Archive for the ‘embedded generator’ Category

Short Term Infrequent Paralleling Issues

Friday, April 9th, 2010

Short term infrequent paralleling is usually applied to standby power supplies that are designed to support a customer’s islanded network and literature review for dissertation allows for seamless transfer following restoration of supply or for load transfer during routine testing.

Acceptable number of, and duration of paralleling time.
G59/1 allows for the infrequent connection of generation with, (at the DNO’S discretion,) a relaxation in protection requirements. No hard and fast definitions are given for “infrequent” but a figure of once per month is suggested. The number of parallel operations should be at the discretion of the individual DNO based on the capability of the network. However if it is required to test more than once a week then this would no longer be considered as infrequent paralleling.

Once operating in parallel, the generator is allowed to remain in parallel for a maximum of 5 minutes. The duration of the parallel should be kept as short as is reasonably practical for change over to take place. Less than 1 minute is normally achievable with longer times only being required if the equipment is incapable of shorter change over. After this time the parallel has to be broken automatically by a timer. This timer should be a separate device from the changeover control system such that failure of the auto changeover system will not prevent the parallel being broken.

Summary: In order for the generator to be considered as operating as “short term infrequent paralleling”, it must not be allowed to connect in parallel for more than 5 minutes in any month, or more frequently than once per week. If the duration of parallel connection exceeds this period, or this frequency, then the generator must be considered as if it is, or can be, permanently connected.


Protection Requirements

Short term occasional parallel operation requires only basic under /over voltage and under /over frequency protection.

A timer separate from the normal changeover control system should be provided to break the parallel if the normal means of changeover should fail.

This protection only needs to be in operation for the time the generator is operating in parallel.

Loss of mains protection in the form of ROCOF and Vector shift are not required, although many G59 multifunction relays now have this function built in as standard.

Similarly additional requirements such as NVD, intertripping and reverse power are not required.

This is based on the assumption that during the year the generator is only likely to be in parallel for a maximum of 1/8760th of the time and therefore the chance of a genuine loss of mains event coinciding with the parallel is unlikely.

If a coincidence did occur, the possibility of the generation supporting the Island also becomes a factor. Under voltage / under frequency is likely to trip the generation off if the load is greater than the generation capacity. Consideration could be given here to applying different settings for short term parallel connection. As this generation will not be expected to give grid Support or contribute to P2/6 security, more sensitive settings e.g. 49.5 Hz -6%V would compensate for lack of LOM protection?

Ultimately if an island was established the situation would only persist for the duration of the parallel operation timer setting before generation was tripped.(Auto Reclose excepted)

Generator Star point Earthing
For HV connected generation ETR113 Fig 5.6 shows that for short term parallel the star point of the generator should remain connected to earth.
It is recommended that for LV generation operating in short term parallel the same should apply for the following reasons.

• Having switches in the generator earthing circuits, that for the majority of the time will need to remain closed creates an unnecessary complication / risk of failure of leaving an unearthed system.

• Multiple earths on the system could result in circulating third harmonic current around the neutral earth path. This could result in heating of the cables, however as this is a thermal rating issue with a relatively long time constant the short period of parallel operation is unlikely to result in any serious overheating.


Very Short Term Parallel

Some manufacturers are now installing their standby machines with Fast acting Automatic Transfer Switches. These are devices that only make a parallel connection for a very short period of time, typically 100 – 200mS. Under these conditions installing a conventional G59 relay with an operating time of 500mS is not appropriate when the parallel will normally be broken before it has a chance to operate. There is however the risk that the device will fail to operate correctly. Therefore a backup timer should be installed to operate a conventional CB if the parallel remains on for more than 1 Sec.

Contribution to fault level
For short term infrequent parallel there is the need to consider the effect of the generation contribution to fault level. If any problems are identified, then the process for controlling this risk will need to be agreed with the Network Operator.

Voltage Rise / Step Change
Networks should be designed such that the connection of a generator under normal operating conditions does not produce voltage rise in excess of the statutory limits. In general this should not be an issue with most short term parallel operation as at the time of synchronising with the mains most sites will normally be generating only sufficient output to match the site load. Therefore the power transfer on synchronising should be small, with the generator ramping down to transfer site load to the mains. If the generator tripped at this point it could introduce a larger voltage step change than would normally be acceptable for loss of a long term parallel generator but in this event it could be regarded as an infrequent event and a step change of up to 10% would be acceptable*
(* assuming this is the figure the DNO agree to).

Breaker out of phase switching capability
For a new metering point the metering circuit breakers should have an out of phase switching capability.

For an existing installation that does not require replacement of the metering breaker for any other reason, then for short term parallel the risks of out of phase opening are low. Therefore replacement of the metering circuit breaker should not be necessary. It is assumed however that the generator synchronising breaker would have out of phase capability.

TECHNICAL – Glossary & terms used for electrical supplies

Wednesday, May 20th, 2009

GLOSSARY
active power – the multiple of the components of alternating current and voltage that equate to true power. Normally measured in kilowatts (kW) or megawatts (MW).
adoption agreement – an agreement between a developer and a DNO, concerning the transfer into DNO ownership of infrastructure supplied and installed by a third party.
approved contractor – a contractor which has been approved by the DNO for carrying out third party connection work.
Balancing and Settlement Code (BSC) – the code which determines the rules governing the Balancing Mechanism and the settlement process for electricity trading in England and Wales as from time to time amended.
capacity factor – a factor, generally applied to renewable energy schemes, which relates the maximum continuous power output of the generator to the expected long run average power output.
committed network – future network configuration for which financial approval has been given.
condition 4 statement – document published by a DNO outlining the basis of charges for connection to the DNO’s distribution system.
connection agreement – an agreement setting out terms relating to a connection with the DNO Distribution System (excluding any bilateral agreement with the transmission licensee).
Connection and Use of System Code (CUSC) – contractual framework for connection to and use of the NGT transmission system.
connection voltage – voltage level at which a site is connected to the transmission or distribution system.
contestable – that part of the connection works which is open to competition.
CDM Regulations – the Construction (Design and Management) Regulations 1994.
Regulations specifying the duties of designers to minimise health and safety hazards involved in the construction of buildings and other installations.
CD&M Regulations – see CDM Regulations.
Declared Net Capacity (DNC) – declared net capacity: the maximum power available for export on a continuous basis minus any power imported by the station from the network to run its own plant.
determination (of disputes) – any dispute arising under certain sections of the Electricity Act 1989 between a DNO and a person requiring a supply of electricity, can be referred to Ofgem for determination. These determinations are then published as a matter of public record, and then form ‘case law’ on the subject.
distributed generator – a generator which is connected to a DNO’s distribution network rather than to the transmission grid. Distributed generation is generally a lot smaller than plant connected to the transmission grid as the maximum operating voltage of the distribution network is 132kV (and 33kV in Scotland).
Distribution Network Operator (DNO) – a holder of a Distribution Licence.
electronic inverter system – an electronic device placed between a generator and the network it is connected to for the conversion of power at one frequency to another (including dc/ac). The output voltage and frequency may be determined by the control equipment associated with the inverter or by the voltage and frequency of the
network it is connected to.
embedded generator – now generally termed distributed generator (see above), although this term is still used in the Distribution Code of Great Britain and the Grid Codes.
extension – It is sometimes necessary to extend the DNO’s distribution network in order to provide a connection for a new user or generator of electricity. Network extensions are often required for generation schemes in remote locations.
fault contribution – the contribution of an electrical source, such as a distributed generator, to the total fault levels in a distribution network.
high voltage (HV) – any voltage exceeding Low Voltage (ie exceeding 1000volts between phase conductors or exceeding 600volts between phase conductors and earth).
induction generator – a type of rotating electrical generator which operates at a speed not directly related to system frequency. The machine is generally excited by reactive power drawn from the network to which it is connected and the output voltage and frequency are determined by those of the network to which it is
connected.
islanding – islands of supply are discrete parts of a distribution system which are capable of generating and maintaining a stable supply of electricity to the customers within those discrete parts without any connections to the rest of the system.
line drop compensation – a voltage control scheme (used for the control of voltage levels in distribution networks) which compensates for the change in voltage drop in a long line as the current in the line changes.
loss of mains – the loss of an electrical connection between a section of a distribution network and the main grid supply, often due to the operation of circuit breakers.
low voltage (LV) – in relation to alternating current, a voltage exceeding 50 volts measured between phase conductors (or between phase conductors and earth), but not exceeding 1000volts measured between phase conductors (or 600volts if measured between phase conductors and earth).
Long Term Development Statement (LTDS) – (sometimes referred to as the LC25 statement). Statement prepared annually by each DNO as required by Standard Condition 25 of the Electricity Distribution Licence.
mains paralleling – the operation of an electrical generator while connected in parallel with the main grid supply.
negative reactance compounding – a voltage control scheme (used for the control of voltage levels in distribution networks) which allows the voltage–regulated system to be fed from two or more transformers in parallel.
Ofgem – the Office of Electricity & Gas Markets (under the Gas and Electricity Markets Authority, established by the Utilities Act 2000).
point of common coupling – the point in the distribution network where the lines or cables which are used solely to provide the supply to one customer (eg a generation scheme) are connected to infrastructure which is also used to provide supplies to other customers.
primary – generic term used by a DNO to indicate the source of the main 11kV or 6.6kV HV distribution network; eg primary substation – 33/11kV or 66/11kV transformation substation infeed to the 11kV network; 11kV primary busbar – source 11kV busbar for an 11kV network.
protection system – the provisions for detecting abnormal conditions in a system and initiating fault clearance or actuating signals or indications.
reactive power – the product of voltage and current and the sine of the phase angle between them which is normally measured in kilovar (kVAr) or megavar (MVAr).
reactor – wound network component generally used to limit reactive power flows and
hence fault levels.
reinforcement – Reinforcement work is usually required to increase the electrical capacity of those parts of the network which are affected by the introduction of new generation or demand. Other work might include upgrading the switchgear at a substation some distance from the proposed generation scheme, due to the increase in fault level caused by the connection of the generator.
Small Scale Embedded Generator (SSEG) – a source of electrical energy and all associated interface equipment, rated up to and including 16A per phase, single or multi phase 230/400V ac and designed to operate in parallel with a public low voltage distribution network.
Static Var Compensator (SVC) – equipment for injecting or absorbing reactive power (Vars) at the point of connection to assist in control of system voltage.
supplier – a person or company providing a supply of electricity. This could be the
local DNO, a second tier supplier or an exempt supplier.
synchronous generator – a type of rotating electrical generator which operates without slip and at a speed that is directly related to system frequency.
thermal rating – the current-carrying capacity of a cable, an overhead line or any other item of electrical infrastructure, which is determined by the heating effect arising from electrical losses.
third party connection – connection provided by a contractor other than the local DNO.
Use of System (UoS) – the use of a transmission or distribution network by a generator, a supplier, a customer or an interconnected party for the purposes of transporting electricity.