ABB RELION 620 Series Technical Manual page 737

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1MRS757644 F

Technical Manual

by a binary input, a horizontal communication input or an internal signal of the

protection relay's program. The influence of the BLOCK signal activation is

preselected with the global setting Blocking mode.

The Blocking mode setting has three blocking methods. In the "Freeze timers" mode,

the operation timer is frozen to the prevailing value. In the "Block all" mode, the whole

function is blocked and the timers are reset. In the "Block OPERATE output" mode,

the function operates normally but the OPERATE output is not activated.

The task of a generator in a power plant is to convert mechanical energy into electrical

energy. Sometimes the mechanical power from the prime mover may decrease so

much that it does not cover the internal losses. The task of an underpower protection

is to protect the generator from very low power output conditions.

Steam turbines become easily overheated if the steam flow becomes too low or if the

steam ceases to flow through the turbine. Hydro turbine of the Kaplan type may be

damaged due to the fact that the turbine blade surfs on the water and sets up axial

pressure on the bearing. Diesel engines may be damaged due to insufficient

lubrication.

If the generator size is very large, it is uneconomical to continue running it with low

generated power. In the reverse power condition, large generators draw a considerable

amount of power from the rest of the system to feed their internal losses. Hence, it is

desirable to disconnect the generator in such situations.

In case of the parallel-connected generators, for example, the load of one generator

may be so low that it is better to disconnect it and let the remaining generators feed the

Where a low value of power setting is required, for example less than

2%, the correction parameters should be used to compensate for the

measuring errors. The manufacturer of the measuring devices is to be

contacted for information on the measuring errors.

If the measuring errors are not compensated for, the underpower

setting should not be lower than the sum of the current-measuring and

voltage-measuring errors.

For example, if the error of the current-measuring device is 2% and

that of the voltage-measuring device is 1%, the minimum setting is (2

Protection functions

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