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