ABB 615 series Technical Manual page 426

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Protection functions

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Typical radial compensated network employed with wattmetric protection

The wattmetric function is activated when the residual active power component exceeds

the set limit. However, to ensure a selective trip, it is also required that the residual current

and residual voltage also exceed the set limit.

It is highly recommended that core balance current transformers are used for measuring

IG when using the wattmetric method. When a low transformation ratio is used, the

current transformer can suffer accuracy problems and even a distorted secondary current

waveform with some core balance current transformers. Therefore, to ensure a sufficient

accuracy of the residual current measurement and consequently a better selectivity of the

scheme, the core balance current transformer should preferably have a transformation

ratio of at least 70:1. Lower transformation ratios such as 50:1 or 50:5 are not

recommended, unless the phase displacement errors and current transformer amplitude

are checked first.

It is not recommended to use the directional wattmetric protection in case of a ring or

meshed system as the wattmetric requires a radial power flow to operate.

The relay characteristic angle needs to be set based on the system grounding. In an

ungrounded network, that is, when the network is only coupled to ground via the

capacitances between the phase conductors and ground, the characteristic angle is chosen

In compensated networks, the capacitive fault current and inductive resonance coil

current compensate each other, meaning that the fault current is mainly resistive and has

zero phase shift compared to the residual voltage. In such networks, the characteristic

angle is chosen as 0º. Often the magnitude of an active component is small and must be

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