National Environmental Standards for Electricity Transmission Activities: technical (3 of 9)
The technical pages provide background and explanations of:
Transmission operation
Voltage, current and power
Electricity is rather like the flow of water through a hose. In the same way that there is pressure in a hose (even if the water is not flowing), there is an electrical pressure, known as the voltage in a transmission line.
Voltage is measured in volts (V) or kilovolts (kV, thousands of volts).
When electricity flows (like water passing through a hose), there is an electric current. The current is measured in amperes or amps (A).
The power transmitted in a conductor (or wire) is a function of the voltage (the electrical pressure) and the current (the flow). In the transmission of electricity, the voltage of a particular transmission line is held relatively stable but the current varies to meet the change in the electricity demand.
Although the voltage of a particular line is held stable, the National Grid is made up of lines of different voltage. For the same current, a higher-voltage line will transmit more power than a low-voltage line.
The core of the National Grid is the 220kV Alternating Current (AC) network, which is located in both the North and South Island, and the High Voltage Direct Current (HVDC) link between them. These terms are explained below. The 220kV lines connect the largest power stations with the main load centres. Smaller load centres and smaller power stations are connected by transmission lines operating at 110kV, 66kV and 50kV. Transpower also has some local lines operating at 33kV and 11kV.
HVDC and AC
When new transmission lines are proposed another transmission method consideration is whether to use HVDC transmission or AC.
In HVDC the flow of electricity through the transmission line is in one direction.
In AC the direction of current is constantly being reversed back and forth at regular intervals. In New Zealand, the frequency of alternating-current electricity is 50 cycles per second (Hertz, Hz).
In New Zealand (and internationally), most of the National Grid carries AC from the generators to the consumers of electricity. This is because it is easier to change the voltage from the high voltage needed for transmission to the low voltage used in the home.
High-voltage AC transmission requires three conductor sets and this determines the design of the support structures of the transmission line (the towers).
A DC transmission system can transmit more power down the same size conductor. DC requires only two conductors (positive and negative), so a much simpler supporting structure suffices as it supports a lighter load. This reduces the cost of the structures and reduces visual impacts compared with an equivalent AC line. DC lines have lower transmission losses (are more efficient) than AC lines. The electrical efficiency and structural differences add up to huge cost savings for a long point-to-point transmission system.
However, it is AC that generators produce and consumers need; it is expensive to convert AC power to DC power and back again. The environmental benefits and savings in power losses and construction costs need to be balanced against the cost of the conversion technology.
View more information on voltage and current of the transmission network.
Outages
Transmission lines are occasionally taken out of service, or de-energised, for example to allow maintenance of a transmission line or power station. The period when a line is taken out of service is referred to as an outage. Outages are carefully planned to avoid loss of supply to electricity users. For example, they may be timed to coincide with periods of low power use, when alternative routes for transmitting power have sufficient capacity to carry the increased power during the outage. Transmission lines are de-energised and the route of electricity transmission diverted at a switching or sub-station.
Spans and conductor sag
There is a limit to the amount of power that can be sent over a transmission line. The limiting factor for many lines is the heating of the conductors. The higher the current, the higher the resistance to the current flowing in the line and the greater the heating. Heating of the conductor causes it to expand, which increases its length and sag. The position of a conductor above the ground will rise and fall depending on the current as well as on climatic conditions such as air temperature and wind speed.
If too much current is allowed to flow, the conductors may sag too close to the ground. This determines the 'thermal limit'. The Electrical Code of Practice NZECP:34 requires Transpower to maintain a minimum safe clearance from the overhead transmission line conductor to the ground, or structures underneath the line, in worst case conditions of high temperatures and high load. This is a mandatory legal requirement.
Last updated: 18 January 2010
On this 'technical' page: 