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...

Rate of Change of Frequency (ROCOF or df/dt) relay is used for fast load shedding, to speed up operation time in over- and under-frequency situations and to detect loss of grid. For better understanding of role and operation of df/dt relay, let us first study the variation of frequency with load for a Grid. Graphical relationship between power and frequency of a Grid is shown in figure below. We can have two things to be noted from the above graph: a)     If the power available in the Grid increases i.e. in other words generation is more than the load, frequency will go up. b)     If the generation is less than load i.e. power deficient in the Grid then frequency will decreases. Large power grids (here large power grid means having large installed capacity in MW) are characterized by a very high stiffness constant which means that a large perturbation (load generation mismatch) is required to cause the grid frequency to change by 1...

The oil can only be filled in the transformer which satisfies the standard specifications mentioned by the manufacturer of transformer. Normally Transformer is supplied with gas filled i.e. the Main Tank of Transformer is filled with Nitrogen gas under some specified pressure to prevent any degradation of the insulation of core and winding. But we cannot keep Transformer Main Tank filled with nitrogen for a period more than three months and hence oil filling becomes important. Oil filling in Transformer also becomes important if we want to store the Transformer for more than six months. The oil filling is done in the main tank under vacuum. The large Transformers are generally designed to withstand the full vacuum for long periods. Oil filling and filtration of oil is carried out simultaneously. Oil sampling is done during the process of oil filtration and if sample result for Dielectric Strength and moisture comes under limit then filtration of oil is stopped. Figure below show...

Variable Frequency Transformer or simply VFT is quite a new technology to connect two asynchronous Grids. Asynchronous Grids mean two power systems operating at two different frequencies. The world’s first VFT was installed and commissioned in the year of 2004 at Hydro- Quebec’s Langlois substation, where it is used to exchange up to 100 MW of power between the asynchronous power grids of Quebec (Canada) and New York (USA).  First thing to note here that though the name suggests it to be Transformer but construction wise it is like a Slip Ring Induction Motor. The basic concept behind the VFT is a rotary transformer with three phase windings on both rotor and stator. A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT. VFT is a controlled bi-directional device allowing flow of power in both directions i.e. from one Grid to another and vice ...

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...

Frequency all over a synchronous Power Grid is the same in steady state. Mind the word Synchronous Power Grid, as there may be two different grids operating at slight different frequencies. It is very important to maintaining a constant frequency or to frequency to vary over a very narrow band in a power system operation. Frequency in a power system is intimately related to the electrical speed of synchronous generators. As we know that the acceleration of a Generator is solely governed by the difference between mechanical and electrical torques, therefore to maintain a constant speed, mechanical input and electrical output power need to be continually matched. Electrical load can vary randomly, but fortunately the total load versus time roughly follows a trend. Frequency of Grid is depends upon many factor like load variation, prime move control of Generator etc. Frequency needs to be maintained near 50 Hz (For India) for the following reasons: 1) Steam turbine blades ...

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...

DC Machines can be tested by three different methods namely Direct Method, Indirect Method and Regenerative Method. Direct Method of testing of DC Machine, also known as Brake Test (if carried out for a DC Motor) will be discussed in this post. Direct method is suitable for small DC machines. In Direct Method, the DC machine is subjected to rated load and the entire output power is wasted. The ratio of output power to the input power gives the Efficiency of DC Machine. For a DC Generator the output power is wasted in resistor. Direct Method of testing when conducted on a motor is also known as Brake Test. Brake Test of DC Motor is carried out as shown in figure below. A belt around the air cooled pulley has its end attached to the spring balance S1 and S2. Using belt tightening hand wheels H1 and H2, the load of motor is adjusted to its rated value. Assuming the spring balance to be calibrated in kilogram, then rated load on the DC motor is given as Motor ...