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A feeder protection relay is a device that protects power system feeders from various types of faults, such as short circuits, overloads, ground faults, and broken conductors. A feeder is a transmission or distribution line that carries power from a substation to the load or another substation. Feeder protection relays are essential for ensuring the reliability and security of power systems, as they can quickly detect and isolate faults, prevent damage to equipment, and minimize power outages.
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One of the most common types of feeder protection relays is the distance protection relay, also known as an impedance relay. A distance protection relay measures the impedance (Z) of the feeder line by using the voltage (V) and current (I) inputs from the corresponding potential transformer (PT) and current transformer (CT). The impedance is calculated by dividing the voltage by the current: Z = V/I.
The distance protection relay compares the measured impedance with a predefined setting value, which represents the maximum allowable impedance for normal operation. If the measured impedance is lower than the setting value, it means that there is a fault on the feeder line, and the relay will send a trip signal to the circuit breaker to isolate the fault. The relay can also display the fault parameters, such as fault current, voltage, resistance, reactance, and fault distance, on its screen.
The fault distance is the distance from the relay location to the fault location, which can be estimated by multiplying the measured impedance by the line impedance per kilometer. For example, if the measured impedance is 10 ohms and the line impedance per kilometer is 0.4 ohms/km, then the fault distance is 10 x 0.4 = 4 km. Knowing the fault distance can help locate and repair the fault quickly.
How Does a Quadrilateral Characteristic Work?
A distance protection relay can have different operating characteristics, such as circular, mho, quadrilateral, or polygonal. A quadrilateral characteristic is a popular choice for modern numerical relays because it offers more flexibility and accuracy in setting the protection zones.
A quadrilateral characteristic is a parallelogram-shaped graph that defines the protection zone of the relay. The graph has four axes: forward resistance (R F), backward resistance (R B), forward reactance (X F), and backward reactance (X B). The graph also has a slope angle called the relay characteristic angle (RCA), which determines the shape of the parallelogram.
The quadrilateral characteristic can be plotted by using the following steps:
Set the R F value on the positive X-axis and the R B value on the negative X-axis.
Set the X F value on the positive Y-axis and the X B value on the negative Y-axis.
Draw a line from R F to X F with a slope of RCA.
Draw a line from R B to X B with a slope of RCA.
Complete the parallelogram by connecting R F to R B and X F to X B.
The protection zone is inside the parallelogram, which means that if the measured impedance falls inside this area, then the relay will trip. The quadrilateral characteristic can cover four quadrants of operation:
First quadrant (R and X values are positive): This quadrant represents an inductive load and a forward fault from the relay.
Second quadrant (R is negative and X is positive): This quadrant represents a capacitive load and a reverse fault from the relay.
Third quadrant (R and X values are negative): This quadrant represents an inductive load and a reverse fault from the relay.
Fourth quadrant (R is positive and X is negative): This quadrant represents a capacitive load and a forward fault from the relay.
What Are Different Zones of Operation?
A distance protection relay can have different zones of operation, which are defined by different setting values of impedance and time delay. The zones are designed to coordinate with other relays in the system and provide backup protection for adjacent feeders.
The typical zones of operation for a distance protection relay are:
Zone 1: This zone covers 80% to 90% of the feeder length and has no time delay. It provides primary protection for faults within this zone and trips instantaneously.
Zone 2: This zone covers 100% to 120% of the feeder length and has a short time delay (usually 0.3 to 0.5 seconds). It provides backup protection for faults beyond zone 1 or in adjacent feeders.
Zone 3: This zone covers 120% to 150% of the feeder length and has a longer time delay (usually 1 to 2 seconds). It provides backup protection for faults beyond zone 2 or in remote feeders.
Some relays may also have additional zones, such as Zone 4 for load encroachment or Zone 5 for overreaching faults.
What Are Other Types of Feeder Protection Relays?
Besides distance protection relays, there are other types of feeder protection relays that can be used for different applications or in combination with distance protection relays. Some examples are:
Overcurrent protection relays: These relays measure only current and trip when it exceeds a preset value. They are simple, inexpensive, and widely used for radial feeders.
Differential protection relays: These relays compare current inputs from both ends of a feeder and trip when there is an unbalance between them. They are fast, selective, and sensitive for short feeders or busbars.
Directional protection relays: These relays measure both current and voltage and determine their phase angle difference. They trip only when current flows in a specific direction relative to a voltage. They are useful for looped feeders or parallel feeders.
Arc-flash detection relays: These relays use light sensors and high-speed overcurrent detection to identify arc-flash events on feeders. They trip faster than conventional relays and improve safety for personnel.
How to Select Feeder Protection Relays?
The selection of feeder protection relays depends on various factors, such as:
The type, length, configuration, loading, grounding, and insulation level of feeders
The availability, accuracy, cost, maintenance, communication, and integration of relays
The coordination, selectivity, sensitivity, speed, reliability, security, and stability of protection schemes
The standards, regulations, codes, policies, and practices of power system operators
Some general guidelines for selecting feeder protection relays are:
Choose numerical relays over electromechanical or static relays for better performance, functionality, flexibility, and diagnostics
Choose distance protection relays over overcurrent or differential protection relays for long or complex feeders
Choose quadrilateral characteristics over circular or mho characteristics for more accuracy and adaptability
Choose low-energy analog sensor inputs over conventional current/voltage inputs for reduced size, weight, and safety hazards.
Choose arc-flash detection relays over conventional relays for faster tripping and personnel safety.
Conclusion
Feeder protection relays are vital devices that protect power system feeders from various types of faults. They can improve power system reliability, security, and efficiency by quickly detecting and isolating faults, preventing damage to equipment, and minimizing power outages.
One of the most common types of feeder protection relays is the distance protection relay, which measures the impedance of the feeder line by using the voltage and current inputs from the corresponding potential transformer and current transformer. It compares the measured impedance with a predefined setting value, which represents the maximum allowable impedance for normal operation. If the measured impedance is lower than the setting value, it means that there is a fault on the feeder line, and the relay will send a trip signal to the circuit breaker to isolate the fault.
