Guidance on National Environmental Standards for Electricity Transmission Activities

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National Environmental Standards for Electricity Transmission Activities: technical (8 of 9)

Work on conductors

What is a circuit?

An electrical circuit transmits electricity from one geographical location to another. Transpower’s alternating current (AC) transmission system uses three-phase circuits.

Reconductoring and adding conductors

'Reconductoring' is when the existing conductor is replaced with a new conductor. Replacement may be required for maintenance reasons (ie, the conductor has reached the end of its life) or to increase the carrying capacity of the circuit (ie, by installing a conductor capable of carrying more current).

Read a description of the figure on the right

Right: some of the different types and sizes of conductor that Transpower currently uses.

Conductors come in a range of types and diameters. The diameter of the replacement conductor may be bigger or smaller than what presently exists. This will depend on the existing conductor type (eg, older copper conductor is normally replaced with modern aluminium or aluminium/steel conductor which requires a larger diameter to achieve the same rating), and also on forecast load flows.

Conductors could be added as part of an additional circuit (see 'Adding circuits' below), or part of adding conductors to an existing circuit (eg, for 'duplexing' an existing simplex circuit).

Adding conductors to an existing circuit enables more current to be carried down that circuit, and thus more power to be delivered.

A 'simplex' transmission circuit is a circuit with only one conductor per phase (there are three phases in a circuit).  For example, a single circuit with simplex conductor would have a total of three conductors (wires) on the transmission line. A double circuit with simplex conductor would have a total of six conductors (three conductors per circuit).

Duplex means that there are two conductors per phase, so a single-circuit transmission line with duplex conductors would have a total of six conductors on the line, but all six conductors would be associated with one circuit.

Read a description of these figures

Simplex (left) and duplex conductors (right, double lines) in farmland.

Adding circuits

Some of Transpower's single-circuit transmission lines carry one circuit, although the transmission structures were designed and built to carry a second circuit. In these cases, the second circuit can be added (‘strung’) to the existing support structures without significant extra work.

The photos show a transmission line of this type before and after a second circuit is added.

Read a description of these figures

The pair of pictures shows hurdles to protect an under-crossing line, before and after the addition of a second circuit to the transmission line.

Implosive jointing

Implosive jointing is a procedure to join two ends of a conductor.  An explosive cordite sleeve is placed over the ends of two separate lengths of conductor and the charge is detonated.  The resultant implosion compresses the sleeve onto each end of the conductor and forms a permanent connection between them.  The activity is a temporary construction activity associated with works on a transmission line.

Noise levels from this activity are assessed using NZS6803:1999 'Acoustics – Construction Noise'. This standard sets out procedures for the assessment of construction works and recommends noise limits. Noise is measured in units of decibel, A-filtered (dB_A).

Transpower has previously undertaken some noise assessment for implosive jointing. Noise levels measured 112dB_A at 151 metres from the implosion at one location and 118dB_A at 110 metres from the implosion at another location. The ground topography between a receiver and the implosion will have a significant effect on the actual noise level at the receiver.

Undergrounding

Overhead lines and underground cables

Underground cables have lower visual effects than overhead lines.  However, there are significant cost and reliability penalties with underground cable, as identified below. This means that this option is rarely preferred.

Electricity flowing through transmission lines produces heat.  With overhead lines this heat is dispersed into the air.  Underground cables must be insulated electrically which makes their manufacture expensive.  The earth doesn't cool cables as well as the air cools overhead lines, so larger insulated cables are needed underground.  With high currents it can also be necessary to cool the cables, which can be done in a number of ways.

Work needed to put overhead lines underground

The greater diameter of underground cables means that they are heavier and stiffer than overhead conductors. This makes cables more difficult to transport, and more difficult to bend around corners. These characteristics limit the continuous length of the cable that can be installed at one time. Such sections of the cable must be joined every few hundred metres.

Underground cables typically involve more earthworks than required for an overhead line. A trench needs to be dug along the whole route, whereas overhead lines typically only need earthworks at the towers. Where the overhead and underground sections meet, a transition station (termination structure) is needed.

A cable route must be kept permanently clear of buildings and other structures that would prevent vehicle access. Any trees should be several metres away from the cables and be shallow-rooted to minimise any adverse effects on the cable operation. This is not necessarily the case with overhead lines where, for example, an overhead line crosses a valley.

Termination structures

The regulations define 'termination structures' as "a tower or pole used for the transition between an overhead and an underground transmission line". Engineers generally refer to these as 'transition structures'.

Security and maintenance

Underground cables are more complex than overhead lines. They can be difficult, time-consuming and costly to repair. Cable faults require excavations and repairs may take as long as three weeks to complete. This makes underground cables less reliable than overhead lines.

Cost

The Grid Investment Test is used by the Electricity Commission to assess Transpower's investment proposals favours a least-cost solution. The installed cost of a length of 220kV underground cable is between five and fifteen times more than the same length of 220kV overhead line. The terrain to be crossed is a big factor in this variation. Costs are much more affected by obstacles like gullies or rivers, which overhead lines can easily span.

Temporary and permanent deviation

Temporary deviations may be used if extensive maintenance or repair work is required on a line, for example by a vehicular impact on a tower location or a flood undermining a tower. The deviation may occur while repairs are being made before the original alignment can be reinstated.

Read a description of these figures

After the tower has been washed away (see debris in the river, left), a temporary structures is being erected (right) to keep the duration of the fault to a minimum.

Permanent deviations are usually motivated by third parties, for example a new state highway alignment requiring an existing transmission line to be shifted. Such deviations would be paid for by the third-party developer. The opportunity for route selection and optimisation are often limited, due to the usually short lengths of transmission line involved; or the short duration of the change in alignment in the case of temporary deviations.

Adding earthwires and earth peaks

Many transmission lines are equipped with overhead earthwires. These are designed to provide shielding from lightning. For some lines, the overhead earthwire runs the full length of the line. In other cases, it is only installed for the first kilometre or so from the substation, to provide some protection for the substation equipment from lightning surges on the transmission line. 

An earthwire usually consists of one or two conductors and is added to the top of the transmission tower. An earthwire may also contain fibre cable for communication purposes around the network.

Read a description of these figures

The pair of pictures shows a tower before (left) after addition (right) of an earth peak and an earth wire.

 

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Last updated: 18 January 2010