International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 12, December 2014)
Transformer Inrush Current and Related Challenges
Saurabh Shrivastava1, Ashfaque Khan2, Amita Mahor3
Factors contributing to the magnitude and duration of
inrush current are Magnitude of residual flux in transformer
core, Nonlinear magnetizing characteristic of transformer
core, Magnitude of source voltage at the switching instant
and Impedance and short circuit power of the source
including VAR abortion capacity.
The key adverse effects include-
Abstract—Energizing of large power transformers is
considered a critical event in the operation of an electric
power system. When a transformer is energized by the grid or
utility, it takes very high value current known as inrush
current, the typical value of this current could be ten times.
This inrush causes many problems like mechanical stress on
transformer & harmonics injection to generator windings,
and system protection malfunction. This transient
phenomenon needs detail analysis for system protection.
A. Mechanical and electrical stresses in windings
The amplitude of inrush current can be equal to that of
the short circuit current and may last longer depending on
system configuration. This can seriously damage the
windings through excessive mechanical stresses [5].
Keywords—Transformer inrush current, transformer
transients, inrush current model of transformer.
I. INTRODUCTION
The energizing of large power transformers is
considered a critical event in the operation of an electric
power system. When a transformer is energized by the grid
or utility, it takes very high value current known as inrush
current, the typical value of this current could be ten times.
This inrush causes many problems like mechanical stress
on transformer & harmonics injection to generator
windings, and system protection malfunction. With the
development of smart grid, distributed generation from
independent power producers is growing rapidly & to
energize the system in a situation like black start has long
been considered as challenging. The main reason for this
criticality is the unpredictable system transients post
energization. In the subsequent sections we will attempt to
focus on phenomenon, reasons, effects and mitigation plans
of this aspect and to ensure both steady state and transient
stability aspects of system.
B. Harmonic resonant over voltages
Transformer inrush currents are rich in harmonics and in
the event of resonance, a sustained Harmonic resonant over
voltages may exist and if these over voltages last for a long
period of time, they may eventually damage the equipment.
C. Mall operation of protective relays
D. Voltage dips:
Due to high magnitude and asymmetrical nature of
inrush current a voltage dip is observed by the system. The
magnitude, duration and unbalance of voltages in the
respective phases are function of system impedance,
source, transformer capacities etc.[6]
III. MITIGATION PLAN
As the phenomenon of transients is dynamic in nature
and largely depends on system configuration. Therefore, it
is not possible to set any rule of thumb to access the precise
behaviour of transients or suitability of system. The only
reliable way is through system simulation (Electromagnetic
modelling) in available electrical software. Still it is
experienced that a unit is capable to charge a transformer of
equal or moderately higher capacity without losing the
system stability. Though there are ways to mitigate the
transient but they are more system specific than a
generalized solution. Some of the mitigation methods are –
II. REASONS & EFFECT
When the transformer is charged, a transient current
known as magnetizing inrush current (of magnitude as high
as 10times of rated current) flows in to the system. This is
due to the nonlinear relationship of flux and magnetizing
current as transformer core is in saturation mode. It is not
only the high magnitude of inrush current but its
composition (rich in DC component and harmonics) and
duration are also the cause of concern which severely
affects the stability of the system.
450
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 12, December 2014)
A. Pre Insertion Resister
One of the solutions to mitigate the risk related to
transformer inrush currents is reducing the magnitude of
these currents. Traditionally it is done by using preinsertion
resistors in the circuit breakers. The voltage drop across the
pre-insertion resistor produce by the inrush current will
decrease the voltage on the transformer windings, which in
turns decreases the magnetic over flux in the core. As a
result, the magnitude of the transient magnetizing currents
will be reduced as well.[5],[6]
Capacitor banks in the high voltage network which are
used to help control voltage under normal conditions
generally cannot be used in black start conditions for two
reasons:
 They are generally sized for normal short circuit levels.
Under black start conditions the short circuit levels are
significantly lower and thus switching the capacitor bank
might cause excessive voltage change at the bus where it
is switched.
 The excessive capacitance may result in generator
self‐ excitation. Under self-excitation conditions, the
high voltage bus could theoretically rise in range of 2 to
4 p.u.
Even allowing for the action of arresters, this level of
overvoltage would be sufficient to damage equipment.
The amount of reactive margin that needs to be
maintained is dependent on the composition of the load,
including both steady state power factor and dynamic
behavior. The minimum loading of a generator is unit
dependent and is typically 30 to 60% of the generator full
load rating.
B. Low Impedance Charging
If system configuration permits then charge the main
transformer through auxiliary transformer. As small
transformer has not only relatively low magnitude inrush
current but duration is also small which will be
considerably lower than charging the main transformer first
Steps for fig – Charge the Ta and T1 (via Ta) on G1
Once inrush is decayed close the breaker of T1 towards G1
IV. INRUSH CURRENT EVALUATION THROUGH ATP
Inrush current analysis performed for 15/6.6 kV, YY
winding transformer, testing data of open circuit and short
circuit used to model the transformer in ATP. External
inductance used to model the saturation characteristics of
the transformer.
In model show blow the transformer is charged through
grid and the value of inrush current is being analysed for
different angle of supply voltage. Transformer discharging
case is also analysed and effect of transient due to residual
flux is being analysed.
As the transformer charging current also depends on the
pole discrepancy, circuit breaker is connected in the
incomer of the transformer and effect of pole discrepancy
on inrush current is included in the results.
Figure 1: Typical Circuit diagram of power station [7]
C. Soft Charging
Another way to energize the generator step up
autotransformer by connecting it prior to starting the
generator. Then the transformer would be energized
together with the generator under excitation control. This
would avoid any inrush effects. Unfortunately this
procedure cannot be followed on transformers which are
not unit transformers.
D. Load Composition
It is recommended to load the generator to its minimum
possible load prior to proceeding with energization of other
system components. Load consists of active & reactive
components and there are limitations on capacity of the
connected generators to supply or absorb reactive power.
Ideally the nature of load should not be highly capacitive
like capacitor banks or long transmission line etc.
Figure 2: ATP model of transformer
451
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 12, December 2014)
V. CONCLUSION
1100
[A]
It is accepted beyond doubts that transformer
energization is a transient phenomenon which not only
affects the system stability but to the system components as
well. No rule of thumb can be assumed. Based on the
details of the event it is a crucial call which largely relies
on the experience of the engineer and his knowledge of the
system.
900
700
500
300
REFERENCES
100
-100
0.00
0.05
0.10
0.15
0.20
[s]
[1] IEC 60071-2, Insulation co-ordination – Part 2: Application Guide.
(Third Edition, 1996)
[2] H. K. Hoidalen, ATPDraw for Windows version 5.5, 2010.
[3] J. A. Martinez and F. C. Aranda, "Tower Modeling for Lightning
Analysis of Overhead Transmission Line," ( Proceedings of 2005
IEEE Power Engineering Society General Meeting, Vol. 2, June 1216, 2005, p.p.1212- 1217).
[4] IEEE Standard for Insulation Coordination—Definitions, Principles,
and Rules, ( IEEE Std. 1313.1-1996).
[5] L.Prikler, G.Banfai, G.Ban and P.Becker, Reducing the Magnetizing
Inrush current by means of Controlled Energization and deEnergization of Large Power Transformer, International Conference
on Power System Transients, IPST 2003.
[6] W. Xu, S.G. Abdulsalam, S.Chen, and X. Liu, A Sequential Phase
Energization Method for transformer inrush current reduction, Part
II: Theoretical Analysis and Design Guide, IEEE Trans. on Power
Delivery, Vol. 20, pp. 950-957, April 2005.
[7] Salman Kahrobaee, Marcelo C. Algrain, Sohrab Asgarpoor,
―
Investigation and Mitigation of Transformer Inrush Current during
Black Start of an Independent Power Producer Plant‖ Electric Power
Division, Caterpillar, Inc., Peoria, USA
0.25
(f ile tr.inrush.pl4; x-v ar t) c:X0030A-TR_15A
Figure 3: Transformer Charging Inrush
7.0
[A]
3.8
0.6
-2.6
-5.8
-9.0
0
5
10
15
20
25
30 [ms] 35
(f ile tr.inrush.pl4; x-v ar t) c:X0030A-TR_15A
Figure 4: Transformer De-energizing Inrush
Value of inrush current at various phase angle of supply
voltage has been analysed and maximum value obtained in
indicated in figure3 and 4, from above figures it can be
concluded that for modelled 4MVA transformer the
maximum value of inrush current is 1100Amps, which is
seven times the nominal current value.
452