Overexcitation Protection Oexpvph - ABB REG670 Applications Manual

1MRK 502 071-UEN -
10.4
10.4.1
10.4.2
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual

Overexcitation protection OEXPVPH

Identification
Function description
Overexcitation protection
Application
When the laminated core of a power transformer is subjected to a magnetic flux
density beyond its design limits, stray flux will flow into non-laminated components
not designed to carry flux and cause eddy currents to flow. The eddy currents can
cause 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
Voltage protection
IEC 61850 IEC 60617
identification identification
OEXPVPH
U/f >
SYMBOL-Q V1 EN
Section 10
ANSI/IEEE C37.2
device number
24
419