Kenda equipment boosts accuracy of National Grid's metering
data
The National Grid Company (NGC) has significantly improved
the accuracy of its real-time
metering data by installing operational
metering summator (OMS) units from Kenda
Electronic Systems at each of the 62 centrally despatched
power stations in England and Wales.
Privatisation of the electricity supply industry generated
a need for more accurate metering of electricity generation and
transmission and especially the need to reduce any discrepancy
between the industry's settlement metering and NGC's operational
metering.
Since even small discrepancies can have significant commercial
implications, a system was required which would replace the transducer
based metering inherited from the CEGB and use the same metering
source as the settlements system.
Following a competitive tender, Southampton-based Kenda Electronic
Systems was awarded the contract to supply the new OMS equipment,together
with all the associated cabling and software.
Consisting of two identical hardware units, the OMS is designed
to pick up pulses from the same source as the Settlements metering
and convert them into analogue outputs for operational metering
purposes. Usually, the front end (FE) unit is installed
close to the power station's metering cubicles, with the processing
end (PE) up to 2km away in the NGC substation.
Although the FE unit can accept up to 32 meter pulse inputs,
the two units are actually designed to communicate over just
two twisted pairs via an RS422 data link. This arrangement
has eliminated the need for additional cross-site cabling since
spare capacity on existing cables has been readily available.
The PE unit decodes the information from the FE, performs
various types of summation and provides up to 16 analogue outputs,
eight pulse outputs and two serial data outputs. Values
of parameters such as active power, reactive power, net station
output and auxiliary load are then transmitted via a SCADA system
back to NGC's Wokingham Control Centre.
"Since installing the new OMS equipment, our operational
metering has remained within fractions of a percent of the Settlements
system," commented Tim Truscott, a Systems Integration Engineer
with NGC's Control Technology Centre. "The only discrepancies
now are unavoidable ones due to the tolerances within the systems."
Spurred on by this success, NGC has also installed the Kenda
OMS equipment at 45 grid supply points around the country to
enable it to obtain accurate real-time values of the power being
drawn from the network by the RECs.
Truscott again: "Having proved the advantages of accurately
measuring the input to the Grid system from generators, we then
extended the programme to enable us to accurately measure the
output from the Grid through transformers to the 132kV distribution
system."
When compared with the power going into the network from the
various generating companies, this information enables NGC to
produce tighter computerised modelling of the system, thereby
helping it to reduce operating costs.
Kenda has continued to develop the system and can now offer
a single-unit OMS for installations that do not require a two-unit
configuration.
Ref. KE78/2 April 1996
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