Transformer Protection: Differential protection (87T)
Input Data
a) 132kV CT ratio : 200/1
b) 33kV CT ratio : 800/1
c) Power transformer rating : 45MVA
d) Power Transformer Primary : 132kV
e) Power Transformer Secondary : 33kV
f) Power Transformer tap changes : -10.00% to +10.00% in step of 1.25%
g) Primary Voltage at +10% max tap : 145.20kV
h) Primary Voltage at -10% min tap : 118.8kV
i) Transformer Vector Group : YNd1
j) Rated FLA at primary side : 196.824A
k) Rated FLA at secondary side : 787.296A
l) Power Transformer & impedance : 13.5%
m) Transformer Primary Fault Current : 1457.96A
n) Transformer Secondary Fault Current : 5831.82A
Calculation
Differential setting:
1) Idiff>
2) Slope 1 and Base point 1
3) Slope 2 and Base point 2
4) Idiff>>
5) Harmonic restraint
6) Vector correction & Zero sequence elimination
1) Idiff>
Differential initial setting should be set above the steady-state magnetizing current of the
transformer. Unbalance current checking in regarding with the tap changer position of highest and
lowest tap position.
Tap change regulation error
Primary FLA at center tap = 196.824A
Secondary current at CT, In = 196.824/200
= 0.984A or 0.984In
Primary FLA at +10% tap = 45MVA/1.732/145.2kV = 178.931A
Secondary current at CT = 178.931/200
= 0.895A or 0.895In
Primary FLA at -10% tap = 45MVA/1.732/118.8kV = 218.693A
Secondary current at CT = 218.693/200
= 1.093A or 1.093In
Differential current at 10% tap = | 0.984In – 0.895In |
= 0.089In------------------------------(1)
Differential current at -10% tap = | 0.984In – 1.093In |
= 0.109In------------------------------(2)
Considering item 1 and 2 we can assume setting 0.2In or 20% differential current setting. But,
Assumed CT errors 5%, and considering safety margin,
Hence, the recommended pick up current, Idiff> = 0.25In
Set I_PcntDiff = 0.25
2) Slope 1 and Base point 1
This is the second section of the tripping characteristic covers the load current range, so that in this
section we must reckon not only with the transformer magnetizing current, which appears as
differential current, but also with differential current that can be attributed to the position of the
voltage regulator.
Tap changer (Total) :20% (10% to -10%)
CT errors :10% (assumed)
Hence the recommended slope 1- setting =30%
Set Slope_PcntDiff = 0.3In
Checking:
Differential initial setting : 0.3In
From the above, bias current : (Iw1 + Iw2)/2
Ibias1 : (0.984 + 0.895) / 2
: 0.9395A
Ibias2 : (0.984 + 1.093) / 2
: 1.0385A
Hence, at +10% tap, with Diff, initial setting = 0.25, the operating current of the relay
: Intial setting + 0.3 Ibias
: 0.25 +(0.3 x 0.9395)
: 0.53A This is higher than diff current
at 10% tap (0.089A)
The operating current of relay at -10% tap : Initial setting + 0.3 Ibias
: 0.25 + (0.3 x 1.0385)
: 0.56A This is higher than diff current
at -10% tap (0.109A)
Hence, the proposed setting, initial setting of Idiff, 0.25 and slope 1, 0.30, is acceptable.
3) Slope 2 and Base point 2
This is the second knee point of the tripping characteristic; it is set to produce stabilization
in the range of high current which may lead to current transformer saturation.
A setting of 70% is selected for slope 2 with relay for Base point 2
Hence the recommended slope 2 setting = 70% at base point 2
Set Slope_PcntDiff = 0.7In
4) Idiff>>
This is a simple instantaneous unrestrained highest differential over current setting. It is not
influenced by restraining current (triple slope characteristics), harmonic restraint,
overfluxing restraint or saturation detector. Also, this setting is to be set just above the
inrush current rms value so that blocking of the triple slope characteristic by second
harmonic restraint or by saturation detector is removed.
Short –circuit current of transformer (primary) = 1457.96A
Normally, High set (Idiff>>) setting should be 1.3 higher than 1.3 x transformer fault current:
1.3 x 1457.96A = 1895.348A
Corresponding CT secondary current = 1895.348/200
= 9.47 In ------------------------------(3)
However, the transformer inrush current can go up to 12 times the rated current:
Maximum transformer inrush current = 12 x rated current
= 12 x 196.824A
= 2361.888A
Corresponding CT secondary current = 2361.888/200
= 11.81In
Hence, the proposed setting is, idiff>> =12In
Set I_InstDiff =12
5) Harmonic Restraint
2nd harmonic blocking Percentage (%)
Energization of a transformer causes a temporary large flow of magnetizing inrush current
into one terminal of a transformer, without this current being seen at other terminals. Thus,
it appears as a differential current and could cause improper relay operation. Magnetizing
inrush currents contain larger amounts of second-harmonic current than do fault currents.
This second harmonic current can be used to identify the inrush phenomenon and to
prevent the relay from misoperation. The P642 measures the amount of second-harmonic
current flowing in the transformer. Transformer simulations show that magnetizing inrush
current usually yields over 30 percent of (IF2/IF1) in the first cycle of the inrush. A setting of
15 percent usually provides a margin for security.
K_2ndH_PcntDiff: 15% 2nd Harmonic blocking percentage setting.
En_CrossBlkDiff: 1 Cross blocking
5th Harmonic blocking percentage (%)
Overexcitation is defined by ANSI/IEEE (C37.91, C37.102) as a condition where the ratio
of the voltage to frequency (V/Hz) applied to the transformer terminals exceeds 1.05 per
unit at full load or 1.1 per unit at no load. Transformer overexcitation produces odd order
harmonics, which can appear as differential current to a transformer differential relay.
Fifth-harmonic blocking is primarily used on unit-generator step-up transformers at
power plants. The transformer voltage and generator frequency can vary somewhat
during start-up, overexciting the transformer. Fourier analysis of these currents during
overexcitation indicates that a 35 percent fifth-harmonic setting is adequate to block the
percentage differential element.
K_5thH_PcntDiff: 35% 5th Harmonic blocking percentage setting.
6) Vector Correction & zero sequence elimination setting
Vector Correction
The power transformer is YNd1. The recommended setting as in manual is:
TRCON : YDAC Transformer connection in YND1
CTCON : DACY CT connection to be connected in Delta-Wye
RZS :Y To remove zero-sequence
