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2. Earth-fault Relay connected in Neutral to Earth Circuit (Fig. 5).
Another method of connecting an earth-fault relay is illustrated in Fig 5. The relay is
connected to secondary of a CT whose primary is connected in neutral to earth connection.
Such protection can be provided at various voltage levels by connecting earth-fault relay in
the neutral-to-earth connection of that voltage level. The fault current finds the return path
through the earth and then flows through the neutral-to-earth connected. The magnitude of
earth fault current is dependent on type of earthing (resistance, reactance or solid) and
location of fault. In this type of protection,
The zone of protection cannot be accurately defined. The protected area is not
restricted to the transformer/generator winding alone. The relay senses the earth
faults
beyond the transformer/generator winding hence such protection is called
unrestricted earth-fault protection. The earth-fault protection by relay in neutral to
earth circuit depends upon the type of neutral Earthing. In case of large generators,
voltage transformer is connected between neutral and earth
(Fig. 5) Earth-fault protection by earth-faultrelay connected
in neutral-to-earth circuit.
v
Combined Earth-fault and Phase-fault Protection
It is convenient to incorporate phase-fault relays and earth-fault relay in a combined
phase-fault and earth-fault protection. (Fig. 4) The increase in current of phase
causes corresponding increase in respective secondary currents. The secondary
current flows through respective relay-units Very often only two-phase relays are
provided instead of three, because in case of phase faults current in any at least two
phases must increase. Hence two relay-units are enough.
v Earth-fault Protection with Core Balance Current Transformers. (Zero Sequence
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Earth-fault Relay connected
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CT)
In this type of protection (Fig. 6) a single ring shaped core of magnetic material,
encircles the conductors of all the three phases. A secondary coil is connected to a
relay unit. The cross-section of ring-core is
(Fig.6) Principle of core-balance CT
for earth fault protection
Ample, so that saturation is not a problem. During no-earth-fault condition, the
components of fluxes due to the fields of three conductors are balanced and the
secondary current is negligible. During earth faults, such a balance is disturbed and
current is induced in the secondary. Core-balance protection can be conveniently used
for protection of low-voltage and medium voltage systems. The burden of relays and
exciting current are deciding factors. Very large cross-section of core is necessary for
sensitivity less than 10 A. This form of protection is likely to be more popular with
static relays due to the fewer burdens of the latter. Instantaneous relay unit is
generally used with core balance schemes.
v
Theory of Core Balance CT
. Let Ia,
Ib and I c , be the three line currents and Φa, Φb and Φc be corresponding
components of magnetic flux in the core. Assuming linearity, we get resultant flux Φ
as,
Φ=k (Ia + Ib + I c )
where k is a constant Φ = K * Ia. Referring to theory of symmetrical components
(Ia + Ib + I c )= 3 I c= I n
Where, Io is zero sequence current and In, is current in neutral to ground circuit. During
normal condition, when earth fault is absent,
(Ia + Ib + Ic) = 0
Hence Φr = 0 and relay does not operate
During earth fault the earth fault current flows through return neutral path.
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For example for single line ground fault,
If = 3Iao = In
Hence the zero-sequence component of I o produces the resultant flux Φr in the
core. Hence core balance current transformer is also called as zero sequence current
transformers (ZSCT).
v Application for Core Balance CT's with Cable Termination Joints
The termination of a three core cable into three separate lines or bus-bars is through
cable terminal box. Ref. (Fig. 7), the Core Balance Protection is used along with the
cable box and should be installed before making the cable joint.
The induced current flowing through cable sheath of normal healthy cable needs
particular attention with respect to the core balance protection.
The sheath currents (Ish) flow through the sheath to the cover of cable-box and then to
earth through the earthing connection between cable-box. For eliminating the error due
to sheath current (Ish) the earthing lead between the cable-box and the earth should be
taken through the core of the core balance protection.
Thereby the error due to sheath currents is eliminated. The cable box should be
insulated from earth.
1. Cable terminal box
2. Sheath of 3 core cable connection to (1)
3. Insulator support for 1
4. Earthing connection passing through 5
5. Core balance CT
Fig (7) Mounting of Core
Balance CT with Cable
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Terminal Box
v Frame-leakage Protection
The metal-clad switchgear can be provided with frame leakage protection. The
switchgear is lightly y insulated from the earth. The metal-frame-work or enclosure of the
switchgear is earthed with a primary of a CT in between (Fig. 8).
The concrete foundation of the switchgear and the cable-boxes and other conduits are
slightly insulated from earth, the resistance to earth being about 12 ohms. In the event
of an earth fault within the switchgear, the earth-fault current finds the' path through the
neutral connection. While doing so, it is sensed by the earth fault relay.
(Fig. 8) Principle of frame-leakage protection
of metal-clad-switchgear
Circulating current differential protection also responds to earth-faults within its
protected zone.
v Earth-fault protection can be achieved by following methods:
1. Residually connected relay.
2. Relay connected in neutral-to-ground circuit.
3. Core-balance-scheme.
4. Frame leakage method.
5. Distance relays arranged for detecting earth faults on lines.
6. Circulating current differential protection.
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10/27/2018, 8:36 PM