ABB REG670 Technical Manual page 126

Section 6
Differential protection
120
IL1_W1 is the fundamental frequency phase current in phase L1 on the W1 side
IL2_W1 is the fundamental frequency phase current in phase L2 on the W1 side
IL3_W1 is the fundamental frequency phase current in phase L3 on the W1 side
IL1_W2 is the fundamental frequency phase current in phase L1 on the W2 side
IL2_W2 is the fundamental frequency phase current in phase L2 on the W2 side
IL3_W2 is the fundamental frequency phase current in phase L3 on the W2 side
Ur_W1 is transformer rated phase-to-phase voltage on the W1 side (setting parameter)
Ur_W2 is transformer rated phase-to-phase voltage on the W2 side (setting parameter)
As marked in equation 1
equation, represents the total contribution from the individual phase currents from the
W1 side to the fundamental frequency differential currents, compensated for eventual
power transformer phase shift. The second term on the right hand side of the equation,
represents the total contribution from the individual phase currents from the W2 side
to the fundamental frequency differential currents, compensated for eventual power
transformer phase shift and transferred to the power transformer W1 side. The third
term on the right hand side of the equation, represents the total contribution from the
individual phase currents from the W3 side to the fundamental frequency differential
currents, compensated for eventual power transformer phase shift and transferred to
the power transformer W1 side. These current contributions are important, because
they are used for calculation of common bias current.
The fundamental frequency differential currents are the "usual" differential currents,
the magnitudes which are applied in a phase-wise manner to the operate - restrain
characteristic of the differential protection. The magnitudes of the differential
currents can be read as service values from the function and they are available as
outputs IDL1MAG, IDL2MAG, IDL3MAG from the differential protection function
block. Thus they can be connected to the disturbance recorder and automatically
recorded during any external or internal fault condition.
On-line compensation for load tap changer movement
A load tap changer is a mechanical device, which is used to step-wise change number
of turns within one power transformer winding. Consequently the power transformer
overall turns ratio is changed. Typically the load tap changer is located within the HV
winding (that is, winding 1, W1) of the power transformer. By operating load tap
changer, it is possible to step-wise regulate voltage on the LV side of the power
transformer. However at the same time the differential protection for power
transformer becomes unbalanced. Differential function in the IED has built-in feature
to continuously monitor the load tap changer position and dynamically compensate
on-line for changes in power transformer turns ratio.
Differential currents are calculated as shown in equation
parameters, the winding location of the OLTC is defined. Also, the voltage change of
each step. Thus, if for example the load tap changer is located within winding 1 the no-
load voltage Vn_W1 will be treated as a function of the actual load tap changer
position in equation 1 and equation 2. Thus for every load tap changer position a
and equation 2, the first term on the right hand side of the
1MRK502052-UEN B
1 and equation 2. By setting
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