Glossary terms beginning with O

Terms used within Transpower
Click one of the letters above to go to the page of all terms beginning with that letter.
obsolete item

Items becoming non-standard and/or superseded by modern equivalent. Normally such items can be allowed to reach replacement criteria before being replaced with the modern equivalent.


A circuit-breaker in which the contacts open and close in insulating oil.


Outdoor buswork and associated supports of a station usually air-insulated and enclosed within a climb-proof fence.

ODV Handbook

Handbook for Optimised Deprival Valuation of System Fixed Assets of Electricity LIne Business (4th edition October 2000, Ministry of Economic Development).


The state of a transformer which is energised but not supplying load, that is the HV or LV circuit-breaker is open.

off-normals list

Computer-generated summary of abnormal plant conditions, including control outputs that have been disabled.

off-voltage tap changer (transformers)

A device fitted to a power transformer whereby the voltage ratio between the windings can be varied only while the unit is off load.


Out of Ground Effect is a performance figure that gives the maximum weight at which a helicopter can hover one rotor diameter or more above ground level. It is used to indicate the degraded performance of a helicopter in a free air hover compared to hovering just above ground level. OGE power would satisfy the requirements for a slow speed or hover, at transmission structure or conductor height.


Overhead earth wire.

oil circuit-breaker

A circuit-breaker in which the contacts open and close in insulating oil.

oil condition tests

A series of oil tests and analysis required to determine the condition of the insulating oil or as specified in the maintenance standards.

oil leakage sump

A receptacle which is intended to receive the oil of a transformer or other oil-filled equipment in case of leakage.

oil pass

The quantity of oil indicated on the transformer or tap-changer nameplate which is circulated through the oil treatment plant.

oil reclaiming

An oil treatment process which removes soluble and insoluble contaminants. Heated oil is vacuum dehydrated then percolated through Fuller’s earth and filtered. Additionally the vacuum process will degas the oil.

oil reconditioning

An oil treatment process where water and solids are removed by filtration, centrifuging and vacuum dehydration. Additionally the vacuum process will degas the oil and remove some of the more volatile acids.

oil regeneration

A proprietary process which uses re-fining and blending techniques to produce insulating oil with essentially new oil characteristics.

oil ring lubricated bearing

A bearing in which a ring encircling the journal and rotated by it, raises oil to lubricate the bearing from a reservoir into which the ring dips.

oil spill

A term used in this standard for minor and major oil spills, oil leaks and oily effluent discharges.

oil spill containment

A method of oil spill management which prevents oil spills from entering adjacent properties and waterways.

oil spray nozzle

A device that sprays oil into a transformer to degasify the oil while under vacuum and to uniformly distribute the hot oil over the core and windings.

oil treatment

A term used in this standard to include the processes of reconditioning oil, reclaiming oil and the de-sludging of transformers.

oil water valve

A valve that automatically closes when an effluent with a quantity of oil flows through it. The oil may be sensed by a change in the specific gravity of the effluent or by an oil-in-water detector.

oil-immersed type transformer

A transformer of which the magnetic circuit and windings are immersed in oil. [IEC 76-1].

oil-jacked bearing

A bearing in which high pressure oil is forced between the running and stationary surfaces to establish a lubricating film.

oily effluent

A mixture of oil and water.

oily tighten

Tightening of oiled windings and major insulation after dry-out and oil impregnation.

on compensate

An alternator which has had its prime-mover de-watered, or de-clutched (or in the case of a thermal turbine, has had the main steam or fuel shut off) and is running synchronised on the power system for the purpose of reactive power compensation as well as the provision of spinning reserve.

on load

In service and carrying current.

on-load tap-changer

Equipment fitted to a power transformer by which the voltage ratio between the windings can be varied while the transformer is on load.

open (verb)

Electrical: to operate a switch, circuit-breaker, disconnector etc, so as to interrupt or prevent the flow of an electric current. Mechanical: to operate a gate or valve so as to cause or permit the passage of a substance.

open end voltage

The voltage appearing at the disconnected end of an energised transmission circuit.

open line test (HVDC)

An open line test enables the operator to apply a direct voltage to an open d.c. line. This is used for energization and testing purposes. The operator can increase or decrease a voltage reference. The firing angle is then directly controlled to maintain this reference. [ABB TR-LK-89 022].


Analogue tone at the trip or acceleration frequency (eg Trip A, Trip B).

operating certificate

A formal document issued by an employer testifying that the named employee is competent to carry out specified operating duties at the stated location(s).

operating competency

Demonstrating the necessary ability and knowledge to perform operating functions effectively.

operating contractor

A company which is under contract to carry out operating work on Transpower equipment.

operating form

A form used for listing an operating sequence or a single action. Note: This can be TP 564, the front side of TP 565 or similar.

operating information /instructions

Information and instructions provided for the proper operation of equipment. Note: This includes Transpower’s operating standards, station operating information and the documents and forms of Transpower and other asset owners.

operating sequence

A plan sequence of operating actions (or a single action) compiled on a designated form.

operating services contract

The contract between Transpower and an operating contractor for the provision of operating services at a specified Transpower station or group of stations.

operating standard

A document specifying requirements to be followed when operating power system equipment.

operation (of the grid system)

The real time co-ordination of the conveyance of electricity accross the grid system, and operate the grid system is to be interpreted accordingly.

operation (of the grid)

The exercising of authority, whether direct or delegated, to control the grid. [GOSP glossary]

operational and maintenance acceptance certificate

The certificate signed by the Transpower inspector once connected equipment is accepted by Transpower.

operational control

The exercising of authority, whether direct or delegated, to control equipment.

operational control (of equipment)

The exercising of authority, whether direct or delegated, to control equipment required for the operation of the grid. [GOSP glossary]

operational request

A request to a service centre for the release of, or changes to the mode of operation or control of, grid or communications equipment. See also release request.

operational state(s)

In service: the state of equipment which is connected to a source of energy or may be connected to a source of energy by an operating action. Removed from service: equipment isolated to the extent that it ceases to perform its designated function.

operator (function)

An employee who provides operating support to a controller.

operator / controller

An employee deemed competent by their employer in terms of the asset owner’s standards to operate at a specific station, or at specific stations, or in a specific area or specific equipment; authorised or appointed by the owner to operate or have operational control at that station(s) or in that area or of that specific equipment. Note:The employee may not necessarily be occupationally designated as an operator or a controller.  Depending on which rule is being considered, and on how an asset owner organises operating functions, an operator/controller may be: an employee undertaking operating actions on site, eg opening disconnectors, applying portable earths; or an employee in a control center at the site or at a remote site who oversees a station or area. Operating actions by such an employee can include putting equipment into or removing it from service and preparing authorisation’s; or an employee who undertakes duties in both the above categories. [SR-EI April 1995].


A metallic conductor used as an earth wire for a high voltage transmission line and containing as an integral part of its internal construction, optical fibres used for communications purposes.

optical cable,

A non-metallic cable used to carry optical fibres. Examples used on lines are ADSS and fibre wrap cable.

optical conductor

Means OPGW or OPPC.

optical fibre ground wire

A metallic conductor used as an earth wire for a high voltage transmission line and containing as an integral part of its internal construction, optical fibres used for communications purposes.


In an optimisation process, surplus assets (overbuild) and excess capacity (over design) are identified and removed in terms of cost/value.  This is a two step process requiring an analysis of the transmission system as a whole and then the assets making up that system.

Optimised Deprival Valuation

ODV is a valuation methodology, the aim of which is to value assets at the level at which they can be commercially sustained in the long term, and no more. They resulting value should be equal to the loss to the owner if they were deprived of the assets and then took action to minimised their loss.


Optical repeater site.


The current master copy of the drawing.  This may be a transparency or another form of drawing storage medium adopted by Transpower.


A form of hunting in which the fluctuations are periodic variations in the speed of a synchronous machine above and belo the normal speed, resulting in periodic phase variations relative to the rest of hte power system.


Occupational Safety & Health Service of the Department of Labour.


Optical Time Domain Reflectometry.


The state of an item of equipment when it is not available to perform its intended function. An outage may or may not cause an interruption of supply to customers.

outage co-ordination

The actions of the grid operator undertaken, in accordance with the Outage Co-ordination Code, to co-ordinate planned outages of equipment forming part of the grid. Note: The equipment may consist of, or include, secondary equipment or sub-systems required for control, protection, communications, etc. [GOSP glossary].

outage duration (or outage time)

The period from the initiation of an outage until the affected item or its replacement once again becomes available to perform its intended function (that is, available for service). Includes the time required for operating plus work time.

outage plan

An overall programme for windows of power system equipment. The Outage Plan is a living document and under continual review.

outage time

The time during which power system equipment is removed from service. It includes switching time and work time.

outdoor circuit-breaker

A circuit-breaker suitable for installation outdoors.

outdoor structure

Outdoor buswork and associated supports of a station usually air-insulated and enclosed within a climb-proof fence.

output profile

A plan for one day of active and reactive mean power for one or more generating sets prepared by a producer in accordance with section 11 of the Dispatch Code. [GOSP glossary].

outstanding access permit

An access permit which has not been returned. That is, any access permit under which the holder is still authorised to work on specified equipment which is under the authorisation holder's control.

over frequency limit

The frequency, above the normal frequency range, at which producers are required to take independent remedial action to minimise damage to equipment. The limit is quantified in Table 1 of the Grid Emergencies Code. [GOSP glossary].

over frequency reserve

The input to an HVAC power system which can be interrupted to counter a contingent event or extended contingent event, the response being fast enough to limit the rise in frequency to within the limits set out in section 2 of the Power Quality Code, or assist in the return of frequency to within the limits set out in section 2 of the Power Quality Code. [GOSP glossary].

over-frequency arming

A facility fitted to some generators in the South Island to trip them in the event of the loss of the HVDC link during northwards transmission. This is to prevent an over-frequency condition occurring in the South Island. [See also over frequency reserve.] [Transpower website glossary].

overhead earthwire (Aerial Earth Conductor or Ground Wire)

Means an aerial conductor which is grounded or earthed at multiple points.

overhead line

Means a transmission line.

overhead service line

Means an overhead line operating at a voltage less than 1 kV owned by an electricity supply authority, and located, or to be located between the electricity supply authority's overhead line and the point of connection to an electrical installation.


A load greater than the maximum continuous rating.