ELECTRICAL EASY

Generator, Generator Transformer and Unit Transformer protections have been classified into Class-A, Class-B and Class-C. Class-A tripping is further classified into Class-A1 and Class-A2. In this post we will discuss each type of tripping classes and their significance. Picture taken from http://universalerectors.com/steam_turbine_generator.html Basis of Tripping Classification: The tipping classification of Generator is based on the need of isolation of Generator on the basis of type of fault. For example, there are some faults like Generator Differential Protection which calls for immediate tripping of Generator Breaker without delay whereas there are some fault like Loss of Excitation, Rotor Earth Fault etc. which do not call for immediate tripping of Generator. Class-A1 Trip: The protections for the faults in the Generator which need immediate isolation are grouped under this Class-A1.  There are a list of faults which are kept under this class. They...

A synchronous Generator / Alternator is expected to supply active power to the system in normal operating condition. If the turbine i.e. prime mover fails the Generator / Alternator connected to the system will continue to operate as synchronous Motor drawing active power from the system. This reversal of power flow due to loss of prime mover can be detected by reverse power relay. The  consequences  of  generator  motoring  and  the  level  of  power  drawn  from  the  power system will be dependent on the type of prime mover as under this condition prime mover acts as a load for synchronous Motor. For steam turbines, the motoring power is around 0.5-3 % of rated power of Generator. Under the failure of prime mover, due to motoring of turbine windage loss will be more in turbine blades as there is no steam to cool it down. Thus it will lead to damage of turbine. Reverse Power element of Numerical Relay calcula...

Dead Machine Protection in a Generator is provided to ensure that Generator is not energized accidentally in standstill condition or when the Generator is on Turning Gear. Accidental energization of Generator when the machine is not running can cause severe damage to the machine. ( How? You will be able to answer after you go through the post.) Suppose the Breaker is closed when the Generator is at standstill condition, the Generator will behave as an Induction Motor with surface of rotor core and rotor winding slot wedges acting as rotor current carrying conductors. This abnormal current in the rotor can cause arcing between the components like slot wedge to core leading to rapid overheating and damage.  Generally, the time to damage the generator stator from the high in-rush currents received during energizing at standstill is in the order of a few seconds. The bearing, however, may be damaged more quickly due to the lack of oil pressure. Thus it is very important to provi...

The rating of AC Machines such as Transformer, Alternators etc. are determined by the heating and hence losses in them. These losses are made up of Ohmic (I 2 R loss ) and Core Loss and a small amount of friction & windage loss. The ohmic loss depends upon the current and the core loss depends upon the voltage, therefore the losses in an electrical AC machine are unaffected by the power factor of the load. In view of this, the rating of AC machinery is determined by volt-ampere of the load not by the load power alone. Therefore the Alternators or Synchronous Generators are rated in kVA or MVA. You may like to read  Losses in Electrical Machine - Core Loss and Eddy Current Loss It shall be noted here that the size of boiler or the fuel requirement is solely determined by the output power and not on the volt-ampere. For example, for 700 MW load at 0.85 power factor, the rating of Alternator or Synchronous Generator will be 823 MVA (700/0.85 = 823) while the fuel requir...

Open Circuit Test and Short Circuit Test are performed on a Synchronous Machine to find out the parameters of Synchronous Machine and hence to have an idea of their performance.  Open Circuit Test of Synchronous Machine is also called No Load, Saturation or Magnetizing Characteristics for the reason which will be clear after going through the post. For getting the Open Circuit Characteristics of Synchronous Machine, the alternator is first driven at its rated speed and the open terminal voltage i.e. voltage across the armature terminal is noted by varying the field current. Thus Open Circuit Characteristic or OCC is basically the plot between the armature terminal voltage E f versus field current I f while keeping the speed of rotor at rated value. It shall be noted that for OCC, the final value of E f shall be 125% of the rated voltage. Figure below shows the connection diagram for performing the Open Circuit Test of Alternator. As clear from the figure a...

We are quite familiar with Synchronous Generator or often called Alternator. There are mainly two types of Synchronous Generator from rotor construction point of view. The two main types of synchronous machine are Cylindrical Rotor and Salient Pole. Saliency simply means projection outward. Therefore from the literal meaning of Saliency one can guess that Salient Pole machines must have poles projecting outward. In general, the Cylindrical Rotor type machines are confined to 2 and 4 pole turbine generators, while salient pole types are built with 4 poles upwards and include most classes of duty. Both classes of machine are similar in Stator construction point of view. Each has a stator carrying a three-phase winding distributed over its inner periphery. Within the stator bore is the rotor which is magnetised by a winding carrying DC current. The main difference between the Cylindrical Rotor and Salient Pole classes of machine lies in the rotor construction. The cylindrical roto...

For better understanding of advantages and disadvantages of Short-pitched winding, we shall first understand the meaning of short-pitched coil and full-pitched coil. It will take some 10 minutes of your precious time to go through the write-up but it will really help you. Full Pitch Coil: A full pitched coil is defined as the coil whose two coil sides are 180 electrical degree apart. As the Pole Pitch, which is defined as the space electrical angle between two consecutive poles is 180 degree, therefore in an alternate manner we can also say that a Full Pitch Coil has its two coil sides one pole pitch apart. This can be seen in the figure below. Short Pitch / Chorded Coil: If it happens that the two coil sides are not lying under the pole, then obviously the space electrical angle between the coil sides will not be 180 degree rather it will be less than 180 degree as shown in figure below. Thus we can define a Short Pitch Coil as the coil having th...