What is the purpose of overhead distribution lines?
The most common means of delivering electricity from the substation to the industrial/ commercial & residential customers
What is the difference between primary and secondary conductors?
- -Primary conductors carry the primary voltage from substation to service transformers
- -Secondary conductors carry secondary voltages from the transformer to the customer
What is conductor sag and what are some of the factors affecting conductor sag?
- Sag is the downward curve of the conductor between poles
- Factors affecting sag:
- -The weight of the conductor itself
- -Tension on the conductor
- -Adverse conditions such as heavy icing
What vertical and horizontal forces are distribution poles subject to?
- Vertical forces:
- -Weight of the conductors, insulators, transformers, other equipment mounted to pole
- Horizontal forces: (transverse loads)
- -Unbalanced conductor tension at corners
- -Side pull of service drops
- -Horizontal weight of components when the pole is not perfectly vertical
What is a dead end?
The end of a primary line
What is a double dead end used for?
- -Facilitates the extension of an existing dead end primary line
- -Can provide for a line cut, or two different circuits back-to-back
- -Reduce strain on conductors, as in an ‘uplifting situation’
What is a dip?
The transition of overhead conductors to underground cables (primary or secondary)
What is a joint-use pole?
Under a joint use agreement, hydro agrees to share space on a pole to accommodate telephone, cable TV, other equipment
What is guy wire used for?
Guy wires oppose destructive forces applied to the pole from conductor tension, angles and corners
What is the purpose of an overhead disconnect switch?
Used on the distribution system for switching and isolating of feeder circuits
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
What is required to operate an in-line disconnect switch?
A load-buster tool
What is a SCADA switch?
- -A switch that can be remotely operated from the Control Centre
- -Provides the Control Centre with info on fault location and which phase faulted
What is a load-buster tool?
- -A tool line crews use to interrupt current on single or three phase overhead circuits up to 34.5 kV
- -The tool is rated at 600 amps(nominal) and 900 amps(max)
What is a recloser and what is its purpose?
- -A self-contained device for automatically interrupting and reclosing a distribution circuit with predetermined settings for opening and closing
- -The purpose is to prevent lengthy outages from transient faults, and provide an isolation point for 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?
- Hydraulic : An over-current(fault) is detected by a trip coil that is connected in series with the line.
- The magnetic force in the trip coil creates the force which operates the recloser and opens the contacts
Electronic: A pole mounted control box controls the opening and closing of the recloser using digital relays (microprocessor based)
What affect does enabling the HLT feature on an HLT recloser have?
- Enabling Hot Line Tag (HLT) puts the recloser on a faster tripping curve which makes it operate uncoordinated with downstream fuses
- Enabling Hot Line Tag (HLT) prevents the recloser from performing a CLOSE.
What is a sectionalizer and what is its purpose?
- -Automatic switches that are controlled by built-in logic systems
- -They work in conjunction with a recloser to help minimize the area affected by a permanent fault
Describe how a sectionalizer works in conjunction with a recloser.
- -A sectionalizer is mounted downstream of a circuit recloser.
- -When there is a downstream fault, the sectionalizer counts the number of reclose attempts made by the recloser.
- -If the recloser is still attempting to reclose a fourth time, the sectionalizer will trip open during the split second de-energized moment.
- -This limits the outage area to the downstream portion beyond the sectionalizer, allowing the recloser to restore customers up to the sectionalizer isolation point
What is the purpose of an overhead distribution transformer?
To supply the majority of BCHydro’s customers with a distribution service
What are the most common single-phase and three-phase secondary voltages?
- Single Phase: 120/240, 347
- Three Phase: 120/208, 347/600
How does BC Hydro protect its overhead transformers and what advantage does this method provide?
- -Pole mounted fused cut-out (with a lightning arrester)
- -This method allows line crews to quickly trouble shoot outages as the fuse will be visibly in the open position
- -(as opposed to the CSP transformer(completely self protected))
Where are field voltage regulators installed?
- -Installed along distribution circuits at points where feeder voltage falls below the minimum allowable value during periods of heavy load.
- -They may be pole mounted or platform mounted
What must be done prior to bypassing a voltage regulator?
Automatic voltage regulation is OFF and the regulator is in the NEUTRAL position
NCO: Neutral / Control Off
What is a bidirectional voltage regulator?
- -A voltage regulator where the source generation could come from either side.
- -It is able to detect the direction of power flow and regulate voltage in either direction
Why are shunt capacitors installed along feeders?
- -The capacitive reactance of a capacitor bank opposes the inductive reactance associated with loads.
- -This improves Power Factor, which in turn allows for greater loads without increasing the size/ capacity of system
- -Shunt Capacitors are connected in parallel and may be switched in/out as necessary
How do field shunt capacitors help voltage?
- -Field Shunt Capacitors are useful in reducing line current, which then reduces the voltage drop on the line
- -Most distribution capacitors are for voltage control rather than PF correction
What are oil switches?
- -Single phase devices for use on pole-mounted capacitor installations
- -The switch consists of electrically operated contacts( plus auxiliary equipment) immersed in oil
What are the methods used to control field shunt capacitors?
- Voltage Control Type
- -distant locations from substation,
- -usually for distribution voltages
- - Low voltage sensors switch bank in, High voltage sensors switch bank out
- Current Control Type
- -close location to substation
- -usually for use on 60 kV Transmission lines for voltage support
- - Connected on the source side of the current sensors
- -Current sensors switch bank in/ out
- VAR Control Type
- -½ to 2/3 location down circuit from substation
- -Operate just like current control type, but capacitor bank is located on load side of current sensors
What is an Independent Power Producer?
A wholesale electricity producer which sells electricity to BCHydro under an Electricity Purchase Agreement
How are Distribution IPPs connected to the power system?
Through distribution feeders ( 12, 25 kV)
Why would a transmission line’s protection trip the feeder circuit breaker associated with an IPP?
- -To ensure that the IPP does not backfeed on the transmission line (and/ or substation equipment)
- -Prevents an IPP from operating in an islanded configuration where it is not capable of carrying the feeder’s load
What provision is in place to prevent an out-of-sync close on a Distribution IPP?
- -Voltage Transformers (VTs) are installed on the load side of the feeder circuit breaker to detect voltage.
- -Allows supervisory closing with voltage supervision/ matching