Hydrogen Cooling in Generators

Hydrogen is a cooling medium used in high capacity generators which generate large quantities of heat. 

Hydrogen has a higher capacity to transfer heat, its specific heat is about 14 times that of air of similar weight.

The density of hydrogen is lesser than that of air.  This reduces the windage losses.  Thus the efficiency is improved.

Hydrogen also improves the life of the insulation as the hydrogen circulated is pure without dirt, moisture, etc.  This reduces the maintenance required.

Alternators which are to be hydrogen cooled have special circulating ducts in the windings to circulate hydrogen along with seals to prevent leakage and pumps.  The hydrogen used for cooling is locally produced using electrolyzers.  These equipments produce hydrogen by the electrolysis of water.  The hydrogen is used in a closed circuit at a pressure of around 6 bar(kg/cm²)

The major risk of using hydrogen is its flammability.  Hydrogen burns readily in the presence of oxygen(air).  To prevent this, the hydrogen is maintained at a purity of more than 70%.  This ensures that there is very little oxygen available so as to prevent combustion.    The high pressure in the hydrogen cooling circuit ensures that no air can enter the circuit.  A small quantity of hydrogen may leak into the atmosphere.  This is compensated by adding hydrogen.

Hydrogen is a odorless gas.  Hence, special gas detection equipment are required to detect its leakage.

Amortisseur Windings

Amortisseur windings are bars which are found in the rotor of synchronous motors.  These bars are short circuited similar to the rotor windings in a squirrel cage motor.  The function of these windings is to dampen the torsional oscillations in the rotor that may occur as a result of load fluctuations.  They are also known as damper windings.

The Amortisseur windings can also be used to start the synchronous motors.  The Synchronous motor is not self-starting.  Hence, motor starts initially as an induction motor through the action of the amortisseur windings.  When the sufficient speed has been attained, the excitation to the rotor of the synchronous machine is switched on.  The motor then runs at the synchronous speed as a synchronous machine. 

Amortisseur windings also play a role in preventing rotor overheating when a synchronous machine is exposed to negative sequence currents.  Negative sequence currents may be induced when a synchronous generator is subject to a short-circuit.  Under these conditions, a negative phase sequence currents is induced in the rotor of the generator.  The amortisseur winding provides a path for the rotor currents and prevents the from flowing through the rotor forging and cause overheating.

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Gas formation in Transformers

Gas formation in Transformer oil is an indicator of problems inside the transformer.  Gas can be formed due to the decomposition of either the oil or the insulation of the windings.

The gas formation in transformers can be gradual or sudden.  Gradual  Oil decomposition can be caused due to minor leakage faults within the transformer, internal short circuits.  Loose connection of the windings can also be a reason for gas formation.   Overheating of insulation is another cause for the formation of gas.

The Gas thus formed tends to get dissolved in the transformer oil.  The dissolved gas can be detected by Dissolved Gas Analysis.  The dissolved gas can get released when the transformer experiences temperature fluctuations during its operation cycle.

Major faults such as flashovers lead to sudden gas formation in Transformers.

The Bucholz relay is the protection in transformer against gas formation.  Minor gas formation will accumulate inside the relay and activate the top most float.  This can be configured for alarm  Sudden discharge of gases due to major faults will cause the bottom float in the relay to operate and trip the transformer.

Thermal Protection in Motors

A bimetallic overload Relay

Thermal Protection is an important protection in motors.  Motors can get heated due to overloading, high ambient temperature, variations in power quality, etc.  Thermal overload can result in stator overheating, faulty operation and in some extreme cases even fire.  Hence, all motors need to be fitted with protection against thermal overload.

Thermal overload protections can be classified into three types  viz. Bimetallic, Magnetic and temperature sensing protection.

Bimetallic Protection

In bimetallic protections, a strip of two metals which are attached to one another is used.  The motor current is made to pass through the strips.  As current passes through the strips, the strips heat up and expand.  Since, the strip is made up of two different metals and these metals have different rates of expansion, the strip bends in one direction.  When the temperature of the strip reaches a particular value, it activates a mechanism which trips the motor.   This kind of protection is widely used and is simple in construction.    However, this method is not suitable in applications which require frequent starting and stopping of the motor.

The bimetallic protection gets reset faster than the motor cooling temperature and it may thus permit the motor to be started again when the motor has not sufficiently cooled from the thermal overload.  

Magnetic Protection

This consists of a magnetic element whose field strength is a function of the motor current.  When the motor current exceeds a preset value, the electromagnet inside the relay operates and trips the motor.  The downside of this kind of protection is that it does not take into account ambient operating conditions such as temperature and ventilation which play an important role in the temperature rise of motors.  

Temperature based thermal overload protection

This is method of protection that is relatively new.  This method involves actually measuring the temperature of the motor and those of the winding hotspots using a temperature sensor such as the RTD (Resistance Temperature Detector).  This method uses direct temperature sensing and is the most reliable and accurate, though, it is more expensive.

 

Stalling in Induction Motors, its effects and prevention

Stalling of the motor refers to a condition, where the motor is unable to rotate.  This condition can be caused either due to any obstruction in the load or due to any problem with the motor such as bearing seizure, etc.

This condition is also known as locked rotor.  

When the speed of the rotor decreases to a very low value or stops completely due to stalling, the slip of the induction motor increases.  This causes higher voltage and consequently higher current to be induced in the rotor windings.  

The stator currents also increase.  The equivalent of the motor stalled condition is that of a transformer whose secondary is short circuited. 

The high current drawn will cause damage to the windings and cause the rotor to heat up.

Stall protection devices work by monitoring the motor current and the speed.  If the motor draws higher current at a preset low speed, the relay is activated. 

Advantages of Three Phase Power over single Phase power

Three phase power transmission has become the standard for power distribution. Three phase power generation and distribution is advantageous over single phase power distribution. Three phase power distribution requires lesser amount of copper or aluminium for transferring the same amount of power as compared to single phase power.

The size of a three phase motor is smaller than that of a single phase motor of the same rating. Three phase motors are self starting as they can produce a rotating magnetic field. The single phase motor requires a special starting winding as it produces only a pulsating magnetic field. In single phase motors, the power transferred in motors is a function of the instantaneous current which is constantly varying.

Hence, single phase motors are more prone to vibrations. In three phase motors, however, the power transferred is uniform through out the cycle and hence vibrations are greatly reduced. The ripple factor of rectified DC produced from three phase power is less than the DC produced from single phase supply.

 Three phase motors have better power factor regulation. Motors above 10HP are usually three phase. Three phase generators are smaller in size than single phase generators as winding phase can be more efficiently used.

Winding resistance Measurement

Winding Resistance is an important measurement in electrical machines.  Winding resistance tells us about the condition of the winding.  Any fault in the winding such as an open circuit or an inter-turn short circuit will be reflected in the winding resistance value.  Besides, winding resistance is used to measure I²R losses in the winding.

The Winding Resistance is measured by winding measurement test kits.  In earlier times, winding resistance was measured using the Kelvin Bridge. The Kelvin Bridge is an arrangment of resistors which enables the measurement of very low resistances.  Winding Measurement Kits work by injecting known current through the winding and measuring the voltage drop across the winding.

The machine to be tested is disconnected from the lines and de-energized.  The measurement are usually taken phase-to-phase.  The three readings should be within 1% of the average value.

Winding resistance can change with temperature.  The measurement are usually taken at the cold temperature known as the cold resistance.  The transformer or the motor is allowed to cool for a few hours and the temperature taken.

Based on the measurement taken at a particular temperature, the resistance at any other temperature may be calculated from the following formula

Where

R_s= Resistance value to be calculated at a specific temperature

R_m= Resistance valued measured

T_m= Temperature at which the resistance was measured

T_s= Temperature at which the resistance is to be calculated

T_k= Winding Material Constant ( 234.5 °C for copper or 225 °C for aluminum)

The windings can store a huge amount of electromagnetic energy when a current is passed through them during measurement. When the test current is stopped, there may be a voltage kickback from the winding.  The test equipment should be able to absorb the voltage kick and safely discharge it.

Cooling of Alternators

Cooling is a vital aspect in the construction and operation of generators.  Cooling in generators can be broadly classified into two types

• Open Circuit and
• Closed Circuit

In Open Circuit cooling, air is drawn into the generator by means of fans and circulated inside.  The air is later released back into the atmosphere.  This is a simple method of cooling which does not require elaborate circulation equipment.  This kind of cooling is suitable for small alternators.

Closed Circuit cooling is used in large-sized alternators.  These alternators cannot be cooled by air as they generate a huge amount of heat.  

Here, hydrogen is usually used as the cooling medium.  The hydrogen is passed through the generators by means of pumps and then drawn back into a chamber.  Special circulation equipments such as radial and axial ducts, seals and pumps for this type of cooling.

Hydrogen can transfer heat better than air as it has a higher specific heat.  It has a low density which results in reduced windage losses for the alternator rotor.   The alternator frame can also be reduced.

Water can also be used as a cooling medium in closed circuit cooling systems.  Water has a better cooling capacity as compared to Hydrogen.  However, the circulation equipment for water are more expensive. Special systems for the purification of water are also required.

Some generators use both hydrogen and water cooling systems.

Making and Breaking Capacity of a circuit breaker

          Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the instant of closing.  The making capacity is considered to the peak value of the first cycle when there is an imaginary short circuit between the phases.

          When there is a short circuit in the line and the breaker is closed, the peak value of the first cycle is the most severe from an electrodynamic perspective.  This value is in kA.  The making capacity is expressed as a peak value as the dc offset during fault conditions is taken into account.

          Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt.  This is also in the order of kA. 

          The making capacity of the circuit breaker is usually greater than the breaking capacity of a circuit breaker as breaking an electric circuit is difficult due to arcing which occurs and which has to be quenched.