Relay sa Distansya nga Impedance Type

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Pagtulun-an ug Prinsipyo sa Impedance Relay (Distance Relay)

Ang impedance relay, usab gikataas nga distance relay, mao ang usa ka voltage-controlled protective device nga ang operasyon niana depende sa electrical distance (impedance) tali sa fault point ug ang installation position sa relay. Ang pagkamahimong niini basehan sa pagsukol sa impedance sa faulty section ug paghatag sa pre-set threshold.

Operasyon

Pagsukol ug Paghatag: Ang relay nagpatuloy sa pagmonitor sa line voltage (pinaagi sa potential transformers, PTs) ug current (pinaagi sa current transformers, CTs) aron masukol ang impedance ( $Z = V/I$ ).
Tubag sa Fault: Kon ang sukolan nga impedance mas baho sa setting sa relay (naa indikasyon og fault sa protected zone), iya na mogihatag og tripping command sa circuit breaker. Sa normal nga kondisyon, ang line impedance mahimong taas (voltage >> current), nga gigihatagan sa relay og inactivity. Kon magkaroon og fault, ang current mosurge ug ang voltage mosud, nagbawas sa impedance ug nakapuyo sa relay.

Prinsipyo sa Operasyon

Sa normal nga operasyon, ang ratio sa voltage-to-current (impedance) gibahin sa taas sa threshold sa relay. Sa panahon sa fault (halimbawa, F1 sa line AB), ang impedance mosud sa sulod sa setting. Halimbawa, kon ang relay gisulay aron maprotektahan ang line AB nga may normal nga impedance $Z$ , ang fault nagbawas sa impedance, naghatag og tripping sa breaker. Kon ang fault adunay posisyon sa wala sa protected zone (halimbawa, gi-aban sa AB), ang impedance mausab taas, ug ang relay walay activity.

Karakteristik sa Operasyon

Ang relay gisulay ngadto sa duha ka pangutana:

Current Operating Element: Nagbuhat og deflecting torque proporsyonal sa current.
Voltage Restraining Element: Nagbuhat og restoring torque batasan sa voltage. Ang equation sa balance sa torque mao kini:k₁I² −k₂VIcos(θ−ϕ)=0 ang phase angle tali sa voltage ug current, ug $θ$ ang maximum torque angle sa relay. Sa impedance diagram, ang karakteristik sa operasyon sa relay makita isip usa ka circle nga anaa sa origin, ug radius sama sa setting impedance. Kini nga circular characteristic nagpadayon sa sensitivity sa magnitude ug phase sa impedance, naghatag og reliable discrimination tali sa in-zone ug out-of-zone faults.

-K3 mao ang spring effect sa relay. Sa normal nga operasyon, net torque = 0 sa mga balor sa V ug I.

Kon ang spring control effect nabatasan, ang equation mao kini

Ang figura nagpakita sa operating characteristics sa voltage ug current; dashed line nagpakita sa constant line impedance.

Ang sumala nga figure nagpakita sa operating characteristic sa impedance relay. Ang rehiyon sa itaas sa characteristic line nagrepresentar sa positive torque, diin ang line impedance mas taas sa impedance sa faulty section, nagtrigger sa operasyon sa relay. Sa uban, ang negative torque region (sa ilalum sa line) nagindikar nga ang fault impedance mas taas sa line impedance, nagpanatili sa inactivity sa relay. Kini nga distinksiyon nagpadayon sa precise fault detection sa paghatag sa measured impedance labi sa pre-set threshold, nagpadayon sa reliable protection sa power systems.

Ang radius sa circle nagrepresentar sa line impedance; ang X-R phase angle nagindikar sa vector position. Impedance < radius = positive torque (relay operates); impedance > radius = negative torque (relay inactive). Kini nga visual distinction nagpadayon sa rapid fault detection sa power systems.

Kini nga relay gisulay isip high-speed relay.

Electromagnetic Induction Relay

Ang torque sa relay mao ang resulta sa electromagnetic interactions tali sa voltage ug current, nga gihatagan sa operasyon. Sa circuit, Solenoid B—powered by a potential transformer (PT)—generates clockwise torque, pulling plunger P2 downward. A spring on P2 applies restraining force, creating clockwise mechanical torque.

Solenoid A, excited by a current transformer (CT), produces clockwise deflecting (pick-up) torque that moves plunger P1 downward. Under normal conditions, relay contacts stay open. During a protective zone fault, surging system current increases Solenoid A’s torque while reducing Solenoid B’s restoring torque. This imbalance rotates the relay’s balance arms, closing contacts to initiate protection. The design ensures rapid response to faults via torque comparison between electromagnetic and mechanical forces.

The force exerted by solenoid A (the current element) is proportional to $I^{2}$ , while that from solenoid B (the voltage element) is proportional to $V^{2}$ . As a result, the relay will activate when the current-derived force exceeds the voltage-derived force.

The constants $k1$ and $k2$ depend on the ampere-turns of the two solenoids and the ratios of the instrument transformers. Relay settings can be adjusted via tappings on the coils.

On the characteristic curve, the y-axis denotes the relay’s operating time, while the x-axis represents impedance. Notably, the relay’s operating time remains constant (indicating instantaneous action) for impedances within the preset protection zone. At the predetermined distance (corresponding to the set impedance), voltage and current values stabilize; beyond this point, the measured impedance theoretically becomes infinite, meaning the relay remains inactive for faults outside its protective scope. This linear relationship between impedance and operating time ensures reliable, rapid fault detection within the defined zone.

Induction Type Impedance Relay

The circuit diagram of an induction-type impedance relay is illustrated below. This relay incorporates both current and voltage elements, featuring an aluminium disc that rotates between electromagnets.

The upper electromagnet contains two distinct windings: the primary winding is connected to the secondary coil of a current transformer (CT), while the secondary winding is linked to a potential transformer (PT). The current setting of the primary winding can be adjusted via a plug bridge positioned beneath the relay, allowing for precise calibration of the relay’s sensitivity. The voltage element, energized by the PT, generates a magnetic field that interacts with the current-derived field from the CT.

This interaction induces eddy currents in the aluminium disc, producing a torque that drives its rotation. Under normal operating conditions, the disc remains stationary due to balanced torques; during a fault, the current surge unbalances the torques, causing the disc to rotate and trigger the relay contacts. This design ensures reliable impedance-based fault detection in power systems.

The electromagnets in the relay are connected in series, with their induced fluxes generating rotational torque that drives the aluminium disc. A permanent magnet provides both controlling and braking torque to stabilize the disc’s motion.

Under normal operation, the force on the armature exceeds the torque from the induction element, keeping the trip contacts open. When a system fault occurs, the current through the electromagnets surges, causing the aluminium disc to rotate. The disc’s rotational speed is directly proportional to the fault current, winding a spring as it turns. This rotational motion gradually overcomes the restraining torque from the permanent magnet.

Once the disc’s rotation reaches a critical threshold (corresponding to the preset impedance), the trip contacts close, initiating the protective response. This design ensures that the relay reacts swiftly to faults while maintaining stability during normal operation, with the permanent magnet providing essential control over the disc’s acceleration and braking to prevent false tripping.

The rotation angle of the relay's disc relies on the armature force, which is directly proportional to the applied voltage. Hence, voltage dictates the rotation angle.

Time-Characteristic of High-Speed Impedance Relay

The figure depicts that the relay remains inactive for values exceeding 100% of the pickup threshold. Curve 1 represents the actual operational characteristic, while Curve 2 offers a simplified model of Curve 1. This design ensures rapid response to faults within the preset range while maintaining stability under normal conditions. The relay's high-speed operation is critical for minimizing damage in power systems, with the simplified curve facilitating easier implementation and analysis in protective relay settings.

Drawbacks of Plain Impedance Relay

The following are the key disadvantages of impedance relays:

Lack of Directional Discrimination
The relay responds to impedance changes on both sides of the current transformer (CT) and potential transformer (PT). This makes it difficult for circuit breakers to distinguish between internal faults (within the protected zone) and external faults (outside the zone), potentially leading to unnecessary tripping or delayed isolation of faults.
Sensitivity to Arc Resistance
The relay’s operation is significantly influenced by arc resistance during faults. Arc resistance introduces additional impedance, which can mask the true fault impedance and cause the relay to either under-react (fail to trip for internal faults) or over-react (falsely trip for external faults).
Vulnerability to Power Swings
Impedance relays are highly sensitive to power swings—periodic oscillations in voltage and current caused by system disturbances (e.g., sudden load changes or generator instability). Power swings can mimic fault conditions by altering the measured impedance, leading to false tripping or delayed operation.
Non-Directional Operation
The relay trips whenever the measured impedance falls below the preset threshold, regardless of the fault direction. This means it cannot differentiate between forward faults (within the protected line) and backward faults (toward the power source), limiting its applicability in complex, multi-source power systems.