Interview Questions With Answers 5v2c

Need for slack bus In a load flow study, real and reactive powers (i.e. complex power) cannot be fixed at all the buses as the net complex power flow into the network is not known in advance, and the system power losses are unknown until the load flow study is complete. It is therefore necessary to have one bus (i.e. the slack bus) at which complex power is unspecified so that it supplies the difference in the total system load plus losses and the sum of the complex powers specified at the remaining buses. By the same reasoning the slack bus must be a generator bus. The complex power allocated to this bus is determined as part of the solution. In order for the variations in real and reactive powers of the slack bus to be a small percentage of its generating capacity during the iteration process, the bus connected to the largest generating station is normally selected as the slack bus. Since load flow problems generate non-linear equations that computers cannot solve quickly, numerical methods are required. The following methods are some commonly used algorithms that solve these nonlinear equations: 1. Gauss Iterative Method. 2. Fast Decoupled Load Flow Method. 3. Gauss-Seidel Method. 4. Newton-Raphson Method. What is meant by sparsity: In case of large network there are 1000(e.g) busbars,one busbar is connnected to adjacent busbars whose numbers are very less(e.g 6-10) as compared to the total no buses in network.So when the Y matrix is formed, most of the elements of Y becomes 0.Zero values of Y are for such elements between two buses which are not physically connected. The term sparse indicate the same fact that,very few elements of Y matrix are nonzero.Thats why Y matrix is called sparse matrix. Sparsity= total no of non zero elements/n2 x 100 Why load flow study is necessary The great importance of power flow or load-flow studies is in the planning the future expansion of power systems as well as in determining the best operation of existing systems. The principal information obtained from the power flow study is the magnitude and phase angle of the voltage at each bus and the real and reactive power flowing in each line. What is meant by compact storage scheme in load flow analysis Only non zero elements are stored and ensured full position identification takes place Jacobian the Jacobian matrix) is the matrix of all first-order partial derivatives of a vectorvalued function

What is reverse power relay? Reverse Power flow relay are used in generating stations's protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay. 13. What will happen if DC supply is given on the primary of a transformer? Mainly transformer has high inductance and low resistance.In case of DC supply there is no inductance ,only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer.So for this reason coil and insulation will burn out. 14. What is the difference between isolators and electrical circuit breakers? What is bus-bar? Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions .Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected.Bus bar is nothing but a junction where the power is getting distributed for independent loads Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why? Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter. What is rated speed? At the time of motor taking normal current (rated current)the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency What are the main divisions of power system? The generating system,transmission system,and distribution system 37. What is Instrumentation Amplifier (IA) and what are all the advantages? An instrumentation amplifier is a differential op-amp circuit providing high input impedances with ease of gain adjustment by varying a single resistor. 38. What is meant by impedance diagram. The equivalent circuit of all the components of the power system are drawn and they are interconnected is called impedance diagram. 39. What is the need for load flow study. The load flow study of a power system is essential to decide the best operation existing system and for planning the future expansion of the system. It is also essential for deg the power system. 40. What is the need for base values? The components of power system may operate at different voltage and power levels. It will be convenient for analysis of power system if the voltage, power, current ratings of the components of the power system is expressed with referance to a common value called base value

Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

Answers:-

:Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse connected in the particular control circuit alone will blow off. :why, when birds sit on transmission lines or current wires doesn't get shock?

Answers::Its true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock.. so if a human touch single one line(phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral)। and in the most of electric lines the neutral is grounded..so that means that human who touch the line closes the circuit between phase and neutral.

Why transformer rating in KVA instead of KW? Any device is rated depending on losses of the machine. In a transformer the losses are iron losses which depends on voltage and copper losses which depends on current so there are no losses which depend on power factor hence transformer is rated in KVA. There are 2 losses in transformer. One is copper loss which depends on current and the other is iron loss which depends on voltage. These two factors are not affected by the power factor. This is why transformers are rated in KVA and not KW. Single phase KVA = Amps x Volts/1000. Single phase KW = Amps x Volts x pf/1000. 3 phase KVA = Amps x Volts x 1.73/1000. 3 phase KW = Amps x Volts x 1.73 x pf/1000 yes there are two type of losses in transformer is that ohmic losses and core losses . ohmic losses due to transformer current and core losses due to transformer voltage. these losses are due to transformer voltage and current. and these losses not depends on load ie kw that's the way its rating in kva..

In an electric power system, switchgear is the combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment