As a general principle, it would be desirable to determine the efficiency of an HVDC converter station by the direct measurement of its energy losses. However, there are practical difficulties that prevent such a measurement, including the following:

1) Attempts to determine the station losses by subtracting the measured output power from the measured input power must recognize that such measurements have inherent inaccuracy, especially if performed at high voltage dc.

Moreover, the losses of an HVDC converter station at full load are generally less than 1% of the transmitted power. Therefore, the difference between the measured input and output power is a small difference between two large quantities and, as such, is not likely to be a sufficiently accurate indication of the actual station losses.

2) In some special circumstances, it may be possible to arrange a temporary test connection in which the two converters are operated from the same ac source and are also connected together via their dc terminals. In this connection, the ac source only provides enough power to make up the losses in the circuit.

However, it must also provide var support and commutating voltage for the two converters. Here again, there are practical measurement difficulties.

Because of the problems described above, it is recommended that the total station losses be calculated from the losses of the individual equipment. The equipment losses should be determined according to Section 4.

Summation of Equipment Losses
The total losses of the HVDC converter station should be arrived at as the sum of the losses of each piece of equipment. It is important to note that the actual losses in each piece of equipment will depend on the ambient conditions under which it operates, as well as on the operating conditions or duty cycles to which it is applied.

Therefore, in order for the summation of the individual losses to be a sufficiently accurate representation of the actual total HVDC converter station losses, the ambient and operating conditions for each piece of equipment must be defined, based on the ambient and operating conditions of the entire HVDC converter station.

For some equipment or components, losses or electrical characteristics are measured at the factory under standardized ambient and operating conditions. In these cases, the results should be related to the actual conditions in the HVDC converter station through well-recognized calculation procedures.

The fundamental principle of this recommended practice is that the determination of the equipment losses is, in fact,
based on physical tests on the actual equipment or components. Therefore, the sum of the equipment losses does give
a dependable measure of the losses for the entire HVDC converter station.

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