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What is the purpose of a distribution substation?
The purpose of a distribution substation is to transform higher transmission voltages (i.e. 60kV, 138kV, and 230 kV) down to more usable distribution voltage levels and deliver power from the substation directly to large industrial/commercial costumers or through service transformers to smaller costumers. A distribution substation also acts as a centralized location where switching can be performed for maintenance and restoration purposes. All this is accomplished while maintaining maximum reliability, flexibility, and continuity of service at the lowest investment cost possible.
What factors affect the location of a distribution substation?
Several factors come into play when deciding the location of a distribution substation. A substation should be located where is can serve the needs of the power system most economically. This means the load area a distribution substation will serve is of the utmost importance. A given load area can be supplied by fewer large substations or more small substations. Another important factor that comes into play is the availability of suitable right-of-ways and access to the site by overhead or underground. Other factors include: Possible objections due to appearance, noise, or electrical effects Atmospheric conditions Public safety Security from theft, vandalism, or damage
What is the purpose of a substation ground grid?
A station ground grid protects personnel and the public from hazardous potential differences, protect equipment from damaging potential differences, and ensure proper electrical system operation.
What is the purpose of a transfer bus?
A transfer bus can be used to carry one or several circuits (depending on its equipment rating) while their CBs are out of service. This scheme allows for continuity of service on feeders while maintenance is being performed on equipment such as CBs, reactors, regulators, etc.
What is the main purpose of substation buildings?
Substation buildings contain and shelter protective relays, control panels, auxiliaries, batteries, compressors, emergency generators, etc.
What is station service?
Station service is the power supply needed to operate the equipment within a substation. Most stations have AC and DC station service supplies working together to satisfy the power supply requirements.
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 from another station
- -A stand-by generator powered by and internal combustion engine
What are the limitations of AC station service?
AC station service is limited by the fact that AC power can not be stored.
How is DC station service supplied?
Normally DC equipment is supplied by AC through a rectifier/battery charger. The battery is connected in parallel with the battery charger so that it is kept charged. In the event of AC failure, DC equipment can be fed by a charged battery.
What is the purpose of DC station service?
- DC station service is required for DC equipment and applications such as:
- Normal operation of control, alarm, and protection circuits
- Switches and lights on control panels and some operating circuits for power circuit breakers
- Emergency lighting
- Emergency pumps and fansUPSs for vital computer aided controls and comm. equipment
What are the standard AC and DC station service voltages?
- AC – 120/208V and 347/600V
- DC – 125V mainly
What is a station service auto transfer scheme?
This is a scheme that transfers station service from one station service transformer to another in the event of failure. SS1 and SS2 are not normally operated in parallel.
Why is rigid bus usually hollow?
Direct current flows throughout the entire cross-sectional area of a conductor in a uniform manner while alternating current has a tendency to flow on the outside surface of a conductor. This action of current being forced to the surface of a conductor is know as “skin effect”. This causes the inner portion of a conductor to be wasted when carrying AC. This is one of the more important reasons why hollow buses are used throughout substations.
- Other reasons hollow buses are used include:
- -Inexpensively and easily manufactured
- -Stronger than solid bars of equal diameter
- -Lighter and use less material, making them less expensive
What type of conducting metal is used for outdoor rigid bus and why?
- Aluminum is used for outdoor rigid bus for the following reasons:
- -Its much lighter (one third the density of copper)
- -Better suited to the industry’s method of manufacturing
- -The lower conductivity of aluminum requires a larger cross-sectional area which in turn slightly reduces skin effect
What are the reasons for using SF6 bus?
SF6 is used in situations where space is insufficient for conventional bus arrangements and for coastal stations where salt contamination of the insulators can cause problems.
What SF6 gas qualities make it highly suited for use as the insulating medium in CGIS?
- SF6 in a pure state is inert and has exceptional thermal stability. It has excellent arc-quenching properties. These main qualities, along with good insulating properties, make SF6 excellent for use as an insulating medium in gas-insulated bus systems as well as an interrupting medium in CBs and switches.
- Other SF6 properties include:
- -No condensation in gas down to -34°C at 0.44 MPa
- -Density is ~ 5 times that of air
- -Extended periods of arcing do not materially affect it’s electrical properties
- -One of the most stable compounds
- -When pure, it’s inert, non-flammable, non-poisonous, odourless, and has no harmful effect on people
How is the dielectric strength of SF6 gas monitored?
Because dielectric strength is a function of density, and density is in turn a function of pressure and temperature, a temperature-compensated pressure switch is used which activates alarms when conditions reach prescribed limits.
The presence of moisture also deteriorates the dielectric strength of SF6. The moisture content is determined by random tests after the compartments have been initially filled.
What is the purpose of a power transformer in a distribution substation?
Power transformers in distribution substations are used to step down voltage from a transmission level to a more usable distribution voltage level (34 kV or less).
What are the basic components of a power transformer?
- The basic components or a power transformer include:
- The tank or enclosure and associated accessories or fittings
- The core/winding assembly
- The insulating and cooling medium
What is the purpose of a conservator tank and how does it function?
A conservator tank’s purpose is to keep the main transformer tank filled with oil at all times, allowing for the expanding and contracting of the oil with changes in temperature. The tank is connected to the transformer via a pipe and has a breather to the outside. It is usually mounted atop the transformer to provide a head of fluid level higher than the top of the main tank. Typically the conservator tank is about half full. Some conservator tanks have air bags to eliminate any oil-ail contact to limit contamination of the oil.
What is the purpose of a silica gel breather?
The silica gel breather is a device used for drying the air that enters the transformer during the breathing process.
What is the purpose of an explosion vent and how does it function?
An explosion vent is used to relieve the pressures caused by a fault in an oil-filled transformer. It consists of a large diameter pipe extending a few feet above the transformer cover that curves towards the ground at the outlet end. The end of the pipe is fitted with a diaphragm that will rupture at a relatively low pressure, relieving any extreme pressures on the transformer tank.
What is the purpose of transformer bushings? Why are the primary and secondary bushings different sizes?
Bushings are used to provide insulation and an oil/weather-tight seal where the conducting materials enter and leave the tank. Different sizes are used because there are different nominal voltages on the primary and secondary windings and higher voltages require more insulation.
How is oil circulation accomplished in oil-filled transformers?
Circulation of oil in a transformer can occur by natural circulation or forced circulation. Natural cooling depends on the thermosyphon effect. The thermosyphon effect occurs when oil heated by the windings naturally rises to the top of the tank and cools against the outside of the main tank as it circulates down the sides and back to the bottom.
Forced circulation is in reference to the pumping of oil in their circulating paths
What are the common methods used for cooling?
Common methods used for cooling include:
- Self cooling – oil cools again outside of tank or radiator
- Fan cooling – equipped with radiators and fans, increasing cooling rate
- Water cooling – forced cooling water through copper tubing mounted inside tank
What are tap changers used for?
Taps changers are used to change the turns ratio of a transformer in order to keep the voltage on the secondary side at an acceptable level.
How is an off-load tap changer adjusted?
Off-load tap changers must have the transformer taken out of service in order to be adjusted. They do not respond automatically.
Why are load tap changers used?
Constant changing of load causes a constant change in voltage. Using a load tap changer allows the transformer to maintain a desired voltage level while on load, allowing for no interruption of service to customers. Load tap changers are used where frequent ratio changes are necessary due to the character of the load connected.
What three methods can be used to operate a load tap changer?
Most LTCs can be operated from a control switch, a voltage sensitive relay, or a hand crank.
What are instrument transformers and what are they used for?
Instrument transformers are designed to represent the condition of the current or voltage and phase position of their primary circuit. They bring voltage or current down to a more usable level in order to take proportional readings for information and protection.
What is the purpose of an instrument transformer’s polarity markers?
Markers are used to indicate the relative instantaneous direction of primary and secondary currents.
How is the primary winding of a potential transformer connected?
The primary winding of a potential transformer is connected in parallel with the circuit whose voltage is being measured or controlled.
Why should the secondary winding of a potential transformer never be short circuited?
Short-circuiting PT secondary windings is the same as short-circuiting any other transformer. This creates almost zero resistance, allowing unlimited amount of current to flow. This would in turn burn out the transformer in a very short amount time.
How is the primary winding of a current transformer connected?
The primary winding of a current transformer is connected in series with the circuit whose current is being measured.
What is the standard rated secondary current of a current transformer?
What is the nominal current ratio of a current transformer?
Typical ratios are expressed as 100:5, 600:5, and 1000:5.
Why must the secondary winding of a current transformer never be opened while the primary is energized?
The 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.
What is the burden of an instrument transformer?
Burden is the active and reactive power consumed by the load on the secondary windings.
What is the purpose of a circuit breaker?
Circuit breakers are used to make and interrupt circuits between separable contacts under normal and abnormal conditions. They protect electrical equipment such as transmission lines, distribution lines, buses, transformers, and generators from electrical faults by disconnecting the affected equipment from the rest of the system. They may also be used to interrupt normal current levels when performing switching to take equipment oos for maintenance.
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?
An electric arc is a self-sustained discharge capable of supporting large currents with a relatively low voltage drop. An arc momentarily bridges a gap formed between separating contacts. As the contacts separate, resistance increases due to the increase in distance between them. While the distance grows, temperature around the contacts increases to a point where the metal will melt. Voltage, current, resistance, and temperature are therefore the main characteristics of an arc.
What are the three basic methods used to increase the impedance of the arc path to extinguish an arc?
- Increasing the length of the arc path
- Decrease the cross sectional area of the arc path
- Cooling the arc path and replacing the ionized gas with a non-conducting material
Why is DC current more difficult to interrupt than AC current?
DC current travels in one direction only while flowing normally. Adversely, AC current constantly changes direction, so it must come to a point of 0 A when it does change direction. i.e. AC current with a frequency of 60 Hz must come to 0 A flow 120 times a second.
Briefly describe recovery voltage and re-strike.
- When there is a short circuit, current increases considerably because of the dramatic drop in impedance and voltage of the breaker decreases. Immediately following the next current zero after the breaker contacts have separated, the current remains at zero while the voltage across the breaker begins to recover toward system-generated voltage. The energy stored in the circuit will then cause the voltage across the contacts to overshoot and oscillate. This is called “Recovery Voltage” and the rate at which the voltage increases is known as “Rate of Rise of Recovery Voltage”
- For a CB to interrupt properly, the insulation between the contacts must rise more rapidly than the voltage, or the arc will re-establish after current zero is passed. Under certain condition a CB may interrupt the arc as current zero is reached, but before the contacts are able to separate adequately, the arc may be re-established or “re-strike”.
Describe the two most common types of circuit breaker contact construction.
- Bayonet type – consists of one moveable or fixed contact which inserts into another
- Butt type – consists of moveable and/or fixed components that connect face-to-face (butt) with eachother
Describe the two types of oil circuit breakers.
- Bulk-oil type
- Components include:
- -Interrupter assemblies
- -Referred to as “Dead Tank” design, where the housing is at ground potential.
- -Slower operation
- -Arc-quenching chamber
- Minimum-oil type
- -Uses same Class 2 mineral oil as bulk type but quantity and viscosity is far less
- -Faster Operation
- -Oil used for contact interruption/dielectric only
- -Oil viscosity lower in colder temps as no heaters can be used
- -Arc-quenching chamber similar to bulk type
- -Referred to as “Live Tank” as all part of porcelains will be above ground potential
What are the advantages of air-blast circuit breakers over oil circuit breakers?
- Cost of insulating medium
- Reduced fire risk
- No draining, filtering or testing of oil
- Less arcing time, which equals less burning of contacts
- Fresh air for each operation with arc products carried to atmosphere
Describe the operation of a puffer breaker.
As the contacts of the breaker open, a portion of the SF6 gas is compressed and forced out of the puffer cylinder in the direction of the arc by an insulating nozzle. The flow of gas stretched and cools the arc. The byproducts of the SF6 from the arcing area are then absorbed by the desiccant.
What is the main advantage of vacuum circuit breakers? What is the main disadvantage?
The long and relatively maintenance free life of a vacuum CB is the main advantage.The main disadvantage is the possible loss of vacuum and the limitation on the positive monitoring of the condition of the vacuum in the interrupter.
What is the purpose of a circuit breaker prime mover?
The prime mover provides the mechanical force required to open and close the CB contacts.
What is the purpose of a circuit breaker control circuit?
The CB control circuit provides 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.
How can motors be used in operating mechanisms?
A motor can be used as the prime mover, where it moves the interrupter contacts through gears, rods, and linkages. It can also be used to wind or charge a spring, compress air, or compress hydraulic fluid which then used to operate the interrupters through the linkages.
What is a trip-free operation?
“Trip-free” is when a breaker has the ability to trip automatically upon receipt of a trip signal before the closing operation is complete.
What is the purpose of a circuit breaker control cabinet?
The CB control cabinet houses the components that make up the control schematic.
How does metal-clad switchgear differ from cubicle switchgear?
- Main switching and interrupting device is removable (draw-out)
- Automatic shutters which cover primary circuit elements when the removable element is in the disconnected, test, or removed position.
- Primary buses and connections are covered with insulating material throughout
What is the purpose of automatic shutters?
To prevent accidental contact with live parts of the primary circuit when the removable element is in the test position, disconnect position, or has been removed.
Why are feeder circuit breakers equipped with automatic reclosing?
To help prevent lengthy outages from transient faults.
What is the purpose of a substation disconnect switch?
- To isolate equipment so that field personnel can work safely
- Some disconnects are capable of interrupting various currents and can be used for switching
What is the advantage of a motor-operated disconnect switch?
The switch can be operated manually or automatically, and can be operated locally or remotely.
What is the purpose of a horn gap and how does it work?
- The purpose of the horn gap is to have the arc form across the arcing horns as opposed to the main contacts. This reduces the wear caused by the arcing across the main contacts.
- As the main contacts separate, the arcing horns remain in contact, carrying the current themselves. As the arcing horns separate, the arc is form across them instead of the main contacts until a sufficient gap is established to extinguish the arc.
What is the purpose of a quick-break attachment and how does it work?
The quick-break attachment speeds up the opening of the contacts via spring action. The allows the contacts to be exposed to the arcing for a shorter amount of time, reducing the burning on them.
How can wind affect the arc control of an air-break switch?
- Effective contact separation may be affected since the arc root will move from the contact tips
- Arc root can travel downward along the blade if it’s at an angle to the wind
- A down draft could force the arc toward grounded objects
Why is a load-interrupter device built as an auxiliary to an air-break switch?
After interruption, the air-break then provides the isolation. Also, the standard air-break switch provides an economical foundation for the device.
What is the purpose of a circuit switcher?
A circuit switcher is a device used to switch electrical currents. It interrupts the flow of current or closes the circuit at high voltage. It is similar to an air interrupter switch in appearance, but like a CB in performance.
What are the components that make up a circuit switcher?
A circuit switcher’s main components are a disconnect switch in series with an interrupter.
How does a circuit switcher differ from an interrupter switch?
A circuit switcher is an interrupter with a disconnect as an accessory while an interrupter switch is a disconnect with an interrupter as an accessory.
How is a circuit switcher similar to an interrupter switch?
Circuit interrupters and interrupter switches are similar in appearance.
How is a circuit switcher similar to a circuit breaker?
Circuit interrupters and circuit breakers are similar in performance.
What common application of the circuit switcher is most seen by the Load Operator?
Circuit switchers are most commonly seen on the high-side of a power transformer.
What does a circuit switcher use as its interrupting medium?
The interrupting medium is SF6.
What is the purpose of a voltage regulator and how are they applied in distribution substations?
Voltage regulators are used to maintain system voltage levels within acceptable limits. At distribution levels, regulators have been used to regulate individual feeders or to regulate substation buses.
What are the basic requirements of a voltage regulator?
- Small changes in voltage quantities that will not be noticeable in the operation of electrical appliances or devices
- Load current must be continuous during regulation
What is the most common type of voltage regulator used in distribution substations? Briefly describe their operation.
The step voltage regulator is the most common type of regulator used in distribution substations. It is essentially a multi-tapped autotransformer fitted with an automatically controlled on-load tap-changing mechanism that raises or lowers the output voltage. Most distribution type regulators give 10% boost and/or buck. By changing the tap position, the regulator can raise or lower the output voltage. You can boost or buck on regulators that have the series winding separated from the exciting winding with the addition of a reversing switch.
What is load bonus?
Some 32 step regulators are designed to carry sustained load currents above their normal maximum, but only if the range of regulation is reduced. This feature is popularly referred to as “load bonus”.
Why are bridging wipers used?
Bridging wipers are used so that there is no interruption in current flow while changing taps. The preventative autotransformer of the wiper prevents circulating currents when the wiper bridges two contacts. It also divides voltage between two bridged contacts, thereby providing twice as many steps of regulation as there are taps.
What is NCO?
Voltage regulators can be taken out of Auto to allow for manual control when required. Turning the control off and placing it in neutral tap position is know as “Neutral/Control Off” or “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.
What is the most common application of a series reactor?
The feeder reactor is the most common application of a series reactor.
Why are reactors installed in series with shunt capacitors?
- To lower the required circuit breaker ratings
- Decrease inrush currents
- Decrease voltage transients
What is percent reactance?
The ratio of the voltage drop across a reactor, at rated current and frequency, to the voltage between line and neutral (3 phase circuits) or between lines (Single-phase circuits).
What are shunt capacitors used for?
- To supply capacitive vars to improve voltage conditions within a system
- Reduce losses
- To a lesser extent they are also used in HVDC filtering, static compensators, wave shaping and transient recovery voltage suppression
Where is it best to locate shunt capacitors and why?
- Locating shunt capacitors as close to load as possible maximizes the benefits from their use, releasing generation, system, and transformer capacity; reducing system losses; and optimizing improvements to voltage regulation.
- Use of shunt capacitors in a substation or in the distribution system can also reduce capital cost by permitting new loads to be served with a smaller system investment and allowing additional transmission to be deferred.
How does a capacitor store energy?
- Capacitors consist of two conducting plates separated by a dielectric medium. When voltage is applied, energy is stored in the electric field between these plates.
- A capacitor will store energy during the first quarter of a cycle, discharge in the second, charge again in the third quarter, and discharge once more in the fourth quarter. It will continually repeat this process from cycle to cycle, charging and discharging 120 times a second in a 60 Hz system.
How does the applied voltage affect a capacitor’s output?
- Electric field storage is a function of the voltage squared of the system:
- A 100 MVAr capacitor will deliver 100 MVAr at its rated voltage.
- Reduce the voltage to 90% of rated and the output is 81% (.9 x .9) of normal or 81 MVAr.
What is the relationship between line loss and power factor?
A decrease in line loss will result in an improved power factor and vice versa.
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.
These banks are generally assembled using capacitor cans connected in a Y configuration. For banks rated 5-10 MVAR, a so called 1-Y configuration is used, while a 2-Y scheme is usually used for banks rated higher than 10 MVAR.
What is a surge diverter?
A surge diverter is a piece of equipment designed to absorb enough transient energy to prevent dangerous overvoltages and overcurrents, and to cut off the flow of current at the first current zero after discharging the transient surge.
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. By discharging any voltage surges to ground, power interruptions are reduced and damage to valuable equipment is prevented or lessened.
What can cause a voltage surge?
Surges result from different functions, such as lightning, switching, etc.
What is the difference between a spark gap and an arrester?
An arrester consists of a spark gap unit a characteristic element. When a characteristic element isn’t used, the device is simply called a spark gap.
What is an arrester counter and why are they used?
An arrester counter keeps track of the number of discharges that occur using a cyclometer counter. It may be desirable to know how many times these arresters have functioned.
Why are spark gaps sometimes used instead of arresters at substations?
Because they are more economical than expensive arresters, especially in locations where lighting and other surges occur less frequently, or where protection is given by shielding and sky wires.
How can the shape of a spark gap help extinguish an arc?
On Horn Gaps for example, as the horns rise, the separation between them becomes greater. Since the arc usually rises along these horns, there is less burning of gap material, and since the arc length is increased, it is more easily extinguished. Spherical Gaps produce an effect very similar to that of Horn Gaps.
What is the purpose of overhead distribution lines?
To deliver electricity on the distribution system from the substation to the many industrial, commercial, and residential customers of BC Hydro.
What is the difference between primary and secondary conductors?
Primary conductors carry primary voltage from the distribution substation to the distribution service transformers.
Secondary conductors are those which carry secondary voltages from the distribution transformer to the customer.
What is conductor sag and what are some of the factors affecting conductor sag?
Sag refers to the downward curve of a conductor between poles, which results from a combination of the weight of the conductor and any pressure on the conductor. This can include conditions such as icing, wind, etc.
What vertical and horizontal forces are distribution poles subject to?
- Normal vertical forces include the weight of conductors, insulators, transformers, and other equipment mounted on the pole. An abnormal force would be when the conductors become loaded with ice.
- Normal horizontal forces include:
- -Unbalanced conductor tension at corners
- -Side pull of service drops
- -Horizontal weight of components when the pole isn’t completely vertical
- Abnormal horizontal stresses include:
- -Wind pressure
- -Conductor breakageFailure of supporting guys
- -Failure of supporting guys
What is a dead end?
The end of a primary line
What is a double dead end used for?
- Facilitate the extension of an existing line
- Provide a primary cut, or two different circuits back to back
- Reduce strain on conductors, as in a conductor-lifting or “uplift” situation
What is a dip?
The transition of overhead primary to underground.
What is a joint-use pole?
A pole carrying telephone cable and access boxes as well as Hydro distribution lines. Typically the pole will carry Cable TV wire, amplifiers, and splitter boxes.
What is guy wire used for?
Guys and anchors are a means of opposing destructive forces acting on a pole, which could otherwise pull the pole over, or even break it.
What is the purpose of an overhead disconnect switch?
Overhead disconnect switches are used for the switching and isolating of feeder circuits on the distribution system.
What System Operating Order describes the capabilities and limitations of the various overhead switch types?
What is a pothead disconnect switch?
A single-pole, non-load break disconnect switch located on poles where the primary underground feeder cables terminate and make the transition to the overhead feeder section, or on a dip/reverse dip.
What is required to operate an in-line disconnect switch?
A 3-person crew
What is a SCADA switch?
Supervisory Control and Data Acquisition switches are those that can be controlled remotely from the control center. They are also equipped with sensors that provide information to the control center and engineering department, such as device status.
What is a load-buster tool?
A load-buster tool is an attachment used to open non-load break switches, allowing them to be opened at a higher current rating. They are rated at 600 amps up to 34.5 kV. This rating is applicable to transformer demagnetizing, parallel and loop switching, and load dropping.
What is a recloser and what is its purpose?
A recloser is a self-contained device for automatically interrupting and reclosing an AC circuit with a predetermined sequence of opening and reclosing operations followed by resetting, hold closed or lockout.
The main purpose of reclosers is to prevent lengthy outages from transient faults. They also permit the isolation of permanent faults.
What Distribution Operating Order describes the various types of reclosers as well as the criteria for blocking automatic reclosing to facilitate Live Line Permits and Assurance of No Reclose Permits?
How does the sensing and operation of a hydraulic recloser differ from that of an electronic recloser?
- For hydraulic reclosers, over-current is detected by a trip coil that is connected in series with the line. They do not have a control box and operate on hydraulic principles. When the minimum trip current flows through the trip coil, it moves a plunger downward into the coil. This plunger movement forces powerful operating springs to separate the contacts of the recloser and open the circuit momentarily. This subsequently stops current flow through the trip coil, removing its magnetizing force. The plunger then moves back up and the operating springs then reclose the contacts automatically.
- Electronic reclosers have a control box which controls the opening and closing of the device. The electronic recloser control provides both phase and ground sensing of over-current faults.
Minimum phase and ground-trip values, timing of tripping and reclosing, number of trips to lockout, and reset timing are adjustable at the control box, without de-energizing the recloser.
What affect does enabling the HLT feature on an HLT recloser have?
HLT is programmed to a very fast time current curve, and it disables the closing function of the control, When HLT is enabled, the recloser may trip faster than a downstream fuse if a fault occurs (aka no coordination).
What is a sectionalizer and what is its purpose?
Sectionalizers are automatic switches that are controlled by built-in logic systems. They work in conjunction with reclosers to help minimize the area affected by a permanent fault.
Describe how a sectionalizer works in conjunction with a recloser.
When a fault occurs downstream of a sectionalizer, it will begin to count the number of operations of the recloser device ahead of it on the circuit. If the fault persists to the point where the recloser is set to go to lock-out on the fourth trip operation, the sectionalizer will be set to trip during the open-circuit time following the third tripping operation of the recloser. If the fault is removed by opening the sectionalizer, the recloser will then be able to hold closed on the fourth reclose operation, allowing continuity of service for customers upstream of the sectionalizer.
What is the purpose of an overhead distribution transformer?
Overhead transformers are used to transform primary distribution voltages to a more useable level required for certain customers (i.e. residential).
What are the most common single-phase and three-phase secondary voltages?
- Single-phase: 120/240V
- 3-phase: 240V delta, 120/208V wye, and 347/600V wye
How does BC Hydro protect its overhead transformers and what advantage does this method provide?
BC Hydro uses separate primary fusing to isolate and protect distribution transformers. This allows faulted transformers to be located far more quickly, resulting in reduced outage times.
How can a three-phase transformer bank be modified to operate with one transformer out of service?
Three-phase transformer banks can be suited with an emergency connection which allows service to be maintained while repairs are being carried out on a faulty transformer. This uses two of the three transformers to form an open wye-delta, open delta-delta, open wye-wye, or open delta-wye bank. Capacity of the bank is reduced to 57% for open delta and 50% for open wye.
Where are field voltage regulators installed?
They are located at points where feeder voltage falls below acceptable levels during heavy load periods.
What must be done prior to bypassing a voltage regulator?
The Voltage regulator must be placed in “NCO”.
What is a bidirectional voltage regulator?
A bidirectional voltage regulator is one that is able to sense the directional of power flow and regulate the voltage either way.
Why are shunt capacitors installed along feeders?
Shunt capacitors are installed along feeders to correct poor power factors and to help raise the circuit voltage when needed.
How do field shunt capacitors help voltage?
Circuit conductors carry both resistive current and inductive current. When a shunt capacitor is switched in, its capacitive reactance opposes any inductive reactance on the circuit. This reduces the inductive current thus reducing total line current. A drop in total line current will then reduce the voltage drop on the line itself.
What are oil switches?
They are single-phase devices used on distribution pole-mounted capacitor installations. They consist of a set of electrically operated main contacts, plus auxiliary equipment, immersed in a tank of oil.
What are the methods used to control field shunt capacitors?
Three methods are Voltage Controlled Type, Current Controlled Type, and VAR Controlled Type.
What is an Independent Power Producer?
An IPP is a wholesale electricity producer that is unaffiliated with franchised utilities in the area in which it is selling power.
How are Distribution IPPs connected to the power system?
Distribution IPPs connected to the system through distribution feeders. Usually protection considerations will require that an IPP only be connected to the distribution system through one specific feeder position in the substation.
Why would a transmission line’s protection trip the feeder circuit breaker associated with an IPP?
To insure the IPP does not back feed the faulted transmission line and to prevent certain IPPs from operating in an islanded situation where they are incapable of carrying the associated feeder load.
What provision is in place to prevent an out-of-sync close on a Distribution IPP?
Voltage transformers are installed on the load side of the feeder CB to detect voltage on the feeder. This will provide voltage supervision for the supervisory closing control of the feeder CB to prevent an out-of-synch close onto the IPP.
What is the purpose of underground distribution lines?
Underground distribution cables deliver electricity on the distribution system from the substation or overhead to the many industrial, commercial, and residential customers of BC Hydro.
What is feeder cable?
Feeder cable is the underground portion of a feeder that goes from the substation to the first switchable field device.
What are the basic classes of common cable construction?
The two basic classes are single-conductor and three-conductor. They can be further divided into unshielded, shielded, and concentric neutral cables.
What is cable splicing and what are the general steps involved?
Cable splicing is the act of joining the ends of cables to the URD apparatus or other equipment. The steps involved include cable preparation, connection, and re-insulation.
What is cable terminating and what functions must a termination perform?
Cable terminating is the installing of a device on the end of a cable that allows the cable to be connected to equipment in the system.
- Terminations must:
- -Provide stress relief at the end of the terminated shield
- -Protect against tracking or burning on the exposed surface of the termination
- -Keep moisture out of the cable
- -Protect against flashover
Mark II switchgear is outfitted with what two devices? What is the purpose of each device?
Mark II switchgear is outfitted with switches and fuses. The switches provide the incoming and the capability to feed through, continuing the circuit. The fuses allow power to be tapped from the switchgear to feed transformers and customer switchgear, as well as providing their usual current interrupting capabilities.
How many compartments is the cabinet of a Mark II switching kiosk divided into to?
What Distribution Operating Order describes the operation of Vista switchgear?
1D-08 “Vista Underground Distribution Switchgear Operation”
Briefly describe some of the enhancements Vista switchgear provides over traditional Mark II switchgear.
- Enhancements that Vista switchgear provides include:
- Both underground and pad-mounted style
- SF6 as an insulating medium
- Vacuum fault interrupters
- Visible indication of switch positions
- Voltage indication and low-voltage phasing
- SF6 gas pressure gauge
- SCADA capability
- Local/Remote switch
- Automatic controllers that transfer on loss of source, etc.Over-current lockout to prevent closing onto a fault
Describe a fault interrupter way.
It is a way with a load-interrupter switch in series with a vacuum fault interrupter.
What is the purpose of a junction box?
They are used to tap off primary distribution and feed underground distribution transformers. They’re also used as switching points to allow the isolation of URD sections or the re-routing of power.
Describe load-break elbows.
Load-break elbows are used to terminate primary cables, providing a connection to junction bars which can be used to break current, typically up to 200A. They can be identified by two white bands on the body of the elbow.
Describe a doghouse transformer.
The term “doghouse” references the metal enclosure to house what is essentially an overhead transformer set at ground level on a pad.
What is a dead-front transformer?
The primary connections in the operating compartment of a dead-front transformer are insulated and the outside of the connections are maintained at ground potential. There is also a sealed tank inside the transformer housing which holds the transformer, primary bus, fuses, and switch. Primary cables connect to dead-front transformers through load-break elbows. These elbows could originally be used for switching and isolation of the primary circuit. They are now no longer used for switching or isolating the transformer while it is live due to safety issues.
How do primary cables connect to dead-front transformers?
Primary cables connect to dead-front transformers through load-break elbows.
What type of service is provided by a Low Profile Transformer?
Single-phase service is provided by LPTs.
What type of service is provided by a Pad Mount Transformer?
Three-phase service is provided by PMTs.
Where in the province is the Dual Radial System unique to?
BC Hydro’s Dual Radial System is unique to Vancouver.
Briefly describe the Dual Radial System.
It consists of a normal source of supply, known as the running circuit, and an alternate source, known as the stand-by circuit. A Dual Radial System actually much resembles a completely underground multi-branch normally-open loop, although it is not usually thought of in this manner.
What is a Double Dual Radial vault?
Double Dual Radial vaults are provided with not one but two running circuits.
What operating drawings are unique to the Dual Radial System?
Dual Radial Standby Drawings are unique to the Dual Radial System.
What Distribution Operating Order describes the Dual Radial switching and isolation procedures?
What Distribution Operating Order describes the issues associated with feeder paralleling between substations through the Dual Radial System?
How is customer vault isolation accomplished on the Dual Radial System?
Customers will isolate under WorkSafe BC OHS Regulations. Costumers performing live-end disconnect switch maintenance must acquire a GOI from the BC Hydro PIC prior to isolating their equipment as per WorkSafe BC OHS Regs.
How are BC Hydro owned street vaults typically isolated on the Dual Radial System?
The circuit isolation will typically include points outside the vault, but will be completed by opening and tagging, usually test and work, the load-side elbows of the disconnect switch.
How are switching instructions on the Dual Radial System issued to Power Line Technicians?
The PIC uses Dual Radial Switching Cards to issue switching instructions on the Dual Radial Systems outside of the substation to PLTs.
What BC Hydro action is required following customer vault maintenance?
Vault inspection is required to verify switches have been restored to proper operating status.
What is a Cable Transition Module?
It is a point on the Dual Radial System designed to provide a means of tying standby cables from different substations together in order to facilitate emergency load transfer. They are normally considered permanent line cuts and therefore are not required as isolation points.
What two network system configurations are found in Victoria and what voltage levels do they operate at?
- Integrated Network – 120/208 V
- Spot Network – 347/600 V
What Distribution Operating Order describes the Victoria Underground Network?
What substation feeds the Victoria Underground Network and how many feeders does the network consist of?
Horsey substation has 7 Network circuits.
What is the main advantage of a network system?
In the event of a transformer failure, service to customers isn’t interrupted because the grid shares the total loading of the network service area.
Outside of Victoria, where in the province is the only other spot network located?
The West End of Vancouver.
What are the required isolation points for a network feeder cable?
The substation feeder disconnect and all the associated network transformer high-voltage disconnect switches on the given circuit are required for isolation.
T or F – Network transformer high-voltage disconnect switches are capable of de-energizing network transformers.
This is false. The switch is not rated to interrupt the magnetizing current of the transformer.
What are the required isolation points for a network transformer?
To perform any work involving the main tank components or affecting the main tank fluid level, isolation is required. This is achieved by de-energizing the feeder cable, and then opening the associated transformer high-voltage disconnect switch and removing the network protector breaker fuses.
Describe the function of a network protector.
The transformer network protector is an automatic device designed to protect and isolate the secondary network and prevent backfeed into the transformer and primary cable. It consists of an automatic electrically-operated air circuit breaker which includes a tripping mechanism, control equipment, and network relays.