Transformer Inrush Current Mitigation Concept for Hybrid Transformers
BURKARD Johannes
Transformer Inrush Current Mitigation Concept for
Hybrid Transformers
J. Burkard and J. Biela
Laboratory for High Power Electronic Systems (HPE)
ETH Zurich, Physikstrasse 3, CH-8092 Zurich, Switzerland
Email: burkard@hpe.ee.ethz.ch
Abstract—Large inrush currents can occur when power transformers are connected to the grid. In the past, a wide range
of countermeasures has been developed whereof the application
of power electronic converters recently received more attention.
With hybrid transformers combining a conventional transformer
with a power electronic converter, the elimination of inrush
currents can be realized without considerable additional hardware effort. For such hybrid transformers, a novel mitigation
procedure is proposed, which is based on the injection of
a synchronous core flux before the transformer is connected
to the grid. In addition, dedicated demagnetization strategies
applicable for grid connected converters with filter elements are
presented. The effectiveness of the proposed procedure is verified
by comprehensive simulations and a comparison to a conventional
method.
I.
Inrush current mitigation concepts
1
Inrush current
damping
2
Optimized
closing time
Inrush current
compensation
Shunt
VSI
Series
VSI
Resistor Reactors
Constant Synchronous
preflux
preflux
HT
LFT
MV grid
VMV
3
LV grid
3
3
MV
LV
Shunt
conv. B
DC
Aux
Series
conv. A
AC
3
When a power transformer is connected to the grid, a large
inrush current could occur due to the saturation of the core.
These large inrush currents impair the transformer lifetime, the
grid power quality as well as the functionality of grid protection devices. Different concepts to reduce such inrush currents
have been developed, which can be subdivided according to
the applied mitigation mechanism as shown in Fig. 1.
Group Œ: By inserting current limiting elements such as
resistors or reactors in series to the transformer windings,
a limitation of the inrush current could be obtained [1],
[2]. Furthermore, a voltage source inverter (VSI) controlled
as a dynamic resistor could be applied [3]. To avoid high
conduction losses, the damping elements have to be bypassed
during normal operation which requires additional switches.
Group : In theory, inrush currents could also be eliminated
by connecting the transformer windings of the three phases
sequentially to the grid at optimal points in time which are
defined by the residual core flux. However, circuit breakers
(CBs) with independent drives for each phase as well as the
measurement of the residual core flux would be necessary
[4]. Furthermore, the scatter of the CB closing time and
flux measurement errors deteriorate the effectiveness of this
approach [5].
Group Ž: In [6] a VSI is connected in parallel to the transformer windings in order to compensate the inrush current by
injecting a current of opposite direction. With this approach,
however, excessive currents and mechanical stresses of the
transformer windings cannot be avoided.
Core flux
manipulation
4
Fig. 1: The inrush current mitigation methods can be subdivided into four
groups based on the underlying mitigation mechanism. In this paper, the focus
is on the synchronous prefluxing concept.
I NTRODUCTION
EPE'17 ECCE Europe
3
VLV
Vser
AC
DC
Back-to-back conv.
Fig. 2: Schematic of a HT allowing the control of VLV as well as active and
reactive power flow.
Group : The last group of mitigation techniques relies on
the manipulation of the core flux before the transformer is
magnetized. This includes the demagnetization of the core
[7] and the injection of a defined constant residual flux by
means of a precharged capacitor. For an effective elimination
of inrush currents, the injection of a constant preflux must be
combined with an optimized grid connection instant (see group
), which entails the mentioned requirements for the CBs [8],
[9]. As an alternative solution of group , the application of
power electronic converters allows the injection of a sinusoidal
preflux whereby inrush currents could be completely avoided.
In contrast to the aforementioned methods, this so called
synchronous prefluxing method does neither require additional
switches nor flux measurements while the effectiveness is
independent of the closing time scatter of the CBs. Therefore,
the power electronics based synchronous prefluxing method
is the most promising solution of the mentioned alternatives.
Although the basic operation principle has been studied in
the literature, the high additional cost of the converter has
prevented a wide practical application so far [10], [11].
For hybrid transformers (HTs), which combine a power electronic converter and a conventional low frequency transformer
(LFT) to enhance grid controllability, the implementation of
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Transformer Inrush Current Mitigation Concept for Hybrid Transformers
BURKARD Johannes
LFT model (Fig. 5)
MV CB
LV CB
LV load
LCL filter Conv. B
DC
VAux C LFB
FB
3
AC
Conv. A
DC
3
3
VMV
3
MV grid
MV
3
LV
Aux
AC
De-/magnetization
control
External turn-on/off
Fig. 3: 3D CAD rendering of a possible assembly of the converter and the
LFT. The HT interfaces the 20 kV MV and 400 V LV grids with a calculated
total HT system efficiency of 98%.
VDC
Fig. 4: The simulation model of the HT includes an equivalent circuit of the
LFT as detailed in Fig. 5, CBs on both transformer sides, a controller and
the shunt connected converter B. Converter A is not required for the inrush
current mitigation and is therefore shown in grey.
TABLE I: Parameters for the model of the 400 kVA, 20 kV-400 V HT.
the synchronous prefluxing technique can be realized without
considerable additional effort. Fig. 2 shows a possible configuration of a HT as presented in [12]. The major share of the
power is transferred via the LFT whereas the converter is only
rated for a fraction of the LFT power (typically 10%). Thus,
the high efficiency and robustness of the LFT and the flexibility
of the power electronic converter are combined. The back-toback converter of the HT consists of a shunt connected side B
and a series connected side A (Fig. 2). Besides the control of
the voltage VLV as well as the active and reactive power flow,
HTs also allow active filtering and load balancing in order to
ensure a high power quality even in the presence of a high
share of renewable energy sources. Due to the connection of
converter side B to an auxiliary (Aux) winding of the LFT, a
direct magnetization of the core is possible. Fig. 3 depicts a
3D CAD rendering of a possible assembly of the converter and
the LFT. Further information about the HT concept is available
in [12] and [13].
In this paper, a novel inrush current mitigation procedure
based on synchronous prefluxing is presented which allows the
elimination of inrush currents without considerable additional
hardware effort if applied to HTs. After the presentation of
the simulation model of the HT in section II, a conventional
mitigation method is investigated as a benchmark case in section III. The concept of the synchronous prefluxing technique
and the proposed procedure including a novel demagnetization
method are explained in section IV. Section V proves the
effectiveness of the presented procedure to eliminate inrush
currents by comprehensive simulations.
II.
S IMULATION M ODEL
The considerations of this paper are exemplarily carried out
for a 400 kVA three-phase HT fed from a 20 kV MV grid,
which supplies a radial 400 V LV grid. This HT consists
of a 400 kVA LFT combined with a 40 kVA back-to-back
converter. The specifications of the HT are given in Tab. I.
The simulation model including an equivalent circuit of the
LFT is shown in Fig. 4. The controller for the magnetization
and demagnetization shown in this figure measures the MV
and Aux side transformer voltages VMV and VAux as well as
EPE'17 ECCE Europe
Parameter
Value
Sn
VMV , VLV , VAux
Controllability
Sconv , Vser
Topology
DC-link
LFB1 , CFB , fsw
400 kVA
20 kV, 400 V, 28 V, RMS, line-line
±10% of nominal VLV , P and Q
40 kVA, 23 V
2-level VSIs, back-to-back
60 V, 6.8 mF
10 μH, 100 μF, 50 kHz
the DC-link voltage VDC and controls converter B as well as
the MV and LV grid circuit breakers (MV CB and LV CB).
An external turn-on/off signal starts the process which connects
and disconnects the HT to and from the grid. Since converter A
of the HT does not influence the presented mitigation methods,
it is not considered in this model and the load of the LV
grid is directly attached to the star-connected LV winding
for simplicity. For the demagnetization and magnetization of
the transformer core, the DC-link of the converter has to be
powered by an external power supply (e.g. a battery) which is
represented by the voltage source VDC . Furthermore, an LCLfilter consisting of the boost inductor LFB , the filter capacitor CFB and the Aux side transformer leakage impedance,
represented by a magnetic reluctance within the transformer
model, is considered (cf. Fig. 4). The LCL-filter is damped by a
capacitor-resistor branch in parallel to CFB designed according
to [14].
In order to simulate inrush currents, the transformer core is
modeled by saturable reluctances with hysteresis. The reluctances Rc,o and Rc,m represent the outer and middle limbs as
shown in Fig. 5. The transformer limbs and the associated
windings are denoted as 1, 2 and 3 for the left, middle and right
limb. Since the residual flux Bres at the beginning of the simulation cannot be directly set with the used simulation software,
an initial voltage pulse is applied to the transformer windings
to magnetize the core to different flux levels before the inrush
mitigation methods are tested. The leakage inductances on the
MV, LV and Aux side of the transformer are represented by
the reluctances Rσ,MV , Rσ,LV and Rσ,Aux . For validating the
transformer model, the MV line currents of phase 1 are given
in Fig. 6 for the case that the transformer is connected to the
MV grid without and with a residual core flux while the LV
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Transformer Inrush Current Mitigation Concept for Hybrid Transformers
Phase 2
Rı09
Rı09
Rı/9
Rı/9
/9
2
Rc,o
Rc,m
Rc,o
09
Phase 3
Initial magn.
to set Bres
Connection of
remaining
phases
Connection of
first phase
1
Phase 1
Rı09
B in T
Phase 1
BURKARD Johannes
0
Rı/9
Phase 2
-1
Rı$X[
Rı$X[
$X[
Rı$X[
-2
Fig. 5: In the LFT model, the saturable reluctances Rc,o and Rc,m represent
the core limbs. The leakage reluctances are modelled by Rσ,MV , Rσ,LV and
Rσ,Aux . The windings attached to the left, middle and right limb are denoted
as phase 1, 2 and 3.
16
10
20
30
40
50
60
Fig. 7: Prospective (dashed) and actual (solid) flux densities if the phases are
energized at the ideal time instants with the ”rapid closing strategy” [4]. After
the connection of the first phase, the residual fluxes in the remaining phases
are also modified.
C ONVENTIONAL I NRUSH M ITIGATION C ONCEPT
Inrush currents are caused by core saturation and occur if the
residual core flux Φres prior to the magnetization deviates from
the ideal DC offset-free prospective flux Φpros defined by the
applied winding voltage vwind . To avoid inrush currents, (1)
has to be satisfied at the point in time tmag when the winding
is connected to the grid. N is the number of turns of the
considered transformer winding.
12
IMV,1 / În
0
t in ms
III.
With res. flux
No res. flux
Phase 3
8
4
= vwind
0
0.2
0.4
t in s
0.6
0.8
1
Fig. 6: Simulated ratio of the MV line current of phase 1 IMV,1 to the peak
value of the nominal current Iˆn for a typical connection of the LFT to the MV
grid with and without residual flux. No load is connected to the LV windings.
For the simulation with residual flux, an initial voltage pulse is applied to
magnetize the core limbs
TABLE II: Parameters of the 400 kVA LFT model.
Parameter
Value
Core material
Sat. flux density Bsat
Rem. flux density Brem
Nom. flux density Bn
Sat. field strength Hsat
Coerc. field strength Hc
Core cross sectional area Ac
Length of inner & outer limbs
Wind. losses MV, LV, Aux
Core losses
Number of turns
Short circuit imp. MV-LV
Short circuit imp. MV-Aux
Cold rolled grain oriented steel
1.7 T
1.5 T
1.26 T = 0.75 · Bsat
127 A m
40 A m
0.025 m2
0.5 m, 1.2 m
2800 W, 1500 W, 500 W (full load)
600 W
NMV = 2858, NLV = 33, NAux = 4
zSC,MV−Aux = 4%
zSC,MV−Aux = 6.5%
circuit breakers CBLV are open. There, the specifications of
the LFT given in Tab. II are used. With the given transformer
model, the peak inrush currents are more than 10 times larger
than the peak line current during nominal operation Iˆn which
is in accordance with amplitudes for inrush currents published
in [5] and [15].
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1 tmag V̂ · cos(ωt) dt
(1)
Φres = Φpros =
N 0
As a benchmark case, the optimized closing time mitigation
concept (cf. Fig. 1 ) is analysed in the following. The
performance of the proposed inrush current mitigation concept
is then compared to this conventional concept in section V.
As a first step of the optimized closing time method, the
winding attached to the limb with the lowest Φres is connected
to the MV grid when the residual and prospective fluxes
of this phase are equal, so that no inrush current occurs.
Thereby, the flux distribution of the other two limbs is also
modified. For the connection of the two remaining phases, the
so-called ”rapid closing strategy” is applied in the following.
With this strategy, the remaining phases are connected at the
next instant when the measured and prospective flux of these
phases coincide [4]. Since the optimal connection time tmag
depends on Φres , a flux measurement is required which can
be achieved by continuously measuring and integrating the
winding voltages. In Fig. 7, the waveforms of the prospective
and actual transformer flux densities are shown for the ideal
case which leads to a magnetization without inrush currents.
!
However, the non-ideal measurement of the flux and the closing time scatter of the CBs impede the ideal energization of the
transformer, so that the inrush currents cannot be completely
avoided. To simulate the performance of this concept including
the non-idealities, the same model as described in section II
can be used but the converter, the filter and the LV grid
are removed. In the considered case, the flux measurement
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Transformer Inrush Current Mitigation Concept for Hybrid Transformers
BURKARD Johannes
4
Initial demagnetization (Fig. 14)
Ɏoffset / Ɏn:
10%
8%
6%
4%
2%
0%
1
0
0
0.5
tdelay in ms
Turn-on
2
CBs off, Bres= 0
1
1.5
Fig. 8: Simulated inrush currents resulting with the conventional optimized
closing time method as a function of the MV CB closing time delay tdelay
reassembling closing time scatter. The relative flux measurement uncertainty
Φoffset
Φn is used as a parameter.
uncertainty is exemplarily reproduced by measuring the ideal
fluxes of the three limbs and adding flux offsets of +Φoffset
and −Φoffset to the ideal measurements of phases 1 and 3. By
delaying all MV CB closing signals by tdelay , the worst-case
closing time scatter is emulated. The adverse effect of these
non-idealities depends on Bres and on the connection instants.
For some points on the flux curve, an unprecise closing leads
to comparably high differences between the actual and the
prospective flux as has been illustrated in [4]. Fig. 8 depicts the
resulting inrush currents for the worst-case Bres , closing time
delays of 0 ≤ tdelay ≤ 1.5 ms and measurement uncertainties
of 0 ≤ Φoffset ≤ 0.1 · Φ̂n . According to [16], CBs specially
designed for this mitigation concept achieve a closing time
scatter below 1 ms. Hence, inrush currents of up to 2· Iˆn have to
be expected if a flux measurement error below 6% is assumed.
In comparison to the connection without any inrush mitigation
method, the maximum excited currents can be significantly
reduced which proves the good performance of this method.
However, the CBs require independent drives for each phase
and a special mechanical design to minimize closing time
scatter. Furthermore, a precise measurement and integration
of the MV grid voltage is necessary to indirectly measure the
core fluxes.
IV.
I NRUSH C URRENT M ITIGATION C ONCEPT FOR
H YBRID T RANSFORMERS
In contrast to methods aiming for a connection at the optimal
time instant, the synchronous prefluxing method (cf. Fig. 1
) induces an AC flux equal to the prospective flux prior to
the transformer connection. After prefluxing, the transformer
can be connected at any time without exciting inrush currents
so that no special CBs with reduced closing time scatter and
independent drives for each phase are needed.
A flowchart of the proposed mitigation procedure for HTs is
shown in Fig. 9. Before the individual steps are detailed in
section IV-A and section IV-B, the basic principle is explained
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Prefluxing
Demagnetization
Close MV CB
Open MV CB
Conv. B off
Conv. B on
Close LV CB
Open LV CB
Start HT control
Stop HT control
Turn-off
Iinrush / În
3
Nominal HT operation
Fig. 9: Procedure of the proposed inrush current mitigation method. Before
the injection of a synchronous flux is possible, a demagnetization of the
core is required. If the HT is connected for the first time or after a sudden
disconnection, an initial demagnetization as detailed in IV-B2 has to be
performed.
in the following. At the beginning of the turn-on process, it is
assumed that the LFT is in the demagnetized state (Bres = 0).
The converter side B of the HT is used to inject a synchronous
preflux by modulating a three-phase voltage in phase with
the medium voltage grid voltage VMV as will be discussed
in section IV-A. Subsequently, the MV CB is closed and the
semiconductors of converter B are turned off which results in
a commutation of the magnetizing current from the converter
to the MV grid. Since (1) is fulfilled, no inrush currents are
excited. After closing the LV CB to supply the load, the
control of the back-to-back converter can be started which
includes synchronizing to the Aux winding voltage with a
PLL and generating the required output voltage Vser . When
the HT is turned off, the steps are repeated in reversed order.
After shutting down the HT control and opening the LV
CB, converter B applies a three-phase voltage synchronous
to VMV across the Aux windings. By opening the MV CB,
the magnetizing current commutates to the converter and a
demagnetization is performed resulting in Bres = 0 as will be
described in subsection IV-B1. If the HT should be connected
to the grid for the first time or after a sudden disconnection,
a dedicated demagnetization has to be performed before the
prefluxing is started as will be described in subsection IV-B2.
A. Synchronous Prefluxing with Power Electronic Converter
Fig. 10 shows a prefluxing strategy which is applied at the
beginning of the turn-on process when the transformer is in
the demagnetized state and all CBs are open (cf. Fig. 9). At
time t0 , the converter starts to generate a three-phase voltage
in phase with VMV with linearly increasing amplitude which
induces a three-phase, DC offset-free flux in the transformer
core. At t1 , the winding voltages reach the nominal value and
the core flux is identical to the prospective flux so that the
MV CBs can be closed without inrush currents at t2 . This
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Transformer Inrush Current Mitigation Concept for Hybrid Transformers
t1
0
-1
H
ĭ in p.u.
1
t
0
t
-1
H
Phase 3 Phase 2 Phase 1
1
t
0
-1
t0
Phase 3 Phase 2 Phase 1
t
0
t2
t1
t0
t1
-1
t2
Fig. 10: Ramp prefluxing concept: Starting from the demagnetized state, the
three-phase Aux winding voltage is ramped up to the nominal value whereafter
the MV CBs are closed.
method is denoted as ”ramp prefluxing” in the following. The
slope of the voltage ramp is a compromise between speed and
effectiveness. For a fast increase of the voltage amplitude, an
accurate prefluxing cannot be ensured since the positive and
negative peak flux values differ significantly due to the ramp
up. However, for a slow increase, the required time and losses
increase.
An alternative magnetization strategy denoted as ”sequential
prefluxing” is depicted in Fig. 11 and makes use of the threelimb construction of common distribution transformer cores.
If a flux is injected in the middle limb of the core, it splits
equally to the two outer limbs as long as the core does not
saturate. At t0 , the prospective flux of the middle limb is
assumed to be zero which coincides with the actual flux since
the transformer is demagnetized. During the following quarter
period between t0 and t1 , the converter applies v1 = v3 = − 12 v2
to the Aux windings where the amplitude of v2 is equal to the
nominal value. At t1 , the actual and prospective fluxes of all
phases coincide and the converter applies the nominal voltages
to all windings. The MV CBs can be closed without inrush
currents at t2 . With this method, the prefluxing is achieved
within a quarter of a grid period which results in a very
low energy consumption. Due to the considered LCL filter
of the back-to-back converter (Fig. 4), oscillations are excited
by the steps of the converter output voltage. For typical filter
designs, however, the occurring overvoltage does not exceed
the winding withstand voltage defined by LFT standards (e.g.
IEC 60076-3, [17]). In [18], the effectiveness of the sequential
prefluxing strategy is experimentally verified for a UPS system
feeding multiple transformers.
For both the ramp and the sequential prefluxing method, a measurement of VMV is required for synchronizing the converter
output voltage. Due to the comparably low magnetization
currents, the voltage drop across the boost inductor LFB can
be neglected during prefluxing and a simple open-loop control
is sufficient.
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1
ĭ in p.u.
V in p.u.
t0
B
1
t2
V in p.u.
B
BURKARD Johannes
t0
t1
t2
Fig. 11: Sequential prefluxing concept: Injection of a synchronous preflux by
sequentially magnetizing first the middle and then the outer core limbs. The
prospective fluxes and voltages are shown in dashed lines.
B. Demagnetization with Power Electronic Converters
In order to induce a defined synchronous flux in the transformer core during turn-on, the residual flux of each limb has
to be known. By demagnetizing the transformer before the
prefluxing step, a defined initial state with Bres = 0 is reached
without an additional flux measurement.
In [7] and [19], a demagnetization method is presented which
uses a rectangular voltage source with constantly decreasing
period or amplitude to demagnetize the transformer. The
current is measured and triggers a reversal of the winding
voltage as soon as a predefined current limit is exceeded. For
power electronic converters connected to the grid, passive LC
or LCL filters are required to comply with EMI standards
during nominal operation (Fig. 4). If a converter with such a
filter is used to generate the rectangular voltage, the presence
of the capacitive filter elements limits the slope of the converter
output voltage during the voltage reversal. This reduced slope
would further increase the absolute value of the core flux
before the zero crossing of the winding voltage is reached.
At the beginning of the demagnetization procedure, where
the residual flux Φres could be close to the saturation level,
the reduced output voltage slope would saturate the core.
Considerable magnetization currents through the converter and
an inaccurate demagnetization would be the consequence. This
prevents the application of this demagnetization method if
capacitive filter elements are present as it is the case for the
HT. A further limitation of the conventional demagnetization
method is that the rectangular voltage is only applied to a
single winding even in case of three-phase transformers. As
a result, a simultaneous demagnetization of all transformer
limbs is not guaranteed. For a more efficient demagnetization,
[20] presents the possibility of additionally measuring the
core fluxes. However, this requires a very precise continuous
measurement and integration of the winding voltages which
is difficult to realize for transformers operating under harsh
field conditions. Hence, advanced demagnetization methods
are required which fulfill the following requirements:
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Transformer Inrush Current Mitigation Concept for Hybrid Transformers
B
BURKARD Johannes
B
1
1
t2
-1
H
ĭ in p.u.
1
t1
t
t2
Phase 3 Phase 2 Phase 1
0
-1
H
1
t
0
V in p.u.
0
ĭ in p.u.
t1
V in p.u.
t0
t
Phase 1 Phase 2
Phase 3
t
0
t0
-1
t0 t1
Fig. 12: Ramp demagnetization concept: After the CBs are opened at t0 , the
converter continues to apply the nominal three-phase voltage across the Aux
winding until t1 after which the amplitude is ramped down to zero and the
core is demagnetized at t2 . Besides the winding voltages and the core fluxes,
the B-H curve of an arbitrary core limb is depicted.
•
Effective demagnetization, independent of the time
instant of disconnection and of the residual flux.
•
Step changes of the winding voltage only at low
transformer flux levels to avoid core saturation.
•
Demagnetization of all core limbs considering the
interdependency of the core fluxes.
•
No flux measurement is required.
•
Fast procedure with low energy consumption.
In the following, three new demagnetization strategies which
fulfill the mentioned criteria are presented and discussed.
1) Demagnetization After Disconnection: Starting from the
normal operation, the transformer flux has to be reduced to
zero during the demagnetization step at the end of the turn-off
process of the HT (Fig. 9). Consequently, the same principles
as for the prefluxing step presented in section IV-A can be
applied but in reversed order.
Fig. 12 demonstrates the ”ramp demagnetization” strategy.
After opening the MV CB at t0 , the converter continues
to modulate the three symmetrical voltages across the Aux
winding of the transformer until it starts to linearly ramp down
the amplitude at t1 . When the amplitude reaches zero at t2 ,
the semiconductors of the converter are turned off. As for the
ramp prefluxing strategy, the slope of the voltage ramp is a
compromise between speed and accuracy.
With the ”sequential demagnetization” strategy shown in
Fig. 13, the converter continues to modulate the nominal
three-phase voltage in phase with VMV after opening the
MV CB at t0 . At t1 , the voltage across the middle winding
v2 is zero whereafter the converter applies v1 = v3 = − 12 v2
to the windings for one quarter period. The transformer is
demagnetized at t2 and the semiconductors of the converter
are turned off. Since the magnetizing currents of the phases
EPE'17 ECCE Europe
-1
t2
t0
t1
t2
Fig. 13: Sequential demagnetization concept: After opening the CBs at t0 ,
the converter continues to modulate the three-phase voltage until v2 = 0 is
reached at t1 . For the subsequent quarter period of v2 , the converter applies
v1 = v3 = − 12 v2 after which the transformer is demagnetized at t2 . Besides
the winding voltages and the core fluxes, the B-H curve of the left core limb
(phase 1) is depicted.
have to decay to zero after t2 , a minor remagnetization occurs
after the disconnection. With this procedure, a demagnetization
within a quarter of a grid period is possible which results in
low losses. As for the sequential prefluxing method, the voltage
steps applied to the outer windings do not occur at flux levels
close to saturation so that both the sequential prefluxing and
demagnetization strategy are applicable for converters with LC
or LCL filters.
A measurement of VMV is required for both demagnetization
strategies as a reference for the open-loop converter control.
2) Initial Demagnetization: An initial demagnetization has to
be performed when the HT is connected to the grid for the
first time or after a sudden disconnection for example due to
a grid fault (cf. Fig. 9). Consequently, the methods presented
in subsection IV-B1 cannot be applied. While the conventional
demagnetization procedure applying a rectangular voltage to a
winding is only applicable if the converter is coupled to the
LFT via a purely inductive filter, the demagnetization method
proposed in the following and depicted in Fig. 14 is also
applicable if capacitive filter elements are present. The basic
principle of this method is illustrated in Fig. 15.
At the beginning of the procedure, it is assumed that the
transformer is disconnected from the grid and has a residual
magnetization Bres = 0. Converter B of the HT is used to apply
phase-to-phase voltages VAux,i (i ∈ {1, 2, 3}) with a variable
amplitude Vramp and variable offsets ΔVi to the Aux winding
system:
⎛
⎞ ⎛
⎞
⎞
⎛
cos(2π fdemagt)
ΔV1
VAux,1
⎝ VAux,2 ⎠ = Vramp · ⎝ cos(2π fdemagt − 2π ) ⎠ + ⎝ ΔV2 ⎠
3
VAux,3
ΔV3
cos(2π fdemagt + 2π
3 )
In step Πthe amplitude Vramp of the sinusoidal three-phase
voltage is linearly increased (cf. Fig. 14 and Fig. 15). The
magnetizing currents of each phase are measured and compared to predefined limits in step . Depending on which limit
ISBN: 9789075815276 et CFP17850-ART
© assigned jointly to the European Power Electronics and Drives Association & the Institute of Electrical and Electronics Engineers (IEEE)
P.6
Transformer Inrush Current Mitigation Concept for Hybrid Transformers
Transformer disconnected, Bres  0
With the variable Voffset , the stepwidth of the DC flux reduction
can be adjusted. By continuously repeating steps Œ,  and
Ž, all transformer limbs are gradually demagnetized. If the
magnetizing currents of all phases exceed the positive and
negative limits within Tdemag , the DC part of the flux has been
removed and all offsets ΔVi are set to zero. Thereafter, the
sinusoidal voltage is ramped down to zero and the transformer
demagnetization is finished (Fig. 14 and Fig. 15 step ).
Vramp= ǻV1= ǻV2= ǻV3= 0
Increase Vramp
1
2
Compare currents to limits, All limits exceeded within Tramp
determine s1, s2, s3
Calculate and
add ǻV1ǻV2ǻV3
3
4
Ramp down Vramp,
Set ǻV1 ǻV2 ǻV3 = 0
Transformer demagnetised, Bres= 0
Fig. 14: Procedure of the proposed initial demagnetization method. A converter
is used to apply a sinusoidal voltage with increasing amplitude Vramp to the
transformer windings (step ). Variable offset voltages ΔVi are added such
that always the phase with the highest residual flux is demagnetized (step )
until the DC flux of all transformer limbs is zero.
B
B
Bres  0
B
Ilim
B
Bres= 0
I
I
-Ilim
-Ilim
Steps 1 & 2
Ilim
Step 3
I
-Ilim
Ilim
Steps 1 & 2
I
-Ilim
Ilim
Step 4
Fig. 15: Principle of the proposed initial demagnetization method. The B-I(H)
curve of an arbitrary transformer limb is shown. The amplitude of the
sinusoidal voltage is increased until a current limit ±Ilim is exceeded (steps
Œ & ). A DC-offset is added to the voltage to reduce the residual flux (step
Ž). When both the upper and lower limits are exceeded within one cycle, the
voltage is ramped down (step ).
is exceeded, a positive or negative DC offset is added to the
sinusoidal voltage of the respective phase for the subsequent
1
in step Ž such that the absolute value
period Tdemag = fdemag
of the DC flux is reduced.
For the following mathematical description, the variables
s1 , s2 , s3 ∈ {−1, 0, 1} are introduced which indicate whether the
magnetizing current of phase i exceeds the positive (si = 1) or
negative (si = −1) current limit. If the current either exceeds
no limit or both limits within Tdemag , the respective variable is
zero (si = 0). With the variables si , the offset voltages ΔVi are
calculated with the offset matrix M according to (2) in step Ž.
⎛
⎞
⎛
ΔV1
m11
⎝ ΔV2 ⎠ = ⎝m21
ΔV3
m31
=M
m12
m22
m32
⎞ ⎛
⎞
m13
s1
m23 ⎠ · ⎝ s2 ⎠
m33
s3
(2)
Since the three voltages VAux,i have to add up to zero, the same
must be true for ΔV1 , ΔV2 and ΔV3 for all values of si which is
equivalent to ∑3i=1 mij = 0 ( j ∈ {1, 2, 3}). The following offset
matrix complies with this requirement and is considered as an
example in the following analysis.
⎛
⎞
1
−0.5 −0.5
1
−0.5⎠
M = Voffset · ⎝−0.5
−0.5 −0.5
1
EPE'17 ECCE Europe
BURKARD Johannes
In case of a delta connection of the Aux transformer winding,
the magnetizing currents which need to be compared to the
limits for this procedure cannot be measured directly. By
neglecting the low frequency filter capacitor currents and
taking into account that the converter does not excite circulating currents in the transformer, the winding currents can be
calculated from the boost inductor currents.
With larger values for the parameters Vramp and Voffset , a faster
demagnetization with lower losses is possible. However, the
accuracy is reduced which could lead to a higher residual
flux after the demagnetization. The maximum demagnetization
frequency fdemag is defined by the maximum peak-to-peak
flux density which is required to exceed both the positive
and negative current limits at the transition to step . If the
voltage drop across the filter inductor LFB is neglected, the
maximum core flux density B̂max which needs to be excited at
this transition can be calculated:
B̂max (Ilim,outer ) =
1
NAux · Ac
π
2
0
max(Vramp ) · cos(2π fdemagt)dt
Since the amplitude of the Aux winding voltages Vramp is limited to the DC-link voltage VDC , the demagnetization frequency
fdemag has to fulfill (3).
fdemag <
VDC
2π · NAux · Ac · B̂max (Ilim,outer )
(3)
In Fig. 16, the Aux side magnetizing currents and the core flux
densities of an exemplary demagnetization with the presented
method are shown. There, fdemag = 40 Hz, Vramp = 15 Vs and
Voffset = 0.4 V are chosen which results in a maximum residual
of Bres < 0.1 · B̂n for all initial residual flux levels after a
demagnetization phase of 4 s. Taking into account converter,
core and winding losses, a total energy of approximately 2 kJ
has to be provided from the DC-link.
Due to the shorter flux path of the middle transformer limb
compared to the outer limbs, the nominal magnetization current of phase 2 is lower than that of phases 1 and 3. Hence,
also the current limit Ilim,middle for phase 2 is lower than the
limit Ilim,outer for phases 1 and 3.
It has to be noted that this demagnetization strategy could
also be applied during the turn-off of the HT as an alternative
to the methods described in subsection IV-B1 although this
would be unfavorable with respect to losses and duration of
the demagnetization.
ISBN: 9789075815276 et CFP17850-ART
© assigned jointly to the European Power Electronics and Drives Association & the Institute of Electrical and Electronics Engineers (IEEE)
P.7
Transformer Inrush Current Mitigation Concept for Hybrid Transformers
Repeating steps 1 , 2 and 3
30
Ilim,outer
Ilim,middle
VMV in kV
IAux in A
t0 Prefluxing t1 t2 t3 Nominal op.t4 t5 t6 Demagn. t7
Step 4
40
Phase 1
10
BURKARD Johannes
Phase 3
Phase 2
0
0
-40
-10
B in T
1
-Ilim,middle
-Ilim,outer
-30
2.4
Limit
exceeded
Phase 1
-1
1.6
1
IMV / În
B in T
0.8
0
Phase 3
-0.8
-1.6
0
Phase 1
Phase 2
Phase 3
0
-1
0
Phase 2
1
2
t in s
3
4
Fig. 16: Aux winding currents IAux and core flux densities B for an exemplary
demagnetization based on the procedure given in Fig. 14. If the magnetizing
current exceeds the predefined limit (±Ilim,outer and ±Ilim,middle for the outer
and middle core limbs), a DC voltage offset is added to the sinusoidal voltage
to push the flux levels towards zero. When all magnetizing currents exceed
their positive and negative limits within one period Tdemag , the sinusoidal
voltage is ramped down and the transformer is demagnetized. The envelopes
of the waveforms are highlighted.
V.
0
S IMULATIVE V ERIFICATION OF THE P ROPOSED
P ROCEDURE
To verify the effectiveness of the proposed inrush mitigation
techniques, a simulation of the complete procedure according
to Fig. 9 is performed both for the ramp and the sequential
prefluxing and demagnetization strategies. The simulations are
performed with the simulation model given in Fig. 4 and the
parameters given in Tab. I and Tab. II.
Fig. 17 shows the waveforms of the MV side voltages VMV and
currents IMV as well as the flux density B for the sequential
method. Starting from the demagnetized state at t0 , the core is
magnetized until t1 by applying the sequential magnetization
method shown in Fig. 11. Due to the step in the voltage
reference, oscillations of the winding voltages are excited
at t0 and t1 . The amplitude and damping of the oscillations
highly depend on the chosen filter topology and the damping
elements. At t2 , the MV CB and at t3 the LV CB is closed
whereupon the nominal operation of the HT starts. The MV
CB is closed without noticeable inrush currents.
At t4 , the opening of the LV CB terminates the nominal
operation. Converter B starts to modulate a three-phase voltage
in phase with VMV across the Aux winding and the MV CB is
opened at t5 . The sequential demagnetization method as shown
in Fig. 13 is performed between t6 and t7 . As for the initial
EPE'17 ECCE Europe
0.02
t in s
0.06
0.08
Fig. 17: Simulated voltage, flux density and current waveforms for the
procedure shown in Fig. 9 with sequential prefluxing and demagnetization.
Starting from the demagnetized state, the sequential prefluxing is performed
and the HT starts the normal operation. At the end, the core is demagnetized
sequentially.
demagnetization procedure presented in subsection IV-B2, the
residual magnetization after demagnetization is Bres < 0.1 · B̂n .
Since the assumed nominal peak flux is B̂n = 0.75 · Bsat , the
core will not saturate if the proposed prefluxing procedure is
applied at the subsequent turn-on of the HT with such a low
residual flux. With the duration of a quarter of a grid period for
the prefluxing and the demagnetization steps, the net energy
which must be supplied from the DC-link including converter,
core and winding losses is approximately 50 J.
In Fig. 18, the same procedure is shown for the ramp prefluxing
and demagnetization method. Between t0 and t1 the core is
magnetized by applying a voltage with increasing amplitude
to the Aux windings. At t2 , the MV CB and at t3 the LV CB
is closed whereupon the nominal operation of the HT starts.
The MV CB is closed without noticeable inrush currents.
At t4 , the opening of the LV CB terminates the nominal
operation. Converter B starts to modulate a three-phase voltage
in phase with VMV across the Aux winding and the MV CB
is opened at t5 . The ramp demagnetization method as shown
in Fig. 12 is performed between t6 and t7 by ramping down
the voltage applied to the Aux windings. Compared to the
first method, this procedure is more time consuming which
also reflects in the increased energy supply of 110 J. However,
a more accurate demagnetization of Bres < 0.02 · B̂n can be
achieved.
The simulation results prove the effectiveness of the proposed
inrush current mitigation concept. In comparison to the conventional method with the non-idealities explained in section III,
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© assigned jointly to the European Power Electronics and Drives Association & the Institute of Electrical and Electronics Engineers (IEEE)
P.8
Transformer Inrush Current Mitigation Concept for Hybrid Transformers
BURKARD Johannes
t0 Prefluxing t1 t2 t3 Nominal op.t4 t5 t6 Demagn. t7
R EFERENCES
VMV in kV
20
[1] R. Harchandani and R. Kale, “Selection of effective mitigation method
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in Engineeing and Science, 2014.
0
-20
B in T
1
[2]
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[3]
A. Ketabi and A. R. H. Zavareh, “New method for inrush current
mitigation using series voltage-source pwm converter for three phase
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[4]
J. H. Brunke and K. J. Frohlich, “Elimination of transformer inrush
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[5]
A. Ebner, M. Bosch, and R. Cortesi, “Controlled switching of transformers - effects of closing time scatter and residual flux uncertainty,”
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[6]
K. Shinohara, K. Yamamoto, K. Iimori, Y. Minari, O. Sakata, and
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0
-1
IMV / În
1
Phase 1
Phase 2
Phase 3
0
-1
0
0.08
t in s 0.16
0.24
Fig. 18: Simulated voltage, flux density and current waveforms for the
procedure shown in Fig. 9 with ramp prefluxing and demagnetization. Starting
from the demagnetized state, the ramp prefluxing is performed and the HT
starts the normal operation. At the end, the core is demagnetized by ramping
down the winding voltage.
the inrush currents can be further reduced significantly from
2 · Iˆn to practically zero. In contrast to the controlled switching
method, standard CBs with a single drive for all phases and
a normal closing time scatter can be applied. For both the
conventional and the proposed concept, a measurement of VMV
is additionally required if not already present at the substation.
For the power electronics based procedure, an external power
supply or a small battery is required to supply the prefluxing
and demagnetization process.
VI.
C ONCLUSION
Numerous inrush current mitigation concepts have been presented in the literature whereof the power electronic based
synchronous prefluxing method is especially promising. In
this paper, it is shown that the hybrid transformer allows the
implementation of this mitigation concept without considerable additional effort. A complete procedure consisting of a
demagnetization and a subsequent synchronous prefluxing step
is proposed. Different alternatives for those steps optimized for
speed or accuracy of the prefluxing and demagnetization are
presented and compared. In addition, a novel demagnetization
strategy applicable for converters coupled to the transformer
via LC or LCL filters is proposed. A simulative comparison
to the conventional mitigation method based on optimized
connection instants is performed which underlines that the
inrush currents can be significantly reduced from 2 · Iˆn for
the conventional method to practically zero for the proposed
procedure.
EPE'17 ECCE Europe
[8] J. Singh, “Prefluxing Technique to Mitigate Inrush Current of ThreePhase Power Transformer,” Int. Journal of Scientific & Engineering
Research, 2013.
[9]
“System, Apparatus, and Method for Reducing Inrush Current in a
Three-Phase Transformer,” United State Patent US 8,878,391 B2, 2014.
[10]
K. Yamamoto, K. Shinohara, I. T., and M. Miyake, “Reduction of
magnetizing inrush current in three-phase transformer with voltage
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[11]
R. Ekstrm, S. Apelfrjd, and M. Leijon, “Transformer Magnetizing
Inrush Currents Using a Directly Coupled Voltage-Source Inverter,”
ISRN Electronics, 2013.
[12]
J. Burkard and J. Biela, “Evaluation of Topologies and Optimal Design
of a Hybrid Distribution Transformer,” in Proc. of the Power Electronics
and Applications Conf. (EPE), 2015.
[13]
——, “Protection of Hybrid Transformers in the Distribution Grid,” in
Proc. of the Power Electronics and Applications Conf. (EPE), 2016.
[14]
R. Erickson, “Optimal single resistors damping of input filters,” in
Applied Power Electronics Conference and Expositio (APEC), 1999.
[15]
N. Chiesa, “Power Transformer Modeling for Inrush Current Calculation,” Ph.D. dissertation, NTNU, 2010.
[16]
J. H. Brunke and K. J. Frohlich, “Elimination of transformer inrush
currents by controlled switching. II. Application and performance
considerations,” IEEE Transactions on Power Delivery, 2001.
[17] IEC 60076-3 Power Transformers - Part 3: Insulation levels, dielectric
tests and external clearances in air, IEC Std., 2013.
[18]
Y. H. Chen, M. Y. Yeh, P. T. Cheng, J. C. Liao, and W. Y. Tsai, “An inrush current reduction technique for multiple inverter-fed transformers,”
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[19]
“IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors,” IEEE Std C57.152-2013, 2013.
[20]
M. Pütter, M. Rädler, and U. B., “Reliable demagnetization of transformer cores,” 2014.
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P.9