ABB RELION RET670 Applications Manual page 633

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
excessive heating and severe damage to insulation and adjacent parts in a relatively
short time.
Overvoltage, or underfrequency, or a combination of both, will result in an excessive
flux density level, which is denominated overfluxing or over-excitation.
The greatest risk for overexcitation exists in a thermal power station when the
generator-transformer block is disconnected from the rest of the network, or in network
"islands" occuring at disturbance where high voltages and/or low frequencies can
occur. Overexcitation can occur during start-up and shut-down of the generator if the
field current is not properly adjusted. Loss-of load or load-shedding can also result in
overexcitation if the voltage control and frequency governor is not functioning
properly. Loss of load or load-shedding at a transformer substation can result in
overexcitation if the voltage control function is insufficient or out of order. Low
frequency in a system isolated from the main network can result in overexcitation if the
voltage regulating system maintains normal voltage.
According to the IEC standards, the power transformers shall be capable of delivering
rated load current continuously at an applied voltage of 105% of rated value (at rated
frequency). For special cases, the purchaser may specify that the transformer shall be
capable of operating continuously at an applied voltage 110% of rated value at no load,
reduced to 105% at rated secondary load current.
According to ANSI/IEEE standards, the transformers shall be capable of delivering
rated load current continuously at an output voltage of 105% of rated value (at rated
frequency) and operate continuously with output voltage equal to 110% of rated value
at no load.
The capability of a transformer (or generator) to withstand overexcitation can be
illustrated in the form of a thermal capability curve, that is, a diagram which shows the
permissible time as a function of the level of over-excitation. When the transformer is
loaded, the induced voltage and hence the flux density in the core can not be read off
directly from the transformer terminal voltage. Normally, the leakage reactance of each
separate winding is not known and the flux density in the transformer core can then not
be calculated. In two-winding transformers, the low voltage winding is normally
located close to the core and the voltage across this winding reflects the flux density in
the core. However, depending on the design, the flux flowing in the yoke may be
critical for the ability of the transformer to handle excess flux.
The Overexcitation protection (OEXPVPH, 24) has current inputs to allow calculation
of the load influence on the induced voltage. This gives a more exact measurement of
the magnetizing flow. For power transformers with unidirectional load flow, the
voltage to OEXPVPH (24) should therefore be taken from the feeder side.
Heat accumulated in critical parts during a period of overexcitation will be reduced
gradually when the excitation returns to the normal value. If a new period of
Section 10
Voltage protection
627