. Test methods and flame propagation of power and control/communication cables.Note the identical EN60332 and equivalent national standard BSEN60332 supersede EN50265 and BS 4066.
IEC 60754	Test of gases evolved during combustion of electric cables.
IEC 61034	Measurement of smoke density of cables burning under defined conditions. Identical EN61034 and national equivalent BSEN 61034 supersede EN50268 and BS7622.
BS 6387	Performance requirements for cable required to maintain circuit integrity under fire conditions.
BS 6724	Electric cables. Thermosetting insulated, armoured cables for voltages of 600/1000 V and 1900/3300 V, having low emission of smoke and corrosive gases when affected by fire.
BS 7211	Electric cables. Thermosetting insulated, non-armoured cables for voltages up to and including 450/750 V, for electric power, lighting and internal wiring, and having low emission of smoke and corrosive gases when affected by fire.
BS 7835	Specification for cables with cross-linked polyethylene or ethylene propylene rubber insulation for rated voltages from 3800/6600 V up to 19 000/33 000 V having low emission of smoke and corrosive gases when affected by fire.
EN 50267	Common test methods for cables under fire conditions. Tests on gases evolved during combustion of materials from cables. Apparatus. BSEN50267 is identical and supersedes BS6425. Similarly French standard NF C 20-454 is superseded.

Toxic and corrosive gases

It is recognized that conventional flame retardant cables having sheathing based upon PVC type materials evolve considerable quantities of halon acid gases such as hydrogen chloride upon burning.

Such materials are not therefore suitable for use in confined spaces where the public are likely to travel, and moreover the fire in the ENEL power station at La Spezia in 1967 showed that in certain circumstances PVC cables will burn completely and contribute to the spread of a fire.

Materials have now been developed for cable oversheaths and bedding which are normally free of halogen based compounds. They consist of a mixture of inorganic filler such as aluminium hydroxide and polymers such as ethylene vinyl acetate, acrylates and ethylene propylene rubbers.

 

Cables manufactured with such materials are known as ‘Low Smoke and Fume’ (LSF) and have acid gas evolution less than 0.5% in comparison to 25–30% for PVC compounds.

IEC 60754-1 specifies a method of determining the amount of halogen acid gas, other than hydrofluoric acid, evolved during combustion of halogen based compounds. The method essentially measures the existence of halogen acid greater than 0.5%, the accuracy limit for the test.

Therefore cables tested having less than the 0.5% limit are generally termed ‘zero halogen’ or ‘low smoke zero halogen’ (LS0H).

Smoke emission

Normal cable sheathing compounds also give off dense smoke when burned and this is of particular concern in underground transport system installations. The generation of large amounts of smoke obscures vision and reduces the ease with which the fire brigade is able to bring members of the public to safety in the event of a fire. LSF cables therefore play an important part in reducing this danger to a minimum.

London Underground Limited (LUL) have developed a test of practical significance which has been designed to measure the density of smoke emission from cables and it has now been adopted by British and IEC Standards. This defines the standard absorbance produced across the opposite faces of a test cubicle and is popularly known as the 3 m cube test.

Paris Metro (RATP) adopts the French Standard UTE C20-452 on smoke emission which determines under experimental conditions the specific optical density of smoke produced by burning material. This slightly different approach is generally known as the NBS smoke chamber test.

Oxygen index and temperature index

‘Oxygen index’ is the minimum concentration of oxygen in an oxygen/nitrogen mixture in which the material will burn. As air contains approximately 21% oxygen it is stated that a material with an oxygen index greater than about 26% will be self extinguishing. In general, a particular oxygen index value offers no guarantee of resistance to the spread of flames.
In practice materials having identical oxygen indices may have widely different burning properties especially if base polymers or additives are of different types.

The ‘temperature index’ of a material is the minimum temperature at which the material supports combustion in air containing 21% oxygen when tested under controlled conditions. The test is useful for the comparison of similar materials but no correlation with flammability under other fire conditions is implied.
Oxygen and temperature indices are to some extent inter-related.

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4 Practical Approaches To Minimize Voltage Drop Problems

What NEC states for max. voltage drop?

The NEC states in an Informational Note that a maximum voltage drop of 3% for branch circuit conductors, and 5% for feeder and branch circuit conductors together, will provide reasonable efficiency of operation for general use circuits.
For sensitive electronic loads, circuits should be designed for a maximum of 1.5% voltage drop for branch circuits at full load, and 2.5% voltage drop for feeder and branch circuits combined at full load.

Four practical approaches can be used to minimize voltage drop problems:

1. Increasing the number or size of conductors
2. Reducing the load current on the circuit
3. Decreasing conductor length, and
4. Decreasing conductor temperature

1. Increase the Number or Size of Conductors

Parallel or oversized conductors have lower resistance per unit length than the Code-required minimum-sized conductors, reducing voltage drop and increasing energy efficiency with lower losses than using the Code-required minimum-sized conductor.

In data centers and other sensitive installations, it is not uncommon to find conductor gauges for phase, neutral, and ground exceeding Code minimums, and a separate branch circuit installed for each large or sensitive load.

To limit neutral-to-ground voltage drop, install a separate, full-sized neutral conductor for each phase conductor in single-phase branch circuit applications.

For three-phase feeder circuits, do not downsize the grounded conductor or neutral. For three-phase circuits where significant non-linear loads are anticipated, it is recommended to install grounded or neutral conductors with at least double the ampacity of each phase conductor.

2. Decrease Load Current

Limiting the amount of equipment that can be connected to a single circuit will limit the load current on the circuit. Limit the number of receptacles on each branch circuit to three to six.

Install individual branch circuits to sensitive electronic loads or loads with a high inrush current.

For residential applications, install outdoor receptacles not to exceed 50 linear feet between receptacles, with a minimum of one outdoor receptacle on each side of the house, and with individual branch circuits with a minimum of 12 AWG to each receptacle.

3. Decrease Conductor Length

Decreasing conductor length reduces the resistance of the conductor, which reduces voltage drop. Circuit lengths are usually fixed, but some control can be exercised at the design stage if panels or subpanels are located as close as possible to the loads, especially for sensitive electronic equipment.



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