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What is the purpose of a distribution substation?
To receive power from one or more high voltage transmission lines, and step down the voltage to a more usable distribution voltage level.
What factors affect the location of a distribution substation?
- -The load area it will serve
- - Locate substations as close to load centres as possible
- -The most cost-efficient method
- -Availability of suitable right-of-ways
- -Appearance, noise, electrical effects
- -Public Safety
What is the purpose of a substation ground grid?
- -To protect personnel and the public from Step and Touch Potential Differences
- -To protect equipment from damaging potential differences
What is the purpose of a transfer bus?
- A Transfer Bus is a ‘Spare’ or second Bus in addition to the Main Bus.
- It allows any circuit to be transferred to the Transfer Bus to remain in service while its normal circuit breaker/ reactor/ regulator is out of service for maintenance
What is the main purpose of substation buildings?
Contain and give shelter to protection relays, control panels, auxiliaries, batteries, compressors, emergency generators
What is station service?
The power supply that is required by every substation to operate the equipment within it.
How is AC station service supplied?
- -Dedicated Station Service transformers connected to local Station buses
- -Tertiary windings of main transformers
- -A line feeding in from another station
- -A standby generator (usually diesel)
What are the limitations of AC station service?
- AC station service is limited in that AC energy cannot be stored.
- DC station service is available for back-up as DC energy can be stored in batteries
How is DC station service supplied?
- DC SS is supplied by the DC Battery bank.
- The DC Battery Bank is connected in parallel with the Battery Charger, which is supplied by normal AC SS.
- If the AC fails, the DC Battery Bank is fully charged
What is the purpose of DC station service?
- In the event that the normal AC source for SS is lost, the DC back-up is capable of supplying SS for at least 8 hours
- (sufficient time to restore the normal AC source)
What are the standard AC and DC station service voltages?
- AC: 120/208V & 347/600V
- DC: 125V typical, 24V & 48V as well depending on equipment requirements
What is a station service auto transfer scheme?
- Most stations have more than one source of Station Service (SS1, SS2, etc)
- In the event of a SS1 failure, the ‘Auto Transfer Scheme’ will automatically switch to the alternate source of SS.
- SS is not normally operated in parallel to prevent a low-side tie on the secondaries (looping currents)
Why is rigid bus usually hollow?
- -The ‘skin effect’ of alternating current promotes a larger diameter hollow tube, taking advantage of the most useful conducting area (A/C tends to flow along the surface of a conductor)
- -less expensive and easily manufactured
- -stronger than solid bars of the same diameter
- -lighter and use less material
What type of conducting metal is used for outdoor rigid bus and why?
- Although only 60% conductivity compared to copper, aluminum is:
- -More suited to the industry manufacturing process
- -larger dimensions required (compared to copper)-->increases the advantage of a lower skin effect
What are the reasons for using SF6 bus?
- -In areas with difficult applications where space requirements are a problem
- -In coastal areas where salt contamination occurs on the insulators
What SF6 gas qualities make it highly suited for use as the insulating medium in CGIS?
- Compressed Gas Insulated System
- In its pure state, SF6 is inert (stable and will not chemically react with other materials)
- Exhibits exceptional thermal stability
- Excellent arc-distinguishing properties
- Excellent insulating properties
How is the dielectric strength of SF6 gas monitored?
- Gas pressure is monitored to ensure there has been no gas leakage
- Ambient temperature is monitored (poor performance below -40 C)
- Presence of moisture is monitored (arcing byproducts can be toxic and corrosive)
What is the purpose of a power transformer in a distribution substation?
Used to step down the voltage delivered to the substation from the transmission system to a lower, more usable distribution voltage
What are the basic components of a power transformer?
- -The tank or enclosure and associated accessories or fittings
- -The core and winding assembly
- -The insulating and cooling medium
What is the purpose of a conservator tank and how does it function?
- -A tank mounted externally above the transformer.
- -It contains a reservoir of oil that keeps the main tank completely filled during oil expansion and contraction with temperature change
- -The main advantage is that the oil in the main transformer is not in contact with the atmospheric air
What is the purpose of a silica gel breather?
When air enters the breather during oil contraction, the silica gel compound absorbs moisture from the air and allows only dry air to pass into the space above the oil.
What is the purpose of an explosion vent and how does it function?
- In an electrical fault occurance, high internal pressures are possible.
- The Explosion Vent provides for a relatively low pressure relief point to prevent a total tank rupture.
What is the purpose of transformer bushings? Why are the primary and secondary bushings different sizes?
- The bushings provide insulation at the point where the electrical conductors enter/leave the transformer.
- The primary bushings are larger, which reflects the higher voltage requiring a higher level of insulation.
How is oil circulation accomplished in oil-filled transformers?
- Oil circulation occurs naturally by convection from the thermosyphon effect
- (Temperature difference between the heated core and the cooler sides)
- Oil pumps may be used to aid in the thermosyphon effect
What are the common methods used for cooling?
- Normal = Natural Convection
- Forced = Oil Pumps or Cooling Fans blowing onto the radiators
- ONAN: Oil Normal, Air Normal
- ONAF: Oil Normal, Air Forced
- OFAN: Oil Forced, Air Normal
- OFAF: Oil Forced, Air Forced
- A transformer could be equipped with multiple combinations and stages of cooling systems, with different ratings for each
What are tap changers used for?
To accommodate system voltage changes while maintaining a consistent voltage to the customer
How is an off-load tap changer adjusted?
- The transformer must be taken out of service before making a manual adjustment
- The ratio change is usually made on the high-voltage winding
- This off load tap changer is for more of a permanent nature
Why are load tap changers used?
- Voltage can be maintained with no impact to customers (no outage required)
- Changes can be made automatically and frequently to compensate for loading changes and interconnection variances
What are instrument transformers and what are they used for?
Metering and Protection
What three methods can be used to operate a load tap changer?
- Control Switch: Remote Supervisory Operation
- Voltage Sensitive Relay: Automatic Operation
- Hand Crank: Local Operation
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 currents
Why should the secondary winding of a potential transformer never be short circuited?
A secondary short circuit will cause the transformer to overheat and fail very quickly
What is the nominal current ratio of a current transformer?
- It is the inverse of the turns ratio
- Typical turns ratios are 100:5, 600:5, 1000:5
What is the purpose of a circuit breaker?
- To interrupt both load and fault current
- Protects equipment under fault conditions and for normal operating purposes for equipment coming OOS
What are the four general categories of circuit breaker ratings?
- Current Interrupting Requirements
- Current Carrying Requirements
- Operating Voltage Requirements
- Operating Speed Requirements
What is an electric arc?
- A self-sustained discharge capable of supporting large currents.
- An arc momentarily bridges the gap formed between the stationary and moving contacts when they are separated
- The arc ionizes the contacts and the medium(air/oil/SF6), which helps break it down
What are the three basic methods used to increase the impedance of the arc path to extinguish an arc?
- Increase the length of the arc path
- Decrease the cross-sectional area of the arc path
- Cooling the arc path with nonconductive materials and de-ionized gas/oil
Why is DC current more difficult to interrupt than AC current?
- In an AC circuit, current alternates and has a magnitude of 0 twice per cycle(120 times per second)
- Since it is already naturally zero, the CB need only prevent current from flowing again in a cycle
In a DC circuit, all of the energy must be dissipated as heat or transferred to electrostatic energy.
Briefly describe recovery voltage and re-strike.
- Recovery Voltage: Immediately after a short circuit, on the very next cycle, current will stay at zero, and voltage will overshoot and oscillate about the steady-state voltage (predisturbance voltage)(due to energy stored in the circuit)
- Re-Strike: Immediately after a short circuit, when the CB operates, the arc that forms between the contacts must be interrupted by the insulating dielectric, or the arc may re-establish (Restrike)
Describe the two most common types of circuit breaker contact construction.
- Bayonet Type: 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 with each other
Describe the two types of oil circuit breakers.
- Bulk-Oil Type: ‘Dead Tank’ design, the housing is at ground potential, thousands of gallons of oil
- Minimum-Oil Type: ‘Live Tank’ design, bushings above ground potential, maybe only 50 gallons of oil
What are the advantages of air-blast circuit breakers over oil circuit breakers?
- Every breaker operation uses ‘new’ air eliminating contamination from the byproducts of previous arc extinguishing
- Fire hazards are reduced( no flammable oil)
- Maintenance issues regarding oil are eliminated(save $$$)
- Arcing time reduced, less burning of the contacts
Describe the operation of a puffer breaker.
- The puffer breaker is an integral part of the moving contact.
- When the breaker opens, gas is compressed in the puffer assembly until the moving contact exposes the blast nozzle.
- The gas is released into the arc and extinguishes it.
What is the main advantage of vacuum circuit breakers? What is the main disadvantage?
- In a near perfect vacuum, there is very little medium to ionize, so only a small arc should form
- Long lasting and relatively maintenance free
- Possibility of the loss of vacuum and limits on monitoring the condition of the vacuum
- Limits on visually inspecting the condition of the contacts
What is the purpose of a circuit breaker prime mover?
Provides the mechanical force to open and close the circuit breaker contacts
What is the purpose of a circuit breaker control circuit?
- -Control the Closing
- -Control the Tripping
How can motors be used in operating mechanisms?
Motors may be used instead of solenoids to control the closing contacts
What is a trip-free operation?
If a breaker can trip automatically upon receiving a trip signal before the closing operation is complete, it is said to be ‘trip free’
How does metal-clad switchgear differ from cubicle switchgear?
- Metal Clad:
- Voltages from 4 to 34 kV
- CB’s can be physically moved (racked out)
- Voltages from 14 to 69 kV and up to 5000 A
- CB’s are stationary
- Separate compartment provided for disconnect switches( with open window for visual inspection)
What is the purpose of automatic shutters?
Prevents accidental contact with the live parts of the primary circuit when the removable element is in the test position, disconnect position, or removed
Why are feeder circuit breakers equipped with automatic reclosing?
Helps prevent lengthy outages from transient faults
Automatic Reclosers can be programmed for tripping characteristics and reclosing times, and can be coordinated with other protective devices on the system.
What is the purpose of a substation disconnect switch?
To isolate equipment so that field personnel can work safely
What is the advantage of a motor-operated disconnect switch?
The proper mechanical force is applied to operate the switch blade reliably and from a safe distance
What is the purpose of a horn gap and how does it work?
- The arc horns are designed to draw the arc away from main contacts.
- This reduces wear in the contacts caused from arcing
What is the purpose of a quick-break attachment and how does itwork?
- The quick break attachment breaks the arc by the rapid separation of contacts, using spring action.
- Arcing contacts are part of the design
How can wind affect the arc control of an air-break switch?
- The wind could blow the arc roots down the contact from the arc tips.
- This could result in unwanted flashover towards grounded objects
Why is a load-interrupter device built as an auxiliary to an air-break switch?
- The air-break switch provides the isolation after the arc interruption
- The standard air-break switch provides an economical foundation for the device
What is the purpose of a circuit switcher?
A device for switching electrical currents
How does a circuit switcher differ from an interrupter switch?
Interrupter switch is a disconnect with an interrupter accessory
- Circuit switcher is an interrupter with a disconnect as an accessory
- -the disconnect is in series with the interrupter
How is a circuit switcher similar to an interrupter switch?
Can be used as an isolation point when locked open
How is a circuit switcher similar to a circuit breaker?
- Capable of breaking load currents
- -but not really fault currents
What common application of the circuit switcher is most seen by the Load Operator?
On the high side of a power transformer
What does a circuit switcher use as its interrupting medium?
What is the purpose of a voltage regulator and how are they applied in distribution substations?
- Maintains system voltages within acceptable limits
- Acts like an autotransformer where output voltage is adjusted by boosting or bucking the input voltage
- Used to regulate individual feeders
- Used to regulate substation buses
What are the basic requirements of a voltage
- -regulation must be performed automatically
- -changes in voltage must be in small amounts (<1%) to not be noticeable
- -load current must remain continuous during regulation
What is the most common type of voltage regulator used in distribution substations? Briefly describe their operation.
- Step Voltage Regulators
- -Essentially a multi-tapped autotransformer fitted with an automatically controlled on-load tap-changing mechanism
- -Typically designed to give a 10% boost or buck
- - On-load design characteristic necessitates placement of the tap contacts in a circle to allow continuous load current
What is load bonus?
- Some voltage step regulators(32 steps for example) are designed to carry sustained load currents above their normal maximum, but only if the range of regulation is reduced.
- This is ‘load bonus’
Why are bridging wipers used?
- Bridging wipers permit continuous current supply during a tap change.
- Load is never interrupted as the contacts move from tap to tap.
- Also provides for twice as many voltage steps as there are actual taps
What is NCO?
- Automatic voltage regulators can be turned off
- Turning an AVR to off and placing it in the neutral tap(neither boost nor buck) is referred to as
- NEUTRAL/ CONTROL OFF: NCO
What is the primary purpose of a series reactor?
- To reduce the current that flows in various parts of a system during a short circuit
- -reduces stress on conductors, supports, switches, and other equipment carrying short-circuit currents
What is the most common application of a series reactor?
The feeder reactor – in series with a feeder line supplying load
Why are reactors installed in series with shunt capacitors?
- -Lower the required circuit breaker ratings
- -decrease inrush currents
- -decrease voltage transients
- In rare cases:
- -To balance load through paralleled transformers when impedances are widely varied
- -To force parallel transmission lines to share current
What is percent reactance?
- The ‘Percent’ reactance is the ratio of the voltage drop across the reactor* to the voltage between the line to ground(3Ø) OR voltage between the line(1Ø)
- -*(when the rated current of the circuit at rated frequency is flowing through the reactor)
What are shunt capacitors used for?
- To supply capacitive VARs to improve voltage conditions within a system
- -Opposes the effects of Inductive loads
Where is it best to locate shunt capacitors and why?
- -As close as possible to the load centres
- -This ‘releases’ generation, system, and transformer capacity, reduces system losses, and optimizes improvements to voltage regulation
- -Improves power factor
- -Permits new loads on a distribution system without the economic burden of significant capital investment
How does a capacitor store energy?
- -Capacitors store energy in their electric field (when a voltage is applied)
- -The amount of MVARs a shunt capacitor can inject into the system is directly related to the amount of energy stored in its electric field
How does the applied voltage affect a capacitor’s output?
- Electric field storage is equal to the voltage of the system squared
- Ex. : Voltage reduced to 90%: .9 x .9 = .81 = 81%
What is the relationship between line loss and power factor?
A reduction in I²R Losses equals an increase (improvement) in power factor
How are substation shunt capacitor banks installed?
- -Large freestanding three-phase banks made up of series/ parallel combinations of capacitor cans
- -Usually mounted in three tiers, one per phase
- -The series/ parallel connection depends on the operating voltage and capacity of the bank desired
What is a surge diverter?
- A piece of equipment which acts as a safety valve to discharge electrical surges
- It protects equipment from damage when extra-high voltage surges occur
How does a surge diverter operate?
- Similar to a pressure relief valve, which opens to release extra-high pressure and closes when normal.
- A surge arrester will find a temporary low impedance path to ground which exists until the voltage returns to normal
What can cause a voltage surge?
- Transient overvoltage conditions caused by switching and induction
What is the difference between a spark gap and an arrester?
- An Arrester is comprised of two elements (A spark gap unit and a characteristic element)
- The characteristic element has a resistance approximately inversely proportional to the voltage across it
- A Spark gap refers to a device where a characteristic element is not used
- The spark gap breaks down at a predetermined voltage, but does not extinguish itself
What is an arrester counter and why are they used?
- Usually incorporated with lightning arresters, it may be desirable to know how many times these have functioned
- The number of discharges is recorded by a cyclometer counter, visible through an inspection window
Why are spark gaps sometimes used instead of arresters at substations?
- Spark gaps are far more economical than surge arresters
- Spark gaps are desirable to use where lightning or other surges occur infrequently
How can the shape of a spark gap help extinguish an arc?
The shape of the spark gap can help increase the length of the arc or direct the arc away from grounded equipment, which makes the arc easier to extinguish
Horn Gaps, Spherical Gaps, Ring Gaps