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1. What is differential protection?
Differential protection operates 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.
2. What is the advantage of using differential protection over simple over-current protection to protect transformers?
The advantage of differential protection over simple over-current protection is that differential protection can detect internal faults because differential protection operates when there is a difference between current going into the transformer and coming out of the transformer that has exceeded a pre-determined amount.
3. What is the limitation of using over-current relays to employ differential protection?
The limitation of using over-current relays to employ differential protection is that CTs don’t preform exactly alike and the ratios don’t always match causing a differential error current to flow during an external fault. The overcurrent relay must be set above any error current that can flow on an external fault. This reduces sensitivity. These settings may compromise the ability to detect low-magnitude internal faults.
4. How do percentage differential relays increase the sensitivity of differential protection?
Percentage differential relays use operate and restraint coil which, during an external fault, high magnitude current flows through the restraint windings and only the differential current flows through the operating coil. The higher the current on a through fault, the more differential current required to operate the relay.
On an internal fault, the high magnitude differential current overcomes the restraint, and the relay operates
they can have increased sensitivity without affecting security.
5. How can magnetizing inrush current pose a problem to differential protection?
Magnetizing inrush current has a high harmonic content, especially 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.
6. How are a gas relay’s alarm and trip outputs triggered?
- An alarm is triggered by detecting an accumulation of gas that displaces the oil in the relay chamber. A float follows the lowered oil level in the relay chamber. At a certain point, a micro-switch operates to trigger an alarm. The intent is to detect minor faults before they become major.
- A trip is initiated upon detection of an internal fault. Arcing caused by a fault will vaporize the oil resulting in a rapid build up of gas pressure. A pressure wave through the oil moves a flexible bellows in the gas relay to operate a micro-switch which initiates a trip.
7. What two conditions in combination will trigger “bullet-hole” protection?
The combination of the low oil alarm and the gas accumulation alarm will trigger “bullet-hole” protection.
8. How does a winding temperature device measure temperature? What is it used for?
- In addition to measuring top oil temperature, a winding temperature device has a heater which is fed from a CT in the transformer winding. Temperature measured by the thermometer depends on the top oil temperature and the current in the transformer winding.
- The winding temperature device is used to protect against overload.
9. How are faults cleared on substation transformers without high-side circuit breakers?
A fault is cleared on substation transformers without high-side circuit breakers by a transfer trip sent to the remote line terminal or a SOG is applied to force operation of the remote line terminal protection.
10. What is transformer auto isolation?
Transformer auto isolation causes transformer disconnect switches to open, isolating the transformer, following circuit breaker tripping to clear the fault. Closing the disconnects is blocked until the lockout is reset. “Block close” condition is removed from the circuit breakers to allow restoration of the lines, buses, etc that were tripped by the transformer protection.
1. What type of relay is generally used in bus protection?
Over-current relays are generally used for distribution feeder bus protection.
2. How must the phase and neutral relays of bus protection be set in terms of coordination?
- Phase relays 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 (load unbalance) and must coordinate with feeder neutral relays.
3. What is the purpose of torque control and how does it function?
- Torque control ensures that bus protection does not operate on load current. It is used on distribution buses that supply many feeders and a significant amount of load.
- Under-voltage or distance elements are added to control the operation of feeder over-current relays. The undervoltage/distance elements operate for faults but not heavy loads.
4. What is the risk in using the under-voltage method of torque control?
Loss of voltage would remove the supervision of the overcurrent element which may cause protection to operate when unintended.
5. Why would a distance relay be used to provide torque control?
A distance relay is used to provide torque control when there is a feeder reactor located on the source side of the feeder breaker. If the source impedance is small compared to series reactor impedance, most of the voltage drop for a bus fault beyond the reactor will be across the reactor. There may be insufficient voltage to operate an undervoltage relay.
6. 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.
1. Describe typical substation feeder relaying.
Typical substation feeder relays consists of four overcurrent relays, one for each phase and one for neutral. Each relay has an inverse time over-current element and an instantaneous over-current element.
- The instantaneous overcurrent relay is used for close-in faults. They must be set with large enough margins to ensure they will not initiate tripping for faults beyond the next fuse.
- For fault currents less than the instantaneous element settings, the inverse time overcurrent element will operate.
2. How is a feeder fault cleared if the feeder circuit breaker fails to operate?
If the feeder circuit breaker does not clear the fault within the setting time, the 95FT (Feeder breaker failure Timer) trips the bus.
3. How does a fused cutout function?
A fused cutout protects and isolates if a particular transformer fails or the primary short circuits. The fuse link melts and the hinged portion opens.
4. What is an expulsion fuse?
Cutout fuses are called expulsion fuses because the heat generated by the melting creates gases that expel the arc products from the fuse tube. When the link breaks upon fusing, the tension is lost and gravity or a spring opens the holder.
5. How does the fault protection provided by vista switchgear differ from that of a switching kiosk?
Vista switchgear protection is a load-interrupter switch in series with a vacuum fault interrupter. Switching Kiosks have separate compartments for the fuses.
6. How are the majority of dead-front transformers protected?
The transformer fuse is connected between the internal primary bus and the transformer. When an internal fuse blows, the primary bus inside the transformer is still energized, only the transformer primary coil has been interrupted.
The majority of single-phase dead-front transformers are protected by a series combination of a replaceable expulsion fuse and a back-up current limiting fuse. The current limiting fuse protects against short circuits on the primary winding and the expulsion fuse protects against overloads and faults on the secondary side.
7. Can a bay-o-net fuse be used to break load in an LPT?
The manufacturer says bay-o-net fuses can break load, BC Hydro practice is not to break load with any pad-mounted transformer fuses.
1. How are shunt capacitor banks primarily protected? What benefits does this provide?
Shunt capacitor banks are primarily protected by individual capacitor fuses.
These fuses protect the individual units.
2. What means of protection can be provided to isolate a shunt capacitor bank 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 the capacitor bank for major bank faults.
3. Why is neutral-shift over-voltage protection applied to shunt capacitor banks?
Neutral-shift over-voltage protection is applied to shunt capacitor banks because a fault will cause the neutral potential to permanently shift proportionate to the change in impedance of the leg having the failed unit.
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