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2014-05-23 11:45:29

Show Answers:

  1. What is the purpose of a distribution substation?
    The purpose of a distribution substation is to have power transformers that step down the power delivered to it by one or more high voltage transmission lines (60kV, 138kV, 230kV, etc) to a distribution voltage level (34kV or less. BC Hydro usually uses 4kV, 12kV and 25kV)
  2. What factors affect the location of a distribution
    • ·     Load area it will serve to minimize length
    • of transmission and distribution lines.

    • ·   Availability of right-of-ways and access to
    • site for both overhead and underground transmission and distribution lines.

    • ·  Public objections due to appearance, noise,
    • or electrical effects

    ·  Atmospheric conditions

    ·  Public safety

    ·   Security from vandalism and theft
  3. What is the purpose of a substation ground grid?
    To protect personnel from the dangers of both step potential and touch potential.

    The ground grid is installed below the surface of the earth throughout the substation to protect personnel and the public from hazardous potential differences, protect equipment from damaging potential differences and ensure proper electrical system operation.
  4. What is the purpose of a transfer bus?
    A transfer or spare bus and spare circuit breaker allows a circuit to be transferred to the transfer bus and remain in service while its normal feeder equipment (ie: circuit breakers, reactors, regulators) undergoes maintenance.
  5. What is the main purpose of substation buildings?
    The main purpose of substation buildings is to contain and give shelter to protection relays, control panels, auxiliaries, batteries, compressors, emergency generations, etc.
  6. What is station service?
    Station service is a power supply that operates the equipment within the substation.
  7. How is AC station service supplied?
    AC power supplies may come from:

    · Dedicated station service transformers connected to station buses

    · tertiary windings of main transformers

    · A line feeding in from another station

    ·  Stand-by generator powered by internal combustion engine.
  8. What are the limitations of AC station service?
    AC energy cannot be stored.
  9. How is DC station service supplied?
    DC station service is normally suppliced from the AC by a rectifier/battery charger. The battery is connected in parallel with the  attery charger so that it is kept in a charged condition to supply the equipment if the AC fails
  10. What is the purpose of DC station service?
    DC is stored in batteries so that if AC fails, the battery will supply essential services for at least 8 hours.

    • A reliable DC power supply is required for several applications:
    • • Normal operation of control, alarm, and protection circuits.
    • • Switches and lights on control panels and some operating circuits for
    • • power circuit breakers.
    • • Emergency lighting to allow safe movement of people in event of a
    • • complete failure of AC.
    • • Emergency pumps and fans.
    • • Uninterruptible power supplies for vital computer aided controls and communication equipment
  11. What are the standard AC and DC station service voltages?
    • The standard AC bus voltage in BC Hydro are
    • 120/208V and 347/600V.

    Typical DC supply is 125V but some may have more than one battery bank with 24V or 48V DC for communication equipment.
  12. What is a station service auto transfer scheme?
    BC Hydro stations have a minimum of 2 station service transformers. There is an automatic transfer from one to the other in the event of failure. SS1 is usually the preferred/normal source.
  13. Why is rigid bus usually hollow?
    Rigid bus is usually hollow because alternating current tends to flow along the surface of the conductor, “skin effect”. Also because they are inexpensive, stronger and lighter.
  14. What type of conducting metal is used for outdoor rigid bus and why?
    • Aluminium because:
    • • It is much lighter (1/3 the density of copper)
    • • Better suited to industry manufacturing process
    • • Less conductivity so slightly lower skin effect.
  15. What are the reasons for using SF6 bus?
    SF6 bus is used for difficult application where space requirements are a problem when using conventional or inverted open-type bus arrangements. They are also used for coastal stations where trouble can occur with salt contamination of the insulators.
  16. What SF6 gas qualities make it highly suited for use as the insulating medium in CGIS?
    • Sulphur hexflouride (SF6) is used for bus systems because:
    • • It is inert
    • • Exceptional thermal stability
    • • Excellent arc-quenching properties
    • • Exceptionally good insulating properites
  17. How is the dielectric strength of SF6 gas monitored?
    The dielectric strength of SF6 gas is monitored by a temperature-compensated pressure switch (dielectric strength is a function of density which is in turn a function of pressure and temperature).
  18. What is the purpose of a power transformer in a distribution substation?
    In a distribution substation power transformers are used to step down the voltage delivered to the substation from the transmission system to a lower, more usable distribution voltage (34kV or less). BC Hydro usually uses 4kV, 12kV, and 25kV.
  19. What are the basic components of a power
    • The basic components of a power transformer are:
    • • The tank or enclosure and associated accessories or fittings
    • • The core and winding assembly
    • • The insulating and cooling medium
  20. What is the purpose of a conservator tank and
    how does it function?
    • A conservator tank keeps the main tank completely filled with oil at all times, permitting expansion and contraction with temperature changes. It keeps oil in the main transformer from coming into contact with the atmospheric air.
    • The tank is connected to the transformer by a pipe, above the transformer, and has a breather to the outside.
    • It allows the transformer to have more than 100% of its oil needs available.
  21. What is the purpose of a silica gel breather?
    Air enters the breather intakes and passes through the silica gel. A silica gel breather absorbs moisture from the air and allows drier air to pass into the conservator.
  22. What is the purpose of an explosion vent and how does it function?
    An explosion vent is a large diameter pipe extending a few feet above the cover of a transformer and curved in the direction of the ground. A diaphragm fitted at the curved end will rupture at a relatively low pressure and release the pressure from the transformer tank. Very high pressures are possible when an electrical fault occurs.
  23. What is the purpose of transformer bushings? Why are the primary and secondary bushings different sizes?
    The purpose of transformer bushings is to insulate electrical power circuits where they enter the tank. They must be oil-tight and weather-tight.

    The higher voltages require addiction insulation in the form of oil and moulded paper so they are larger.
  24. How is oil circulation accomplished in oil-filled transformers?
    • In the case of oil-filled transformers, cooling is accomplished by:
    • • Passage of the warm oil over a cool surface by either natural (convection) or forced (pumps) means.
    • • Passage of the warm oil over surfaces cooled by water. These surfaces are water-cooled coils installed either in the transformer tank or external to the tank.
  25. What are the common methods used for transformer  cooling?
    • Natural cooling via “thermosyphon effect”.
    • o Heat from the windings and core is conducted to the cooler medium (oil)
    • o As the oil temperature increases, the density of the oil decreases
    • o The higher temperature oil circulates up replacing it with the heavier (cooler) oil
    • o The oil is cooled along the sides of the transformer tank or in the tubes of the pipes of the radiator, becomes dense, and flows back to the bottom of the tank
    • o The cycle repeats circulating the oil.
    • Pumps may be used to assist the thermosyphon flow. (forced circulation)

    • External metal tubes welded to the side of the tank or external radiators attached to the tank provide additional surface area for cooling (self cooled)

    • Fan Cooled
    • • Air from the fans is forced over the radiators increase the cooling rate
    • • The oil to be cooled moves in an up-and-down path through the cooler, thus there is no natural convection flow and movement of the oil is dependant on the pumps. If the fan or pumps fail, the transformer must be removed from service.

    • Water Cooling
    • • Forced circulation of cooling water through coils of copper tubing mounted inside the tank.
    • • In external type of water cooler, hot oil flows through an external separate water-cooled device (oil flow may be natural thermosyphon or oil pumps)
  26. What are tap changers used for?
    Tap changers are used to accommodate system voltage changes. Off-load tap changers are used when the voltage change is more permanent. For rapid voltage changes due to loading and unloading of circuits, a load tap changer (LTC) is used.
  27. How is an off-load tap changer adjusted?
    An off-load tap changer is adjusted in the high-voltage winding by dividing it into two halves and providing several tapping points on both sides of the divided winding. The tapping points are brought from the winding to a terminal board.

    Either the tank must be opened and the liquid lowered to expose the terminals or by using a tap changing switch. The switch is operated by rods passing from the operating wheel or handle and through a liquid-tight gland in the transformer cover or side. Various combination of the windings (usually 2.5% steps) from zero to 10% down from the full winding can be obtained.
  28. Why are Load tap changers used?
    Load tap changers are used to maintain a voltage at a desired level without removing the load from the line.
  29. What three methods can be used to operate a load tap changer?
    • • From a control switch
    • • From a voltage sensitive relay
    • • Hand crank
  30. What are instrument transformers and what are they used for?
    They are used for both metering and protection.
  31. What is the purpose of an instrument transformer’s polarity markers?
    One primary and secondary terminal of each instrument transformer is marked to indicate the relative instantaneous direction of primary and secondary current. When current flows “in” at the marked primary, it flows “out” at the marked secondary.
  32. How is the primary winding of a potential transformer connected?
    The primary winding is connected in parallel with a circuit whose voltage is to be measured or controlled for a potential transformer.
  33. Why should the secondary winding of a potential transformer never be short circuited?
    The secondary windings of a potential transformer should never be short circuited because the windings will burn out in a very short time.
  34. How is the primary winding of a current transformer connected?
    The primary winding is connected in series with the line for a current transformer
  35. What is the standard rated secondary current of
    a current transformer?
    The standard rated secondary current of a current transformer is 5A.
  36. What is the nominal current ratio of a current transformer?
    The nominal current ratio of a current transformer is the inverse of the turns ratio.
  37. Why must the secondary winding of a current transformer never be opened while the primary is energized?
    If the secondary is open-circuited while the primary current is flowing, the entire primary current acts as the exciting current, causing core saturation and inducing high voltages in the secondary winding that may reach levels dangerous to both personnel and insulation.
  38. What is the burden of an instrument transformer?
    The burden of an instrument transformer is the active and reactive power consumed by the load on the secondary winding.
  39. What is the purpose of a circuit breaker?
    A circuit breaker is a device for making and interrupting circuit between separable contacts under normal and abnormal circuit conditions.
  40. What are the four general categories of circuit breaker ratings?
    • • Current Interrupting Requirements
    • • Current Carrying Requirements
    • • Operating Voltage Requirements
    • • Operating Speed Requirements.
  41. What is an electric arc?
    An electric arc is a self-sustained discharge capable of supporting large currents with a relatively low voltage drop. An arc momentarily bridges the gap formed between the stationary and moving contacts when these contacts are separated.
  42. What are the three basic methods used to increase the impedance of the arc path to extinguish an arc?
    • There are three basic methods employed in current-interrupting devices to increase the impedance of the arc path to extinguish an arc:
    • • Increasing the length of the arc path; by increasing the distance that the arc must travel, the amount of energy required to establish and maintain the arc increases.
    • • Decreasing the cross sectional area of the arc path; Constricting the path that an arc must travel also increases the amount of energy required to establish and maintain the arc.
    • • Cooling the arc path and replacing the ionized gas with a nonconducting material; replacing the ionized gas with a non-conducting material, such as a de-ionized gas or oil, greatly increases the amount of energy required to maintain the arc.
  43. Why is DC current more difficult to interrupt
    than AC current?
    AC current and voltage has a magnitude of zero twice during each cycle or 120 times a second (when the frequency is 60Hz). All the current interrupting device needs to do is prevent current from flowing again once it has reached a natural zero in a cycle.

    DC current and voltage does not naturally come to zero so all the magnetic energy stored in the DC circuit must be dissipated as heat or transformed into electrostatic energy.

    If DC circuits are opened too quickly, the voltage across the switch may rise to dangerous levels as a result of the stored energy of the inductance. The arc dissipates the energy and controls the rate of decrease of current.
  44. Briefly describe recovery voltage and re-strike.
    Recovery voltage is the overshoot and oscillation about the steady-state voltage after a breaker opens at zero voltage after a short circuit. The short circuit causes an increase in current and a decrease in voltage. At zero current, and the breaker contacts have parted, the voltage across the breaker terminals begins to rise.

    Re-strike is when the electrical arc is re-established after a current zero because the increasing voltage was too high and there was not enough insulation between the contacts of the breaker. If a re-stike occurs in the switch or breaker
  45. Describe the two most common types of circuit breaker contact construction.
    • Bayonet type - This type of contact consists of one part (movable or fixed) which inserts into another part of the contact.

    • Butt type - This type of contact consists of movable and/or fixed components that connect face-to-face (butt) with each other.
  46. Describe the two types of oil circuit breakers.
    • Minimum oil circuit breaker:
    • • Faster operation speed
    • • Much less oil
    • • Oil is used for contact interruption/dielectric only.
    • • Oil must be low viscosity because no tank heaters are used

    • Bulk-oil Interrupter:
    • • Much more oil
    • • Slow operation speed

    • Arc-quenching chamber. An enclosed chamber that allows the pumping action to build up pressure inside the chamber. The increased pressure aids in oil circulation and the quick extinction of the arc

    • • Contacts are parted under oil. When the contacts are opened, the oil in the immediate vicinity of the arc will be decomposed, creating a spherically shaped hydrogen bubble around the arc. The path of the arc in hydrogen recovers its dielectric strength rapidly during each natural current zero.
    • • Oil functions after the arc is extinguished ,when it decomposes.
  47. What are the advantages of air-blast circuit breakers over oil circuit breakers?
    • • Lower cost
    • • Reduce fire hazards since there isn’t large quantities of oil
    • • Draining, filtering and testing oil not needed so reduces time and expense
    • • Arcing energy is reduced and the arc energy is not as small as in oil breakers so less burning of contacts
    • • Fresh air is available for every operation and the arc products are carried away by the atmosphere
  48. Describe the operation of a puffer breaker.
    • A puffer breaker mechanically generates its own blast (or puff) of SF6 whenever it is operated.
    • • Arc is formed as the contacts separate
    • • SF6 gas in the breaker head is compressed
    • • The gas is forced out of the puffer cylinder and directed at the arc by an insulating nozzle
    • • Flow of gas cools and elongates the arc
    • • Decomposition products of the SF6 gas from the arcing area are absorbed by the desiccant.
  49. What is the main advantage of vacuum circuit breakers? What is the main disadvantage?
    The main advantage of a vacuum circuit breaker is the long and relatively maintenance free life. The main disadvantage is the possibility of the loss of vacuum and the limitations of the positive monitoring of the condition of the vacuum in the interrupter.
  50. What is the purpose of a circuit breaker prime mover?
    The prime mover is that part of the operating mechanism that provides the mechanical force to open and close the circuit breaker contacts.
  51. What is the purpose of a circuit breaker control circuit?
    The function of the closing control circuit in all circuit breaker designs is to supply controlled energy to the close coil which will activate the prime mover energy to physically close the interrupters and begin power flow.

    The circuit breaker operating mechanism control circuits provide the necessary capability to operate the breaker safely. They enable the breaker to trip or close manually or automatically, as well as provide the stored energy to the prime mover for breaker operation.
  52. How can motors be used in operating mechanisms?
    Motors can directly move the interrupter contacts through a series of gears, rods, and linkages (motor would be the prime mover).

    A motor can wind or charge a spring, or compress air, or compress hydraulic fluid, which is then used to operate the interrupters through the linkages.
  53. What is a trip-free operation?
    Trip-free operation is the ability for a breaker to trip automatically upon receiving a trip signal before the closing operation is complete.
  54. What is the purpose of a circuit breaker control cabinet?
    The circuit breaker control cabinet houses the components that make up the control schematic.
  55. How does metal-clad switchgear differ from cubicle switchgear?
    Metal-enclosed switchgear is a switchgear assembly completely enclosed on all sides and top with sheet metal.

    Metal-clad switchgear has removable breakers.

    Cubicle switchgear has the main components in seperate metal housing and the circuit breakers are stationary.
  56. What is the purpose of automatic shutters?
    Automatic shutters cover primary circuit elements when the removable element is in the disconnected, test, or removed position.
  57. Why are feeder circuit breakers equipped with automatic reclosing?
    Feeder circuit breakers are equipped with automatic reclosing to prevent lengthy outages due to transient faults.
  58. What is the purpose of a substation disconnect switch?
    • Disconnect switches are used in substations primarily to isolate equipment so that field personnel can work safely.
    • Provide a visible separation.
    • Some disconnects are capable of interrupting various currents and can be used for switching purposes.
  59. What is the advantage of a motor-operated disconnect switch?
    Motor-operated disconnect switch (MODS) can be operated remotely or automatically.
  60. What is the purpose of a horn gap and how does it work?
    Horn gap (or air-break) switches are mainly used on 15kV circuits and above and are limited to breaking very light currents.

    They are the same as high-voltage disconnects with the addition of arcing horns at which the arc has formed when the main contacts have parted.

    The arc rises because of the heat generated and the movement of the horns until the resistance in the length of the arc causes its current flow to stop. The wear caused by arcing in the main contacts is reduced.
  61. What is the purpose of a quick-break attachment and how does it work?
    The switch depends on spring action to break the arc by the rapid separation of contacts. Depends on spring action.

    The only advantage of a quick-break attachment is that the critical arc length can be attained in less time.
  62. How can wind affect the arc control of an air-break switch?
    The effective contact separation may be affected since the arc roots will move from the contact tips.If the switch blade is at an angle to the wind, the arc root can travel downward along the blade. A down draft could force the arc towards grounded objects.
  63. Why is a load-interrupter device built as an auxiliary to an air-break switch?
    • The load-interrupter device is built as an auxiliary so the air-break switch then provides the isolation after the arc interruption.
    • The standard air-break switch provides an economical foundation for the device.
  64. What is the purpose of a circuit switcher?
    A circuit switcher is a device for switching electrical currents. It interrupts the flow of current or closes the circuit at high voltage. It looks like an interrupter switch but preforms like a circuit breaker.
  65. What are the components that make up a circuit switcher?
    The circuit switcher is an interrupter with a disconnect as an accessory.
  66. How does a circuit switcher differ from an interrupter switch?
    • A circuit switcher outperforms an interrupter switch.
    • • As the disconnect blade approaches the full open position, the brain recloses and re-cocks the interrupting unit contacts, unlike the interrupter switch, which must re-cock at the most adverse loading point in its closing cycle.
    • • As the brain closes the blade, inrush current (or fault current) is picked up at the jaw - not in the interrupting unit. Unlike the interrupter switch, the blade closes at high speed, unimpeded by having to reclose and re-cock an interrupting unit.
  67. How is a circuit switcher similar to an interrupter switch?
    • A circuit switcher looks like an air interrupter switch but
    • capable of interrupting higher currents.
  68. How is a circuit switcher similar to a circuit breaker?
    • A circuit switcher performs more like a circuit breaker but costs much less.
    • Capable of switching and fault current interruption.
  69. What common application of the circuit switcher is most seen by the Load Operator?
    On the high side of a power transformer.
  70. What does a circuit switcher use as its interrupting medium?
    The interrupting medium is SF6.
  71. What is the purpose of a voltage regulator and how are they applied in distribution substations?
    Voltage regulators are devices which are used to maintain system voltage levels within acceptable limits. It is essentially an autotransformer in which the output voltage is brought to a desired value
  72. What are the basic requirements of a voltage regulator?
    • • Regulation must be performed automatically
    • • Changes in voltage must be in small quantities, something less than 1% per change or stop, so that such changes will not be noticeable in the operation of electrical appliances or devices
    • • Load current must be continuous during regulation.
  73. What is the most common type of voltage regulator used in distribution substations? Briefly describe their operation.
    The most common type of voltage regulator used in distribution substations is the step voltage regulator.

    An automatic step voltage regulator is essentially a multi-tapped autotransformer fitted with an automatically controlled on-load tap-changing mechanism which raises or lowers the output voltage.
  74. What is load bonus?
    A load bonus feature restricts the tap changer from moving beyond a certain range, limiting the amount of current carrying turns, and thus reducing I^2*R losses and heating. Putting a regulator into “load bonus” will increase its continuous rating.
  75. Why are bridging wipers used?
    Bridging wipers are used to offer a continuous supply when the voltage is raised and lowered in steps and not have the load interrupted each time the contact wiper passes from one tap to another.
  76. What is NCO?
    NCO is “Neutral/Control Of” which is turning a voltage regulator’s control off and placing it in the neutral tap position.
  77. What is the primary purpose of a series reactor?
    The series reactor (or current-limiting reactor) limits the flow of current under fault conditions. This reduces stress on conductors, supports, switches, and other apparatus carrying short-circuit currents.
  78. What is the most common application of a series reactor?
    The feeder reactor (in series with the line feeding the load) is the most common application of a current-limiting reactor.
  79. Why are reactors installed in series with shunt capacitors?
    • • Lower the required circuit breaker ratings
    • • Decrease inrush currents
    • • Decrease voltage transients.
  80. What is percent reactance?
    Percent reactance is the ratio of the voltage drop across the reactor (when the rated current of the circuit at rated frequency is flowing through the reactor) to the voltage between line and neutral on three-phase circuits, or the voltage between the lines on single-phase circuits.

    For example, in a single phase circuit, a 5% reactor in a 6600 volt circuit means that the reactor will have a drop of 5%, or 330 volts, when rated current is flowing.

    In a three-phase, 25 kV (line-to-line) circuit with 5% reactors in each phase, the voltage drop across each reactor will be:25,000/√3 x 0.05 = 721 volts when normal current is flowing.
  81. What are shunt capacitors used for?
    Shunt capacitors are mainly used to supply capacitive vars to improve voltage conditions within a system. As an added benefit they are reduce losses.
  82. Where is it best to locate shunt capacitors and why?
    Locating shunt capacitors as close as possible to load maximizes the benefits from their use since they release generation, system, and transfer capacity, reduce system losses, and optimize improvements to voltage regulation.
  83. How does a capacitor store energy?
    When a voltage is applied, capacitors store energy in their electric field. Capacitors consist of two conducting plates separated by a dielectric medium. Purely capacitive circuits cause reactive power with the current waveform leading the voltage waveform by 90 degrees.

    A capacitor will store energy in the first ¼ cycle of the voltage waveform and return energy to the system during the next ¼ cycle.
  84. How does the applied voltage affect a capacitor’s output?
    Electric field storage is a function of the voltage squared of the system

    Q = E^2/X
  85. What is the relationship between line loss and power factor?
    Improved power factor, reduces the reactive power and line current, increased the voltage at the load, therefore decreasing I^2*R losses and I^2*X losses.
  86. How are substation shunt capacitor banks installed?
    Capacitors are usually mounted in three tiers, one per phase. The series-parallel connection depends on the operating voltage and capacity of the bank desired
  87. What is a surge diverter?
    A surge diverter is a piece of equipment which acts as a safety valve to discharge extra high voltage surges. It absorbs enough transient energy to prevent dangerous overvoltages and overcurrents and cuts off the flow of current at the first current zero after discharging the transient surge.
  88. How does a surge diverter operate?
    When a power surge hits an arrester or spark gap, it finds a temporary low impedance path to ground which exists until the voltage returns to normal. The voltage surges are discharged to ground so power interruptions are reduced and damage to valuable equipment is prevented or lessened.

    The device conducts energy to ground during the surge and insulates the apparatus from ground at all other times.
  89. What can cause a voltage surge?
    • • Lightning
    • • switching
  90. What is the difference between a spark gap and an arrester?
    An arrestor consists of two elements: a spark gap unit and characteristic element.When a characteristic element is not used, the device is a spark gap.

    The spark gap breaks down at a predetermined voltage but does not usually extinguish itself. The interruption of power flow current must usually depend on some other current limiting device (circuit breaker, fuse)
  91. What is an arrester counter and why are they used?
    An arrestor counter provides information on how many times the arrestor has functioned.
  92. Why are spark gaps sometimes used instead of arresters at substations?
    Spark gaps are less expensive. Fuses or station protective relaying and circuit breaker in the substation can interrupt the current.
  93. How can the shape of a spark gap help extinguish an arc?
    A spherical gap has an effect of lengthening the arc path thus making it easier to extinguish.