When B∅ circuit comes into contact with the A∅ or C∅ circuit
When one of the phases comes into contact with ground
In order to maintain Power System stability and continuity of customer service, some form of is needed
What are the most common devices designed to detect fault current and isolate fault
Direct-acting trips on CBs
Combine the function of fault detection and circuit clearing in one device
Characteristics of fuses
They are single phase devices.
They maintain a fast-operating speed.
The tripping time is non-adjustable.
They must be replaced after an operation
Fuses are best suited for where they do not operate in conjunction with other protective devices
branch circuit protection
In the fuse the complete fuse assembly must be
replaced each time it blows
The fuse is constructed so the casing can be easily dismantled and the blown link can be replaced with a new fuse link of proper amperage and voltage ratings
Non-current limiting fuse, with the link contained in an explosion chamber - used in Station Service
The is a non current-limiting fuse which consists of a glass tube, a fuse element and an arc extinguishing liquid
The fuse element is surrounded by a tube liner of compressed boric acid
Boric Acid Fuse
Fuses used extensively for service with potential transformers and control power transformers.
Silver Sand fuse.
are normally used in underground distribution circuits where voltages do not exceed
Oil fuse cutouts
is one in which the breaker is tripped by direct mechanical action when the current reaches a pre-determined value
A direct-acting trip device
Direct acting trips and fuses can only be employed for protection
Relays can be used to protect against undervoltage,
overvoltage, unbalanced phases, low frequency, current differential, and phase angle comparison
Measuring instruments equipped with auxiliary contacts
that operate when the quantities flowing through them either exceeds or drops below a predetermined value
Relays may not be used to locate a fault by measuring the
line impedance (distance) from the relay to the fault
Used where an instantaneous operation is required in a relay with an adjustable pick-up.
Plunger type relay
A relay that consist of a disc positioned between two magnetic poles that is not instantaneous
Induction-Disc type relay
A relay does three things in order to accomplish its purpose. It must:
A relay does three things in order to accomplish its purpose. It must
The sense function in a relay input circuit. If the input circuit is connected to a source of voltage or current, the relay is monitoring an electrical quantity.
The function of a relay, is accomplished by mechanisms within the relay (relay movement). This mechanism is adjusted to respond to a specific level or degree of input
The function is done by closing relay output contacts that most often are used to open or trip circuit breakers
What the four basic characteristics of relay
Sensitivity - able to distinguish between normal/abnormal conditions
Selectivity - recognize a fault and trip the appropriate number of CBs
Adjustable - trip CBs within the proper time
Reliability - must operate correctly and with certainty
A device located under the bushing, with the primary side connected to the source to be monitored and the secondary side is connected to the relay circuit
Reduce bus or line voltage to 120 volts.
A short in the circuit may prevent the relay from seeing the fault
A DC short in the relay circuit may cause a to blow
are installed in the positive side of the DC to allow relays to be manually blocked
The Circuit Breaker status signals, and the Relay Trip Circuit, are made up through pallet switches, known as the A & B
pallets for the circuit breaker
is used in system protection schemes to monitor relay tripping frequency signals
Carrier Equipment includes such items as:
Provides the path for the terminals of a monitored
line section to communicate, The two most common methods used by the Department are to transmit an audio-frequency tone over a leased telephone line or use the line itself
Wave Trap filters the audio-frequency tone, thus preventing the
carrier signal from going beyond the circuit is it monitoring
Used in Receiving Stations (RS) for transmission and sub-transmission line protection and as back-up protection on banks in Substations
When overcurrent relays are used to protect lines, only the Receiving Station end of the line will be tripped by the relay
When overcurrent relays are used to protect banks, normally
only the bank will trip
high side circuit breakers
overcurrent relays have no intentional time delay, they are used for tripping when extremely high current values are present
overcurrent relays have a time delay element, the time delay is needed so that tripping can be coordinated with other relays or fuses.
Protects lines in Distributing Stations, Trips only the local end of the faulted line
Directional overcurrent uses an overcurrent relay with a directional element that requires both current and voltage
Protects, a particular piece of equipment such as, a
transformer, condenser, generator, or bus Trips all sources and loads to the faulted circuit.
Sudden pressure relays are commonly used to protect transformers
One of the most common sudden pressure relays used on the Power System is the relay
compare the voltage or current on both sides of the equipment being protected
is a back-up protection used to clear a fault if a circuit breaker fails to trip
What of relay trips the bank low side circuit breakers when the frequency drops below a predetermined value
Under-frequency relays are used at most receiving and switching stations.
Bank low side circuit breakers trip on under-frequency. The load is dropped, but the Transmission System and Station Service are maintained.
protects capacitors and trips the capacitor source CB
Protects Transmissions lines and trips both ends of the faulted line
Distance relays are directional, which means they look in only one direction for a fault
When a fault occurs a long distance away, the bus current increase is than when the fault is close in. Conversely, when the fault is a long distance away the bus voltage will be less than when the fault is close in.
Distance relays are not designed to operate on line to ground faults because the impedance changes constantly
The impedance of a transmission line depends on the length of the line the size of the conductor the material used to make the conductor the spacing between conductors the height above ground
Phase comparison relays are similar to differential or pilot wire relays, phase comparison relays compare the phase angle and current magnitude of the line at each terminal
If a receiver stops getting the Guard Frequency without immediately receiving a trip frequency, a "loss of guard" or "transfer trip signal" alarm is initiated
What are the transfer trip relays used by DWP
Direct Under-reaching Transfer Trip
Permissive Under-reaching Transfer Trip
Permissive Overreaching Transfer Trip
In this method, the relays are set to protect 80 to 90% of the line. If a fault is detected, a trip signal is sent to the local circuit breaker and the transmitter sends a trip signal to the remote end.
Direct Under-reaching Transfer Trip
The transfer trip signal and must be received simultaneously to initiate a remote trip
permissive relay signal
This method of transfer trip protection uses directionally controlled relays set to overreach the remote terminal.
Permissive Overreaching Transfer Trip
Relays that operate in order to assist another relays or devices
Once the auxiliary relay is energized by the protective relay, it is .
The general philosophy of relay application is to divide the power system into .
The Screw-Base Plug fuse and the Cartridge fuse. The Cartridge fuse is the type most often used by the Department.
Why must the CTs must never be energized with the secondary open
This will result in very high impedance across the open.