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PRACTICAL ELECTRICAL ENGINEERING BASICS

PRACTICAL ELECTRICAL ENGINEERING BASICS. Fundamentals. Most common power generators in power stations generate AC voltages Follow sinusoidal waveforms alternating between positive and negative peaks around zero axis

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PRACTICAL ELECTRICAL ENGINEERING BASICS

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  1. PRACTICAL ELECTRICAL ENGINEERING BASICS Fundamentals

  2. Most common power generators in power stations generate AC voltages Follow sinusoidal waveforms alternating between positive and negative peaks around zero axis This waveform of voltage/ current is normally referred to as AC voltage and AC current in Electrical distribution Alternating Current System

  3. Typical AC Sinusoidal Wave Form • 0 to 360 degree in the picture corresponds to one cycle of wave form, which is represented by one full circular motion

  4. Why AC? Almost entire power generation and transmission is by AC (alternating current) AC lends itself to voltage changes easily Voltage can be chosen for optimum efficiency and optimum capital cost Thus better economy of power system operations

  5. Alternating current system • Power generators generate AC voltages, which follow sinusoidal waveforms • Mathematically expressed as V = Vp Sin(2 × f × t) • This voltage/current waveform normally referred to as AC voltage and AC current • V – Instantaneous magnitude of voltage • Vp– Peak value of voltage • F – Frequency • T – Time Typical AC sinusoidal wave form

  6. AC Waveform Voltage or current in AC circuits varies cyclically number of times per second This number is called frequency (f) Time for one cycle is 1/f Variation follows sine relationship and waveform is called sine wave Electrical system can be of single or 3 phase type

  7. Single Phase AC waveform 7

  8. Single and three phase generators

  9. Three phase AC Universally adopted because of lower equipment cost per unit power handled Helpful while interconnecting several generating sources (sources tend remain in phase or stay synchromised) Three phase AC motors (which account for most of the energy used) have simple design and are self starting

  10. Benefits of 3-phase ac power system • Higher power output for a given size of motor • Better conductor economy per unit power • Self starting induction motors • Better synchronising torque makes parallel operation of alternators possible

  11. Electrical Power and Energy In DC circuits Power = Voltage x Current (Watts) Energy = Power x Time (Watt Hours) In AC circuits instantaneous voltage and current keep changing as they follow a sine curve Power is computed using Root Mean Square (RMS) voltage and RMS current

  12. AC Power Fundamental Definition of Power: In an AC circuit both are sinusoidal against time Sinusoidal Voltage and Current are defined in RMS magnitudes where: 12

  13. Power Triangle S Q f P 13

  14. Majority of electrical circuits are inductive in nature A standard inductive circuit is normally represented with parameters called resistance R and inductance L, which are basically the measure of resistance offered by the circuit preventing/ limiting the flow of current Typical Standard Inductive Circuit

  15. Formulae for Power Apparent power S = V*I Active Power P = V*I*Cos f Reactive Power Q = V*I* Sin f Cos f is known as the Power Factor and f is power factor angle The angle depends on ratio of resistance to the reactance in a circuit

  16. AC voltages/currents are normally represented in vector forms Instantaneous positions of voltage and current in relation to each other are vectorially indicated as below with both V and I go on rotating full 360 degrees for each completed cycle V & I for an Inductive Circuit

  17. Three line diagram

  18. Singlelinediagram

  19. Advantages of a single line diagram • It requires much lesser space and hence leaves room for designer for providing other useful information in the drawing • It is very versatile and comprehensive because it can depict very simple DC circuits, or very complicated three-phase system • Simple, hence requires much lesser time for reader to understand the basic system design

  20. Differences between single line and the 3-line diagram

  21. Power network • Generation • Transmission/sub-transmission • Distribution • Utilisation

  22. Three phase power network All AC generation, Transmission and Distribution is through 3 phase systems Exception: Single phase/SWER distribution systems Utilisation can be 3 phase (motors and rectifiers for drives, furnaces) or single phase (commercial and lighting) Ability to transmit larger amount of power for a given voltage/conductor volume Availability of rugged 3 phase cage motors with self starting capability

  23. Power system Power generation plants are located based on fuel availability Other considerations like water, pollution issues etc. Often they are in remote locations Loads are situated in population centers Transporting generated power to population centers for use requires a power system

  24. A 400 kV Transmission line and Structure

  25. Substations A facility incorporating Transformers Switching/isolation equipment Control/protection/measuring Auxiliary power equipment Indoor or Outdoor type Air-insulated or gas-insulated Different configurations

  26. Outdoor 330 kV switchyard (Transmission) Typical indoor distribution substation

  27. Voltage classification BS and IEC • Circuits supplied at nominal voltage up to and including 1000 V a.c. or 1500 V d.c. are said to be in the low voltage range, and equipment rated for voltages above these values are classified as high voltage (HV) equipment.

  28. Voltage classification as per IEEE 141:1993 • Low voltage (LV) • Systems of nominal voltage up to 1000V • Common usage: 380V, 415V, 480V • Medium voltage (MV) • Systems of nominal voltage 1000V and above but less than 100000V • Common usage: 4160V, 6900V, 12000V, 13800V, 34500V, 69000V • High voltage (HV) • Systems of nominal voltage 100000V and above and up to 230000V • Common usage: 115kV, 138kV, 230 kV

  29. Basis of voltage selection • System segment • Generation: MV • Transmission: HV and Extra High Voltage (230kV+) • Sub-transmission: HV/MV • Distribution: MV/LV • Utilization: MV/LV

  30. Distribution and utilisation voltages • Domestic, commercial and small industry loads at LV • Utilities distribute power at MV and also at LV • Large industries have a mix of MV drives, Transformers and LV loads • Distribution and utilisation can involve more than one voltage

  31. Distribution voltage options for industries • Receiving and distribution at the same voltage • Avoids need for transformer • Difficult to regulate voltage • Independent earthing of plant system is not possible • Receiving at a higher voltage • Involves a transformer • Internal voltage independently variable through OLTC • Can choose system earthing independent of the utility • Transformer acts as a buffer (harmonics, voltage dips)

  32. A typical utility system Supply from a higher voltage Normally open point Supply from a higher voltage

  33. Power distribution Distribution is done at lower voltage levels Distances are much shorter than transmission Voltage levels are typically 33 kV and less Bulk of consumers draw power at lower voltages This requires distribution at two different levels Distribution substations convert power from transmission to distribution voltages

  34. Governing principles of Power Distribution • Safety of operating personnel and plant assets • Reliable power supply (continuity of power) • Adequate quality of power • Prevention of load generated problems

  35. Electrical safety and power security • Electricity-a good servant but a bad master • Safety is a predominant issue • Legislative requirements • Standards for ensuring quality and reliability • Good O&M practices to ensure power security

  36. Industrial distribution components • An incoming section • In-plant emergency/standby generation and associated distribution system • A primary distribution system, usually medium/high voltage • A step down transformer section • A secondary low voltage distribution system • Controlgear for individual loads • Supervisory control system

  37. Main equipment types • Circuit isolation devices • Circuit switching devices (circuit breakers) • Transformers • HV and LV distribution equipment • Motor control panels • DC supply system • Power system protection • Cable (or overhead line based) distribution systems • Earthing system • Power quality improvement equipment

  38. Types of distribution Radial Radial with redundancy Selective Looped or Ring type Ring type with spur lines Mesh connections

  39. Radial distribution A single incoming feeder Failure causes total disruption Restoration only after the problem is rectified Not desirable where critical production processes are involved If at all used, internal standby source to be planned also

  40. Redundancy Ensuring alternative supply such that Any one of the sources/incoming feeders may take up full load (100% redundancy) Only partial load can be taken up (partial redundancy) Reduces period of interruption Two types of redundancy (selective and looped)

  41. Redundancy Selective Redundancy: Two feeders to each load One feeder used at a time and can be selected Automatic restoration schemes to switch feeders Reduces time of interruption Results in under-utilisation (because of 100% redundancy) Loop (Ring) type redundancy

  42. Loop (Ring) type redundancy

  43. Ring type distribution Favoured by power distribution utilities Fault in a ring section of a closed loop cleared from both ends Selective closing of isolators to detect fault Easy to extend the ring for inclusion of additional loads Supply can be restored to all consumers without waiting for repair of faulty section Automation possibilities for restoration

  44. Mesh type system Similar to ring type Additional connections between some of the nodes Usual in transmission systems Distribution systems provide such connections for additional security Complexity of protection

  45. Industrial example of Incoming supply level redundancy

  46. Industrial example of Main Transformer level redundancy

  47. Industrial distribution-Typical

  48. Distribution Systems in Oil and Gas Facilities • Large facilities have power demands ranging from 30 MW and upwards to several hundred MW. • Voltage levels for high, medium and low voltage distribution boards are in the range of 13- 130kV, 2-8 kV and 300-600 V respectively. • Generated power is exchanged with mains or other facilities on the HV distribution board and relays are used to provide protection.

  49. Distribution Systems in Oil and Gas Facilities

  50. Distribution Systems in Oil and Gas Facilities • Electrical power needed for gathering station induction engines, injection steam generators & wells with units of mechanical & centrifuge pumping is supplied by 15 -150 kVA substations and 13.8/0.48 kV transformers. • Building facilities, external lighting, communication and automation are powered by 575/480/380-220 V, 5 kVA to 15 kVA transformers. • 13.8 kV, 150 -1200 kVA capacitor banks are connected to the electrical distribution feeders to regulate voltage & correct power factor.

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