Substation Faults and Protection

TABLE OF CONTENT

1.0 Introduction

2.0 General Analysis and Background Theory
2.1 Protection Against Direct Over Voltage
2.2 Protection Against Travelling Waves

3.0 Method of Study

4.0 Results

5.0 Discussion

6.0 Recommendation

7.0 Conclusion

8.0 References

SUBSTATION FAULTS AND PROTECTION

1.0 INTRODUCTION:

The purpose of this work is to establish the type of faults that may result when substations are sited in the region of the Niger Delta due to the prevailing climatic conditions. This will aimed at ensuring as far as possible a small probability of damage to substation insulations.

For most transmission lines, relatively large numbers of yearly flashovers are permitted but such number of insulation damages is absolutely not allowed for substations. Flashover of insulation at substation means a short circuit on the busbar which even with the modern means of relay protection can cause most sever system damages. Substations unlike lines have very low probability of damage therefore a quantitative idea of the probability, the so called index of lightning resistance of a substation is used. It is equal to the calculated number of years during which a voltage dangerous for the substation installation does not occur. For modern high voltage substations, the index of lightning resistance is calculated as hundred or even as thousand years which is a proof of attempts made by designers to ensure the largest degree of lightning resistance of substation (Rao,2008,U.S Dept of Agri.2001,Martinez and Castro 2003).

Substations must be protected from the direct lightning strokes and voltage waves travelling from the line as well as switching surges. In transmission lines the induced strokes (indirect strokes) due to lightning are important for 11kv lines only. For high voltage transmission lines (up to 220kv) the surges due direct lightning strokes determine the line insulations design. For extra high voltage (400kv and above) the severity of swiching surges is much more than that due to lightning (Gupta, 2008). Whether external or internal over voltage, for reliable operations of substation proper protection is necessary. There are factors that may not produce over voltage but are capable of affecting the operation of the equipments as well as lowering the flashover, these are humidity, rain temperature, pressure and to some extent contamination on the surface of insulator and equipments (outdoor).

For proper consideration of these factors with the external and internal over voltages, a proper selection of protective devices may be made for the safe operation of substation

2.0 GENERAL ANALYSIS AND BACKGROUND THEORY:

Substations can simply be seen as a combination of apparatus that transforms the characteristics of electrical energy from one form or level to another form with the provision of facilities for switching. There are various types of substations and are classified according to their services, design, voltage level and functions. Whatever function a substation is meant for, it is the most sensitive part of the supply system.

For any reliable operation of substation the basic functions must be performed.

- Protection against direct stroke
- Protection over travelling surges
- Proper earthing scheme must be maintained

2.1 Protection Against Direct Over Voltage:

The effectiveness of lightning protection schemes for high and extra high voltage (ehv) stations depends upon the degree of overhead shielding against direct strokes to the station area. Quantitatively, information on shielding efficiency in terms of shielding failure exposures as a function of shielding angle has been given in several literatures.

The two ways of protecting the equipment from direct stroke is one of the following ways:

(i) Overhead shielding screen (earthed) covering the outdoor substation and the overhead lines approaching the substation.
(ii) Lightning masts installed at strategic location in the station. Lightning masts are preferred for outdoor stations up to 33kv. For 66kv and above the lightning masts become too tall and uneconomical. The overhead shielding wires are preferred because they give adequate protection and the height of structures in the station provided with overhead shielding wires is comparatively less than that of lightning masts (Rao, 2008 Mousa 1991).

The height above the surface of ground at which the leader discharge finally orients itself on one of the objects on earth is called “the height of orientation of lightning H”, which in the first instance depend on the height of the lightning conductor h. It is customary to consider that for lightning conductor up to a height of 30m will be, H = kh, where the proportionality k has a value of 10-20 (Razevig 2003 Uppal and Rao 2009)