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POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE DET310

POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE DET310. CHAPTER 8: BATTERY. 8.1 Introduction

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POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE DET310

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  1. POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE DET310 CHAPTER 8: BATTERY

  2. 8.1 Introduction • Batteries are used to ensure that critical electrical equipment is always on. There are so many places that batteries are used it is nearly impossible to list them all. Some of the applications for batteries include: • Electric generating stations and substations for protection and control of switches and relays. • Industrial applications for protection and control. • Back up of computers, especially financial data and information

  3. 8.2 PRIMARY AND SECONDARY BATTERIES • Primary Battery • The chemical reaction is not reversible. • When all the original active materials have undergone reactions, the cell will not produce no more free electrons. • Secondary Batteries • The reactions is reversible so that the battery can be recharged. • The active materials can be restored to their original composition by passing current through the cell in the opposite direction of discharge.

  4. 8.2.1 RECHARGEABLE BATTERIES. • LEAD ACID – Flooded, lead calcium, lead-antimony - VRLA ( Valve Regulated Lead Acid) - Flat Plate, Turbular Plate • NICKEL CADMIUM – Flooded, Sealed Pocket Plate, Flat Plate • NICKEL-HYDROGEN • LITHIUM

  5. 8.3 BATTERY STRUCTURE • An electrochemical device that generates electricity by a chemical reactions. • Two active materials react chemically to release electrons. • Electrolyte: A medium that aids electron transfer between active materials. • Separator: A barrier that prevents direct contact between active materials. • Casin: Holds everything together. • Cathode and anode : Positive and negative plate

  6. 8.4 Principles of Battery Operation Figure 2 shows a simple galvanic cell. Electrodes (two plates, each made from a different kind of metal or metallic compound) are placed in an electrolyte solution. When external connection is made through a load, the anode will released negative charge anode and positive charge ion. The electron will combine with material at positive terminal causing the released of negative charge metal oxide ion. At the interface of the electrolyte, this ion causes a water molecule to split into a hydrogen ion and a hydroxide ion.

  7. Figure 2: Conceptual diagram of galvanic cell

  8. 8.4 Principles of Battery Operation (continue) The positively charged hydrogen ion combines with the negatively charged metaloxide ion and becomes inert. The negatively charged hydroxide ion flows through the electrolyte to the anode where it combines with the positively charged metal ion, forming a water molecule and a metal-oxide molecule. In effect, metal ions from the anode will “dissolve” into the electrolyte solution while hydrogen molecules from the electrolyte are deposited onto the cathode.

  9. 8.4 Principles of Battery Operation (continue) lead sulfate, PbS04, is formed at each electrode and adheres to it.

  10. 8.4 Principles of Battery operation (continue) • The acid is depleted upon discharge and regenerated upon recharge. Hydrogen and oxygen form during discharge and float charging (because float charging is counteracting self-discharge). • In flooded batteries, they escape and water must be periodically added. In valve-regulated, lead-acid (sealed) batteries, the hydrogen and oxygen gases recombine to form water. • In VRLA, absorbed glass mate or gel contained traps the hydrogen and oxygen during discharge. No water is needed to be added compared to flooded lead acid batteries. • Since the process of discharge converts sulphuric acid into water, the specific gravity of the electrolyte falls

  11. BATTERY CAPACITY. • Lead Acid and nickel cadmium cells are usually rated in ampere-hour capacity and voltage, end of voltage (EOD) • - Ampere-hours - the battery can supply for a specified time before voltage falls below a minimum value. • Example : 250AH, 100 AH • Also can be express as C rate • Example for 250AH at 8 hour rate will be express as C8 • Similarly for 10-h, 3h, 30 min will be express as C10, C3 and • C0.5 respectively

  12. Discharge and Charge Rate. • Is the magnitude of current being which can be drawn or charge from or to a battery. • Example: • - Battery 250 Ah, C10 discharge at 25 A. • - Battery will be able to deliver 25A for 10 hours • In C rate: 25/250C10 or 0.1C10 • -Similar battery discharge at 450 A • In C rate: 1.8C10

  13. Voltage • The voltage of the battery at initial condition/nominal voltage • Usually for lead acid – 2 Volts • in UK, substation batteries are usually 110V nominal (55 cells) or 24 V nominal. • 110 V batteries are usually employed for circuit breaker tripping, closing and protection purposes. • 48 V batteries are employed for telecommunication and control purposes. • Usually depicted as 110 V/ 48 V.

  14. Battery Charging modes • Float Charge • Boost Charging • Equalised Charge • Float charging – to keep the battery at 2.25 V/cell – for supplying continues load • Boost Charging – to charge the battery to 2.7 V/cell from 2.25V/cell – usually to supply during momentary load • Equalised Charging- to keep the battery at 2.23V/cell

  15. End of Voltage (EOD) End of Discharge voltage is the level to which the battery string voltage or cell voltage allowed to fall to before affecting the load. The low voltage limit is the voltage below which the battery can no longer supply usable energy to the load For example : Battery specification, EOD 1.75 V, 400 AH, 0.1C10 Battery can supply continuous ampere of 400 A at 10 hours before it voltage is drop to 1.75 V

  16. End of Voltage (EOD) –continue Example: Determine the cell end of voltage for 55 cells, 110 V if minimum allowable system voltage is 96.25 Volt. Solution: Calculate minimum system voltage 96.25/55 = 1.75 Volt/cell

  17. Discharge Characteristics Usually if a battery which is rated as 1400AH,0.1C7. EOD 1.75 V. If the battery is discharge at 7 hour rate, discharge current is 200 A, and the voltage at the end of discharge time is 1.75 V The same rated battery if discharge at 4 hours, it only produce discharge current of 300A with end voltage of 1.75 V. This is known as discharge/capacity factor. The actual amount of current available from the cell battery can be referred from discharge characteristics

  18. Figure 3: Discharge characteristics

  19. Discharge Characteristics (continue-) Battery manufacturers publish typical discharge data for the cells they manufacture. The cell discharge rates are expressed in amperes, ampere-hours, or watts for various discharge times to various end voltages at the standard cell temperature. Most often, the data is a part of the catalog cut for the cell and may be in the form of a table or curve.

  20. Discharge Characteristics (continue-) Discharge curves are developed by a battery manufacturer by discharging a cell(s) at various currents and measuring the voltage at a number of points during the discharge. These data are plotted as cell voltage vs. time at each of the discharge currents. A typical plot for a lead-acid cell type is shown in Figure 3. Once these data are obtained, they are expressed on a common base of amperes per positive plate.

  21. Discharge Characteristics (continue-) In addition, for some discharge curves, the ampere-hours removed from the cell at various times are also computed and expressed on a base of Ah per positive plate. These data are then plotted as a curve known as a discharge curve. Two commonly used curves are the fan curve and an S curve.

  22. CAPACITY FACTOR When sizing a battery using constant current, the sizing may be based on positive plates or ampere-hours. Sizing using positive plates requires the use of a capacity rating factor, RT, and sizing using Ah uses a capacity rating factor of KT. In North America, lead acid battery manufacturers typically publish values of RT, and nickel-cadmium manufacturers publish values of KT.

  23. CAPACITY FACTOR USING Rt Based on no of positive plate Nickel Cadmium = equal positive and negative no of plates Lead Acid = one negative plates more then positive plates Positive plate sizing – If there is 25 nos plate in battery cells, the number of positive cells are = (25-1)/2 = 12 Nos Usually can be determine by using Fan Curve an S curve

  24. Figure 4: Discharge Curve: Fan curves

  25. CAPACITY FACTOR USING Rt (continue-) Figure 4 is an example of a fan curve. The curve is for a single cell. The x-axis is labeled amperes per positive plate, while the y-axis is labeled ampere-hours per positive plate. There are a number of radial lines from the origin that represent discharge times (in minutes or hours) and curves for various end-of-discharge voltages. At the top of the curve, there is a line labeled “initial voltage.”

  26. Capacity Rating Factor (Rt) continue • Example: Determine the capacity rating factor, RT , for the 60-min rate to 1.75 Vpc at 25°C. • Solutions: • find the point at which the radial line for 60 min intersects the curve for 1.75 FV. • From point A, drop a vertical line that intersects the x-axis and read the RT. This is identified as point B and is 70 amperes/positive plate. • Related Calculations: If this cells have 9 no of plates, then the discharge capability of one cell with 4 nos of positive plate is 4 x 70 A = 280 A dc

  27. Capacity Rating Factor (Rt) continue Figure 5

  28. Capacity Rating Factor using Kt If ampere-hour sizing will be used, the KT capacity rating factors for the cell must be obtained. Some manufacturers have tables or curves (Fig. 6) available; however, they may be calculated directly from the cell rating and discharge data, using the following formula, Kt = C/IT where C is the rated capacity of the cell in Ah, I is the discharge current, and T is the discharge time period.

  29. Capacity Rating Factor using KT (continue-) Example: Determine capacity rating factor Kt for 30 min discharge rate to 1.75 Vpc for Cell Type KM369P KT= 369/266 = 1.3872

  30. Capacity Rating Factor using KT (continue-) Effect of capacity rating factor to battery capacity Battery capacity (corrected) = Battery Capacity/ (capacity rating factor)

  31. FIGURE 6: Typical KT capacity rating factors. (C&D Technologies, Inc.)

  32. SELF-DISCHARGE All charged batteries (except thermal batteries and other batteries specifically designed for a near-infinite shelf life) will slowly lose their charge over time, even if they are not connected to a device. Moisture in the air and the slight conductivity of the battery housing will serve as a path for electrons to travel to the cathode, discharging the battery. The rate at which a battery loses power in this way is called the self-discharge rate.

  33. SPECIFIC GRAVITY WHAT IS SPECIFIC GRAVITY? Define as the ratio of the weight of a given volume of electrolyte to the weight of an equal volume of distilled water. In other words it is the measure of sulphate in the acid.The SG of water is 1.0. Lead-acid batteries use an electrolyte which contains sulfuric acid. Pure sulfuric acid has a specific gravity of 1.835, since it weighs 1.835 times as much as pure water per unit volume.

  34. SPECIFIC GRAVITY (CONTINUE-)

  35. HOW TO CHECK SPECIFIC GRAVITY • USE HYDROMETER • SG IS TEMPERATURE DEPENDANT- NEED TO DO TEMP CORRECTION • FOR EVERY 1.5 DEG CELCIUS ABOVE 15 DEG CELCIUS, ADD 0.001 TO THE READING. • FOR EVERY 1.5 DEG CELCIUS BELOW 15 DEG CELCIUS, SUBSTRACT 0.001 TO THE READING.

  36. Example: HYDROMETER READING AT 27 DEG CELCIUS = 1.197 AFTER CORRECTION: 1.197+[(27-15)X0.001]/1.5 = 1.205

  37. Specific Gravity and Open Circuit Voltage A fully charged lead-acid cell has an open circuit voltage (OCV) of approximately 2.05 to 2.15 V; the exact voltage varies with the electrolyte specific gravity and temperature. The OCV increases as the specific gravity increases and decreases as the SG decreases. The OCV varies with electrolyte specific gravity by the following relationship: OCV = Specific gravity + 0.845

  38. Specific Gravity and Open Circuit Voltage Example: The OCV of a cell with an electrolyte specific gravity of 1.215 is 2.06 V (1.215 +0.845 = 2.06). The OCV of a cell with an electrolyte specific gravity of 1.300 is 2.15 V (1.300 +0.845 = 2.15).

  39. Effect of temperature on discharge current As the power/energy produced within a battery cell is the result of a De-rate Factor electrochemical reaction, any change in the electrolyte temperature has an effect on the efficiency/rate of reaction. i.e. an increase in temperature increase the efficiency/rate were a decrease in temperature reduces the efficiency/rate of the reaction. As a result of this all battery manufactures discharge data will be specified a recommended temperature (typically 20°C) with temperature corrections provided for operation above & below these valves. Discharge current = Discharge current (at 20° C) x correction (at corrected temp) factor

  40. Typical Temperature Correction Factors for VRLA Example: For a battery which can provide 50 Amps at 20 °C to supply a load, Determine the output from the same battery if operated at 0°C Solutions: 50 A x 0.86 = 43 Amps

  41. AGEING FACTOR – The performance of a battery comes down with time. Therefore, to deliver its rated capacity, the ageing factor must be taken into consideration. Ah capacity (after correction factor) = Ah Capacity/correction factor

  42. Battery Sizing and Selection The designer of a backup power system has to determine the battery size. Simply stated, how big a battery is needed? The battery can carry more load or perform longer as it is made larger, but a larger battery is more expensive and requires more floor space. For this reason, there should be a basis for the battery size.

  43. Battery Sizing and Selection (continue-) Selecting a battery size is simple in concept, but often difficult in practice. The following design inputs are needed to determine a battery size: • Discharge capability of selected battery type • Load requirements, including duration • Minimum and maximum voltage limits • Temperature, aging, and design margin allowances

  44. Battery Sizing and Selection (continue-) Discharge capability of selected battery type- refer to discharge curve and capacity rating factor Load requirement- Based on Duty Cycle Battery Duty Cycle - The load that the battery is expected to supply for a specified period of time (sum of all individual loads)

  45. Battery Sizing and Selection (continue-) Load categories Momentary load – closing tripping switchgear, Motor starting currents, Inrushcurrents (solenoids, relaycoils, etc) .usually categorised as load which does not exceed 1 min Continuous load- current for lamp indicator, supervisory control equipment, continuously operating motor, inverters and UPS. Non- continuous load- emergency pump motors that operate when required, ventilation system which operate on and off.

  46. Battery Sizing and Selection (continue-) Example of Duty Cycle

  47. Battery Sizing and Selection (continue-) Plot the load profile

  48. Battery Sizing and Selection (continue-) Minimum and maximum voltage The maximum and minimum allowable system voltages determine the number of cells in the battery.

  49. Battery Sizing and Selection (continue-) Minimum and maximum voltage Example:If the battery is installed in a 125 V system, the electrical devices in the system are often rated for proper operation between 105 to 140 V. The float/equalised charge or maximum voltage percell is 2.33 V Maximum no of cells = 140/2.33 = 60.08 cells Use 60 no of cells. Minimum cell voltage = 105/60 = 1.75 Vpc

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