9.4
Overexcitation protection OEXPVPH
IP14547-1 v3
9.4.1
Identification
M14867-1 v3
Function description
IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Overexcitation protection
OEXPVPH
U/f >
SYMBOL-Q V1 EN-US
24
9.4.2
Application
M13785-3 v6
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 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) 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 should
therefore be taken from the feeder side.
Section 9
1MRK 505 343-UEN B
Voltage protection
478
Application manual
Summary of Contents for Relion 670 series
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