Overexcitation Protection Oexpvph; Identification; Application - ABB RET650 Applications Manual

Section 8
Voltage protection
8.4
8.4.1
8.4.2
156

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
IEC 61850 IEC 60617

identification

identification
OEXPVPH
U/f >
SYMBOL-Q V1 EN-US
Transformer protection RET650 2.2 IEC
1MRK 504 169-UEN A
IP14547-1 v3
M14867-1 v3
ANSI/IEEE C37.2
device number
24
M13785-3 v6
Application manual