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Power transformer transient modeling considering the effects of on-load tap
changer
Conference Paper · October 2017
DOI: 10.1109/ICEPE-ST.2017.8188947
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WK,QWHUQDWLRQDO&RQIHUHQFHRQ(OHFWULF3RZHU(TXLSPHQW6ZLWFKLQJ7HFKQRORJ\;L¶DQ&KLQD
Power Transformer Transient Modeling Considering
the Effects of On-Load Tap Changer
Asad Ahmad, Wanliang Fang, Jun Liu* Member IEEE, Xudong Hao
Shaanxi Key Laboratory of Power Systems, Xi'an Jiaotong University, Xi'an, 710049, China.
e-mails: geminiasad66@yahoo.com, eewlfang@126.com, eeliujun@mail.xjtu.edu.cn, xjtuhxd@stu.xjtu.edu.cn
$EVWUDFW—On-load tap changer (OLTC) is essential in
regulating the operation status of power transformers employed
in modern power systems. While exploring power flow analysis,
electromagnetic and electromechanical simulations by using the
on-load tap changer, it is found that inaccuracy problem may
occur with the assumption in conventional transformers that tap
positions only affect the turns ratio parameter; however, the real
transformers specifications, such as the resistances and
inductances of windings, also fluctuate with different tap
positions. In this paper, a novel inductance matrix model for
both single and three phase transformer is proposed, which
illustrates that winding resistances and inductances are also
varying with the tap positions. Case studies are performed to
assess the performance of the proposed transformer model in
case of symmetrical and un-symmetrical faults. The novel model
is developed using PSCAD/EMTDC software, and the
simulation results are compared with traditional transformers.
It can be concluded that the parameters of the proposed
transformer become more sensitive on the voltage and current
during various transient processes. Thus it's vital to provide an
applicable attention to the analysis and simulation with the
usage of this more accurate mathematical transformer model,
which permits us to obtain the more precise behavior of power
systems without any risk.
,QGH[7HUPV-- On-load tap changer; Transformer; Inductance
matrix model; Symmetrical
PSCAD/EMTDC.
and
un-symmetrical
faults;
NOMENCLATURE
Ni
Gmi
Gm12
ri
u
i
Li
Lsi
M
V
Number of turns of winding i (i =1,2)
Permeance of leakage magnetic path of winding
i (i =1,2)
Permeance of mutual magnetic path of windings 1
and 2
Resistance of winding i (i =1,2)
Voltage of winding i (i =1,2)
Current of winding i (i =1,2)
Self-inductance of winding i (i =1,2)
Leakage inductance of winding i (i =1,2)
Mutual inductance of windings 1 and 2
Impedance correction coefficient
Turn ratio
Reactance coupling relationship between i and j
Subscript’s’ denotes that the variable is system base, and
subscript’t’ denotes that the variable varies with tap positions.
k
X ij
I.
INTRODUCTION
Power transformer is the most complex structured element
of power network while studying the load flow and transient
phenomenon. Highly efficient power could not be possible to
transmit without smart transformers in power grid because
power transformer is the only component which transforms
the AC voltage magnitude from one to another level without
interference. The representation of transformer parameters is
very tangled because of core designs and some also are
nonlinear and frequency dependent. Winding resistance,
mutual and self inductance, skin and proximity effects in
windings, magnetic core saturation, hysteresis and eddy
current losses in core are the tangible features betting on
frequency may have to be properly delineated by a
transformer model [3]. Varied intricacy models are reinforced
to know the transient behaviour of transformers. Transformer
accouter with on-load tap changers to perform the voltage
regulation according to load while not interrupting the load
current [4]. Therefore the OLTC is also one of the influential
elements of power transformer.
According to the records of electricity boards, declared that
numerous amounts of defalcations of transformers are caused
because of malfunction of OLTC’s in power transformers. A
lot of contributions have been made to reduce the failures of
OLTC’s and transient interaction phenomena’s between the
high voltage equipment and transmission line by introducing
various high frequency transformers models which
cooperates to reduce these convulsions. Transformer model is
one of the feeble elements of advance transient simulation
software because of its convoluted construction which
includes the windings and core. Most of the proposed designs
of varying complications have been executed in simulation
environments to study the expected behaviour of proposed
transformer models but mostly designers don’t have the
applicable attentions regarding the internal variations of
transformer windings.
‹,(((
Aim of this paper is to representing the summary of transient
single phase double winding transformer and three phase
three winding transformer models in a power system
equipped with on-load tap changers on transformers high
voltage sides considering the influence of taps and its effects
on winding resistance and inductance with respects to on-load
tap changer. Also the fault analysis results and comparison of
proposed and traditional model is discussed in detail.
II.
C. Fault Analysis of ThreePhase Model
Transformer is one the exorbitant element of power grid.
Usually transformers are highly assured but maloperations
normally occur in protection devices. In order to study such
condition, in case of fault directly shot the power transformer
another three phase model is designed having symmetrical
and un-symmetrical faults just before the designed
transformer model.
DELINEATION OF TRANSFORMER MODELS IN
POWER SYSTEM
A. Single Phase Model
Dynamic impacts of tap changer on power system have
completely different behaviors on voltage and current within
the event of variation of tap positions. These changes in
voltage and current conjointly turn out the consequences on
power transformer winding X/R ratio and inductance values.
Single phase step down transformer model 70 MVA 132/33
kV is integrated in a power network as shown in figure to
understand the impacts of tap changer on single phase
transformer as shown in Fig. 1.
Figure 1. Single Phase Grid Interconnected Scheme
B. Three Phase Model
Challenges in power network having the three phase
transformer with tap changer are totally different from single
phase transformer models. To grasp the impact of tap changer
on three phase three winding transformer another grid has
been connected to it having the 13kV generation supply
voltage. This model is shown below in Fig. 2.
Figure 3. Three Phase Grid Interconnected Scheme In case of Fault
III.
TRANSIENT MODELING OF SINGLE AND THREE PHASE
TRANSFORMER
A. Inductance Matrix Model for Single Phase Double
Winding Transformer
In previous paper single phase double winding
transformer model was represented with deviations as the tap
changes. In this paper, single phase double winding
transformer is simulated using the mathematical model which
provides us more accurate results to implement this model.
The magnetic circuit diagram of a single phase double
winding transformer, in which the arrows denote the
reference direction for voltage, current and magnetic flux [1],
[2].
I
i1
i2
u1
I2s
I1s
u2
Figure 4. Magnetic Circuit Diagram of Single Phase Double Winding
Transformer
The self-inductance of winding 1 and 2 calculated as
L1
Gm1 N12 Gm12 N12
N1
Gm12 N1 N 2
N2
Ls1 kM
(1)
M
(2)
k
Therefore, the voltage equations can be reformulated as
L2
Figure 2. Three Phase Grid Interconnected Scheme
Ls1 ­
°°u1
®
°u
°̄ 2
This work was supported in part by National Natural Science Foundation
of China under grant 51507126.
Ls 2 di1
di
M 2
dt
dt
di
M di2
M 1
r2i2 ( Ls 2 )
k dt
dt
ri
1 1 ( Ls1 kM )
(3)
B. Inductance Matrix Model for Three Phase Three
Winding Transformer
The three-phase transformer can still be regarded as the
multi-branch resistance and inductance coupling circuit, but
the mutual inductance between the three phase windings due
to the common magnetic circuit should be considered. This
means the coupling problem existing between different
windings of different phases. Conceptually, from a single
phase transformer to three-phase transformer, each winding
in single-phase transformer should change to three windings
respectively in iron core winding I II III of three-phase
transformer [2],[3],[4]. The short circuit reactance is
calculated as
X 11'
X 13s
ª X ss13
« s
« X m13
« X ms 13
¬
X ms 13
X ss13
X ms 13
X ms 13 º
»
X ms 13 »
X ss13 »¼
maximum values of voltage and current but having the certain
differences in values.
(4)
Describes the influence caused by datum conversion after
transformer connect system and the tap joint. The method of
1
is same with single phase transformer and
forming matrix LMC
Figure 5. Voltage comparison of traditional and proposed three phase model
three winding transformer but each element is a 3 u 3 same
value matrix and it is shown by Equation (5).
1
LMC
ª I 3u3
1«
k I
V « 12* 3u3
«¬ k13* I 3u3
k12* I 3u3
2
k12*
I 3u3
k12*k13* I 3u3
k13* I 3u3 º
k12*k13* I 3u3 »»
2
k13*
I 3u3 »¼
(5)
Where, the expression of V is same as it of the single-phase
transformer. The inverse inductance matrix of the double
winding transformer is shown by Equation (6).
LM1
IV.
1
1
LMP
u LMC
(6)
RESULTS AND DISCUSSION OF PSCAD MODELS
This transformer model is equipped with on-load tap
changers (OLTC) which permits the voltage regulation by
varying the turn ratio under load conditions. So, in this
transformer model a special feature is designed to allow the
user online control of input data, as well as the ability to
record and display output data in runtime mode. Tap changer
is designed to have 9 taps with a default position. When the
tap position of the transformer is changed from 1 to 9 in
runtime mode its three phase voltage and current magnitude
continuously vary as compare to traditional model according
to the values of on load tap changer without interruption of
load which also influences its resistance and inductance as
shown below in waveform every step of waveform represent
the tap position is changed which proves that the assumption
of considering the constant original impedance parameter in
traditional models is not correct. Also there is a comparison
between the traditional and proposed model every tap
position shows a certain difference in values from tap 1 to 9.
At position 9 it is observed both models are having the
Figure 6. Current comparison of traditional and proposed three phase model
V. SYMMETRICAL AND UN-SYMMETRICAL FAULTS
ANALYSIS OF THREE PHASE TRANSFORMER CONSIDERING THE
INFLUENCE OF TAP POSITIONS
The performance dependability of transformers and their
OLTCs is therefore extremely important and must be kept at
a high level throughout their entire lifetime. A power
transformer is usually affected by internal and external faults.
Among the numerous faults, about 70% of advance
transformers breakdowns is because of internal winding faults
and this is often probably extend because of transformer
loading to their ultimate capability is becoming normal
routine [4]. Most of the work already done to protect the
transformer from internal factors affecting transformer but
very less research work have been published regarding the
external factors directly affecting the transformers. In
addition to fault conditions within the transformer,
abnormal conditions due to external factors result in stresses
on the transformer e.g. Overloading, System faults,
Overvoltage, and Under-frequency operation. So the main
focus of this paper is to enlighten the one of the external
factor “System faults” causing the damage to transformer
directly [5],[6]. For analyzing the faults in proposed power
system fault logic is designed to having 10 positions every
position represents a different fault.
A. Symmetrical Faults
Ideally when the symmetrical faults occurs, symmetrical
fault current of abnormally high magnitude flows through the
network equal in magnitude and displaced 120º from each
other and the voltage at fault point is reduced to zero. But
practically here it is observed that voltage is not reduced to
zero in between the time interval 0.500 to 0.560. When a
symmetrical fault occurs at any point in a system, the
short-circuit current is limited by the impedance of the system
upto the point of fault. During the interval of fault current is
abnormally high because the value of resistance is reduced
abruptly and displaced by 120º as shown in Fig.7.
Figure 8. Single Line to ground fault at default position of OLTC
C. Double Line to Ground Fault (L — L —G)
Consider the double line-to-ground fault involving B–C
lines and earth. Fault dial positions 4, 5 and 6 are having the
Phase AB to ground, Phase AC to ground and Phase BC to
ground. Here just the position 6 is discussed because other
two positions of fault dial also having the same effect [8], [9].
When the fault occur at Phase BC to ground so ideally fault
conditions are I A 0 and VB Vc 0 . During the interval of
fault current of Phase BC is abnormally high because the
value of resistance is reduced abruptly and current of Phase A
is almost equal to zero while the voltage of faulty phases is
not reduced to zero as compare to healthy phase A as shown
in Fig.9.
Figure 7. Symmetrical Fault at default position of On-Load Tap Changer
B. Single Line to Ground Fault (L —G)
Consider single line to ground fault occur just before the
three phase three winding transformer. Fault dial positions 1,
2 and 3 are having the Phase A to ground, Phase B to ground
and Phase C to ground. At any of these positions the effects
of this fault can be known. Here the fault at position 1 which
is Phase A to ground is only discussed because the other
phases also have the same effect of this type of fault. When
the fault occur at Phase A so ideally fault conditions are VA 0
and I B
IC 0 . During the interval of fault current of Phase A
is abnormally high and current of Phase B and Phase C is
almost equal to zero. The voltage of the faulty phase is
reduced but not equal to zero as shown in Fig.8.
Figure 9. Double Line to ground fault at default position of OLTC
D. Line to Line Fault (L — L)
Consider a line-to-line fault between the Phase A and
Phase B. A fault dial position 8 is having the fault between
the Phase AB. Here just the position 8 is discussed because
other two positions of fault dial also having the same effect
[7]. When the fault occur at Phase AB to ground so ideally
fault conditions are I A I B 0 , Ic 0 and VA VB . During the
interval of fault current of Phase A and B is abnormally high
but the current summation of both phases is equal to zero and
current of Phase C is approximately equal to zero while the
voltage of the both faulty phases is disturb but not purely
equal to each other as shown in Fig.10.
VII. CONCLUSION
In conclusion, it can be summarized that the conventional
transformer in power grid transient stability and steady state
analysis, is commonly modeled as consistent impedance
connected to a non-standard turn ratio. It is found that the
genuine parameters of transformers, such as the resistance
and inductances of transformer winding also have variations
with alternation of tap positions. Case studies on this advance
model is also performed, the results have shown which
proves the every step is having the divergence which produce
inaccuracy in power system analysis.
ACKNOWLEDGMENT
Figure 10. Line to Line fault at default position of OLTC
Here only the results of default position is shown only but
when the tap position is changed from 1 to 9 its fault current
and voltage of primary side and secondary is also increased
which also having the impact on its winding resistance and
inductance. In both symmetrical and un-symmetrical faults,
all the variations in between the interval 0.500 to 0.560 are
because this model is having the Fault ON resistance of 0.01
ohm but ideally this resistance is infinity. Three phase three
winding transformer reduces the unbalancing in primary due
to the unbalancing in three phase loads also in fault analysis it
redistributes the flow of fault currents, so it provides the more
practical results in contrast to traditional model.
VI.
I would like to take this opportunity to express my
gratitude to my parents, supervisors and Xian Jiaotong
University. Their support and encouragement provides me the
opportunity to complete this research work successfully. I
believe this way of guidance is really unique and would be
very helpful for further studies.
Mr.Kaleem Ahmad & Mr.s Tahira Kaleem (Parents)
Prof. Wanliang Fang & Prof. Jun Liu (Supervisors)
REFERENCES
[1]
[2]
[3]
COMPARISON OF PROPOSED AND TRADITIONAL MODEL
According to the comparison of proposed and traditional
transformer models it has been found that both models have
severe differences in values which may be produce the
inaccuracy in power system analysis if it’s to continue to use
the traditional transformer models. Almost the 18 tap
positions of proposed and traditional model are compared
having both the positive and negative values of tap changer.
Difference between the both models is shown below
in Fig. 11.
[4]
[5]
[6]
[7]
[8]
[9]
Figure 11. Comparison Trend of Proposed and Traditional Model
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