Inrush Current Transients During Energization of an Unloaded

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INRUSH CURRENT TRANSIENTS DURING ENERGIZATION OF AN UNLOADED
TRANDSFORMER ON THE ESKOM NETWORK
Mfundo Bukubukwana* and Rastko Zivanovic**
*Eskom Distribution, Network Services Project Engineering, P.O. Box 8610, Braamfontein, 2000,
South Africa
E-mail: [email protected]
**Tshwane University of Technology, Department of Power Engineering, P.O. Box X680, Pretoria, 0001,
South Africa
Email: [email protected]
INTRODUCTION
Eskom has experience a high numbers of power transformer
failures as from the period of 1996 – 2004. The high
percentage of these power transformer failures are
contributed by internal failures. Eskom is busy carrying out
research studies on the influence of through faults in the
power transformers. This paper concentrates mainly on the
inrush currents transients during energization of the
unloaded transformers. The field testing was carried out and
the data was analysed and presented for debate. A short
overview of the simple controlled switching techniques was
presented as the solution to reduce inrush currents.
DEREGULATION IN
SUPPLY INDUSTRY
THE
ELECTRICITY
Deregulation in the Electricity Supply Industry (ESI) has
introduced significant pressure on the utilities to focus on
Return-on-Investment. This means that they must balance the
demands of reliable delivery of energy with economic
performance [1]. Taking into account the increase demand of
served energy, there is an urgent need to increase the
availability and reliability of the energy supplies with an
ageing electrical infrastructure. Adaptations are required to
operate in deregulated, competitive market where switching
operations tend to be more frequent. This has introduced
constraints in obtaining outages for HV equipment, which has
resulted in poor or inadequate maintenance. This results in a
noticeable increase in the number of unpredicted transformer
failures. This is not desirable due to the huge cost associated
with such failures, which apart from the capital cost of repair
or replacement of the failed transformer, includes costs of
unserved energy, possible environmental pollution etc [1].
An analysis of transformer failure statistics by the Eskom
Plant Performance Department as from 1996 -2004. Table 1
shows a number of failures for each cause of failure.
CIRED2005
Session No 1
TABLE 1: Causes of failures
Cause of failure
Bushing
Tapchanger
Cable fault
Insulation
Lightning
Protection
Unknown
Total number of
failures
10
1
6
72
22
1
11
Look at table 1 above there is a high number of insulation
failures causes of the transformers. This suggests that inrush
currents could be one of the major causes of the insulation
failures [2]. The inrush current magnitude usually does not
exceed the fault current withstand capability of the
transformer, however the duration of the inrush current
stresses are significantly longer and occurrence is more
frequently than that of the short circuit which is cleared by the
relay protection within some tens of milliseconds.
The inrush currents are 8 -10 times the transformer normal
load current [2].
RANDOM SWITCHING OF TRANSFORMERS
Random switching of transformers is well known to produce
high inrush currents which can reduce their residual life and
may also lead protective relay mis-operation and power
quality reduction. Transformer energization at a non-optimal
point on the voltage cycle can create large core flux
asymmetries and operation above saturation levels of the core
of the transformer. The main cause of this phenomenon is the
presence of a residual magnetic flux in the transformer core.
This, in turn, may result in high magnitude currents that are
rich in harmonic content and have a high direct current
component. These currents can cause electrical and
mechanical stresses in the transformer and, depending on the
prevailing power system conditions, may also cause severe
temporary overvoltages (TOV’s). In the most severe cases,
TOV’s may exceed the energy absorption capabilities of
surge arresters and expose the equipment in the substation to
overvoltages exceeding their withstand levels [3].
i.
The one main system is consisting of the differential
protection, with differential protection relay taking care
of the high fault level faults.
ii.
The restricted earth fault relay and the Buchholz relay
close to the conservator tank. The Buchholz relay is also
catering for the high fault level faults, including all
phase-to-phase faults, which are expected to be quite
heavy in nature.
TRANSFORMER PROTECTION PHILOSOPHY
IN ESKOM
Transformer protection requirement encompasses a number of
aspects. Of prime importance for an Eskom is the requirement
to ensure correct, or best possible, operation of the overall
fault clearance system, even in the event of the failure of one
of the components within the protection scheme itself, or the
failure of an external component within the overall fault
clearance system [4].
Protection regarded as essential for protection of power
transformer of normal rating up to including 10MVA
a)
b)
c)
d)
e)
f)
g)
h)
i)
HV Inverse Define Minimum Time (IDMT) Earth Fault
protection (HV winding earthed).
HV IDMT and Instantaneous Phase Overcurrent
protection.
HV Restricted Earth Fault (REF).
HV Breaker fail.
LV IDMT Phase Overcurrent protection (if no HV IDMT
OC).
LV IDMT Earth Fault protection.
LV REF.
Sustained Fault Timer (SFT Timer)
Surge devices, pressure devices, oil and winding
temperatures.
The restricted earth fault also cover the internal earth
faults, including the low fault level faults not detected by
the Buchholz relay.
iii. The capability of the overall protection scheme to cater
for high fault level faults is further supplemented by the
inclusion of HV instantaneous overcurrent protection and
inputs from tank pressure device, both of which are
duplicated.
iv. The winding/oil temperature trips are realized as single
inputs but results in the issuing of a trip output to both
circuit-breaker trip coils as a result of cross-tripping.
The usual HV and MV IDMT overcurrent and earth fault
protection are included to offer local and remote back-up.
v.
Duplicated breaker failure protection for each breaker is
provided [4].
INRUSH CURRENT MEASURED ON THE
ESKOM NETWORK
Inrush current measurements were performed at switching –
on of the 275/88kV 315MVA transformer. The measured
inrush currents shown in Figure 1.
Protection regarded as essential for protection of
distribution power transformer of normal rating above
10MVA
Inrush Current - Red Phase
b)
c)
d)
e)
f)
g)
h)
i)
Biased differential protection - HV IDMT and
Instantaneous Phase Overcurrent protection.
HV IDMT Earth Fault protection (HV winding earthed).
HV REF
LV IDMT Phase Overcurrent protection (if no HV IDMT
OC).
LV IDMT Earth Fault protection.
LV REF.
Tertiary IDMT Phase Overcurrent protection (three
windings).
Tertiary IDMT Earth Fault Protection (three windings).
Surge devices, pressure devices, oil and winding
temperatures.
The philosophy requires each main protection system to be
able to cater for all types of faults.
CIRED2005
Session No 1
20
10
I (pu)
a)
0
-0.1-10
0.1
0.3
-20
-30
Time (sec)
0.5
0.7
b) Effects of circuit breaker prestrike.
c) Errors in the measurement of residual flux
d) Transformer core or winding configurations that
prevent an optimal solution [5].
I (pu)
Inrush Current - Blue Phase
SIMULATION RESULTS
20
0
-0.1 -20
-40
-60
0.1
0.3
0.5
0.7
Time (sec)
Three simulation tests where performed on the single phase
transformer model at norminal magnetizing current,
Im ax = 4A using MATLAB Student Version to evaluate the
magnetizing inrush currents. The first simulation test was
done closing the circuit breaker at applied voltage phase
angle, α = 0 o . The second simulation test done closing the
circuit breaker at applied voltage phase angle, α = 45 o . The
Inrush Current - White Phase
I (pu)
40
20
0
-20
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time (sec)
third simulation test done closing the circuit breaker at
applied voltage phase angle, α = 90 o .
The evaluation of the simulations revealed the following:
1. @ Applied Voltage phase angle, α = 0 o .
a) Magnetizing inrush current was extremely high
at, 10Imax = 40A .
b) The transformer was very highly saturated as shown
by the B-H curve in figure 2.
Figure 1 Inrush currents measured in each phase
SHORT OVERVIEW OF
SWITCHING TECHNIQUE
CONTROLLED
It is well known that the optimal closing for shunt capacitors
is at the instant when the source voltage is equal to the
voltage on the capacitor. For the case of controlled closing of
transformers the “trapped charge” is analogous to the residual
flux and, for optimum energization, the “induced” flux at the
instant of energization must equal the residual flux. Whilst
there is no induced flux prior to energization the source
voltage has the prospect to create an induced flux. If the
source voltage is considered as a virtual flux source, then the
optimal instant to energized a transformer is when the
“prospective” flux is equal to the residual flux.
These can be achieved by controlling the phase angle of the
source voltage and the inrush current can be reduced to 80%
as the experimental test prove that [2] [3].
For practical application the following factors need to be
considered to achieve the goal of controlled switching:
a)
CIRED2005
Session No 1
Deviations in circuit breaker mechanical
closing time
Figure 2 Instant of energization at applied voltage phase angle, α
= 0o
2.
@ Applied Voltage phase angle, α = 45 o .
a)
Magnetizing inrush current was slightly smaller
at, 0.08Imax = 0.32A
b) The transformer was slightly saturated as shown by
the B-H curve in figure 3.
Figure 4 Instant of Energization at Applied Voltage Phase Angle,
α = 90
o
CONCLUSIONS
1.
Figure 3 Instant of Energization at Applied Voltage Phase Angle,
a)
@
α = 45
o
2.
Simulation test results proved that magnetizing inrush
currents can be greatly reduced during the instant of
energization of power transformer by controlling the
phase angle of the applied voltage.
Reduction of 80 – 90% of magnetizing inrush current
from worst case can be achieved.
Applied Voltage phase angle, α = 90 o .
•
•
Magnetizing inrush current was completely
reduced to minimum at, 0.02Imax = 0.08A
The transformer was unsaturated as shown by
the B-H curve in
Figure 4.
REFERENCES
[1] Anita Oommen, 2004, “Transformer Life-expectancy
Reduction: The influence of the Number of Through Faults,
Downstream Network Protection and ARC Selections”
Proceedings Southern African Power System Protection
Conference, JHB, South African,
[2] John H. Brunke and Klaus J. Frohlich, 2001, “Elimination
of Transformer Inrush Currents by Controlled Switching –
Part I: Theoretical Considerations.” IEEE Transactions on
Power Delivery, Vol. 16, No.2.
CIRED2005
Session No 1
[3] CIGRE WG A3.07, “Controlled Switching of HVAC
Circuit-breakers – Guidance for Further Applications
including Unloaded Transformer Switching, Load and Fault
Interruption and Circuit-Breaker Uprating.”, 09.2003.
[4] Eskom Distribution guide specification number:
SCSAGAAG0 Rev 3 “Transformer Protection Philosophy”
[5] John H. Brunke and Klaus J. Frohlich, 2001, “Elimination
of Transformer Inrush Currents by Controlled Switching –
Part II: Application and Performance Considerations.” IEEE
Transactions on Power Delivery, Vol. 16, No.2.
CIRED2005
Session No 1
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