The
non-grounded system is used in plants where power failure must be prevented for
operation. Particularly, in many petrochemical plants, where explosive gas may
catch fire owing to ground fault current and serious accidents may be caused,
this system is used to reduce the ground fault current. On non-grounded
circuits, the ground fault current is remarkably low even if one line falls
down on the earth, and the current cannot be easily detected. Therefore, note
that the protection effect of ELCB may not be expected only by installing it.
Circuit insulation detector (MEGMONITOR)
Mitsubishi
circuit insulation detector, MEGMONITOR, has an excellent feature that enables
constant monitoring of circuit without necessity of power interruption because
it can monitor the whole electric circuit in the live state with lowvoltage small
current and detect deterioration of insulation of circuit to the earth. Article
40 of interpretation for Guide Book of Electrical Equipment requires
installation of ground fault circuit breakers on non-grounded circuits. In the
past, appropriate devices for ground fault protection were not available, and
the protection was provided by grounding capacitors and installing ELCB.
However, to normally actuate ELCB, the capaci tor capaci ty should be considerably
increased. This can generate sparks upon occurrence of ground fault and is
unfavorable for the purpose of non-grounded circuits.
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| Fig. 9. 31 Explanation of operation |
MEGMONITOR
can detect insulation deterioration corresponding to ground fault current of
several mA and is suitable for detection of ground faults on non-grounded circuits.
A ground fault circuit breaker can be easily configured by combining it with
breakers. The operation is explained based on Fig. 9. 31. As an AC content,
charging current flows through the earth capacitances (CA, CB and CC) on the
circuit from the system power supply (commercial frequency) by the transformer
(Tr). The signal detector (D) of the amplifier and the capacitor C are connected
in series so that the signal by this current does not enter the amplifier
(AMP). Therefore, the current flowing to the earth capacitances (CA, CB and CC) is
bypassed to the capacitor (C) to avoid giving input signals to the amplifier (AMP),
and the AC signal is not amplified. The DC content biases the lines of the
phases (A, B and C) through the windings of the transformer (Tr) from the DC power
supply (DC) installed in the device. If the insulation of the circuit is higher
than a certain value (normal), the DCdoes not leak to the outside of the
circuit, and DC current does not flow into the detector. As stated above, since
this device is biased by DC, charging current flows to the capacitances (CA, CB and CC) only
just after MCCB are closed even if the circuit is long and the earth
capacitances (CA, CB and CC) are large. After this, it will stabilize, and DC will not flow.
Therefore, even on a long circuit, it will not be affected by the earth
capacitances (CA, CB and CC). Against transient inrush current generated when MCCB is closed, a
special circuit for prohibiting operation is provided to prevent malfunction. Then,
if the insulation at the point A of T phase has deteriorated and the insulation
to the earth is degraded, DC current from the DC power supply will flow to the
resistor (R2), grounding point (E),
fault point (A), transformer neutral point (N) and resistor (R1). After the
completion of charging of the capacitor (C) connected in series with the
detector (D), all DC current will flow to the detector (D), the signal of the
detector (D) will be amplified by the amplifier (AMP), and insulation
deterioration will be detected.
With this device, insignificant decrease of circuit insulation to 400kW or less can be detected. The sensitivity can be switched in 6
stages, 10 – 20 – 50 – 100 – 200 – 400k.
Method by combining grounded capacitors and ELCB
Capacitors
are connected to the secondary side of non grounded insulating transformer, the
neutral point is grounded, and, when one-line ground occurs, ELCB detects the
ground fault and protects the circuit.
(1)
3-phase 3-wire Non-grounded insulating transformers are generally Δconnected. The relationship between grounded capacitor
capacity and ground fault current in this case is shown below.
 |
| Fig. 9. 32 |
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Fig. 9. 33
|
In
Fig. 9. 32, the capacitors with capacitances CA, CB and CC are star-connected, and the neutral point is grounded. This circuit is redrawn in Fig. 9. 33
to make it easier to
understand the connection. In the normal state, the capacitors are a star-connected load consisting of capacitors when viewed from the power supply
side, but, when a
ground fault occurs, the resistance Rg (including the fault resistance of device, earth resistance of device and grounded resistance of capacitor) of the
ground fault circuit is
considered to be connected in series with the ground fault capacitor CA of phase A, and the current flowing to Rg is detected by the ZCT. On condition that the capacitor capacities of the phases are
identical and CA = CB = CC = C, the
relationship between capacitor capaci ty C and ground faul t cur rent Ig can be determined by the following formulas.
 |
Fig. 9. 34
|
At
first, determine the maximum limit of ground fault current (related to the
current sensitivity). The current should be about twice the rated current
sensitivity of ELCB to be used. Therefore, the required capacitor capacity C
can be obtained from the formula (2) by determining the maximum detected ground
fault current, the current sensitivity, and
estimating
the ground fault resistance Rg.
<Example>
Determine
the capacitor capacity required to protect the circuit from ground fault with
ELCB by grounding the capacitors as shown in Fig. 9. 34. The line voltage is
440V, frequency is 60Hz, and the total resistance Rg of the ground fault
circuit is 150 Ohms. In this example,
assume that ELCB with sensitivity of 200mA is used, and the ground fault
current is 0.5A. From the formula (2),
Therefore, if the capacitor capacity in
Fig. 9. 34 is 1.8mF (actually,
a capacitor with standard capacity of 2mF is used), current of 0.5A will flow when the grounded circuit
resistance reduces to 150W, and the current
sensitivity of 500mA is allowable. However, actually, the sensitivity is set to
200mA with a little margin for more reliable operation. If a highsensitivity circuit
breaker is used, ground fault current can be detected even when the grounded
circuit resistance is high. For example, when ELCB with sensitivity of 30mA is
used, Rg can be determined by the formula (2) as shown below.
That
is, when the grounded circuit resistance reduces to 8500W, ELCB operates and protects the circuit. This offers the advantage of
detection of ground fault at an early stage at which the degree of fault is
minor. After the capacitor capacity is determined, it is necessary to select a
capacitor having a withstand voltage (rated voltage) appropriate to the circuit
voltage. Although the voltage applied to the capacitor is V / 1.732 in the
normal state, use a capacitor having two times higher withstand voltage to
allow a margin. Since 44 / 1.732 _ 2 _ 509, use a 2-mF capacitor having the standard voltage of 600V.
(2)
Single-phase 2-wire, neutral point grounding
 |
| Fig. 9. 35 |
From
the formula (6), the required capacitor capacity can be obtained.
(3)
3-phase 3-wire (star connection)
From
the formula (8), the required capacitor capacity can be obtained.
4)
Simplified calculation formula of capacitor capacitance
If
the ground fault resistance Rg is negligible, the capacitor capacitance can be
determined by the following simplified calculation formula. In the case of
3-phase 3-wire (Δ connection)
Quick
chart of capacitor grounded capacity on nongrounded electric circuit for the
purpose of detection of earth
leakage
Fig. 9. 37-1
Example:
To obtain a ground fault current of 200mA at ground fault resistance of 500W, the capacitor capacity shall be 3.5mF.
(The
ground fault current shall be at least twice the rated current sensitivity.)
 |
Fig. 9. 37-3
9. 7. 3 Method by grounding transformer
This
method is designed to detect zero-phase voltage and break the circuit owing to
ground fault. When the devices are connected as shown in Fig. 9. 38, the
voltage applied to the primary winding is EA + EB + EC, and the corresponding voltage
is induced to each phase of the secondary delta winding. The voltage applied to
^4 is the vectorial sum of the voltages of the
phases, EA + EB + EC. From Fig. 9. 39,
NA = EA + NE
NB =
EB + NE
NC =
EC + NE
The
both sides of these formulas are added to obtain the following formula.
NA +
NB + NC = EA + EB + EC + 3EN
As
is evidenced from Fig. 9. 39, N is the center of the triangle, and the left
side is 0. Therefore,
EA + EB + EC = −3NE = 3EN
Since
EN is zero-phase voltage, three times larger zerophase voltage appears in ^4. This voltage is detected, and the voltage relay is operated to
trip the circuit breaker to protect the circuit from ground fault.
 |
| Fig. 9. 38 |
 |
Fig. 9. 39
This
method is similar to the method stated in (2) “grounding transformer.” The
degree of ground fault and the grounding phase can be defined by the indicator.
Fig. 9. 40 shows the connection diagram of model LM-11NGD grounding detector
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Fig. 9. 40
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