ABB RELION REX640 Technical Manual page 893

Protection and control

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1MRS759142 F

Figure 499: Example of an inverse time counter operation if START occurs when

BLK_RESTART is inactive while COOL_ACTIVE is active.

If the laminated core of a power transformer or generator is subjected to a

magnetic flux density beyond its designed limits, the leakage flux increases. This

results in a heavy hysteresis and eddy current losses in the non-laminated parts.

These losses can cause excessive heating and severe damage to the insulation and

adjacent parts in a relatively short time.

Overvoltage, underfrequency or a combination of the two, results in an excessive

flux density level. Since the flux density is directly proportional to the voltage and

inversely proportional to the frequency, the overexcitation protection calculates the

relative V/Hz ratio instead of measuring the flux density directly. The nominal level

(nominal voltage at nominal frequency) is usually considered as the 100 percent

level, which can be exceeded slightly based on the design.

The greatest risk for overexcitation exists in a thermal power station when the

generator-transformer unit is disconnected from the rest of the network or in the

network islands where high voltages or low frequencies can occur.

Overexcitation can occur during the start-up and shutdown of the generator if

the field current is not properly adjusted. The loss-of-load or load shedding can

also result in overexcitation if the voltage control and frequency governor do not

function properly. The low frequency in a system isolated from the main network

can result in overexcitation if the voltage-regulating system maintains a normal

Overexcitation protection for the transformer is generally provided by the generator

overexcitation protection, which uses the VTs connected to the generator terminals.

The curves that define the generator and transformer V/Hz limits must be

coordinated properly to protect both equipment.

If the generator can be operated with a leading power factor, the high-side voltage

of the transformer can have a higher pu V/Hz than the generator V/Hz. This needs

Technical Manual

Protection functions