Medium Voltage Switchgear | Medium Voltage Protection

Medium Voltage Switchgear | Medium Voltage Protection

Medium voltage switchgear, ranging from 3 KV to 36 KV, includes various types such as metal-enclosed indoor switchgear, metal-enclosed outdoor switchgear, and outdoor switchgear without metal enclosures. The interruption medium for these switchgears can be insulating oil, SF6 gas, or vacuum. The primary requirement of a power network is to interrupt current during fault conditions, regardless of the type of circuit breaker used in the MV switchgear system. Additionally, medium voltage switchgear should be capable of,

1. Normal ON/OFF switching operation.
2. Short circuit current interruption.
3. Switching of capacitive currents.
4. Switching of inductive currents.
5. Some special application.

All the above-mentioned functions must be carried out with a high degree of safety and reliability.

Short Circuit Current Interruption

The primary focus of circuit breaker design is to ensure that all circuit breakers can reliably and safely interrupt short circuit currents. The frequency of faulty trippings during a circuit breaker's lifespan depends on factors such as system location, system quality, and environmental conditions. If tripping occurs frequently, a vacuum circuit breaker is ideal as it can handle up to 100 faulty trippings with short circuit currents up to 25 KA without requiring maintenance. In contrast, other circuit breakers typically need maintenance after 15 to 20 faulty trippings with the same short circuit current. Substations in rural areas are usually of the outdoor type and are often unattended. Therefore, maintenance-free, outdoor medium voltage switchgear is most suitable for these applications. Porcelain-clad vacuum circuit breakers meet this demand better than conventional indoor kiosks.

Switching of Capacitive Current

In medium voltage power systems, capacitor banks are employed to enhance the system's power factor. Additionally, unloaded cables and overhead lines generate capacitive charging currents. Disconnecting the capacitor bank and unloaded power lines from the system must be done safely to prevent re-ionization, which can lead to overvoltage in the system. Vacuum circuit breakers are well-suited for this task, meeting the requirement effectively.

When activating a capacitor bank, a surge of high current passes through the contacts of the circuit breaker. Circuit breakers using liquid quenching mediums and tulip contacts may encounter problems with contact pins due to this. Vacuum circuit breakers in medium voltage switchgear are preferable because they experience low electric arcing during the brief pre-arcing period.

Switching of Inductive Current

In the past, older vacuum circuit breakers (VCBs) had a current chopping level of 20 A, requiring special surge protection devices when switching transformers. However, modern VCBs have significantly lower chopping currents, typically around 2-4 A. This makes them suitable for switching unloaded transformers without the need for additional surge protection. VCBs are particularly well-suited for very low inductive load switching.

Special Application of Medium Voltage Switchgear

Arc Furnace

Switching an electric arc furnace on and off frequently presents a unique challenge. The current involved can range from 0 to 8 times the rated current of the furnace. Moreover, the furnace needs to be switched on and off at its normal rated current, which can be up to 2000A, approximately 100 times per day. Standard SF6, air, and oil circuit breakers are not economical for this level of frequent operation. In contrast, a standard vacuum circuit breaker is the most suitable alternative for such high-current, frequent operation.

Railway Traction

Another application of medium voltage switchgear is in single-phase railway track systems. The primary function of the circuit breaker associated with railway traction systems is to interrupt short circuits on the overhead catenary system, which occur frequently and are transient. Therefore, a circuit breaker used for this purpose should have a short breaking time for small contact gaps, a short arcing time, and quick breaking capabilities, making a vacuum circuit breaker (VCB) the best possible solution. Although arcing energy is higher in single-phase circuit breakers compared to three-phase ones, it is still much lower in a vacuum circuit breaker than in conventional circuit breakers. The number of short circuits occurring in the overhead catenary system is much higher than those in electrical transmission systems. Thus, medium voltage switchgear with vacuum circuit breakers is the most suitable for traction applications. In conclusion, in medium voltage systems with a high tripping rate, MV Vacuum Switchgear is the most appropriate solution.

FAQs on Medium Voltage Switchgear

1. Q: What is medium voltage switchgear?

   - A: Medium voltage switchgear is a type of electrical equipment used to switch, control, and protect electrical circuits operating at medium voltage levels, typically ranging from 1 kV to 36 kV.

 

2. Q: What are the main components of medium voltage switchgear?

   - A: The main components include circuit breakers, current transformers, voltage transformers, protection relays, measuring instruments, electrical switches, fuses, and surge arresters.

 

3. Q: What is the purpose of medium voltage protection?

   - A: Medium voltage protection safeguards electrical equipment and systems from faults such as overcurrent, short circuits, earth faults, and other abnormal operating conditions.

 

4. Q: What are the common types of circuit breakers used in medium voltage switchgear?

   - A: Common types include vacuum circuit breakers, SF6 circuit breakers, air circuit breakers, and oil circuit breakers.

 

5. Q: Why are vacuum circuit breakers often preferred for medium voltage applications?

   - A: Vacuum circuit breakers are preferred for their fast operation, high reliability, and low maintenance requirements, making them suitable for frequent switching operations.

 

6. Q: What role do protection relays play in medium voltage protection?

   - A: Protection relays detect abnormal conditions in the electrical system and send signals to the circuit breakers to interrupt the current flow, thus protecting the system from damage.

 

7. Q: How do current transformers and voltage transformers assist in medium voltage protection?

   - A: Current transformers measure the current in the system, while voltage transformers measure the voltage, providing essential data for protection relays to monitor and detect faults.

 

8. Q: What are some common protection schemes used in medium voltage systems?

   - A: Common protection schemes include overcurrent protection, differential protection, distance protection, and earth fault protection.

 

9. Q: Why is medium voltage protection important in electrical systems?

   - A: Medium voltage protection ensures the safety, reliability, and continuity of electrical power systems by detecting and isolating faults, thus preventing damage to equipment and minimizing downtime.

 

10. Q: What considerations should be made when selecting medium voltage switchgear and protection systems?

    - A: Considerations include the specific requirements of the application, the reliability and performance of the equipment, compliance with standards and regulations, and cost-effectiveness over the lifecycle of the system.