XXIV Symposium
Electromagnetic Phenomena in Nonlinear Circuits
June 28 - July 1, 2016 Helsinki, FINLAND
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MODIFICATION OF THE OPERATING POINT OF RESIDUAL CURRENT
TRANSFORMERS FOR HIGH FREQUENCY
EARTH FAULT CURRENTS DETECTION
Stanislaw Czapp, Krzysztof Dobrzynski, Jacek Klucznik, Zbigniew Lubosny, Robert Kowalak
Gdańsk University of Technology, Faculty of Electrical and Control Engineering
ul. Narutowicza 11/12, 80-233 Gdańsk, Poland, e-mail: stanislaw.czapp@pg.gda.pl
500
RCD1
400
tripping current (mA)
Abstract - For protection against electric shock in low voltage
systems residual current devices are commonly used. However,
their proper operation can be interfered when earth fault current
with high frequency components occurs. Serious hazard of
electrocution exists then. One of the most important element of
residual current devices is a residual current transformer with
iron core. Tripping characteristic of residual current devices
strictly depends on the operating point and properties of the
current transformer. This paper presents problems of the
operation of the residual current transformer in presence of high
frequency current components.
300
200
RCD2
100
30
0
50 100
200
300
400
500
600
700
800
900
1000
frequency (Hz)
I. INTRODUCTION
Protection against electric shock in low voltage systems is
very often realized with the use of residual current devices
(RCDs) [1]. They are applied in various types of electrical
circuits also in circuits where there is earth fault current with
high frequency components. It leads to the problem of RCDs
compatibility with such a type of current [2-5]. Voltage and
current harmonics are mainly produced by power electronics
converters [6]. An example oscillogram of earth fault current
from circuit with frequency converter is presented in Fig. 1.
10 ms
10 ms
i (t)(t)
EE
0.5 A
50 Hz
150 Hz
0.1 A
0
2.5
3 kHz
5
6 kHz
7.5
10
12 kHz
12.5
(kHz)
15
17.5
20
22.5
25
Fig.1. Oscillogram of earth fault current in the case of fault in the output
terminals of the frequency converter, and spectrum of this current
The waveform presented in Fig. 1 comprises harmonics,
especially high-order harmonics. The order of harmonics
corresponds with the applied PWM frequency of the
converter.
Typical residual current devices are not compatible for the
detection of earth (residual) current comprising high
frequency components. For such earth fault currents, tripping
current of residual current devices can be very high [2]. This
negatively influences the effectiveness of protection against
electric shock. Figure 2 presents tripping current of the
example RCDs as a function of earth fault current frequency.
Tripping current of these RCDs rises with frequency rising. In
case of RCD1 there is no tripping if the frequency exceeds
400 Hz.
Fig.2. Tripping current of the selected RCDs (In = 30 mA) as a function of
earth fault current frequency
It is important to perform analysis of the operation of main
components of RCDs, especially their current transformer,
when high frequency residual current flows.
II. ANALYSIS OF THE OPERATION OF RESIDUAL
CURRENT TRANSFORMERS
Residual current devices comprise an iron core current
transformer (CT) and an electromechanical relay (ER) with
a permanent magnet (Fig. 3). The secondary current is(t),
transformed by the current transformer, in the one half-wave
amplifies the magnetic flux of the permanent magnet but in the
second half-wave that flux is reduced. If the current is(t)
reaches a predetermined level, the magnetic flux derived from
that current is high enough to reduce the magnetic flux of the
permanent magnet, and RCD opens the main circuit. In order to
ensure sufficient value of secondary current is(t), sufficient
value of secondary voltage es(t), especially peak value, should
be induced.
Secondary voltage (rms value) is defined by the following
formula:
Es  4.44  f  Ns  Φ  4.44  f  Ns  B  sFe
(1)
where: f – earth fault current frequency, Ns – number of turns
of the secondary winding,  – magnetic field strength, B –
magnetic induction, sFe – current transformer core crosssection.
Unfortunately, in some cases this voltage does not rise to
frequency proportionally (Fig. 4). For primary current equal
to the rated residual current In of a RCD, induced voltage
es(t) rises only slightly. If frequency rises 20 times (from 50
to 1000 Hz) the peak value of es(t) rises about 1.35 (instead
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137
of 20). It means that the tripping threshold of a RCD rises
rapidly and protection against electric shock may not be
effective.
PE
L1 L2 L3 N
RCD
is (t)
CT
ER
es (t)
test
RT
i(t) = ip (t)
load
In the case of this iron core, the proper operation of a RCD
(for various frequencies) can be achieved when the operating
point of the current transformer (i.e. iron core) is situated
close to the saturation point for the highest expected
frequency (for example fs1000 if the highest frequency is
equal to 1000 Hz).
A process of designing of residual current devices for
circuits of high frequency residual current should take into
account the properties of an iron core of residual current
transformers in presence of high frequency waveforms.
Operating point of current transformers should be modified
then. Figure 6 presents induced secondary voltage obtained
for the proposed modified operating point of the current
transformer. The expected rise in voltage occurs for primary
current equal to 5In. It is observed that the most important
value of the induced secondary voltage – peak value –
significantly rises with frequency rising. This result may be
achieved by modification of the windings of the current
transformer.
Fig.3. Simplified diagram of a RCD: CT – current transformer,
ER – electromechanical relay, RT – test resistance, i(t) – residual (earth fault)
current, ip(t) – primary current of the current transformer, is(t) – secondary
current of the current transformer, es(t) – induced secondary voltage of the
current transformer
Fig.6. Measured (for example residual current device) secondary voltage es(t)
for primary current ip(t) equal to the residual current 5In;
frequency: 50, 150, 1000 Hz
III. CONCLUSIONS
Fig.4. Measured (for example residual current device) secondary voltage es(t)
for primary current ip(t) equal to the rated residual current In;
frequency: 50, 150, 1000 Hz
One of the main sources of the results presented in Fig. 4 is
a very wide hysteresis loop of the iron core of the current
transformer for high frequency (Fig. 5). To achieve the same
level of induction in the current transformer core for higher
frequencies, a higher value of earth fault current is necessary.
50 Hz
induction
required level
of induction
150 Hz
1000 Hz
fs50
fs150
fs1000 field strength
Fig.5. Hysteresis loops of the selected current transformer for various
frequency primary current. Required magnetic field strength: fs50 for 50 Hz,
fs150 for 150 Hz, fs1000 for 1000 Hz
Proper operation of RCDs for earth fault current with high
frequency components strictly depends on the properties of
residual current transformer to be applied in the RCDs.
Process of selection of this transformer and its coordination
with other elements of RCDs should take into account the
frequency range of the earth fault current to be detected.
REFERENCES
[1] HD 60364-4-41:2007, “Low-voltage electrical installations – Part 4-41:
Protection for safety – Protection against electric shock”.
[2] S. Czapp, “Comparison of residual current devices tripping
characteristics for selected residual current waveforms”, Elektronika ir
Elektrotechnika, No. 4, pp. 7-10, 2010.
[3] F. Freschi, “High-frequency behavior of residual current devices”, IEEE
Trans. on Power Deliv., vol. 27, No. 3, pp. 1629-1635, 2012.
[4] T. M. Lee, T. W. Chan, “The effects of harmonics on the operational
characteristics of residual current circuit breakers”, Int. Conf. on Energy
Management and Power Delivery, Proc. of EMPD’95, vol. 2, pp. 715719, Nov. 1995.
[5] X. Luo, Y. Du, X. H. Wang et al., “Tripping characteristics of residual
current devices under nonsinusoidal currents”, IEEE Trans. on Ind.
Appl., vol. 47, No. 3, pp. 1515-1521, 2011.
[6] J. Schoneck, Y. Nebon, “LV protection devices and variable speed
drives”, Cahier technique No. 204, Schneider Electric, 2002.
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Proceedings of EPNC 2016, June 28 - July 1, 2016 Helsinki, FINLAND