Unit 03 – Electrical Transients
Syllabus: Source of Transient Over Voltages: Atmospheric and Switching transients, motor starting transients, pf correction capacitor switching transients, ups switching transients, neutral voltage swing, etc.
Devices for over Voltage Protection.
Transients:
Transients are disturbances that occurs for a very short duration of time (even less that a cycle) and electrical circuit is quickly restored to original operation provided no damages due to the transients.
Sources of Transient Over Voltages:
1. Atmospheric Phenomena:
- Lightning is a powerful source of impulsive transients.
- Figure shows some of the places where lightning can strike that results in lightning currents being conducted from the power system into loads.
- The most obvious conduction path occurs during a direct strike to a phase wire, either on the primary or the secondary side of the transformer.
- This can generate very high over-voltages.
- This is the most common way that lightning surges enter load facilities and cause damage.
- When lightning current flows along the ground conductor paths, at that time also, similar transient over-voltages can be generated.
- There can be numerous paths for lightning currents to enter the grounding system (most common path are shown by dotted lines).
- Lightning strikes to the primary phase are conducted to the ground circuits through the arresters on the service transformer but in return, a lot of transients are observed at the load side.
- A direct strike to a phase conductor generally causes line flashover near the strike point.
- It not only generates an impulsive transient, but it causes a fault with the accompanying voltage sags and interruptions.
- Lightning arresters, when properly applied, can provide protection against lightning- induced low voltages. Arresters have a well-defined conduction voltage below which they are ineffective. This voltage depends on the rating of the arrester itself. For optimum protection, the arrester voltage should be matched to the lightning impulse withstand of equipment being protected.
2. Switching Transient:
- Switching normal loads in the facility can produce transient.
- The majority of plant’s load drawn large amount of current when initially turned ON.
- Transformer are used in transmission and distribution system for step-up, step-down and for isolation purpose.
- When Transformers are turned ON, they draw inrush current that ranges between 10-15 times their normal full-load current.
- Magnetic relays are protective devices that are widely used in the electrical distribution system in order to protect the electrical loads from transients and over voltages.
- However, these devices generate switching transients and noise during normal operation. These switching transients can be high enough in magnitude to damage equipment and cause equipment malfunctions.
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