Master
student Bykova O.V., Ph.D. Dyakov I.A.
Tambov state technical University, Russia
PROTECTION FROM SINGLE PHASE EARTH FAULT IN NETWORKS 6-35 KV
To
ensure the sensitivity of protection from single-phase ground faults in Europe
widely used arc suppression reactors (DR) type with ASRC shunt resistor RS,
which when single-phase ground faults automatically connects via the power
contactor in the secondary winding of this resonance voltage of 500.
The
mode of neutral grounding via arc suppression reactor with open slip resistor
(SHR) provides a selective and sensitive protection against single-phase ground
faults using a simple overcurrent relay, the operating current which was built
up from the maximum of own capacitive current accessions.
Short-connection of SR with a
single-phase earth fault active current loop creates a thread that only covers
a broken connection. The magnitude of this current is chosen based on
considerations of ensuring the sensitivity of protection from single phase
earth fault.
However, the effect of protection against single-phase
ground faults depends on the action under automation switch SHR to the
secondary winding of this resonance. The failure of this automation can lead to
serious consequences for the electrical installations subject to mandatory shutdown
at single-phase ground faults.
The calculations of resistances.
When
combined with the neutral grounding resistance of high-voltage resistor is
determined from the expression (1):
,
(1)
where UÂÍ – linear voltage;
ΔI is the current deviation compensation, not exceeding
(ΔI = 5).
Widely adopted permanently included high resistance
and low resistance resistors high voltage. However, such resistors have a
significant size, it is rather difficult to install and mount, and their cost
is quite high.
To avoid these disadvantages in networks with a
combined neutral grounding, it seems appropriate to connect to the secondary
winding of this resonance low-voltage resistors.
Resistor RN1 (figure
1) is designed to provide the sensitivity non-directional current protection
against single-phase earth faults. The resistor RN2 in normal mode is bridged
by a contactor K. the resistance is formed by series-connected resistors RN1
and RN2, RN2 after landing limits the overvoltage to a value 2Uômax and current single-phase ground faults the value
not exceeding 10 A.
Figure
1 - Diagram of grounding of a neutral and parallel connection of the resistor
RN2 at single-phase earth faults
The low resistance of
resistor RN is determined by the expression:
, (2)
where KDGR is the transformation ratio of this resonance.
The
resistance of the resistor RN1 is determined from the condition of ensuring the
sensitivity of protection from single-phase ground fault:
, (3)
where KÇ – is the safety factor;;
K×min – is the minimum coefficient sensitivity;
IÑÇmax – Maximum
operating current non-directional current protection against single-phase short
circuits to earth, built from its own capacitive current connection.
The resistance of the resistor RN2 is
determined by the expression:
RN2 = RN – RN1
. (4)
The
use of resistors RN1 and RN2, it is advisable not only by the action of
protection against single-phase earth fault on the signal, but also to limit
the overvoltage under the action of protections on disconnection of faulty
connection, as a network with arc single-phase earth fault is experiencing a
surge. This increases the likelihood of transition single-phase ground fault in
double circuit and multi circuit before disconnecting the single-phase ground
fault.
To avoid the need to install a resistor RN1 and
implementation of schemes of automation of parallel connection you can use
devices of protection with definite time characteristic, operating from
non-industrial frequency currents.
It is known that in the event of single-phase
earth-fault the most current harmonics when the number of connections more than
two passes on the damaged accession, while the remaining accessions passes a
current determined by its own conductivity. Because damaged accession with the
largest current flows is, it turns off with the least delay time according to
the selected definite time-current characteristic. Application on all the
connections of terminals with a single definite time characteristic, the
operating current of industrial frequency or of current non-industrial frequency,
with a minimum trip current of joining the group will give the possibility to
provide selectivity of protection and substantially increase its sensitivity.
Application on all accessions, which should be more than two, of terminals with
a single definite ha-the characteristics of these currents positives of joining
the group will allow in most cases to provide the selectivity and sensitivity
of protection from single phase earth fault.
When single-phase earth fault using the damaged
accession flows total capacitive current non-industrial frequency section.
Because damaged joining this current is more than intact, the protection is
triggered before. After tripping it will return the protection on the rest of
the accessions.
List of
references
1. Smirnov D. A., Antipov K. M. Reference book power. 5th
edition., recycled. and supplemented. – M, Energoatomizdat, 1987.
2. Electrical engineering Handbook: In 3 t. T.
2 Electrical products and devices / Under the General editorship of professors
MEI (editor-in-chief I. N. Eagles) – 7-e Izd. – M, Energoatomizdat, 1986.
3. Guide to electricity and electrical equipment / Under
General editorship by A. A. Fedorov. In 2 volumes – M., Energoatomizdat, 1986.
4. Gurin N. And., Yanukovych And Electrical
equipment of industrial enterprises and installations. Diploma engineering: a
tutorial – Mn.: Higher school, 1990.
5. Titenkov, S. S., Pugachev A. A. Modes of
neutral grounding in 6-35 kV networks and the organization of relay protection
against single-phase earth fault // Energoexpert. 2010. No. 2..