Rêlaya Dizî Negatîf

Pêşnûmara

Relayî negatîf sequence, ya ku heta dikare relayî phase unbalance têne, çêkirin ye ku sisteman elektrîkî yên derbas bibe ji komponentên negatîf sequence. Wateya herêmî ya wê ya ku jenerator û motoran ji bi baran neyêrast bibirê, ku tevahî di vêgera faultên phase - to - phase de digehin. Ji ber vêgera faultan, komponentên negatîf sequence dikarin rewşên xebitiyan û stress mekanîkî yên zêdetir bikeyên elektrîkî bicîhbekin, ku dibe ku heke nebehat bikin ên tune nebehat çareseribike.

Çalakî û Xezan

Relayî negatîf sequence pê kanîna spesifikî yên filter îda ku tenha li ser komponentên negatîf sequence yên di sisteman elektrîkî de hatine reaksiyon bide. Ji ber ku heta qeymtirina nûha yekîngîn ji overcurrent yên dihewîn ji komponentên negatîf sequence re dest pê kirin, relay îda bikar anîn bi settingê ya currentê ya bêdarbîjî. Ev wateya herêmî ya wê ya ku wereşîne û reaksiyon bide bi celîl bi imbalansên nûha yekîngîn pir rawest in ên ku werbigerin da problemên mezin bikin.

Heta ku relayî negatîf sequence wekî eardîye, lê eardîyekî ya ku heta tenha li ser faultên phase - to - earth piştgirî bike. Lakin, ev nebehat çareseribike faultên phase - to - phase; tenha rolê ya ku komponentên negatîf sequence yên ku symptomên vêgera faultan ne becîne û protective actions sazane bike.

Bunyadkirina

Bunyadkirina relayî negatîf sequence di şekila li jêr de hate nirxand. Di wê de pedimpedansan ne, ku bi Z1, Z2, Z3, û Z4 name kirin, ku interconnecte ne di configurationa bridge de. Impedansan ji transformerên currentê yên ku currenta elektrîkî ji sisteman di navbera protection de sample bikin, energy bike. Coilî operating ya relay îda connecte ye bi midpointsa bridge circuit. Rêbazmendîya bi tevahî ya wê ya ku relay îda sense bide presence û magnitude ya komponentên negatîf sequence bi analizkirina voltage differences across the bridge arms, operation reliable û precise bike ji bo protection systeman elektrîkî.

Di circuita relayî negatîf sequence de, Z1 û Z3 pure resistive characteristics hene, lê Z2 û Z4 properties resistive û inductive hene. Values of impedances Z2 û Z4 accurately adjusted ne ku currents passing through them consistently lag behind the currents flowing through Z1 û Z3 by an angle of 60 degrees.

Ji ber ku current reaches junction A, it divides into two branches, namely I1 and I4. Significantly, the current I4 lags the current I1 by precisely 60 degrees. This specific phase - difference relationship is fundamental to the proper functioning of the negative sequence relay, enabling it to accurately detect and respond to negative sequence components within the electrical system.

Similarly, current from phase B split at junction C into two equal components I3 and I2, I2 lagging behind I3 by 60º.

The current I4 lags behind I1 by an angle of 30 degrees. Likewise, I2 lags behind IB by 30 degrees, while I3 leads IB by the same 30 - degree margin. The current flowing through junction B is equivalent to the algebraic sum of I1, I2, and IY. This precise angular relationship and current summation at junction B are critical for the proper functioning of the negative sequence relay, ensuring its ability to accurately detect unbalanced conditions within the electrical system by analyzing the phase and magnitude differences among these currents.

Flow of Positive Sequence Current

Phasor diagram depicting the positive sequence components is illustrated in the figure below. In a scenario where the load is in a balanced state, the negative sequence current remains absent. Under such circumstances, the current passing through the relay can be described by the following equation. This relationship between the balanced load condition, the absence of negative sequence current, and the current through the relay is fundamental to understanding the normal operation and protective functions within the electrical system.

Operation Under Balanced Conditions

Consequently, the relay remains active during the operation of a balanced electrical system, ensuring continuous monitoring and readiness to respond to any potential anomalies.

Flow of Negative Sequence Current

As illustrated in the figure above, currents I1 and I2 exhibit equal magnitudes. Due to their equal and opposing nature, they effectively cancel each other out. As a result, only the current IY traverses the operating coils of the relay. To safeguard against the detrimental effects of even minor overloads, which can rapidly escalate into severe system issues, the relay's current setting is deliberately kept lower than the normal full - load rating current. This sensitive calibration allows the relay to promptly detect and react to unbalanced conditions caused by negative sequence components.

Flow of Zero Sequence Current

In the case of zero sequence current, currents I1 and I2 are phase - displaced from one another by an angle of 60 degrees. The resultant of these two currents aligns in phase with the current IY. Consequently, the operating coil of the relay experiences a total current that is precisely twice the magnitude of the zero sequence current. It is important to note that by connecting the current transformers (CTs) in a delta configuration, the relay can be rendered inoperative for zero sequence currents. In this delta connection setup, zero sequence currents do not flow through the relay, providing a means to selectively filter out or bypass certain types of fault currents depending on the system's protection requirements.

Induction Type Negative Sequence Relay

The construction of an induction type negative phase sequence relay closely resembles that of an induction type overcurrent relay. It features a metallic disc, typically fabricated from an aluminium coil, which rotates between two electromagnets: an upper electromagnet and a lower electromagnet.

The upper electromagnet is equipped with two windings. The primary winding of the upper electromagnet is linked to the secondary side of the current transformer (CT) connected to the line under protection. Meanwhile, the secondary winding of the upper electromagnet is connected in series with the windings of the lower electromagnet.

Owing to the presence of a centre tapping, the primary winding of the relay has three terminals. Phase R, with the assistance of CTs and an auxiliary transformer, energizes the upper half of the relay, while phase Y energizes the lower half. The auxiliary transformer is specifically adjusted such that its output lags by an angle of 120º rather than the conventional 180º.

Operation with Positive Sequence Currents

When positive sequence currents are present, currents IR and IY flow through the primary windings of the relay in opposite directions. The currents I’R and I’Y have equal magnitudes. This balanced current flow ensures that the relay remains in an inactive state, as there is no net force to trigger its operation.

Operation with Negative Sequence Currents

In the event of a fault, negative sequence current I is induced to flow through the primary winding of the relay. This negative sequence current disrupts the equilibrium within the relay, setting in motion a series of events that lead to the relay's activation and subsequent protective action.

The relay will initiate its operation once the magnitude of the fault current surpasses the pre - set value of the relay. This means that when the fault current becomes large enough to exceed the specific threshold determined for the relay, the relay is triggered into action to perform its protective function within the electrical system.