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What is the purpose of protection?
- Protection does not prevent disturbances, rather it reacts to them after they have occurred.
- The purpose of protection is to detect faults and remove them from the system before they pull the entire system down. They do so by removing the smallest portion of the faulted system possible by clearing with fault removal devices (circuit breakers).
What is the difference between a protection system and a protection scheme?
- Protection System: Each individual protective arrangement which individually protects against a certain risk.
- Protection Scheme: Coordinated combination of protection systems.
With regards to protective relaying, what is meant by the term – Selectivity?
Selectivity: Only a minimum portion of the power system should be disconnected to isolate a fault.
With regards to protective relaying, what is meant by the term – Security?
Security: Schemes must be able to discriminate between signals which require action, and those that do not.
With regards to protective relaying, what is meant by the term – Reliability?
Reliability: Ability to operate without failure whenever a fault or abnormal condition exists.
With regards to protective relaying, what is meant by the term – Coordination?
Coordination: Schemes must relate to each other so they operate in the right order.
Provide two reasons why two sets of redundant protection are used on a piece of equipment. What are they named?
- Using two sets of redundant protection achieves:
- 1. Increased dependability
- 2. One protection may be removed from service for maintenance and the equipment will remain protected.
- They are called primary protection and standby protection.
Typically in a fault condition, current will __ while voltage ____.
current will rise while voltage falls
What are the two main reasons for utilizing instrument transformers?
- Instrument transformers
- 1. Provide useable quantities of voltage and current for metering and control.
- 2. Provide useable quantities of voltage and current for protection relays.
What is a VT? What purpose does it serve? What is its typical output?
- Voltage transformer (VT) provides relays, meters and control circuits with a low value of voltage proportional to system voltage.
- 120V is the typical standard secondary rating (output) of a VT.
What is a CT? What purpose does it serve? What is its typical output?
- Current transformer (CT) provides relays, meters and control circuits with a low value of current proportional to system current.
- 5A is the standard secondary rating (output) of a CT.
If a CT has a ratio of 2000:5, what is the primary current if the secondary current is 2.5 amps?
Primary current = 2.5A * 400 = 1000A
With respect to a CT, what is burden? How does exceeding the burden impact the operation of a CT?
- Burden (ohm) is the amount of load connected to the secondary of the CT.
- If the burden is exceeded, the CT saturates and can no longer accurately reflect system current.
With respect to a VT, can the secondaries be opened with the VT energized? Explain any consequences.
VT’s may have their secondaries open circuited (no burden on VT). The only consequence is for the protective relays utilizing the voltage reading from the VT.
With respect to a CT, can the secondaries be opened with the CT energized? Explain any consequences.
The secondaries of a CT must never be open circuited (no burden on CT). Extremely high open circuit secondary voltages may result.
What is a protection zone? How is it typically defined?
- A protection zone is the section of the power system that a particular protection covers.
- It is typically defined by the location of the CT’s.
What is a tripping zone? How is it typically defined?
A tripping zone is the section of the power system that is tripped by protection. It is determined by the circuit breaker locations.
With respect to protection, what is meant by the term overlap?
To prevent the possibility of unprotected area and to provide backup, protection zones are overlapped.
What is breaker fail protection?
Breaker fail protection operates to trip the adjacent zone if the circuit breaker protecting the faulted zone fails to open. Breaker fail protection initiates tripping if fault current is still flowing through the breaker after a time elapses following the protection zone operation.
What is the purpose of relay targets?
Relay targets are included in protective relay systems in order to provide a record of their operation.
They can be flags that must be reset or LEDs on the front of the replay.
What are symmetrical components? What is meant by positive sequence? What is meant by negative sequence? What is meant by zero sequence?
- Symmetrical components are commonly used for analysis of three-phase electrical power systems. If the phase quantities are expressed in phasor notation using complex numbers, a vector can be formed for the three phase quantities.
- Positive sequence means the current and voltage has a positive phase rotation (A,B,C). Each phase is equal in magnitude with a 120° angle between them. Under balanced conditions only positive sequence exists.
- Negative sequence may exist in addition to positive if conditions are unbalanced due to unbalanced loads or a fault. Unbalanced load or fault produces unbalanced currents which produces unbalanced voltages. Negative sequence voltages and currents are similar to positive except the rotation is negative (A,C,B).
- Zero sequence exists when conditions are unbalanced and there is a path to ground. Positive, negative, and zero sequences will exist. Zero sequences have phase quantities with equal magnitude and phase.
What two relays are used to detect stator phase to ground faults?
- Zero-sequence overvoltage relays
- Third harmonic voltage differential relays
What relay is used to detect stator phase to phase faults?
Differential relays (87) are used to detect phase to phase faults.
With respect to overspeed of an electrical generator, how is the unit protected?
Overspeed tripping (12) is provided by centrifugal speed switches or permanent magnet generator and initiates a trip of the generator to bring the unit to speed no load should the governor fail to limit the overspeed.
With respect to motoring of an electrical generator, how is the unit protected? Are there any exceptions?
A reverse power relay (32) is used to detect power flow into the generator and initiate tripping. This relay is blocked if the unit is running in synchronous condense mode.
What does SCADA stand for? Explain SCADA.
- Supervisory Control and Data Acquisition (SCADA) monitors, controls and alarms plant or regional electric systems from a central location.
- • SCADA is at substations and generating plants to provide supervisory control of switchyard equipment, generators and spillway facilities from FVO/SIO.
- • Provides telemetry (kV, Amps, MW, MVar, and MVA) from the facilities to the Control Centers.
- • Sends alarms from the substation back to the Control Center when equipment changes status or violates a limit.
- SCADA is a part of the larger Energy Management System (EMS).
With respect to generating station step-up transformer, what protective relays are in place to detect fault conditions? Name both electrical protection and non-electrical protection.
- Differential relay (87) detect windings and thermal faults. A current transformer is installed at each terminal of the protected equipment to measure the current going in and the current going out. A relay is connected so it sees only the difference in in the currents.
- Ground differential protection. Detects an earth fault in the winding.
- Gas relays (63) detects the slow accumulation of gas which may be given off by a developing fault.
- Sudden pressure relays (63) detects a violent disturbance of pressure resulting from a developed fault
- Conservator low oil device
- Oil temperature
- Winding temperature
- Pressure relief devices relieve pressure in the transformer tank should a fault occur.
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