POWER SYSTEM ANALYSIS INTRODUCTION • General perspective & Overview of Power System. • Need for system planning and operational studies OVERVIEW OF POWER SYSTEM • GENERATION • TRANSMISSION & • DISTRIBUTION Components of power system • Generator • Transformer • Transmission Lines • Distribution Lines • Loads & • Compensating Devices (like Shunt, Series, Static VAR Compensators) Function of power system Analysis Need to monitor the voltages at various buses, real & reactive power flow between buses. To design the circuit breakers To plan future expansion of the existing system. Analyze system fault under different conditions (3Φ fault, L- G, L-L, L-L-G faults.) Study of small & large disturbances (sudden changes in load) Need of power system analysis in planning & operation • System planning & operation covers the whole period ranging from the incremental stage of system development • Required area, spacing, RLDC & NLDC for dispatching power • Total demand = Sum of real power generation • System must be reliable & uninterrupted . • Required to maintain over all transmission & distribution) facilities system (generation, Block diagram of Planning & Operation of PS No undesirable deviation Planning Implementation of new plans Monitoring, comparing plans with result Corrective Actions Undesirable deviation Dealing of power system analysis • Load (or) Power Flow Analysis • Short Circuit Analysis • Stability Analysis LOAD (or) POWER FLOW ANALYSIS: System stability. To determine the voltage, current , active & reactive power flows in the PS Operating conditions can be analyzed such as loss in generation, transmission lines & transformer or load. Equipment overload & unaccepted voltage levels. To fix optimum size & location of the capacitors for improving power factor. LFA, is used to evaluate the effect of various loading conditions of a system. Short Circuit Analysis: • Multiple increase in current & forms abnormal system • To find the magnitude of current flow in the system at different interval of times, until it meets steady state. • Types of fault (Unsymmetrical ) • Rescue the system from fault condition. • To analyze SC, symmetrical network are used . components & sequence Stability Analysis: • Type- Steady state & Transient stability. • Stability depends on power flow pattern, generator characteristics, system loading level & line parameters, etc.. • Steady state Stability – PS remains Synchronism & follows relatively slow load change. • Transient stability – PS remains Synchronism under large disturbance Infinite bus bar • A large system whose voltage and frequency remain constant, independent of the power exchange between synchronous machine and bus, and independent of the excitation of the synchronous machine. Per phase analysis. • A balanced three phase system is always analyses on per phase basis by considering one of the three phase lines and neutral. Structure of power system • Generator – Converts mechanical energy to electrical energy [6.6KV, 10.5KV, 11KV, 13.8KV & 15.75KV] • Transformer – Transfer energy for one circuit to another circuit without changing the frequency (for increasing & decreasing the Voltage level) • Transmission Lines – Power transfer from one location to another • Distribution Lines – Distributes power to commercial, domestic & small consumers Primary Transmission Lines –11KV/110KV or 132KV or 220KV or 400KV or 765KV Secondary Transmission Lines – (step down value) 66KV or 33KV or 22 KV or 11KV. Primary Distribution Line- 3Φ, 3 wire system, 33 KV or 66KV feeder Secondary Distribution Line- 400 V (for 3Φ) or 230 V (for 1Φ) SINGLE LINE DIAGRAM (OR) ONE LINE DIAGRAM: In practice, electric power systems are very complex and their size is unwieldy. It is very difficult to represent all the components of the system on a single frame. The complexities could be in terms of various types of protective devices, machines (transformers, generators, motors, etc.), their connections (star, delta, etc.), etc. The purpose of power system analysis, a simple single phase equivalent circuit is developed called, the one line diagram (OLD) or the single line diagram (SLD). The purpose of the single line diagram is to supply in concise form of the significant information about the system. It is to be noted that a given SLD will contain only such data that are relevant to the system analysis/study under consideration. For example, the details of protective devices need not be shown for load flow analysis nor it is necessary to show the details of shunt values for stability studies. Symbols of Single Line diagram • A single line diagram is diagrammatic representation of power system in which the components are represented by their symbols and interconnection between them are shown by a straight line even though the system is three phase system. The ratings and the impedances of the components are also marked on the single line diagram. Impedance Diagram:To calculate the performance of a system under load conditions or upon the occurrence of fault, the SLD is used to draw the single-phase equivalent circuit of the system. The following assumptions are made in obtaining the impedance diagrams. The single phase transformer equivalents are shown as ideals with impedances on appropriate side (LV/HV), The magnetizing reactance of transformers are negligible, The generators are represented as constant voltage sources with series resistance or reactance, The transmission lines are approximated by their equivalent -Models, The loads are assumed to be passive and are represented by a series branch of resistance or reactance and Reactance Diagram With some more additional and simplifying assumptions, the impedance diagram can be simplified further to obtain the corresponding reactance diagram. The following are the assumptions made. Additional assumptions: The resistance is often omitted during the fault analysis. This causes a very negligible error since, resistances are negligible Loads are Omitted Transmission line capacitances are ineffective & Magnetizing currents of transformers are neglected. Note: These impedance & reactance diagrams are also referred as the Positive Sequence Diagrams/ Networks.
