Dissolved Gas Analysis

Dissolved Gas Analysis is an extremely useful testing procedure that is used to give information about the condition of the windings inside a transformer. It is also an effective tool in diagnosing abnormal conditions that may arise during normal operation

During a fault inside the transformer, gases are created as the polymeric compounds of the oil and the winding insulation, usually cellulose, are broken down to form simpler gases such as ethylene, methane, carbon monoxide, nitrogen, etc.

The type and the quantity of the gas evolved is dependent on the nature of the fault inside the transformer.

The reasons for the evolution of fault gases can be classified into three types ,

1. Pyrolysis or the breakdown of polymers due to high temperature
2. Arcing, caused by short-circuits or earth faults in the alternator
3. Partial Discharge or Corona

The type and quantity of the gas evolved is dependent on the nature of the fault.

Pyrolysis,
Pyrolysis refers to the splitting of large organic compounds in oil and the insulating cellulose due high temperature.The pyrolysis of oil leads to the generation of gases such as methane, ethane, ethylene and hydrogen. When cellulose is pyrolysed, it results in carbon-di-oxide or carbon monoxide.

Arcing
Arcing is caused due to electrical faults in the alternator such as short-circuits or earth faults. Hydrogen and Acetylene are the gases which are generally evolved during faults of this nature.

Corona
Corona or Partial discharge is caused by the ionization of gas or oil around a high voltage conductor. Corona in transformers causes the creation of gases such as hydrogen, carbon monoxide and carbon di-oxide.

Dissolved Gas Analysis (DGA), as the name suggests, refers to the analysis of the various gases which are created during a fault conditions and are present in a dissolved form in the oil used for cooling inside the transformer. A sample is extracted from the oil of the transformer. The gases dissolved are extracted for analysis. The nature and quantity of each gas is analysed.

By identifying the type of the gas and the quantity, it is possible to identify the nature of the fault which could have led to that particular gas.

A periodic or continual analysis of the evolved gas helps in the monitoring of the health of the transformer. Hence, gas analysis occupies an important place in the maintenance schedule of the transformer.

Ripples in dc systems

Ripples are the constantly varying voltages found in the output of rectifiers. The output of a rectifier produces a pulsating voltage, which rises from zero to maximum Vp and then to minimum. This voltage is not suitable to be used in electronics as it contains ripples. The ripples need to be filtered by a filter circuit which is usually a capacitor connected in parallel to the power source.










Ripple factor
The amount of ripples in a dc source is indicated by the ripple factor which is defined as the ratio of the rms value of the ripple voltage to the absolute value of the dc voltage.

Ripple factor(γ)= (V_ripple(rms)/V_dc)*100

Thus if we have 10V dc supply with a variation between 9.5 to 10.5 volts, the ripple factor would be (.5/5)*100 which indicates a ripple factor of 5%.

The peak-to-peak value of the ripples at the output of a full wave rectifier is given by

V_pp=I/2fC

In the case of a half wave rectifier, the peak voltage is given by

V_pp=I/fC

where I is the current in the circuit, f is the frequency and C is the value of capacitance that is connected in parallel to filter the ripples

Measuring Ripples
Ripples can be measured in the field by an ordinary multimeter. Set the multimeter to measure AC voltage, and check the voltage at the output of the power supply. Any ripples would reflect as an AC voltage. Now, set the multimeter to dc voltage and measure the actual dc output. The Ripple factor is the ratio of the ac voltage to the dc voltage.

Effects of Ripples
Ripples can cause failure of components such as capacitors and can cause heating and failure in certain electronic components.
In audio circuits, the ripples can be reflected as noise, as the frequency of the ripples is within the audio band.
Ripples can also interfere in TV displays

Using Broken Delta Protection for Earth Faults

The Broken Delta configuration is used to protect against earth faults. It is a configuration that works by monitoring the vector sum of the phase voltages.

It consists of a delta connection on the secondary of a potential transformer that is open at one point as in the figure. In such a construction, when a balanced three phase voltage is present in the star connected primary, the voltage across the broken point of the delta connection would be zero, as it would be a vector sum of all three voltages.

In the event of a ground fault in one phase, the phase-to-ground voltage in the remaining two phases is now equal to the phase-to-phase voltage with a displacement of 60 degrees. The voltage at the broken delta becomes 3Vo or three times the phase-to-ground voltage.

This voltage can be measured by a relay and can be used to trip the power system. A resistor is usually connected across the broken delta connection to prevent Ferro resonance, a condition that occurs when the line capacitance and the inductance in the potential transformers reach a state of resonance.

Broken delta transformers are usually marked as 11kV/√3:110 volts. It means that the transformer is designed that in the event of a ground fault in the primary side, the secondary output will be 110 V.