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Newsletter Issue 96
October 2004
FERRORESONANCE DAMPING DEVICE SOLVES AMTRAK’S PROBLEM
Daniel W. Durbak
Executive Consultant
daniel.durbak@shawgrp.com
The development of the ferroresonance damping device (FDD) described in this article was
spawned by the catastrophic failure of a voltage transformer (VT) during the testing of
Amtrak’s newly constructed overhead catenary system (OCS) between Boston
Massachusetts and New Haven Connecticut. Amtrak asked us (Dan Lawry, Dave Smith
and this author) to analyze these electric circuits in order to determine the cause of the
failure. Our analysis using the Electro Magnetic Transients Program (EMTP) showed that an overcurrent from
ferroresonance was the root cause of the failure.
Ferroresonance has long plagued electric power transmission and distribution systems. Ferroresonance was
also a problem for European rail systems several decades ago, but was previously not of concern in
electrified railroads in North America. Amtrak needed a solution so we analyzed and discussed a number of
mitigation measures. In other types of electrical systems, damping of the ferroresonance oscillations has been
achieved by the use of artificial resistive loads connected to the secondary or tertiary of transformers. Amtrak
tested the use of resistors, but reasonable values were not effective. Large artificial loads could have been
effective, but the energy consumed would have been expensive. The use of other mitigating measures was
stalled due to various system constraints or safety and reliability concerns.
This author conceived of a new device to dampen ferroresonance. EMTP simulations showed that the device
was feasible and further simulations were made to determine a suitable set of electrical parameters for the
device. Amtrak commissioned Kuhlman Electric Corporation to construct of a set of prototypes based upon
this author’s specifications. The prototype tests were satisfactory, and a number of production units were
ordered and permanently installed. The photos below show the ferroresonance damping device under a pole
mounted distribution transformer that serves wayside power needs.
Unmounted
Mounted under a transformer
Track and catenary view
Power Technology
October 2004
Ferroresonant oscillations can result in temporary or sustained overvoltages that can stress equipment
insulation and surge arresters. Also, ferroresonance can produce overcurrents that overheat transformer
windings, as in Amtrak’s case. Ferroresonance is possible only after a circuit breaker opens to de-energize
the catenary for one of the tracks while the catenary for the adjacent track remains energized, which is a
standard switching operation. The ‘de-energized’ catenary receives energy from the energized catenary
through the electrostatic coupling (capacitance) between the parallel set of wires. The capacitively coupled
energy flows into the non-linear magnetizing inductances of the transformers connected to the de-energized
catenary, which can form a ferroresonant circuit. Ferroresonance can occur when the electrical parameters of
the circuit are such that the resonance phenomenon will be triggered. The parameters of influence include:
capacitances, number of transformers connected, transformer saturation characteristics, pre-switch voltage,
current chopping in the circuit breaker, and circuit losses. Gantry mounted VTs as well as distribution
transformers are susceptible to ferroresonance.
The objectives of the FDD are:
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To have negligible impact on the transformer to which it is applied or on the performance of the
overall electric system during normal operating conditions.
To be energy efficient and have low energy consumption during normal operating conditions. To selfactivate without the need an external power source, or a control circuit, or a trigger circuit.
To dampen ferroresonance and the associated overcurrents and overvoltages.
To be simple in design.
To be relatively inexpensive to manufacture.
To be robust and operate for many years with high reliability.
To require little or no maintenance.
To be relatively simple to install and can be applied indoors or outdoors mounted on a pole or gantry.
The ferroresonance damping device is a special saturable (non-linear) reactor that connects to the secondary
or tertiary winding of a transformer (115 V or 240 V). The ferroresonance damper device conducts a negligible
current in normal steady state operation and consumes very little energy. Its connection does not alter the
normal steady state operation of the transformer or of the circuit.
The FDD self-activates when a circuit breakers opens to de-energize the OCS. In the transient period
following a switching activity, the FDD saturates at an instant before the voltage or distribution transformers in
the circuit. The current through the FDD creates a temporary loss that dampens the overall circuit and the
ferroresonance oscillations are not sustained.
The FDD is specially designed and constructed with regard to achieving specific electrical parameters related
to the transformers and system to be protected. The non-linear voltage versus current (saturation)
characteristic of the FDD must saturate at a flux level that is lower than that of the transformer winding to be
protected. In other words, when the FDD saturates, it draws a higher current than the exciting current of the
transformer winding to which it is connected. The FDD draws current through the transformer and takes
enough energy out of the electrical circuit so that the ferroresonance mode of oscillation decays (that is,
effectively damped). The electrical resistance of the FDD affects the rate of decay of the ferroresonant
oscillations. Thus the resistance of the device is a key parameter and must fall within a suitable range.
Ferroresonance is a system problem and the FDD is a system solution. Consider a particular system with two
transformers. If a FDD is used on one of the transformers in that system, the effect will be to reduce the
ferroresonance for both the transformer to which the FDD has been attached as well as the other transformer
in the system which does not have one. The exact effect on either transformer in the system will be
determined by a number of factors, including: the proximity of the transformers to one another, the saturation
characteristics of the transformers, the saturation curve of the FDD, and the other electrical components
which make up the particular system. For maximum effectiveness in preventing ferroresonance effects,
damping devices may need to be placed on every transformer in a system. In a multiple transformer
configuration, each FDD will contribute to the damping of the overall system.
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