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DISTURBANCES IN POLISH
15 KV NETWORK BY GROUND
SHORT-CIRCUITS
Aleksandra RAKOWSKA
Jozef LORENC
Witold HOPPEL
Poznań University of Technology
Institute of Electrical Power Engineering
Polish MV network contains mainly overhead lines
but every year the percentage share of cables
in
electrical
power
network
is
increasing.
This tendency can be observed especially for
medium voltage system.
MV power lines serviced by Utilities
at the end of 2001:
OHL- 169 629 km (76%)
CL- 54 345 km (24%)
The percentage share of various types of cable insulation
in the total length of MV cables in Polish network
80
total length[%]
70
60
50
40
30
20
10
1989
1991
1993
1995
year
1997
1999
Relative failure rate for MV overhead and cable lines
– data for all Utilities
Relative
failure rate
Overhead
lines
Cable
lines
Average
1,0
2,5
0,45
3,7
7,4
1,3
Maximum
Minimum
Types of MV lines for 5 exemplary utilities
Percentage share of MV lines [%]
100
OHL
CL
80
60
40
20
0
1
2
3
4
5
Utility
* Five Polish Utilities participate in survey on failures
for cable (11 308 km) and overhead (47 929 km) lines
Shares of cable types installed in the MV network
and cable failures for different type of insulation
in 5 exemplary Utilities at the end of 2000
PE
10%
XLPE
13%
PAP
77%
PAP
36%
PE
63%
XLPE
1%
The first generation of design – taped screen
Percentage share of cable and overhead
lines failure rates – total for 5 Utilities
100
Wu OHL
Wu CL
Failure rate [%]
80
60
40
20
0
1
2
3
Utility
4
5
The 90’s saw the admittance criteria, developed in the
Institute of Electric Power Engineering of Poznan
University of Technology become commonly operated.
Large part of them are in analog technique (90’s),
but for new substations - there are always digital
admittance earth fault protection systems.
With admittance earth fault protection systems some
Utilities achieved that described protection systems
allow to detect at least 95% of all earth faults .
The admittance earth fault protection systems
can be divided with respect to criteria on which
they rely:
-
excess admittance Yo>,
-
excess conductance Go>,
-
conductance-directional Gok,
-
susceptance-directional Bok,
-
comparative-admittance Yyo.
Admitance criteria were described during CIRED Conference
Birmingham - IEE Publication No 438, 1997
Near 80% of Polish HV/MV power substations work
with neutral point, using Petersen coil (about 2000 substations).
There is a need to assure proper conditions for good protection
function – shunting of Petersen coil with resistor during
protection operation. It is the most popular configuration.
There are configurations with resistor switched on permanently, too.
Transient switching on means current about 15-25 A on primary
winding – better service conditions for protection system.
Resistor switched on permanently means also additional
suppression of overvoltages (it is important especially for cable
lines, with aged insulation or with first generation of extruded
insulation). That’s why some Utilities use such configuration but it
has to be said, that it lowers protection conditions in OH lines –
risk of electric shock.
Based on service experiences – it was found that
admittance earth fault protection are very effective for
intermittent fault, too.
In present time – for a new substation – all equipment
suppliers are asked by Utilities to use the admittance
earth
fault
protection
(now
foreign
well-known
companies have been asked to put also this element
additionally to their systems).
Widely carried out research and construction works, having
resulted
in
analogue
and
modern
digital
solutions,
combined with providing professional training and expertise
made admittance protections - a widely used protection
system, implemented in nearly 10 thousand MV line fields in
Polish distribution networks.
Based on service experience achieved in the power
substations we can conclude, that described protection
systems allow to detect at least 95% of all earth faults.
Main aim:
continuous supply of electrical
energy for the CUSTOMERS