Monday, April 30, 2012

Ferroresonance Prevention Tutorials deal with strategies that electric utilities and end-users could implement to minimize occurrence of the power quality phenomenon. These practices are based from the fact that ferroresonance usually happens with lightly loaded three-phase transformer, having one or two phases open either intentionally or accidentally. Common strategies for managing ferroresonance include: Preventing open phase condition, limiting overvoltages, damping resonance with secondary load, limiting cable length, switching at transformer terminals and grounding transformer primary.

Prevention of Open Phase Condition

Ferroresonance is often the result of linemen performing single-pole switching in lateral lines or blown fuses in one or two of the phases due to short-circuits. Therefore, a logical effective measure against ferroresonance would be to use three-phase or three-pole switches and fault interrupters instead of fuse cut-outs. However, this would be very costly and electric utilities could not afford to do this at every riser pole, except in special cases where there are frequent fuse blowings and sensitive end-users.

In addition, if single-pole switching can’t be avoided, opening or closing all three phases should be executed as quickly as possible.

Limiting Cable Length

Another strategy for ferroresonance prevention is to limit the length of cable runs. This is because when cable capacitance reaches critical value, it could resonate with the transformer inductance. For delta primary connections cable length should be less than 100 feet, while the WYE(G) – wye(g) could tolerate a few hundred feet of cable without exceeding 125% overvoltage during open phase conditions. Moreover, allowable cable length is dependent on the voltage level – shorter cable for higher system voltage.

Switching at Transformer Terminals

The location of switching when energizing or deenergizing a transformer can play an important role in ferroresonance prevention. 
Transformer Switching: (a) At transformer terminals; (b) At remote tapping point
Diagram (a) shows switching at the transformer terminals (switch R) after the underground cable is energized thru switch L. Occurrence of ferroresonance is reduced since the capacitance seen from an open phase after each phase of switch R closes is only the transformer’s internal capacitance. The second diagram (b) illustrates remotely energizing the transformer. Consequently, the likelihood of ferroresonance is increased as the capacitance seen from switch L includes the cable capacitance. Thus, one of the common ways of ferroresonance prevention during cable switching is to switch the transformer by pulling the elbows at primary terminals.

Secondary Load

Ferroresonance can be suppressed by the presence of secondary resistive loads. The amount of load needed is dependent on the cable length and the transformer design. In a typical open phase, a 10% resistive transformer load can significantly lessen the effects of ferroresonance.

In contrast, cases with two phases open are harder to dampen with load especially if coupled with having very long cable runs (1 km or more). The transformer would have to be about 25% resistive loaded just to restrict overvoltages to the commonly accepted threshold of 1.25 pu. Several electric utilities have reported that line crews carry resistive load banks in their vehicles for use in cable-switching activity when the transformers are unloaded or lightly loaded.

Resistance Grounding of Y-Connected Primary

Resistance grounding can prevent ferroresonance. Nonetheless, this solution may create conflicting constraints in that a resistor must be chosen to be high enough to avoid the bank from acting as a low-impedance ground-current source but low enough to prevent ferroresonance.

Caution: Neutral point of primary windings should be isolated from persons for safety purposes.

Limiting Overvoltages

Surge arresters are effective protection for the overvoltage effects of ferroresonance, particularly for transformers with ungrounded primary connections where the voltages can go as high as 3-4 per unit of nominal. These devices could limit the voltages to less than 2.0 pu.

However, surge arresters could fail when subjected to long periods of ferroresonance. If linemen come across a transformer with arresters in ferroresonance, they should always deenergize the unit and wait for some time to cool the arresters. Overheated arresters could fail hazardously if immediately reconnected to a source with high short-circuit capacity.

Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.).
IEEE C57.105-1978. Guide for Application of Transformer Connections in Three-Phase Distribution Systems

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.