One last consideration in such a case is how to increase the reliability of the riser pole where tariff metering is often found. The transient model shows that this can be accomplished by tying the riser pole ground to the station ground. Since station ground grids are generally below 1 ohm, this is the optimal means to manage riser pole reliability.
Voltage at main breaker of station switch gear with 100kA stroke near riser Voltage at main breaker of station switch gear with 31kA stroke near riser Voltage at main breaker of station switch gear with 5kA stroke near riser
Conclusions
Given the increase in distributed generation throughout the world, great care must be taken to mitigate both internally and externallygenerated surges. As in these two examples, several‘ rules of thumb’ can be applied:
Figure 7: Voltage levels in switchgear of power plant.
1. Install arrester-capacitor combinations in front of generator windings to offer protection to these especially vulnerable windings.
2. Tie the pole ground of the first pole( where the tariff metering is installed) to the station grid if possible. If not practical, ensure it has the lowest possible ground resistance.
3. Apply arresters to protect switchgear from outside surges that inevitably will come in on the export line.
4. Consider the end point on collector systems since the voltage will double and easily cause even a relatively low incoming surge to become one that leads to insulator failure. Apply end point arresters to avoid this.
Voltage at switchgear with arrester and capacitor protection
Voltage at switchgear with arresters at the riser pole and at the breaker
Figure 8: Voltages at the main breaker in the switchgear with arresters at riser pole and at main breaker. Also shown( green) is with additional capacitor at breaker.
Voltage at the end point without arresters
5. If in doubt, adding an arrester is the most cost effective surge damage insurance.
6. Whenever necessary, conduct a study that will result in a detailed understanding of the transient environment for all types of surges. �
Voltage at the end point with arresters
Figure 9: Voltage levels at the end point with and without arresters.
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