Yadda ake kawo hanyoyi na tsarin gaba-gaban abuwar tasiri a cikin EHV

Kawo Fasahar Da Shunt Reactors

Dalilin:

Shunt reactors na amfani da su domin kammala da capacitance ta hanyar daɗi mai yawa, wanda ya taka da overvoltages da masu iya shiga reactive power. Sun ba da fadada biyu da dama da zai ci gaba da switching overvoltages idan an haɓa, amma ba zai ce dalilin mafi yawan da utilities ke yi waɗannan. Dalilin mafi yawa da shunt reactors ke da su sun hada da:

• Capacitance Compensation: Hanyar daɗi mai yawa suna da capacitance mai yawa, musamman a matsayin Extra High Voltage (EHV). Wannan capacitance zai iya taka da overvoltages, musamman a lokacin da takalma ya fi yawa ko idan hanyar ta daɗi mai yawa ta fi buƙata. Shunt reactors sun taimakawa wajen kammala wannan overvoltages tare da reactive load wanda ya ci gaba da capacitive effects.

• Switching Overvoltage Reduction: Idan ba ce dalilin mafi yawa, shunt reactors zai iya kammala switching overvoltages. Idan circuit breaker ta buƙace ko ci gaba, transients overvoltages zai iya faru. Shunt reactors zai iya ci gaba da baya cikin transients, saboda haka kammala tsarin overvoltages.

Amfani:

• Shunt reactors suna haɓa a substation daɗi mai yawa a hanyar daɗi mai yawa, musamman a EHV systems inda capacitance effect yana da muhimmanci.

• Ba ake haɓa ne domin kammala switching overvoltages ba saboda wasu masu amfani (kamar closing resistors ko controlled closing) suna da muhimmanci a wannan dalili.

Closing Resistors

Dalilin:

Closing resistors suna amfani da su domin kammala voltage rise a receiving end ta hanyar daɗi mai yawa idan an haɓa. Dalilin mafi yawa shine taimakawa zuwa voltage within acceptable limits, musamman around 2 per unit (p.u.), don kammala equipment damage da kuma taimakawa system stability.

Operation:

• Idan hanyar daɗi mai yawa an haɓa, surge mai yawa ya iya taka da voltage rise mai yawa a receiving end, wanda ya taka da overvoltage conditions.

• Closing resistors suna haɓa a series with the circuit breaker a lokacin closing operation. Sun kammala initial current surge da kuma ci gaba da transients, saboda haka kammala voltage from exceeding 2 p.u.

• Idan transients ya ci gaba, resistors suna bypassed, da hanyar ta yawa a normal.

Fadada:

• Voltage Limitation: Yana ci gaba da receiving-end voltage within safe limits, taimakawa equipment da kuma taimakawa stable operation.

• Transient Suppression: Yana kammala tsarin switching overvoltages, wanda ya zama muhimmi ga EHV systems.

Staggered Pole Closing

Principle:

Staggered pole closing na nufin closing the individual poles of a three-phase circuit breaker one-half cycle apart. The idea is to allow transients in the first phase to attenuate before the next phase is closed, thereby reducing the likelihood of severe overvoltages.

Operation:

• In a three-phase system, each phase is closed sequentially, with a delay of half a cycle (10 ms at 50 Hz or 8.33 ms at 60 Hz) between each phase.

• By staggering the closing, the transients generated by the first phase have time to decay before the next phase is energized. This reduces the cumulative effect of transients and minimizes the risk of overvoltage events.

Benefits:

• Transient Attenuation: Allows transients from the first phase to dissipate before the next phase is closed, reducing the overall severity of overvoltages.

• Simplified Implementation: Does not require complex control systems, making it a relatively simple and cost-effective method for mitigating overvoltages.

Line Terminal Arresters

Purpose:

Line terminal arresters are installed at the ends of transmission lines to protect against overvoltages caused by lightning strikes or switching operations. They limit the overvoltages at the points where they are installed to the protective level of the arrester.

Operation:

• Arresters are designed to conduct excess energy away from the system when overvoltages exceed a certain threshold. They clamp the voltage to a safe level, preventing damage to equipment and ensuring the integrity of the transmission system.

• Typically, arresters are placed at both ends of the transmission line (sending and receiving terminals). However, they only limit overvoltages at those specific locations and do not provide protection along the entire length of the line.

Benefits:

• Overvoltage Protection: Effectively protects equipment at the line terminals from overvoltages caused by lightning or switching.

• Targeted Protection: Provides focused protection at critical points in the system without the need for additional equipment along the entire line.

Controlled Closing

Principle:

Controlled closing is an advanced mitigation measure that uses a dynamic controller to analyze the differential voltage across the circuit breaker, predict future voltage minima, and close the breaker at the optimal moment to minimize overvoltages. The entire process must be completed within less than 0.5 seconds to be effective.

Operation:

• A dynamic controller continuously monitors the voltage difference across the circuit breaker.

• It identifies the minimum voltage points and predicts when future minima will occur.

• The controller then closes the breaker at the predicted minimum voltage point, ensuring that the closing occurs during a low-voltage period and minimizing the risk of overvoltage.

• This method requires fast and accurate control algorithms, as well as precise timing to ensure the breaker closes at the optimal moment.

Benefits:

• Minimized Overvoltages: By closing the breaker at the optimal voltage point, controlled closing significantly reduces the magnitude of overvoltages.

• Improved System Stability: Helps maintain system stability by preventing excessive voltage surges during line energization.

• Advanced Technology: Offers a more sophisticated and effective solution compared to traditional methods like staggered pole closing or closing resistors.

Overvoltage Profile in EHV Long Lines

The figure showing the overvoltage profile in an EHV long line demonstrates the effectiveness of various overvoltage limitation options. Each method has its own impact on the overvoltage levels, and the choice of method depends on the specific requirements of the system.

• Fast Insertion of Shunt Reactors: Reduces overvoltages due to line capacitance and provides some reduction in switching overvoltages.

• Closing Resistors: Limits the receiving-end voltage to 2 p.u., effectively controlling overvoltages during line energization.

• Staggered Pole Closing: Reduces the cumulative effect of transients by allowing them to attenuate between phase closings.

• Line Terminal Arresters: Protects the line terminals from overvoltages but does not provide protection along the entire line.

• Controlled Closing: Minimizes overvoltages by closing the breaker at the optimal voltage point, offering the most effective control over transient overvoltages.

Each of these methods can be used individually or in combination to achieve the desired overvoltage mitigation in EHV long lines, depending on the system's specific needs and constraints.