ABB RELION REX640 Technical Manual page 538

Protection and control

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Protection functions

Figure 311: Typical radial compensated network employed with wattmetric

The wattmetric function is activated when the residual active power component

exceeds the set limit. However, to ensure a selective operation, 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 Io 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 earthing. In

an unearthed network, that is, when the network is only coupled to earth via the

capacitances between the phase conductors and earth, the characteristic angle is

chosen as -90°.

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 0°. Often the magnitude of an active component

is small and must be increased by means of a parallel resistor in a compensation

coil. In networks where the neutral point is earthed through a low resistance, the

characteristic angle is always 0°.

As the amplitude of the residual current is independent of the fault location, the

selectivity of the earth-fault protection is achieved with time coordination.

1MRS759142 F

Technical Manual