LOAD TERM 2 LOE

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.

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)

Overcurrent relays must be set above any error current that could flow(ex. saturated CTs)on an external fault. This reduces sensitivity.

• Sophisticated percentage differential relays improve sensitivity without affecting security
• -improves ability to to detect low-magnitude internal faults
• -employs operate and restraint coils

• -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.

• Alarm
• – 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
• Trip
• – 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

• 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

• 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

Either a transfer trip is sent to the remote line terminal or a SOG is applied to force operation of remote line terminal protection

• 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)

Over-current relays

• -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

• 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

Loss of voltage to the under-voltage relay may cause bus protection misoperation

• 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

Loss of voltage to the distance relay will prevent bus protection operation for a bus phase fault

• 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

• 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

• 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

Expulsion fuses are cutout fuses that when melted, expel the arc byproducts from the fuse tube

• 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

They are protected by a series combination of a replacable expulsion fuse and a back-up current limiting fuse

Although the manufacturer says bay-o-net fuses can break load, it is not a BCHydro practice

• 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

Phase fuses or phase and ground over-current relays may be used between the system and capacitor bank

• 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