Transformer models in EMTP/ATP

Transformer models in
EMTP/ATP
Okt 29, 2012, Bp
Single phase Transformer
Ac
i 1
i 2
+
N
V
1 +
1 V 2
-
N 2
-
ϕ

Core design of 3-phase transformers
Triplex core (3 x single phase trf)
3-legged stacked core
5-legged stacked core
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Transformer models in ATPDraw
• Saturable Transformer
• BCTRAN
• Hybrid Transformer
BCT
Y
Y
Y
Z
SAT
XFMR
• Ideal
n:
1
P S
YY
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Transformer models in ATPDraw
Selection Object name ATP card Description
Ideal 1 phase TRAFO_I SOURCE Single-phase ideal transformer.
type 18
Ideal 3 phase TRAFO_I3 SOURCE
type 18
3-phase ideal transformer.
Saturable
1 phase
TRAFO_S
BRANCH
TRANSFORMER
Single-phase saturable transformer.
Saturable
3 phase
SATTRAFO
≥v. 4.0
BRANCH
TRANSFORMER
General saturable transformer.
3-phase. 2 or 3 windings. All phaseshifts,
zigzag- and Auto-couplings. High
and low reluctance models.
# Sat. Y/Y
3-leg
TRAYYH_3
BRANCH
TRANSFORMER
THREE PHASE
3-phase saturable transformer. High
homopolar reluct. (3-leg). 3-ph node.
Preprocessing of manufacturer data.
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BCTRAN BCTRAN BRANCH
Type 1...9
Hybrid model XFRM BRANCH
Type 1...9
Direct support of Bctran transformer
matrix modeling.
Advanced model based on: Design, test
report or typical values. Direct calculation
of the A-matrix. Sophisticated core
model.
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Saturable Transformer
H1
R1
X1
R2
X2
X1
Rmag
Xmag
H2
X2
H3
X3
H0
Wye - Delta Transformer

Saturable Transformer Model
• Uses a star-circuit representation
• User could include saturation data
• Good for single-phase transformers
• Should not be used for 3-phase high
reluctance transformers (if no delta winding
present)
• The model requires the following data:
The voltage rating of each winding (winding!)
The leakage impedance of each winding
The transformer connectivity information
BCTRAN
• Admittance matrix model
• Represents a linear relationship between the primary
and secondary voltages and currents
• [R] and [ωL] -1 must be separated
• BCTRAN supporting routine generates 3-phase
coupled R-L {Type 1,2,3,... .PCH file}
• Data Required
MVA rating
Nominal L-L voltage (terminal and not! winding)
Winding connections
Short circuit impedances, losses
No-load current and loss

Magnetizing core modeling
• Must be considered in the following studies
• Transformer energization studies
• Load rejection studies
• Switching of transformer terminated lines
• Ferroresonance studies
• CT saturation
• CCVT modeling
• No-load losses include:
• Eddy current loss and
• Hysteresis loss
• Could be represented with a resistance Rm in parallel with the
nonlinear magnetizing inductance, or
• As external, nonlinear (hysteretic) inductance connected to a
low voltage winding
Hybrid model - XFMR
• The user can construct the transformer model on
three sources of data:
– Design parameter: specify geometry and material
parameters of the core and windings.
– Test report: standard transformer tests.
– Typical values: typical values based on the voltage and
power ratings.
• The model includes:
– an inverse inductance matrix for the leakage description,
– frequency dependent winding resistance,
– capacitive coupling,
– and a topologically correct core model with individual
saturation and losses in legs and yokes.
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Hybrid transformer model
BCTRAN
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Hybrid transformer model
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Winding resistance R(f)
• Their dependence on the frequency is due to
- Skin effects
- Proximity effects
- Eddy currents
• The frequency-dependency of R is represented using
Foster equivalent circuit (two cells)
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Capacitive effects
• Capacitances between high and low voltage windings
and core
• Capacitance between high voltage phases, outer legs,
and grounded elements
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Core representation
• Attached to the fictitious N+1th winding
• Topologically “correct” core model, with
nonlinear inductances representing
each leg and limb
– Triplex
– 3- and 5-legged core
• Flux linkage-current relation by Frolich
equation and relative lengths and
areas.
• Fitting to Test Report
λ
i
λ =
a'
+ b'
⋅|
i |
i
R o
L l R l
L o
R y
L y
L l R l
R y
L y
L l R l
R o
L o
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Transformer modeling at high
frequency
• Transformer models in EMTP are valid for
frequencies up to 2 kHz
• To study phenomena with characteristic frequency in
range of 2 - 30 kHz, capacitive coupling among
windings and to ground must be added
• Above 30 kHz a more detailed representation of
internal winding arrangement is required
– Interwinding and interturn/disks capacitances must be taken
into account
• Above 100 kHz the presence of ferromagnetic
material has just secondary importance.

Transformer modeling guidelines
PARAMETER/
EFFECT
Low Frequency
Transients
Slow Front
Transients
Fast Front
Transients
Very Fast Front
Transients
Short-circuit
impedance
Very important Very important Important Negligible
Saturation
Very important
Very
Important (1)
Negligible
Negligible
Iron losses Important (2) Important Negligible Negligible
Eddy currents Very important Important Negligible Negligible
Capacitive
coupling
Negligible
Important
Very
important
Very important
(1) Only for transformer energization phenomena, otherwise important
(2) Only for resonance phenomena
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Application: CT modeling
ip’ Rp Lp Rs Ls
Es
is
im
imr
Rm
imx
Lm
Rl
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Application: CCVT modeling
HV Bus Bar
C
1
LC
SDT
C
2
LP
FSC
ZB
PLC
L d
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