(1) ZCT (zero-phase current transformer)
The
ZCT is a current transformer for detecting minute ground fault current and
shall be distinguished from general current transformers (CT). For the ZCT,
mainly permalloy, a special material with high magnetic permeability, is used.
It consists of a permalloy core, a primary conductor which feeds the main
circuit current and a secondary winding on the core. The magnetic fluxes
generated by the currents of the phases of the primary conductor are vector synthesized
by the core, and electromotive force is generated on the secondary winding by
the magnetic flux according to the difference among the magnetic fluxes of the
phases. Therefore, if the vector-synthesized current of the phases is 0, the
magnetic fluxes cancel with one another in the core, and electromotive force is
not generated on the secondary winding regardless of the magnitude of the
primary current. On the other hand, if a ground fault occurs, the current
balance among the phases is disturbed, the core is excited by the magnetic flux
corresponding to the magnitude of the ground fault current, and electromotive
force is generated on the secondary wiring.
(2) Electronic circuit
The
circuit diagram is shown in Fig. 7. 5. The control power of almost all ELCB is
100 to 440VAC for facilitating selection, storage and maintenance. In addition,
models with 100 to 200VAC, 100 to 230VAC, 200 to 415VAC and 200 to 440VAC and
with fixed voltage of 100VAC are available.
1Detection of ground fault on secondary side of inverter
As
an example method of detecting the ground fault current on the inverter primary
side and secondary side with ELCB installed on the inverter primary side, below
is explained the method of detecting ground faults on the inverter secondary side
where the waveform distortion is the largest.
a. Spectrum of leakage current on inverter secondary side
Fig.
7. 6 shows the spectrum (200V, Δ connection, one-line grounding, the same hereinafter) of ground
fault current caused by the resistance on the inverter secondary side in the
case of use of Mitsubishi inverter FR-Z220. The spectrum consists of commercial
frequency, inverter operation frequency and carrier frequency components and their
harmonic components. The spectrum contains commercial frequency and inverter
operation frequency components at the same rate as that of the content of carrier
frequency components. Fig. 7. 7 shows the spectrum of ground fault current on
the inverter secondary side including the leakage current caused by earth capacitance
in the case of use of FR-Z220. This example simulates a case where the electric
circuit on the inverter secondary side is long and its earth capacitance is
large. Since the earth impedance caused by capacitance is inversely
proportional to the frequency, the content of harmonic components in the
carrier is higher compared to in Fig. 7. 6. This content increases in
proportion to the earth capacitance and carrier frequency. The content of
commercial frequency and inverter operation frequency components is identical
to that in Fig. 7. 6.
b. Concept of detection of ground fault on inverter secondary side
To
detect ground fault on the inverter secondary side, it is necessary to reduce the
influence of leakage current caused by earth capacitance on the secondary side.
For this purpose, we used a method to remove the carrier frequency and harmonic
content of the carrier which change depending on the earth capacitance and may
cause unnecessary operations and unstable sensitivity current with a low pass filter.
The filter characteristics are shown in Fig. 7. 7. IEC 60479-2 presents the
frequency characteristics of current value at which ventricular fibrillation is
caused by the current passing the human body shown in Fig. 7. 8. This curve
shows that, at frequencies higher than 1kHz used as the inverter carrier frequency,
the current value at which ventricular fibrillation occurs to expose the human body
to hazardous situation is 14 times or more the value at 50/60 Hz and there is a
low risk of electrical shock. Therefore, it is possible to realize stable
detection of ground faults while ensuring the safety of human body against electrical
shock by removing carrier frequency and the harmonic content of the carrier
upon detection and detecting ground faults based only on the fundamental wave components.
On inverters, the fundamental wave content is approx. 70% in the ground fault
current. This percentage is less affected by the earth capacitance and is in
proportion to the magnitude of ground fault current. Therefore, stable detection
of ground faults can be realized by judging based on the fundamental wave
content.
c. Structure and operation of ground fault detection circuit
As
the low pass filter for removing carrier frequency and harmonic components of
carrier, a digital filter was used. On ELCB
applicable to higher harmonics and surge, signals from the ZCT are input to the
input circuit and converted from analog to digital by the A/D converter. The
digitalized signals are input to the low pass filter that is a digital filter. The
digital filter is used because the filter provides sharp at tenuat ion necessary
for fundamental f requency components and carrier frequency (depending on the inverter
type, approx. 800 Hz on low-frequency products) and filter characteristics
without attenuating minute ZCT signals, and the filter constant for obtaining a
low cutoff frequency can be set without influence on stability of ZCT and electronic
circuit characteristics. Fig. 7. 9 shows the structure of electronic circuit,
and Fig. 7. 10 shows the digital filter block diagram. The ground fault current
discriminating circuit detects the magnitude of ground fault current and the
duration of signal. Fig. 7. 11 shows the function block diagram for explaining
the operation. When the ground fault signal level exceeds the detection level, charging
of the capacitor is started, and the occurrence of ground fault is detected
after a lapse of a certain time. Therefore, relatively small surge current
components which are leaked by the earth capacitance are removed. These circuits
are contained in one chip as an IC for ELCB. Fig. 7. 12 shows the effect of
digital filter in detection of ground fault current. Comparing the waveforms
before and after the digital filter, it is found that the fundamental wave components
can be effectively extracted from the leakage current on the secondary side
masked by the high frequency components. That is, not only on the primary side,
but also on the secondary side, ground faults can be stably detected through
the digital filter. However, for a circuit containing harmonic components, the
filter shall be used with load device leakage current distortion of 10kHz or
less and at 3A or less because the zero-phase current transformer (ZCT) of the
circuit breaker is overheated by iron loss. For circuit breakers with frame
size of 800A and above, it is necessary to use the filter with load device leakage
current distortion of 5kHz or less and at 3A or less.
Technique for prevention of unnecessary operations caused by
surge
As
an example method of detectings the ground fault current on the ino higher harmonics
and surge, signals from the ZCT are input to the input circuit and converted
from analog to digital by the A/D converter. The digitalized signals are input
to the low pass filter that is a digital filter. The digital filter is used
because the filter provides sharp attenuation necessary for fundamental
frequency components and carrier frequency (depending on the inverter type,
approx. 800 Hz on low frequency products) and filter characteristics without
attenuating minute ZCT signals, and the filter constant for obtaining a low
cutoff frequency can be set without influence on stability of ZCT and
electronic circuit characteristics. Fig. 7. 9 shows the structure of electronic
circuit, and Fig. 7. 10 shows the digital filter block diagram. The ground
fault current discriminating circuit detects the magnitude of ground fault current
and the duration of signal. Fig. 7. 11 shows the function block diagram for
explaining the operation. When the ground fault signal level exceeds the detection
level, charging of the capacitor is started, and the occurrence of ground fault
is detected after a lapse of a certain time. Therefore, relatively small surge current
components which are leaked by the earth capacitance are removed. These circuits
are contained in one chip as an IC for ELCB. Fig. 7. 12 shows the effect of
digital filter in detection of ground fault current. Comparing the waveforms
before and after the digital filter, it is found that the fundamental wave
components can be effectively extracted from the leakage current on the
secondary side masked by the high frequency components. That is, not only on
the primary side, but also on the secondary side, ground faults can be stably
detected through the digital filter. However, for a circuit containing harmonic
components, the filter shall be used with load device leakage current
distortion of 10kHz or less and at 3A or less because the zero phase current
transformer (ZCT) of the circuit breaker is overheated by iron loss. For circuit
breakers with frame size of 800A and above, it is necessary to use the filter with
load device leakage current distortion of 5kHz or less and at 3A or less.
Fig.
7. 14 shows the improvement of the performance to prevent unnecessary operations
of ELCB applicable to higher harmonics and surge comparing with that of a
conventional model. It was confirmed that no unnecessary operation was caused
in any case of gap-less surge absorber and discharge gap type surge absorber.
According to the waveforms verified in Fig. 7. 14, the performance to prevent
unnecessary operations is improved as stated below. (1) Resistance to leakage
current caused by surge: Three times or more as peak value
(2)
Leakage electric power energy (I2t) caused by surge: 100 times or more
 |
| Fig. 7. 14 Performance to prevent unnecessary operations caused by surge |
Improvement of leakage protection function by type A leakage
characteristics
Recently,
more machines are provided with inverters and servos to improve the performance
and accuracy of drive control. Inverters and servos have rectifier circuits,
and if the rectifier circuits go down, leakage current with half-wave rectified
waveform or phase-controlled waveform may occur. To detect this leakage current
and trip the circuit breakers to prevent electric shock and fire caused by
earth leakage, type A (specified by IEC 60947-2) leakage protection characteristics
for detection of half-wave rectified and halfwave phase controlled waveforms of
leakage current shown in Fig. 7.15 must be provided. Then, we enlarged the leakage
protection range by adding a function with the type A leakage characteristics
to CE-marked and UL-listed circuit breakers with frame size of 250A and smaller
(except some models).
Since
electric shock may cause loss of life, it is necessary to check the operation
of circuit breaker. The test device forms a ground fault simulation circuit as
shown in Fig. 7. 17. Current is applied to the circuit by pressing the test button
to make sure that the circuit breaker can operate surely upon occurrence of
ground fault. All ELCB circuit breakers are provided with this test device.
Earth leakage indication device
After
ELCB operates owing to earth leakage, the earth leakage indication button shows
that it has operated not owing to short circuit caused by overload. As shown in
Fig. 7. 18, the earth leakage indication button is lower than the surface in
the normal state and after the circuit breaker operates owing to overcurrent,
but it is protruded when the circuit breaker operates owing to earth leakage. The
button is reset automatically by the handle. The button will not be damaged
even if it is pushed down accidentally. Furthermore, it is designed not to
hinder operation owing to earth leakage even if it is pushed down for any
reason.
Trip button
Models
with a trip button can be mechanically tripped by pressing this button. With an
alarm switch (AL), it is possible to check the operation of the alarm circuit
for tripping owing to overcurrent and, if it has an operation handle, check that
the circuit breaker has been reset by the operation handle.
Sensitivity switching device
With
the sensitivity switching device, the rated sensitivity current can be easily
and reliably switched. Some models can switch in 3 stages, 100, 200 and 500mA, and
others in 2 stages, 100 and 500mA. JIS prescribes that a device which can
switch the sensitivity between high (30mA or 15mA, operation within 0.1 s) and
medium (50mA to 1000mA) levels should not be provided. ELCB of 100AF and
smaller do not have this device. Fig. 7. 20 shows an example of the sensitivity
switching device circuit. The sensitivity can be switched by switching the
adjusting resistances on the secondary size of the ZCT. The sensitivity on the
high level is set by the adjusting resistance R1, and the sensitivity on the low level is set by connecting the
adjusting resistances R2 and R3 in series with the switching device. This
system ensures safety against contact failure between R2 and R3 owing to nonconformity of the switching device because the sensitivity
current is determined by R3 and ELCB
operates with high sensitivity.
Operating time switching device
The
time delay circuit breakers have an operating time switching device in addition
to the sensitivity switching device, which can switch the operating time to
three stages, 0.45, 1.0 and 2.0 s or to two stages, 0.3 and 0.8 s. With this device,
a ground fault protection coordination system can be easily configured.
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