3rd International Conference on Mechatronics, Robotics and Automation (ICMRA 2015)

PSCAD simulation study on the distribution network voltage transformer

failure

Huadong Wang 1, a ,Xu Cai 2, b ,Lili Liu 3, c*

1 Electronic Information and Electrical Engineering, Electrical Engineering, Shanghai Jiao Tong
University, Shanghai, China
2 Electronic Information and Electrical Engineering, Electrical Engineering, Shanghai Jiao Tong
University, Shanghai, China
3Jiangsu Key Laboratory of Large Engineering Equipment Detection and Control, Xuzhou Institute
of Technology, Xuzhou, Jiangsu Province, China
a
huadong.wang@shsdec.com, bxucai@sjtu.edu.cn, cdxwlll@sina.com

Keywords:Transformer voltage fault; Arc suppression coil; Neutral point ungrounded system;
PSCAD/EMTDC
Abstract: In the distribution network, the single phase grounding fault of potential transformer (PT)
caused by burning phenomena occur. PT transient characteristics of the primary side current, On the
basis of single-phase grounding fault and the equivalent model simulate PT transient characteristics
of the primary side current. The main causes of PT failure are deduced for distribution network
single-phase ground capacitor current value caused by a larger. The simulation of a group of 6kV
PT by using power system simulation software PSCAD/EMTDC, and the comparison of the three
solutions: the PT of one side through the non-linear resistor (harmonic elimination device), Open
triangle parallel resistance and the system neutral grounding via arc suppression coil. Finally
obtains that the neutral point through the arc suppression coil grounding is the most the right way to
solve this problem in the distribution network.

Introduction
The 6/10kv distribution network is one of neutral point ungrounded system, bus mounted
electromagnetic voltage transformer are Y0/Y0 open delta connection. When the system
single-phase grounding, will cause Blown fuse o the high side bus PT, or even burned phenomenon
PT. Because most of the PT failure after a single-phase ground fault disappears, field technicians
habitually think happened PT ferromagnetic resonance. In this paper, the equivalent model grid
single-phase ground fault disappears before and after analysis of the transient characteristics of PT
primary current, pointing out the main factors which leading to the distribution network failure of
PT. The single phase to ground fault simulation model has been improved, with PSCAD/EMTDC in
three to single-phase custom V-I characteristic curve of volume three UMEC (Unified Magnetic
Equivalent Circuit) module and the three single-phase transformers module, respectively, simulation
6kV bus PT actual operation of the transformer and the ground, the actual operation situation makes
the model can to reflect more accurately the power grid. At the same time, further simulation
comparison of PT primary side through nonlinear resistance grounding, open delta and resistance
and the neutral grounding via arc suppression coil three solutions, choose the most suitable for the
actual situation of distribution network in solving measures.

© 2015. The authors - Published by Atlantis Press 1401

The analysis of the bus PT failure reason
The 6/10kv distribution network is neutral point ungrounded system, so the high voltage side of
the neutral point grounding busbar PT becomes the only place the entire 6kV grid. System of
single-phase ground fault process can be divided into single-phase ground stabilization and ground
fault disappears in two stages. Firstly, the first phase, to facilitate analysis, assuming that the
single-phase grounding fault phase occures metallic grounding. Non-fault B, C phase voltage rises
to the line voltage, the ground capacitance C0 will be charged and at the same time the electric
charge of this portion of the capacitive current generated by the power supply and the ground form
a loop. Because magnetizing impedance PT is large, the current value flowing high-voltage side
winding at that time is very small, PT at this stage is generally not high pressure side fuse failure.
When the second phase of the single-phase ground fault disappears, Non-fault phase recovery
phase from line voltage to normal voltage level. However, due to the elimination of the fault,
cutting off the passage of charge to earth ground, at this relatively non-fault stored charge
capacitance constitute only released to the earth through the primary side of the neutral point PT(as
shown in Figure 1 from one phase analysis).Therefore, the current single-phase ground fault
disappears after PT primary side should consist of two parts: system power production into
non-mandatory components and charge decay through PT neutral point discharge formation decay
component.

Figure 1 charge discharge circuit after single-phase grounding fault disappears

The single-phase grounding fault elimination of closing the switch can be arranged from the
figure 1:
d 2 uc du
LC0 2
+ RC0 c + uc = 0
dt dt (1)
duc
i = -C0
dt (2)

uc (0+ ) = uc (0- ) = U k (3)

ic (0+ ) = ic (0- ) = 0 (4)

R < 2 L / C0
Uk is the capacitor voltage When single-phase ground fault elimination. Because ,

U k w0 - d t w
uc = e sin(wt + arctan )
Solution of equation (1) is w d
In the formula:
d = R / 2L
w0 = 1/ LC

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 w = w0 2 - d 2

U k -d t
i= e sin(wt )
According to equation (2) wL
From equation (4) shows the amplitude of current I high side PT is mainly concerned with.
When ground capacitance value of the system is larger, the smaller the free oscillation frequency,
the greater its corresponding current amplitude, while the low-frequency oscillations under the
current role of PT's core will quickly saturate the excitation current increases rapidly, then the
high-pressure side of the blown fuse can cause severe winding insulation will endanger the PT.
Similarly when the capacitance value C0 is very small, although it will also lead to an oscillating
current, but this time it is much higher, free-running currents have much smaller, but Peterson
resonance condition C0 is small, the system may occur in accordance with the fundamental
frequency , crossover and high frequency resonance. However, combined with the actual situation
of coal production - ground and underground cables are powered by multiple mines capacitive
current test, the value of up 90A, also the smallest 35A, away from the resonance parameters
Peterson conditions, it is considered low-saturated over-current is the main factor causing mine PT
grid failure.
Voltage value Current value Voltage value Current value

0.557 0.600 1.501 3.067

0.866 0.867 1.631 4.387
1.000 1.000 1.732 6.000
1.097 1.207 1.847 8.667
1.212 1.540 1.905 10.20

Table 1 PT V-I characteristic parameters

Note: The values in Table 1 are the unit value, the voltage reference value is 6000 / 3V , the current reference value is 1.5mA.

Build simulation models and simulation calculations

Actually the transient after single-phase ground fault eliminated is very complex, and is
generate din the three-phase circuit, and if it is converted into a single-phase circuit to analysis, it
will actually produce some differences. Therefore, this paper using common international power
system analysis software PSCAD/EMTDC, it carried out a detailed simulation analysis of the whole
process of single-phase ground fault and measures to eliminate faulty PT.
Establish bus PT model
As noted above in the role of the low-frequency oscillating current, the inductance of PT has a
sharp decline, so the field characteristic of PT have a direct impact on its failure occurred. In the
previous simulation analysis, we usually use nonlinear inductance in series with a resistor to
simulate the high-voltage side coils of PT, but ignores the iron loss and leakage reactance PT itself,
making the simulation error is large. We can use three custom V-I characteristic single-phase
three-volume UMEC (Unified Magnetic Equivalent Circuit) module in PSCAD to simulate actual
running 6kv bus PT (Fig. 2) In this article, rxis the harmonic resistance values in the high-pressure
side of neutral point [4].In the process Simulation, we measured zero sequence voltage at the
opening of the triangle (N, L) from the model, it will be more efficient, more accurate to simulate
the operating characteristics of the bus PT. The V-I characteristic parameters of UMEC Module are
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the measured data of JDZJ-6-type voltage transformer. (Table1).

Figure 2 bus PT simulation model

Establish grounding transformer model
The distribution network of 6/10 kV is small current neutral grounding system, the reason of
which is that the side of 6/10Kv is delta connection, as a result, there is no neutral point connected
by Petersen coil, therefore, an artificial neutral point made of special grounding transformer should
be applied to connect Petersen coil. The connection mode of grounding transformer is Zn,yn11, y
side load of grounding transformer is not considered in this paper, three single-phase transformers is
packed to form a grounding transformer of 400KVA in PSCAD.（As shown in Figure 3）
S_IN
C
A

B
#1

#1

#1
#2

#2

#2

O_OUT

Figure 3 grounding transformer model

Simulation
Taking a 6 /10kv distribution network ungrounded power grid model, PT high voltage side of the
neutral point directly to ground, assuming that the system under normal operating conditions, 0.2SA
phase single-phase ground fault occurs, the transition resistance of 50Ω, after the failure to
eliminate persistent 0.2S, this process of zero-sequence voltage and high-voltage side of the
excitation current PT simulation analysis.
Equivalent model of the system is shown in the Figure 4，Which 110kV power through a Yd, 11
connected to the wiring of the main transformer 6kV bus, each cable outlet bus with centralized
parameter indicates，By setting the ground capacitance value of each outlet, and then change the
current system capacitance to ground zero sequence reactance, through the test of capacitive
current, the measured mine 6kv power single-phase ground is about 60 A, the system ground
capacitance value of about 17μF.In the figure, G_T ground transformers, bus PT open delta switch
in parallel by the resistance of the resistor 2Ω.
In simulation solutions, this paper respectively: ①PT the primary harmonic elimination by
grounding; ②PT open delta paralleled resistance; ③ neutral point arc suppression coil grounded
three solutions were simulated and compared, to find a solution mine grid the best way for this
problem.

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 Figure 4 distribution system simulation

The simulation results and analysis

Analysis of simulation results
When the bus arc suppression coil is not put into operation, After 0.4 seconds ,single phase- to -
earth fault vanishes and the excitation current waveform of the faulted phase PT、fault phase voltage
waveform and zero-sequence voltage waveform at that moment are showed In Figure5, It can be
seen that when the fault disappears the faulty phase voltage waveform is superposed by supply
voltage components and zero-sequence voltage component [3.11]，and the zero-sequence voltage
frequency is about 2.2Hz , Under the action of this voltage , PT core reaches capacity quickly and
high side excitation current’s Maximum peak reaches to 1A , phase voltage level is high and fall
time is long.

Figure 5 single-phase ground fault simulation waveforms

Analysis and comparison of the effect of three kinds of solutions
PT high side series nonlinear resistance (Resonance eliminator)
Neutral series nonlinear resistance equivalent to increase of the resistance of the resistor in
Figure1, When the single-phase grounding fault disappear, The low-frequency saturation current
component flows into the earth through resistor , resistor undertakes the bulk of the voltage drop,
Greatly limits the low-frequency Saturated over current. When the discharge circuit in figure1 is
in over damped state, Low frequency oscillation process turn into a non oscillatory discharge

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process[3-9]. Figure6 show the fault phase voltage of bus、the high-pressure side of the excitation
current waveform of PT after PT primary side of the neutral point connects nonlinear resistor，it can
be seen that the PT Saturated over current and overvoltage are suppressed well, but recovery time of
phase voltage is too long.

Figure 6 PT primary side of the neutral point connects nonlinear resistor

At the same time there are some drawbacks to cascade non-linear resistor: Firstly , if the quality
of nonlinear resistor is poor, Although it is possible to suppress saturation overcurrent better by
larger Resistance, but it also let PT open delta zero-sequence voltage value become too small, Thus
affecting the operation of selecting device, the situation happened in Jincheng Mining to install line
selection device; Secondly, when single-phase ground fault is eliminated the current flows through
the non-linear resistor is great, resistance heat capacity is also an important issue.
PT secondary-side shunt resistor
PT secondary-side shunt resistor, equivalent the high-voltage winding PT parallel resistance, the
main purpose is to suppress the occurrence of resonance. Such existing devices are mostly based on
detect the different frequency components in the zero-sequence voltage, to determine whether the
system occurs a single-phase ground fault or resonance failure, if the judgment is resonant fault we
should control the resistance input, such a judgment process will make resistance into a certain
delay [5], As shown in figure 7, assuming when the ground fault is eliminated after 0.1s, parallel a
2Ω resistor in the opening triangle the fault phase voltage and PT high-voltage side of the excitation
current waveform.

Figure 7 the way of PT secondary-side shunt resistor

It can be seen from Figure 7 that even after input the resistor, there also be about a half cycle
(0.1s) low-frequency overcurrent. At the same time, a current Presence (I peak current for 27A and
for 0.5 seconds) at the open delta current open delta.

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 The system neutral grounding via arc suppression coil
Petersen coil capacity is large, and the impedance is much smaller than the PT field excitation
impedance of the inductor, It is equivalent to the high voltage side coil PT is temporarily shorted
after Single-phase ground fault is eliminated. Low-saturation current constitute loop by Petersen
coil and earth. So as not to cause a PT due to excessive current caused by high voltage side fuse or
itself burnt phenomenon. Also Petersen coil can also be compensated single-phase ground
capacitive current time. Figure 8 the arc suppression coil is in the state of overcompensation. Figure
8 Petersen coil in overcompensation state, off Degree of 6.28%, the inductance value 0.199H,
parallel resistance is 600Ω, fault phase voltage and high-voltage side excitation current waveform
after the failure to eliminate.

Figure 8 the system neutral grounding via arc suppression coil method

Conclusion
We made a detailed simulation analysis on the PT failure which is caused by failure elimination
of6 /10kv distribution network single-phase ground fault and solving measures by PSCAD /
EMTDC, simulation results can be drawn from 6 / 10kv distribution grid PT failures caused by a
single-phase ground mainly by the failure to eliminate low frequency after overcurrent caused.
Using secondary-side shunt resistor method can not be resolved by the elimination of single-phase
ground fault overcurrent caused PT saturation problems, priority should be using the system neutral
point arc suppression coil grounding.

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