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Article On Ferro-Resonance

Voltage Transformer and Ferro-resonance Failure

Introduction


The failure of single phase transformers (VTs) operating in unearthed power system has remained a mystery for many years to designers as well as system engineers. The phenomena known as “FERRO-resonance” or “neutral inversion” or “Neutral instability” causing such failure is discussed in detail in this article.

Ferro-Resonance Phenomenon


The VTs with primary winding connected in star and earthed in an otherwise unearthed system, interacts with the distributed capacitance to earth of the circuit following voltage disturbance due to either switching operation or temporary arcing fault. The interaction causes oscillations, which may be of several frequencies and will persist if the energy losses are not sufficiently high .these oscillations could result in to over heating of primary winding of VT. These oscillations are required to be damped by higher losses.

Fig.1 shows equivalent circuit of a capacitor and coil. The resultant Volt-Amp. Characteristics is shown in Fig.2










The Volt-Amp curve has 3 regions,

1. Zone OC: Initially the circuit is capacitive and current leads .The current increase linearly in proportion to it. The circuit is in stable state.

2. Zone CD: The leading current decreases with rising voltage. This is a source of instability under suitable condition and neutral may remain in state of persistent of oscillations.

3. Zone DE: Current lags to the voltage and increases very rapidly with rise of voltage and circuit is in stable state.

At point ‘D’ the capacitor current and the coil current are equal and opposite, the resultant current is zero, and system is in parallel resonance. It is known that exciting current taken by a transformer on switching in, may be a large multiple of the normal value, according to the point on the line voltage wave at witch the switch is closed. Consequently the transformer and earth capacitance may easily put on unstable part CD or part DE of the characteristics, resulting in inversion and considerable voltage rise. In other words the effect of saturation is to reduce the maximum voltage but to increase the range of circuit parameters which may cause inversion i.e., probability of inversion occurring is greater with saturation than without.

Factor Affecting Ferro-Resonance


The phenomena explain above for 1-phase system also holds good for 3-phase system under normal circumstances the line to neutral voltages of the power system are sinusoidal and balanced. Momentary switching surges upset this balance and cause the line to neutral voltage to rise to higher than voltage values. The VTs become saturated and the degree of saturation depends upon the residual core densities. This condition is between no linear exciting impedance of the transformers and the capacitance to earth of circuit. The voltage transformers, while in this stage, go through the period of saturation during which their impedance is low and they draw pulses of extremely high current. These pulses of current if the Ferro-resonance condition is allowed to persist and the primary fuses do not blow (which is normally the case), will overheat the VT primary winding and may eventually cause insulation break down. Generally the primary fuses for VTs are not intended to give complete thermal protection to VT but are usually selected to give short circuit protection to the power system with enough margin to prevent their being blown by the magnetizing inrush currents. Hence the VT primary fuses may or may not protect the transformer from thermal damage, depending upon the magnitude and duration of the current pulses.

It may be noted that not every disturbance will cause Ferro-resonance oscillations, as the phenomena depends on (1)Initial state of magnetic flux in core of VTs (2)The saturation characteristics of cores (3)The amount of capacitance to ground of the circuit (4) The energy losses in the oscillatory circuits.

Remedial Measures


Avoidind Saturation Of Vt Cores

It is observed that the saturation of magnetic core is the key factor in initiating the Ferro-resonance phenomena. If the design of VT is made in such a way so as not to allow it to be saturated even for line to line voltage then the ease with which Ferro-resonance occurs decreases .This amounts to selection of lower working flux density for given rated voltage.

Range Of Capacitance


The circuit consideration indicates the phenomena may be prevented by decreasing the capacitance to well below that necessary to sustain the triple frequency phenomena or to increase it well above the limit for sustaining the one half frequency ,phenomena is sustained makes this suggestion impracticable to some extant.

Damping Of Oscillations


This is the most useful way of eliminating hazard of Ferro-resonance phenomena to VTs. The damping resistor of suitable value can be connected either in the secondary of VT or by connecting it across the broken delta corner of additional secondary winding. Karlic and Taylor have recommended the value of broken corner delta resistance for more critical frequency by empirical equation.

RB = 100 X La / N square

Where
RB = broken corner delta resistance in ohms.
La = primary inductance during saturation, mH
N = transformer turn ratio.
It may be noted that accuracy of VT should not be affected by the additional burden.

Use Of 5-Limb Vt

This does not directly help in eliminating or damping the phenomena, however, it makes the magnetic circuit independent, unlike in the case of a phase 3-limb VT. The earth fault on one phase does not affect the voltages on unfaulted phases.

Aknowledgement

The author wishes thank the management of Parana Electrotech pvt.ltd. For giving permission to publish this article.

Reference

Ferro-resonance of a grounded PTs on ungrounded power system by Karlick and Tailor-Jr.AIEE 1959, pp 607.

This Article is written by Mr.N.K.Gajjar, M.tech. (ELECTRICAL)