Aleksandra RAKOWSKA J o zef LORENC Witold HOPPEL

1. 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

2. Polish MV network contains mainly overhead lines but every year the percentage share of cablesin 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- 169629 km (76%) CL- 54 345 km (24%)

3. The percentage share of various types of cable insulation in the totallength of MV cablesin Polish network 80 70 60 ] % [ 50 h t g n 40 e l l a t 30 o t 20 10 1989 1991 1993 1995 1997 1999 year

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

5. 100 OHL CL 80 60 Percentage share of MV lines [%] 40 20 0 1 2 3 4 5 Utility Types of MV lines for 5 exemplary utilities * Five Polish Utilities participate in survey on failures for cable (11 308 km) and overhead (47 929km) lines

6. Shares of cable types installed in the MV network and cable failures for different typeof insulation in 5 exemplary Utilities at the end of 2000 PE XLPE 10% 13% PAP 77%

7. The first generation of design – taped screen

8. Percentage share of cable and overhead lines failure rates – total for 5 Utilities 100 Wu OHL Wu CL 80 60 Failure rate [%] 40 20 0 1 2 3 4 5 Utility

9. 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 .

10. 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

11. 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.

12. Based on service experiences – it was found that admittance earth fault protection are very effective forintermittent 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).

13. 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.

14. Main aim: continuous supply of electrical energy for the CUSTOMERS