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What is differential protection?
Differential protection will operate when it has determined that the difference between the current going into the transformer and coming out of the transformer has exceeded a pre-determined amount.
What is the advantage of using differential protection over simpleover-current protection to protect transformers?
The differentially connected over-current relay should not operate on either an external fault or normal load (designed to operate only on an internal fault)
What is the limitation of using over-current relays to employdifferential protection?
Overcurrent relays must be set above any error current that could flow(ex. saturated CTs)on an external fault. This reduces sensitivity.
How do percentage differential relays increase the sensitivity ofdifferential protection?
- Sophisticated percentage differential relays improve sensitivity without affecting security
- -improves ability to to detect low-magnitude internal faults
- -employs operate and restraint coils
How can magnetizing inrush current pose a problem to differentialprotection?
- -Magnetizing inrush current has a high harmonic content and is especially rich in second harmonic
- -This magnetizing current appears to a differential relay as an internal fault as it does not flow out the other side of the transformer
- -The more sophisticated relays are designed to block tripping if the second harmonic content exceeds a certain amount.
How are a gas relay’s alarm and trip outputs triggered?
- – Faults of minor nature cause a slow accumulation of gas, gas displaces oil in the relay chamber, which at a certain point operates a micro-switch
- – A major internal fault will vaporize oil and and cause a rapid build-up of pressure, causing a pressure wave that will operate a micro-switch
What two conditions in combination will trigger “bullet-hole”protection?
- The combination of the low oil alarm and the gas accumulation alarm provide indication of loss of oil
- -these alarm contacts are connected in series
How does a winding temperature device measure temperature? Whatis it used for?
- This device has a heater which is fed from a CT in the transformer winding, thus, temperature measured by the thermometer depends on top oil temperature and the current in the transformer winding
- It initiates alarms, cooling and tripping
How are faults cleared on substation transformers without high-sidecircuit breakers?
Either a transfer trip is sent to the remote line terminal or a SOG is applied to force operation of remote line terminal protection
What is transformer auto isolation?
- After a Circuit Breaker tripping to clear a fault, the transformer disconnect switches are automatically opened to isolate the transformer.
- Closing of these disconnects is blocked until the lockout is reset (Block Close)
What type of relay is generally used in bus protection?
How must the phase and neutral relays of bus protection be set interms of coordination?
- -The phase relays (A,B,C) must be set above the maximum load current and must coordinate with feeder phase over-current relays
- -The Neutral relay must be set above the maximum neutral current and must coordinate with feeder neutral relays
What is the purpose of torque control and how does it function?
- Differentiates between Fault Current and High Load Current by monitoring for an acceptable voltage
- -Torque control ensures that bus protection does not operate on load current
- -used on distribution buses that must supply many feeders and significant load
- Under-voltage/distance elements are added to work in coordination with feeder bus over-current relays to ensure that the proper protection operates first
What is the risk in using the under-voltage method of torque control?
Loss of voltage to the under-voltage relay may cause bus protection misoperation
Why would a distance relay be used to provide torque control?
- In stations where the feeder reactor is located on the source side, most of the voltage drop will be across the reactor for a bus fault.
- This means there may be insufficient voltage to operate an under-voltage relay
- In this case, a distance relay is used to detect the bus fault and provide torque control
What is the risk in using the distance method of torque control?
Loss of voltage to the distance relay will prevent bus protection operation for a bus phase fault
Describe typical substation feeder relaying.
- Feeder relaying consists of four over-current relays (One for each phase and a neutral)
- -usually contain an inverse time over-current and instantaneous overcurrent element
- -Instantaneous elements must be set with large enough margins to avoid misoperation
- -Feeder relays are set to coordinate with downstream reclosers and fuses
How is a feeder fault cleared if the feeder circuit breaker fails tooperate?
- Bus Protection would then operate to remove the fault.
- This would result in multiple feeders out.
- OO 1T-29B describes the policy for restoring a Main Feeder Bus
How does a fused cutout function?
- The hinged portion of the door is held closed by a replacable fuse link.
- When there is fault current, the fuse melts and the door swings open by spring action and gravity
What is an expulsion fuse?
Expulsion fuses are cutout fuses that when melted, expel the arc byproducts from the fuse tube
How does the fault protection provided by vista switchgear differ fromthat of a switching kiosk?
- In a vista, the ‘fault interrupter way’ operates in series with a vacuum fault interrupter (there are no replacable fuses and is 3 phase ganged)
- In a Kiosk, the fuses are replaced and are single pole switched
How are the majority of dead-front transformers protected?
They are protected by a series combination of a replacable expulsion fuse and a back-up current limiting fuse
Can a bay-o-net fuse be used to break load in an LPT?
Although the manufacturer says bay-o-net fuses can break load, it is not a BCHydro practice
How are shunt capacitor banks primarily protected? What benefitsdoes this provide?
- Individual capacitor fuses, which protect the individual units
- -maintains service continuity, removing only a failed can
- -prevents damage to adjacent capacitors and equipment
- -provides visual indication of a failed unit
What means of protection can be provided to isolate a shunt capacitorbank from the system in the event of a major fault?
Phase fuses or phase and ground over-current relays may be used between the system and capacitor bank
Why is neutral-shift over-voltage protection applied to shunt capacitorbanks?
- The neutral potential will be permanently shifted proportionate to the change in impedance of the leg of the failed unit
- If the shift exceeds a predetermined threshold, breaker tripping is initiated
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