Sources of over voltages

Sources of over voltages:

^ü   Some of the causes of transient over voltages on power systems are,

o Lightning – either direct strokes or by induction from nearby strokes.

o Switching surges 

o Switching of utility capacitor banks

o Phase to ground arcing 

o  Resonance and Ferro resonance conditions on long or lightly loaded circuits.

1. Over voltages due to Lightning:

^ü   Lighting is an electrical discharge in the air between clouds between clouds, between different charge centre within the same cloud, or between cloud and earth.

^ü   Even through more discharges occur between or within clouds, there are enough strokes that terminate on the earth to cause problems to power systems and sensitive electronic equipment.

2. Over voltages due to Network switching:

^ü   Switching operations within the distribution network are a major cause of oscillatory transient over voltages.

^ü   Such operations include switching of utility capacitor banks,

^ü   Switching of circuit breakers to clear network faults and

^ü   Switching of distribution feeders.

2.1 Utility capacitor switching:

This is one of the most common switching events on utility systems; it is one of the main causes of oscillatory transients. This transient can propagate into the utility’s local power system, pass through its distribution transformer, and enter into the end user’s load facilities. A common symptom that directly relates to utility capacitor switching over voltages is that the resulting oscillatory transients appear at nearly identical times each day. This is because electric utilities, in anticipation of an increase in load, frequently switch their capacitors by time clock.

3. Ferro Resonance:

Ferro resonance is a special case of series LC resonance where the inductance involved is nonlinear and it is usually related to equipment with iron cores. It occurs when line capacitance resonates with the magnetizing reactance of a core while it goes in and out of saturation.

Ferro resonance is a general term applied to a wide variety of interactions between capacitors and iron core inductors that result in unusual voltages and or currents. In linear circuits, resonance occurs when the capacitive reactance equals the inductive reactance at the frequency at which the circuit is driven.

Iron core inductors have non linear characteristics and have a range of inductance values. Therefore, there may not be a case where the inductive reactance is equal to the capacitive reactance, but yet very high and damaging overvoltage occurs.

In power system the ferro resonance occurs when a non linear inductor is fed from a series capacitor. The non linear inductor in power system can be due to,

^ü   The magnetic core of a wound type voltage transformer

^ü   Bank type transformer

^ü   The complex structure of a 3 limb three phase transformer.

^ü   The complex structure of a 5 limb three phase power transformer.

The circuit capacitance in power system can be due to a number of elements. Such as,

^ü   The circuit to circuit capacitance

^ü   Parallel lines capacitance

^ü   Conductor to earth capacitance

^ü   Circuit breaker grading capacitance