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EMERGENCY POWER SUPPLIES

EMERGENCY POWER SUPPLIES. Problems of Power Failure in Industries. Primary intention of Power Utility - Provide uninterrupted power Causes for interruption: Equipment failures and line faults System instability leading to tripping

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EMERGENCY POWER SUPPLIES

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  1. EMERGENCY POWER SUPPLIES

  2. Problems of Power Failure in Industries • Primary intention of Power Utility - Provide uninterrupted power • Causes for interruption: • Equipment failures and line faults • System instability leading to tripping • Deliberate tripping (usually automatically) to save system collapse • Natural causes - Hurricane, earthquake, flood

  3. Problems of Power Failure in Industries • No power system can be guaranteed to be free from interruptions • Transmission line faults • Equipment malfunctions • Weather related failures • Failures due to other external causes in exposed parts of system • Human errors • System instability due to major disturbances

  4. Problems of Power Failure in Industries • Outages more likely in a system without adequate generation reserve • No buffer storage possible for Electrical energy • Frequency drop due to system overload • Complete system collapse due to inadequate system reserves

  5. Implications of Power Failure • Accidents involving death or injury • Damage to equipment • Creation of potentially hazardous conditions • Loss of production (not only for duration of interruption, also time required to bring process to its pre-failure state) Accidents that can result from a sudden interruption • Dropping of loads lifted by electromagnets • Release of toxic materials • Spillage of hot metal • Explosions • Runaway reactions

  6. Implications of Power Failure Can cause Potentially Hazardous situations • Loss of control power • Loss of lighting in operational areas or exit routes • Loss of ventilation/exhaust systems • Loss of signaling and alarms • Loss of fire-fighting systems

  7. Production behavior following Interruption of Power Supply

  8. Costs of Power Interruption • Direct Costs • Costs due to lost man-hours • Direct expenses due to death/injury • Cost of repairs to damaged equipment • Lost production • Indirect Costs • Legal costs - Accidents involving third parties/ quality problems/ non-fulfillment of contract commitments • Loss of good will

  9. Power Failure Problem Mitigation • Problem mitigation • Installation and use of adequate reserves • Deliberate tripping by islanding schemes • Advance preparation during natural calamities • Proper equipment selection, maintenance • Power supply restoration • Auto-reclose operations (self clearing of transient faults) • Switching to redundant feeders • Starting standby generating sources

  10. Solutions for Power Failure • Distributed generation capacity - Owned by Utility, Industries or third parties • Engine generators, gas turbines - Quick start, can take up peak demands Advantages • Closer to load, not affected by transmission circuit problems • Capable of being started, brought up to load faster • Serve as standby sources • Provide uninterrupted power when synchronized with utility • Improve voltage profile in end-of-line distribution circuits

  11. Industrial Plant - Emergency Generation • Capacity to feed critical loads only • Break before make Switch – Brief interruption during transfer

  12. Tolerance for Interruptions & Voltage/Freq. Excursions • Electrical equipment designed with tolerances in input voltage parameters – Can take care of limited variations • Desktop computers - Tolerate voltage fluctuations to some extent • By virtue of large capacitances • Internal regulation circuitry • Tolerance for voltage fluctuations - General industrial equipment • –10 to +6% for slow/sustained variations (sag/swells) • Considerably more for short time disturbances

  13. CBEMA, ITIC and ANSI - Voltage Sensitivity Curves • Random data errors in computers due to voltage variations • Standard Curves • Indicating voltage limits causing no ill effects, plotted against time • Earliest curves developed by Computer and Business Equipment Manufacturers Association (CBEMA)

  14. CBEMA Voltage Sensitivity Curves

  15. ITIC Voltage Sensitivity Curves(Information technology industry council)

  16. Uninterrupted Power, Emergency Power and Standby Power • Uninterrupted Power - Guarantees continuous power without even a momentary break to connected loads • Emergency Power - Minimum backup power for emergency applications (emergency lighting, emergency shut down systems, alarm systems, elevators, life safety and security systems). Brief interruption can be tolerated • Standby Power - Provision of substantial power to maintain all production, business processes during short/ long power outages

  17. Different Solutions for Different Needs Needs of Equipment • Careful consideration for power quality or continuity needed • Questions to be asked • Can the load tolerate power interruption? If yes, for how long? • What is the effect of a power failure? • Are there associated costs? • Are there safety hazards associated with a failure?

  18. Static UPS Systems • Widely used for feeding lower capacity, low voltage loads - Instrumentation equipment, process control computers, general computing equipment, critical lubrication drives and emergency lighting • Improves power quality to critical loads • Prone to component failures like any other electronic system • Redundant modules for critical loads to improve reliability

  19. Static UPS System - General Configuration

  20. Subsystems of typical Static UPS • Backup source (storage battery) • Rectifier (also used as charger for battery) • DC bus (link between rectifier output, battery and inverter input) • Inverter for synthesis of AC output from DC • Regulation and conditioning of mains power supply • Static transfer switch or static bypass (optional) • Maintenance bypass (optional) • Isolation transformer (optional)

  21. Static UPS System • Storage battery as power source - Critical component for reliability • Rectifier • Converts power from mains into DC - Charges storage battery • Configured to cause minimum harmonics in mains AC • Provided with filters to ensure ripple free output

  22. Static UPS System • Battery - Maintains voltage of DC bus when mains power fails • Voltage keeps falling as battery continues to discharge • Low DC voltage alarm • Cutout to trip inverter to avoid over-discharging

  23. Static UPS System • Inverter converts voltage of DC bus to synthesized AC output • Low capacity inverters - Partial square or trapezoidal waveform • Higher capacity inverters - Pulse-width modulated sinusoidal AC output • Input protection - Surge protection devices to protect sensitive components • Output protection - Fuses/MCCBs with overload protectors, surge protection devices

  24. Static UPS System • Voltage regulating device between AC mains and UPS where very wide fluctuations of input voltage expected • To keep battery from going into discharge mode at low mains voltage conditions (sustained sag) • To regulate output voltage when mains power is directly connected to loads • Static switch - Automatic load transfer from mains power to inverter or vice versa • Filter - Control communication of noise, harmonics from power mains to loads

  25. Static UPS System • Maintenance bypass • Optional component • Manually operated external switch • For undertaking UPS maintenance • Isolation transformer - To avoid surges, common mode noise from being conveyed to loads

  26. Types of Static UPS Systems • Passive (offline) • Line interactive • Double conversion (online)

  27. Passive UPS Power flow under normal (mains power available) condition Power flow under mains failure condition

  28. Passive UPS • Definite delay involved in changeover - 8 to 10 ms • Loads must be able to tolerate break of supply • Mechanical relay instead of static switch in cheaper designs – Slower than static switch • Needs attention while selecting UPS

  29. Passive UPS • Advantages • Cost effective - Rectifier sized only for battery current, Inverter sized only for short duration • Limited losses in inverter (only during its operation) • Generally restricted to lower capacities (2 kVA or less) • Unsuitable where break in output not acceptable • Limited power quality improvement

  30. Passive UPS • Some UPS designs - Changeover not just for mains failure conditions, also during supply voltage, frequency drift Disadvantages • Frequent battery discharge-charge cycles - Reduced battery life • Lower backup time during actual mains failure

  31. Line Interactive UPS • Static switch positioned differently from Passive UPS • Not involved in switching of load from mains to inverter

  32. Line Interactive UPS • Static switch connects mains supply to load when it is present • Inverter module connected to load in parallel • When mains power fails, inverter module operates as inverter, power flow direction is reversed • Static switch opens to prevent flow of power back to mains • Limited to smaller ratings only

  33. Double Conversion UPS

  34. Double Conversion UPS • Primary mode of power flow from mains to rectifier to inverter and to loads • Rectifier supplies power for loads, also charges battery • When mains power fails battery supplies power • No break of supply at UPS output • Higher rectifier current – Battery charging current plus load current • Most designs provided with static switch - Automatic transfer of loads from inverter to mains in case of inverter module failure

  35. Double Conversion UPS • Static switch omitted in some low output UPS designs • Reduced UPS cost • Carries risk of power loss if inverter fails

  36. Double Conversion UPS Advantages • Harmonics, surges and noise not get reflected at output • Inverter can operate at own frequency independent of mains frequency Disadvantages • Higher cost • Rectifier for load current plus battery charging current • Inverter rated for continuous operation • Increased power losses

  37. Double Conversion UPS • Standby batteries normally sized for providing backup for short periods only - 15 minutes to 1 hour • Higher duration power outages - Appropriate standby power source for UPS before battery gets fully discharged • UPS designed for ensuring safe, orderly shutdown of equipment in event of mains failure • Batteries charged at high rate to ensure quick restoration after power interruption

  38. Other types of UPS Systems • Passive UPS with Ferro-resonant output transformer • 3-winding Ferro-resonant transformer at UPS output • Two windings for input (one from mains, one from inverter) one common output winding • Hybrid passive UPS system

  39. UPS with Ferro-resonant Transformer

  40. UPS with ferro resonant transformer • Similar in concept to passive UPS system • Loads fed from mains through static switch, Ferro-resonant transformer when mains available • Static switch opens, inverter takes up load through transformer when mains fails • Ferro-resonant transformer - Acts as output filter, can regulate slow voltage changes • Can minimize break of output when mains fail and inverter takes up load • Problems • Unstable output when load has power factor correction capacitors • Low efficiency at part loads • May interfere with fuse protection • May call for surge protection on output side

  41. Hybrid Passive UPS

  42. Hybrid passive Configuration • Load directly connected to mains when it is present, switched to battery when mains power fails • Inverter always in circuit • Switching delay between mains and battery eliminated by proper setting of regulator • Behaves like double conversion UPS except there are two different rectifiers, one for charging battery and another for supplying load • Disadvantage - Any failure of inverter module can cause failure of UPS output

  43. Hybrid UPS System Combines advantages Static and Rotary Systems

  44. Redundant UPS Configuration • UPS systems also prone to failure just like any other electronic equipment • Failures also caused by battery problems • Redundancy to prevent interruptions due to failure

  45. Redundant UPS Configuration • Usually referred to as N+1 redundant configuration • N modules can supply total power requirement by equal load sharing • One additional parallel module for little extra capacity Continued operation when one module fails • Redundancy not just for electronic components but also for battery • Additional static switch fed from separate source to switch to alternate supply during multiple module failure

  46. UPS - Installation • To be done by experienced personnel • Read, understand manufacturer’s instructions fully before installation and commissioning • Follow Safety procedures strictly to avoid danger to personnel and equipment • Batteries are heavy – Take care during handling, transportation • Acid is corrosive – Take care in handling, storage

  47. Batteries Different Types, Pros and Cons • Energy storage device used for UPS • Energy stored in form of chemical energy during charging, converted back to electrical energy during discharge phase • DC of battery converted to AC by UPS and delivered to loads • Most commonly used battery types • Lead acid battery (Lead Calcium and Lead Antimony) • Nickel Cadmium battery

  48. Lead Acid, Nickel Cadmium Batteries • Lead Acid Battery - Use lead electrodes, Sulphuric acid as electrolyte • Lead Calcium Type • Not suitable for charge-discharge cycling applications • Can undergo 30-50 deep discharges over life • Lead Antimony Type • Can tolerate up to 300-500 deep discharge cycles • Nickel Cadmium Battery - Electrodes made of nickel- cadmium, potassium hydroxide as electrolyte • Lead acid and Nickel-Cadmium batteries designed both in flooded and valve regulated versions

  49. Batteries • Flooded cell batteries - Have liquid electrolyte like sulphuric acid • Valve regulated type (maintenance free batteries) • Do not have free liquid electrolyte • Do not require specially designed room with dedicated temperature control, dedicated exhaust and ventilation system or hazardous area equipment • Less maintenance intensive • Can be oriented in any direction without electrolyte spillage problems • Incorporate following technologies • AGM - Absorbed glass matte technology • Gelled electrolyte technology • Oxygen recombination technology

  50. Batteries • AGM (Absorbed glass matte technology) - Electrolyte completely absorbed in glass matte • Gel cell - Electrolyte in gel form • Valve regulated battery • Smaller in size compared to flooded cell battery. Typically AGM battery has app. 40% less electrolyte than flooded cell, Gel cell about 15% less electrolyte • Also use recombination technology - Gases evolved during charging recombine to form water (eliminates need for adding water)

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