DISTRIBUTION TECHNOLOGY, PRIVATE BAG 1074, GERMISTON 1400 FAX 011-871-2352

DISTRIBUTION TECHNICAL BULLETIN

06-08-2004 Enquiries: G Stanford Tel: (011) 871-2431

TECHNICAL BULLETIN: 03TB-34

Distribution Standard: Part 4 Medium Voltage

TITLE: INSULATION CO-ORDINATION AND BONDING FOR MEDIUM VOLTAGE LINES

Purpose The purpose this bulletin is to revise technical bulletin 99TB-005 written by Phil Crowdy. The original bulletin aimed to explain the purposes of bonding and installing BIL down-wires on MV lines. To make everyone aware of the requirements and the recommended practices in all areas. The requirements do not differ from the MV Standard. The bulletin also gave a method of calculating average number of outages that can be attributed to direct lightning strikes. This can be utilised by performance and maintenance staff to estimate the portion of the line outages that can be reduced by improving the design or maintenance of the line and associated equipment. MV/LV shared structure requirements, alternative methods of ensuring insulation co-ordination and figures of typical installation practices have been added.

Overview The insulation co-ordination as laid out in the MV standard is designed to ensure that installation are safe, have the lowest probability of outage and the lowest failure rate for the capital spent. For optimum performance it is necessary to follow the guidelines laid out below. The selection of the bonding and BIL rating for the line is dependent on the lightning activity in the area and the pollution levels. The installation of bonding and BIL down-wire is a capital expense and this expenditure must be weighed against the probability and costs of damage to the system and the performance requirements for the system. As the performance requirements for the system, safety and repair costs become increasingly important to Eskom more importance is being placed on the reduction of equipment failure and improvement of line performance. For MV wood pole lines the standard arrangement is that the hardware of all three phases are bonded and a BIL down-wire is installed. A gap of approximately 500mm is left between the bonding wire and the BIL down-wire. A circumferential strap is utilised either side of the gap, connected to the BIL down-wire and bonding wire respectively. For MV/LV shared structures the same arrangement as above can be used excepting that an additional 50 mm gap be made below the LV. Report DISREAAH7 called Insulation co-ordination on MV/LV shared wood structures details the decision on MV/LV shared structures and is published on the Distribution Technology website. The gap construction should be the same as that described for the MV gap above. Figure 1 shows the insulation co-ordination practices for medium voltage and shared structures. Unless there is a strong reason for doing otherwise these configurations are to be adopted in all regions on new lines. On existing fully insulated lines, where the installation of BIL down wires is not an option, an alternative practice is to install line arresters or double arrestors at the equipment. This is believed to be less effective than the preferred BIL down wire practice. (i.e. more lighting current will reach the terminal equipment and the impulse voltage rise on the system will be higher). The procedures for the installation of line arresters (SCSPVADG8) and double arresters on equipment (SCSPVADG3) are available on the Distribution Technology web site.

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A guide presented later gives areas where it is possible to still get a reasonable performance by omitting either the bonding or the BIL down-wire based on the expected climatic conditions of the area. Utilising the options of no BIL down-wire or no bonding should be chosen with care and a consideration of all the consequences must be undertaken.

Figure 1: Insulation co-ordination practices to be applied on structures

The standard arrangements utilised, provide the following: a. A insulation level of 300kV or more between the phase conductors and ground resulting in reduced

line outages due to lightning induced surges. b. Reduced probability of pole top wood burning and pole degradation resulting in reduced operational

costs and improved safe conditions. (a leakage current problem) c. Reduced probability of damage to attached equipment resulting in an improved quality of supply and

reduced operational costs. This especially applies to the distribution transformers that are lost due to lightning related failures.

3 x 3.35 Galvanized steel BIL downwire

500 mm BIL wood gap

3 x 3.35 Galvanized steel BIL downwire

200 mm of 3 x 3.35 Galvanized steel BIL downwire

LV

Telephone line

Medium Voltage

100 mm

100 mm

Shared Medium Voltage and Low Voltage

50 mm LV gap

750 mm (min)

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In general it can be stated that the number of outages on an MV line due to direct strikes is difficult to lower and as such is approximately fixed for a specific line. Outages due to induced lightning surges, equipment damage and equipment failures can however be mitigated by sensible design and maintenance.

General Insulation co-ordination is achieved when the insulation strengths of all components of the electricity system are adequate to withstand the electrical stresses of service within selected reliability margins. The conditions that system components are designed to meet are: a. To withstand indefinitely the normal and maximum system operating voltages at supply frequency. b. To withstand temporary supply frequency over-voltages up to the rated short duration power

frequency withstand voltage. c. To withstand lightning impulse over-voltages up to the rated withstand level d. To restore the insulation level after a flashover.

Supply frequency voltages and over-voltages The design, material and specific creepage length of phase insulators influence the performance of a system in terms of (a) and (b) in the paragraph above. SCSSCABI8 is used as the guide for the selection of phase insulators.

Lightning impulse over-voltages CSIR and TSI studies have concluded that the effect of lightning on the performance of MV overhead power lines can be minimised if the 300kV BIL insulation co-ordination philosophy is adopted. The disturbances due to lightning can be due to either direct strikes or induced voltages from adjacent strikes.

Direct strikes Direct strikes to an unshielded line nearly always cause flashover to earth of one or more conductors at the pole closest to the strike. If insulation levels are low, flashover may occur at several structures while if the insulation levels are high, flashover may occur at only one structure. The damage to a pole will depend on the surge current and arc path followed. The greater number of poles flashing over and the containment of the arc to the pole surface will result in the lowest probability of damage to poles. Severe impulse voltage may be transmitted to attached equipment depending on the line insulation value. The amount of energy of the impulse may exceed the capability of the surge arrestor at the attached equipment point and hence result in damage to the attached equipment. The probability of damage to attached equipment (transformers, autoreclosers, etc.) is high for a direct strike to the line pole where the equipment is attached. The higher the insulation level of the line the greater will be the probability of damage to the attached equipment and line poles.

Induced voltages Induced voltages rarely exceed 200 kV with a maximum order of 250 kV. As the induced voltages in different phases are of similar amplitude and identical waveshape, flashovers between phases are not expected. Flashover to ground will occur if the insulation strength to ground is below that of the induced voltage. The amount of flashovers due to induced voltages will depend on the actual insulation values. The lower the insulation value, below 300kV, the higher the number of flashovers due to induced voltages.

Pole and cross-arm damage The type and extent of damage that occurs depends on factors such as the moisture content and the arc penetration into the wood. Wood poles and cross-arms suffer the least damage when the arc can be restricted to the surface of the wood where superficial splintering may occur. This is achieved by not having a wood path gap in the earth wire of greater than 500mm and applying circumferential strapping at the termination points of the BIL down-wire and bonding-wire on either side of the gap. (the circumferential strapping prevents the arc from penetrating the pole to bolts) Avoid using staples on strapping as this will allow the arc to penetrate the surface of the wood.

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Line hardware damage

Most damage to line hardware is caused by the power arc that develops after flashover (0,85 probability), however surges and flashover may also cause damage. The surge or impulse will cause an initial breakdown of the air and ionisation, which will allow a follow through arc to develop. The arc current, will cause erosion of the line hardware at the arc root. Hardware damage will be reduced by rapid operation of the protection equipment and removal of the supply voltage.

Damage to attached equipment The attached equipment to a power line will be damaged if the surge voltage exceeds the BIL of the equipment. This will be the case for the majority of direct strikes and the higher induced surges. To reduce the equipment damage surge arresters are utilised on the equipment. These surge arresters will clamp the voltage to a value lower than the equipment BIL. Provided the surge energy is less than the arrester capability and the equipment BIL has not deteriorated the arrester will protect the equipment. The energy in a direct strike will normally exceed the energy capability of a surge arrester. Should the strike occur at the pole of the attached equipment the probability of destroying the arrester and damaging attached equipment is high. Should a direct strike be to the line away from the attached equipment there is still a high probability of the energy in the traveling wave exceeding the surge arrester rating should no BIL down wire be applied to the intermediate poles.

Bonding

The practice of electrically connecting all the hardware and insulator dead ends is known as bonding. Small leakage currents (due to pollution contamination on the insulation surface) may cause the burning or degradation of unbonded cross-arms and poles. Bonding the dead ends of phase insulators and stay wires can prevent this. In polluted environments this process is more rapid. The bonding wire coupled with the circumferential strapping will also act as part of the BIL system and reduce the probability of cross-arm and pole splitting. Separate bonding designs are provided for all applicable structures in the Distribution Medium Voltage Standard drawing D-DT-0310.

Stay insulators The insulation level of the conventional porcelain stay insulators is low. Porcelain stay insulators may be considered as adding value to the system only in terms of providing protection to the public against a live stay condition from a broken conductor coming into contact with the stay. The porcelain stay provides very little additional lightning surge insulation. Strain structures are not fitted with BIL downwires since the stay-wire fulfills this purpose. To achieve a higher BIL on a strain structure than the porcelain insulator provides, glass fibre longrod type stay insulators may be used, or stays fitted in such a way that there is a suitable wood path in series. Standard porcelain insulators for new LV structures provide sufficient BIL on strain structures for adequate performance. For MV structures the decision to use the preferred, high BIL glass fibre longrod type stay insulators, is based on high lightning conditions. The porcelain type of stay insulator can be employed on MV lines with low lightning conditions. Stay wire insulators can be found on the buyer guide drawing D-DT-3144. The assembly drawings for the stay wire insulators are D-DT-0341 and D-DT-0341.

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Insulation co-ordination and line performance Typically, the following performance can be expected from lines insulated as follows: High BIL lines. (1 MV to 2 MV) with no bonding Direct strike flashovers, Low incidence of flashover Low protection operations from indirect strikes, High stress on attached equipment with a resultant high probability of equipment failure, Some extended outages and safety hazards due to pole and cross-arm damage caused by leakage currents or lightning initiated currents. Low BIL lines (

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Table B.1 Insulation co-ordination and bonding guide

1 2 3 Lightning activity (strikes/km2/year)

Pollution level Low (2 or less) High ( more than 2) Low/medium

(Few / no pollution related

incidents)

No bonding of insulator required

No BIL downwire required

Use a 1,2 MV BIL

Provide a 500 mm gap between the BIL downwire and lowest/nearest MV insulator

No bonding between insulator hardware is required for horizontal structures. For vertical or staggered vertical structures bonding is required.1

Use a 300 kV BIL.

High/very high

(Frequent pollution related

incidents)

Bond between insulators

No BIL downwire required

Use a 1,2 MV BIL

Provide a 500 mm gap between the BIL down wire and the lowest/nearest MV insulator

Bond between insulators

Use a 300 kV BIL

Note 1 Bonding is not generally required on horizontal structures. This will take advantage of the arc quenching properties of the extra wood on cross-arms. Vertical structure can not capitalize on this advantage due the distances between hardware being too short. The short distances between hardware means that the risk of the wood pole splitting when an impulse jumps between hardware is high. Bonding is therefore required for vertical structures. If one methodology is needed to be standardized throughout a region, bonding of all structures would be recommended.

Table B1 is provided as a guideline to be utilised should a specific area wish to consider not utilising bonding or BIL downwires. It is not to say that should a low pollution condition exist there will be no leakage currents but simply that the probability of damage due to the formation of leakage current is low. The costs of bonding in this type of area may be greater than the cost of pole and cross-arm replacement and the importance of the line. The default condition in all cases is to bond and provide a 500mm gap. This will also only be true for the MV lines, since with the greater consequence of failure and cost of rectification of HV lines bonding should always be utilised.

Line auto-reclose operations

Utilisation of the following formula will give the auto-reclose operations that can be expected from a line due to direct strikes in a specific area. The number of strikes (Ns) to a line may be calculated as follows: Ns = Ng (28 H 0.6 + W) * L * 10-3 strikes per year Ng = ground flash density (km-2 yr-1) L = line length (km) H = average tower height (m) W = line width (m) For exposed MV lines the approximate number of auto reclose operations per annum is given in the following table. The line length is the total line length, including spurs.

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Direct lightning strikes per year

Line length Ng Ground flash density /km2/yr Km 2 4 6 8 10

25 6 11 17 22 2850 11 22 33 45 5675 17 33 50 67 84

100 22 45 67 89 112125 28 56 84 112 139150 33 67 100 134 167175 39 78 117 156 195200 45 89 134 178 223

The ground flash densities for various stations are given in the Distribution Standard and should be used as a guide to determine the expected outage rate in an area. Should a line exhibit outages in excess of the value shown in the table above then an investigation should be undertaken to determine the cause. Factors other than lightning, such as fires or equipment failure will also contribute to the number of outages. Generally should the number of outages greatly exceed the value calculated then BILs will be found to be low on several structures and rectification will need to be undertaken.

Signed Signed COMPILED BY: APPROVED BY: G Stanford A Bekker Plant Technologies Senior Engineer Plant Technologies Manager Distribution Technology Distribution Technology