System Power Losses

An estimate of the level of system power loss occurring at the time of the ACS Peak GB Demand for the years 2006/07 to 2012/13 against the SYS background is given in Table 7.4. The losses shown are those incurred on the system between the power station generating unit and the grid supply points and are made up of:

• 'Variable' (I²R) transmission heating losses in the overhead lines, underground cables and other equipment on our transmission system but excluding grid supply transformers at the GSPs;
• 'Fixed' losses made up of corona losses on outdoor transmission equipment and iron losses in transformers;
• 'Variable' (I²R) heating losses (copper losses) in grid supply transformers at the GSPs; and
• 'Variable' (I²R) heating losses (copper losses) in generator transformers.

It is stressed that the losses shown in Table 7.4 are indicative only. They correspond to the time of ACS Peak Demand and have been evaluated against the 'SYS background'. The 'fixed' losses, like the 'variable' losses, can also vary to a certain extent. Accordingly, the exact losses on the day can vary for a number of reasons including:

• the outturn demand and/or in-merit generation pattern being different resulting in changed power flows and consequential changes to the variable losses which are a function of the square of the power flow (I²R); and
• weather conditions being more or less adverse than forecast. For example if 'heavy rain' or 'wet snow' prevails across Great Britain then the so called 'fixed' losses (e.g. corona) could be some 100MW or more higher.

Total system power losses are shown in line 5 of Table 7.4 and these have also been expressed as a percentage (line 7) of the forecast ACS peak demand stream given in Table 2.1 less station demand and less transmission losses (line 6). The demand forecast given in Table 2.1 reflects the demand seen at the metering points at the power stations and accordingly includes both transmission and distribution system losses. As some metering is on the high voltage side of the generator transformers and some on the low voltage side, generator transformer copper losses are only partially taken into account.

The transmission heating losses (line 1) are a function of the power flow pattern around the system and the reduction in 2010/11 is due to the commissioning of new plant in the south which 'backs off' the north to south flows.

Fixed losses (line 2) are fairly constant over the period. Grid Supply transformer heating losses (line 3) display a modest increase over the period in step with the growth in forecast ACS Peak Demand (line 6). Generator Transformers heating losses (line 4) display a modest decrease over the period.

Less significant perturbations, perhaps not obvious in the results displayed in the table, are caused by a number of factors including: increased transmission capacity (through reinforcement rather than reprofiling) which reduces transmission heating losses; or embedded Large power stations closing, decommissioning or otherwise becoming non-contributory which can increase grid supply transformer heating losses.

Relative to the system power losses reported in the 2005 GB SYS, there is a small decrease in total losses. This is also reflected in the corresponding reduction in transmission heating losses, which are the dominant component of system power losses. This reduction is due, in part, to a relative increase in the number of transmission reinforcements included in the SYS background.