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
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.
References:
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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