Aleksandra RAKOWSKA Jozef LORENC Witold HOPPEL Poznań University of Technology Institute of Electrical Power Engineering DISTURBANCES IN POLISH 15 KV NETWORK.

Aleksandra Aleksandra RAKOWSKARAKOWSKAJJoozef LORENCzef LORENCWitold HOPPELWitold HOPPELPoznań University of TechnologyInstitute of Electrical Power Engineering

DISTURBANCES IN POLISH DISTURBANCES IN POLISH 15 KV NETWORK BY 15 KV NETWORK BY

GROUND SHORT-CIRCUITSGROUND SHORT-CIRCUITS

DISTURBANCES IN POLISH DISTURBANCES IN POLISH 15 KV NETWORK BY 15 KV NETWORK BY

GROUND SHORT-CIRCUITSGROUND SHORT-CIRCUITS

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

1989 1991 1993 1995 1997 1999

tota

l len

gth

[ %]

year

20

40

60

80

70

50

30

10

Relative failure rate

Overhead lines

Cable lines

Average 1,0 3,7

Maximum 2,5 7,4

Minimum 0,45 1,3

Relative failure rate for MV overhead and cable lines – data for all Utilities

Types of MV lines for 5 exemplary utilities

0

20

40

60

80

100

1 2 3 4 5

Utility

Per

cen

tag

e sh

are

of

MV

lin

es [

%] OHL

CL

* Five Polish Utilities participate in survey on failuresfor cable (11 308 km) and overhead (47 929 km) lines

PAP77%

PE10%

XLPE13%

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

PE63%

PAP36% XLPE

1%

The first generation of design – taped screen

0

20

40

60

80

100

1 2 3 4 5

Utility

Fai

lure

rat

e [%

]

Wu OHL

Wu CL

Percentage share of cable and overhead lines failure rates – total for 5 Utilities

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