MV Switchgear Operation Insights: Balancing Safety and Efficiency

Medium Voltage (MV) switchgear plays a pivotal role across power systems – from generation and transmission to distribution, energy conversion, and consumption – by facilitating power switching, control, and protection. Its safe and efficient operation is directly linked to the stability of the entire power grid and the safety of personnel. This article will delve into the classification of MV switchgear, its core safety mechanisms, and a series of key operational considerations, aiming to help operators better understand and safely utilize MV switchgear.

Overview of Medium Voltage Switchgear

MV switchgear refers to an assembly of electrical equipment rated from 1kV to 35kV, primarily designed for connecting and disconnecting circuits, as well as providing protection, metering, and control for power lines and associated equipment. Based on its construction and insulation medium, MV switchgear can be broadly categorized as follows:

By Main Component Installation Method: MV switchgear can be classified into Fixed-Type Switchgear and Withdrawable Switchgear (commonly known as “trolley type”). Withdrawable switchgear is further categorized by the trolley’s position into floor-mounted and center-mounted types.

By Safety Level: Key categories include Medium Voltage Metal Clad Switchgear, Compartmented Switchgear, and Cubicle-Type switchgear. These designs offer varying levels of safety assurance in isolating live components and preventing incorrect operations.

By Internal Main Insulation Medium: This includes Air Insulated Switchgear (AIS) and Gas Insulated Switchgear (GIS). Medium voltage gas-insulated switchgear commonly uses SF6 gas, but other types utilizing solid or hybrid insulation are also available.

Core Safety Assurance: Mechanical Interlocks in MV Switchgear

The design of MV switchgear incorporates stringent mechanical interlock devices. These interlocks serve as a critical defense line against incorrect operations, ensuring both personnel and equipment safety. Operators should thoroughly understand the logic behind these interlocks and adhere to them during daily operations to guarantee operational safety:

Prevention of On-Load Switching/Withdrawal:

• When the circuit breaker trolley within the MV switchgear is in the “Test” or “Isolated” position and is closed, the interlock prevents the trolley from moving into the “Service” position.
• Similarly, when the VCB circuit breaker is closed and operating in the “Service” position, the interlock prevents the trolley from being withdrawn or moved to the “Test” or “Isolated” position.
• Purpose: To ensure that all current interruptions are performed solely by the circuit breaker, which is equipped with arc-extinguishing capabilities. It strictly prohibits connecting or disconnecting circuits under load using medium voltage disconnectors or by moving the trolley, as this would cause severe arcing, leading to equipment damage and personal injury.

Prevention of Closing onto an Earthed Circuit:

• When the earth switch in switchgear is in the closed (earthed) position, the circuit breaker (whether withdrawable or fixed-type) cannot be closed.
• Purpose: This is a core safety interlock designed to prevent operators from inadvertently energizing equipment or lines that are already earthed, thereby avoiding short-circuit faults, protecting equipment, and ensuring the safety of maintenance personnel.

Prevention of Access to Live Compartments:

When the circuit breaker within the MV switchgear is in the closed (energized) state, or if the main circuit is not earthed (meaning it could be live), the rear access doors or certain compartment doors of the switchgear are mechanically interlocked by the earthing switch, preventing them from being opened.
Purpose: To eliminate the risk of personnel accidentally entering live compartments and directly contacting live MV components, significantly enhancing safety during operation and maintenance.

Prevention of Earthing a Live Circuit:

• The earthing switch cannot be closed (engaged) if the circuit breaker within the MV switchgear is in the closed (energized) state, or if the main circuit is otherwise determined to be live.
• Purpose: To prevent the inadvertent earthing of an active circuit, which would result in a severe short-circuit fault, leading to catastrophic damage to equipment.

Key Aspects of MV Switchgear Safe Operation

Standardized operation is the cornerstone of ensuring the safe functioning of MV switchgear. The following are detailed key operational points:

Personnel Qualification and Teamwork

Emphasis on Qualification Requirements: Personnel operating MV switchgear should possess the relevant electrical operation certifications, such as electrician’s licenses or grid access operation permits, and hold specific authorization for the equipment in question.

Regular Training and Assessment: Operators are required to regularly attend professional theoretical and practical training. They must pass rigorous assessments before being authorized to work, ensuring their knowledge and skills are continuously updated.

Recommended Two-Person Rule: MV switchgear operations are recommended to follow the “two-person rule,” where one person is the primary operator and the other serves as a safety monitor and reminder. This approach helps ensure operational accuracy and safety, preventing oversights and accidents that might occur during single-person operations.

Theoretical Knowledge and Skill Enhancement

Familiarity with Equipment Parameters: Operators should master the technical parameters, ratings, and operational characteristics of the MV switchgear they operate.

Understanding Electrical Principles: A deep understanding of the switchgear’s single-line diagrams (SLDs), control circuit schematics, protection logic, and interlock relationships is essential.

Mastery of Operational Points: Memorize all operational steps, sequences, and precautions, especially the procedures for handling emergency situations.

Fault Identification and Management: Proficiently understand the methods for identifying common MV switchgear faults (e.g., insulation breakdown, partial discharge, abnormal temperature rise) and initial response measures. Know when to cease operations and seek expert assistance.

Pre-Operation Safety Preparation and Protection

Work Permit and Planning: Before operation, it is crucial to confirm that a valid work permit has been obtained and to conduct a thorough risk assessment and operational planning for the task.

Personal Protective Equipment (PPE): Wear standard-compliant PPE, including insulated gloves, insulated boots, safety helmets, protective eyewear or face shields, and flame-retardant work clothing.

Safety Isolation and Lockout/Tagout (LOTO): Strictly adhere to the “Five Safety Preventions” (De-energize, Verify De-energized, Ground, Tag, Lockout) safety regulations. Establish barricades in the operating area and display prominent warning signs such as “Medium Voltage Danger, Do Not Energize” or “Personnel Working, Do Not Energize.”

Voltage Verification: Before operation, use a reliable voltage detector or the Voltage Presence Indicator System (VPIS) on the switchgear panel to confirm that the operating compartment is indeed de-energized.

Environmental Check: Inspect the operating area to ensure it is dry, clean, free of obstructions, and adequately lit.

Operational Steps and In-Process Monitoring

Adherence to Procedures: Operators should strictly follow each step outlined in pre-approved operating tickets or standard operating procedures, avoiding skips or unauthorized sequence changes.

Sequential Panel Access: Open and close switchgear doors and partitions in the sequence prescribed by the manufacturer, ensuring that interlocks function correctly.

Continuous Monitoring: During operations, closely observe all instrument indications on the switchgear (e.g., ammeters, voltmeters, SF6 gas pressure gauges where applicable). Listen for any abnormal sounds (e.g., discharge noises, unusual clicks) and watch for any unusual odors or smoke.

Position Confirmation: After each operation is completed, it is essential to confirm the correct execution of the action using mechanical indicators, electrical indicators, or open/closed status lights.

Handling Anomalies and Emergency Response

Immediate Stop and Isolation: Should any anomaly be detected during operation (e.g., loud noises, sparks, smoke, burning smell, or abnormal instrument readings), immediately cease operation and promptly isolate the equipment safely (e.g., quickly opening the circuit breaker), cutting off relevant power.

Initial Assessment and Reporting: Based on personal knowledge and experience, make an initial assessment of the anomaly and immediately report it to the shift supervisor or relevant technical personnel.

Adherence to Emergency Plans: Strictly follow the power system’s established emergency response plans. Do not attempt to resolve faults that are beyond one’s authorized scope of knowledge or capability.

Preserve the Scene: Secure the incident scene to allow for subsequent investigation and analysis by expert personnel.

Operation Records and Regular Maintenance

Detailed Record Keeping: After each operation, it is advisable to meticulously document the operation, including the time, operators involved, specific actions, equipment status, instrument readings, and any observed anomalies.

Establishing Maintenance Records: Regularly inspect and maintain MV switchgear. This includes:

For SF6 Gas Insulated Switchgear: Check SF6 gas pressure to ensure it is within normal ranges, and promptly address any pressure anomalies or leaks.

Inspect insulating rods, insulators, busbars, and other insulating components for cracks, damage, contamination, or signs of discharge.

Periodically conduct insulation strength tests (e.g., DC withstand voltage tests, partial discharge tests) to assess insulation condition.

Check the mechanical operating mechanisms of key components such as circuit breakers, disconnectors, and earthing switches for flexibility and reliability. Also, inspect contacts for signs of overheating or burning.

Conclusion of Operating MV Switchgear

Safe operation of medium voltage switchgear is a systematic undertaking that demands a high degree of responsibility, professional knowledge, and a rigorous attitude from operators. By deeply understanding key operational points, fully utilizing the inherent safety interlock mechanisms designed into the switchgear, and combining these with regular maintenance and continuous knowledge updates, it is possible to maximize the safe and stable operation of power systems and extend the equipment’s lifespan.

As one of the specialized MV switchgear manufacturers, Liyond deeply understands the importance of standardized operation. Our products, from their design to manufacturing, strictly adhere to international and domestic standards, incorporating advanced safety interlocks and reliable operational mechanisms. Our goal is to provide users with high-quality, easy-to-operate, safe, and reliable MV electrical equipment solutions. Should you encounter any issues during operation or require further technical support, please feel free to contact Liyond’s online customer service or professional technical team via this website.