Calculating and Compensating for Power Transformer and Cable (or Line) Losses – Standard Methods
the open circuit test; and
the short circuit test.
rated apparent power of the power transformer ( );
rated primary voltage of the power transformer ( );
rated secondary voltage ( );
no-load (iron) watt losses ( ) – the Active Power consumed by the transformer’s core at rated voltage with no load current (during the open circuit test)
full load (copper) watt losses ( ) - the Active Power consumed by the transformer’s windings at full load current for rated VA (during the short circuit test);
% excitation current - the ratio of the no-load test current (at rated voltage) to the full load test current; and
% impedance - the ratio of the full load test voltage (at rated current) to the rated voltage.
the (series) resistance of the cable or line;
the (series) inductive reactance of the cable or line;
the (shunt or parallel) capacitive reactance of the cable or line; and
the (shunt or parallel) leakage resistance17 of the cable or line.
resistance (r) per unit length ( per conductor (at a given temperature)
reactance (x) per unit length ( per conductor
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7 Section X Annex X-1 define a Boundary Point as a point at which any Plant or Apparatus not forming part of the Total System is connected to the Total System.
8 Section X Annex X-1 defines the Total System as the Transmission System, each Offshore Transmission System User Assets and each Distribution System.
9 Section X Annex X-1 defines System as the Transmission System, a Distribution System or Offshore Transmission System User Assets.
10 Section X Annex X-1 defines Systems Connection Point as a point of connection (whether consisting of one or more circuits) between two or more Systems excluding: (a) a point of connection between Distribution Systems in the same GSP Group; and (b) a point of connection between Offshore Transmission System User Assets and the Transmission System.
11 ‘Code of Practice for the metering of circuits with a rated capacity exceeding 100MVA for Settlement purposes’
The test voltage can be applied to either the higher voltage winding or the lower winding to do this test
12 The test voltage can be applied to either the higher voltage winding or the lower winding to do this test.
13 Either the higher voltage winding or the lower voltage winding can be shorted to do this test and a small voltage applied to the other winding.
14 The Meter may also need to be compensated to account for any measurement transformer ratio and phase angle errors.
15 Special care must be taken when calculating magnetic core losses if the voltage level on the primary winding changes, for example, due to a tap change. A tap change will alter the maximum flux level in the power transformer iron core and this will alter the eddy current and hysteresis losses. If the tap position doesn’t normally change the nominal tap can be assumed.
16 i.e. the current flowing in the conductors which make up the primary winding of the current transformer and voltage of the conductors connected to the primary winding of the voltage transformer.
17 This refers to the dielectric losses in insulated cables and on higher voltage lines the losses due to current leaking into the surrounding air (coronal discharge). Dielectrics also suffer from hysteresis losses.