Medium voltage switchgear is an essential component in the interconnection of large solar PV systems. It serves as a central hub for the distribution and control of electrical energy generated by solar panels. In this article, we will discuss the various types of medium voltage switchgear, their functions, and how they are used in solar interconnection systems.
Medium voltage switchgear is a type of electrical equipment that is used to control, protect, and isolate electrical equipment. It is used in medium voltage power systems, typically between 1 kV and 38 kV. The main components of medium voltage switchgear include circuit breakers, power fuses, fusible switches, disconnects, isolators as well as control and metering equipment. These components work together to protect the power system from damage and to ensure a safe and efficient flow of electrical energy.
One of the most common types of medium voltage switchgear used in DER interconnection systems is Metal-enclosed switchgear (MES). The main function of Metal-enclosed switchgear (MES) in large solar interconnections is to provide a safe and reliable means of distributing electrical power from the solar panels to the utility grid. It also provides protection against electrical faults and power outages, by automatically (or manually) disconnecting the power in the event of an abnormal condition. MES also provides a means of isolating the power supply for maintenance or repair work, by providing a main disconnect switch.
MES in large solar interconnections consists of several key components, including a main service disconnect switch, a set of circuit breakers or fuses, and protection relays. The main service disconnect switch is used to disconnect power to the solar power plant, while the circuit breakers or fuses are used to protect the electrical equipment and wiring from overcurrents and short-circuits. Protection relays are used to provide protection and control of the solar interconnection to the grid.
One of the main advantages of MES in large solar interconnections is its ability to provide a high degree of protection and safety. The metal enclosure provides a barrier against electrical hazards and weather, while the main service disconnect switch allows for easy isolation of the power supply in the event of an emergency. Additionally, MES is typically designed to comply with various industry standards and safety codes, such as the National Electric Safety Code (NESC), National Electrical Code (NEC), ANSI, OSHA, NEMA, IEEE, local codes, and utility which helps to ensure the safety and reliability of the equipment.
Another advantage of MES in large solar interconnections is its flexibility and adaptability. It can be easily configured to suit the specific requirements of the solar power plant, such as the number of circuits and the type of load. MES can also be easily expanded or upgraded as the electrical demand of the solar power plant changes.
MES also has a relatively low maintenance requirement. The components of MES are designed to have a long service life, and regular maintenance, such as cleaning and tightening connections, can help to prolong the lifespan of the equipment. The equipment is also designed to have a high level of reliability and availability, which can help to reduce downtime and minimize the impact of power outages.
Medium voltage switchgear is used in DER interconnection systems to control the flow of electrical energy generated by DER. The switchgear is typically located at the point of common coupling (PCC), which is the point at which the solar power system is connected to the utility grid. The switchgear is used to control the flow of energy between the solar power system and the utility grid, ensuring that the solar power system is operating safely and efficiently. The switchgear is also used to protect the solar power system from damage. For example, if there is a fault in the system, the switchgear will automatically isolate the faulted section of the system, preventing further damage and ensuring a safe and efficient flow of electrical energy.
In addition to controlling and protecting the solar power system, medium voltage switchgear is also used to monitor the performance of the system. It is equipped with various sensors and monitoring devices that measure the voltage, current, and power flow in the system. This data is used to optimize the performance of the solar power system and to identify any potential problems.
Medium voltage switchgear serves as the primary point of control and protection for medium voltage circuits. It is used to control and switch electrical power in medium voltage systems, typically ranging from 2.4kV to 38kV. The most important components of medium voltage switchgear include the circuit breaker, disconnect switch, and protective relay. Each component plays a vital role in the operation and protection of the electrical power system.
The Circuit Breaker is the primary component of medium voltage switchgear, used to open and close the circuit under normal and abnormal conditions. The circuit breaker is designed to interrupt the current flow in the event of a short circuit or ground fault, protecting the electrical system from damage. Circuit breakers can be either air-break or vacuum-break type. Air-break circuit breakers use compressed air to extinguish the arc, while vacuum-break circuit breakers use a vacuum to extinguish the arc. Air-break circuit breakers are generally less expensive and easier to maintain than vacuum-break circuit breakers, but vacuum-break circuit breakers have a longer life and are more reliable.
The Disconnect Switch is another important component of medium voltage switchgear. It is used to disconnect the circuit breaker from the electrical system, allowing maintenance and repair to be performed safely. The disconnect switch is typically located near the circuit breaker and is operated manually or remotely. Disconnect switches can be either fusible or non-fusible, depending on the application. Fusible disconnect switches are designed to open automatically in the event of an overcurrent, while non-fusible disconnect switches are designed to be opened manually.
The Protective Relay is the third component of medium voltage switchgear. It is used to monitor the electrical system and detect abnormal conditions, such as short circuits or ground faults. The protective relay is typically located near the circuit breaker and is connected to the electrical system through a set of contacts. When an abnormal condition is detected, the protective relay will send a signal to the circuit breaker, causing it to open and interrupt the current flow. Protective relays can be either electromechanical or microprocessor-based, depending on the application.
In addition, protection systems such as arc flash protection and ground fault protection are also important for the safety of personnel working on or around the switchgear. Arc flash protection systems are designed to detect and mitigate the risk of arc flash hazards, which can cause severe burns or even death. Ground fault protection systems are designed to detect and interrupt ground faults, which can also be a serious safety hazard
There are several types of bus-bars used in medium voltage switchgears, including single-phase bus-bars, three-phase bus-bars, and multi-bus-bar systems. Single-phase bus-bars are typically used in low-voltage systems, while three-phase bus-bars are more commonly used in medium voltage switchgears. Multi-bus-bar systems are used in large power stations and substations, allowing for multiple power sources to be connected to the system. The main function of bus-bars is to transmit electrical power from one point to another within a switchgear system. They are connected to other electrical components, such as circuit breakers and transformers, and act as a central hub for the distribution of power throughout the system. Bus-bars are also responsible for maintaining the voltage level within the system, ensuring that it remains within safe operating limits.
The main bus and taps typically have minimum continuous current rating of 600 or 1200 amperes, as required by the load (or generation for DER). Bus-bars are made from conductive materials, such as copper or aluminum, which are able to withstand high temperatures and electrical loads without degrading. This makes them more durable than other components, such as wires and cables, and reduces the likelihood of system failure. Bus-bars are also customizable to suit the specific requirements of any medium voltage switchgear system. They can be made in different shapes and sizes, and can be coated with various materials, such as paint or plastic, to protect them from corrosion and other environmental factors.
The bus-bar clearance refers to the distance between the conductive parts of the switchgear, such as the busbars, and any other conductive parts, such as the enclosure or other equipment. The bus-bar clearance is important for safety, as it helps to prevent electrical arcing and potential for electrical shock. The bus-bar clearance is typically specified in millimeters, and is determined by the voltage level and the type of switchgear being used. For example, in a medium voltage switchgear, the bus-bar clearance is typically between 50 mm and 100 mm.
Fuses are an important component in medium voltage switchgear, as they protect the electrical system from overloads and short circuits. The size of the fuse is determined by the maximum current that the electrical system is expected to carry. For example, a medium voltage switchgear may use power fuses from Cutler-Hammer Company “RBA-400” (13.2 kV & 33 kV) and S&C Electric Company “SM-5S” (13.2 kV & 33 kV), “SM-40” (13.2 kV). In regards to the electronic fuses, utility comany may approve S&C Electric Company “Fault Fiter” (13.2 kV). The fuse size is also determined by the voltage level of the system and the type of switchgear being used.
The metering compartment is another component of medium voltage switchgear. This is a separate compartment within the switchgear that is used to house the metering equipment, such as the CTs and PTs (2). The metering compartment is typically designed to be easy to access, so that the metering equipment can be easily maintained and replaced if necessary.
Current transformers (CTs) and potential transformers (PTs) are also used in medium voltage switchgear. CTs and PTs are used to measure the current and voltage in the electrical system, respectively. The CT and PT ratios are determined by the voltage level and the type of switchgear being used. For example, a medium voltage switchgear may use CTs and PTs with ratios of 600/5A or 1200/1A.
Surge arresters are devices that protect electrical equipment from damage caused by overvoltage transients, also known as surges or lightning strikes. These transients can occur due to a variety of causes, including lightning strikes, switching operations, and faults in the power system. In medium voltage metal-enclosed switchgear, surge arresters are used to protect the switchgear itself as well as the equipment connected to it.
Surge arresters are essentially voltage-limiting devices that are designed to allow normal voltage levels to pass through while diverting or absorbing overvoltage transients. This is typically achieved through the use of non-linear voltage-dependent components, such as metal oxide varistors (MOVs) or gas-filled tubes. When a surge arrester is installed in a medium voltage metal-enclosed switchgear, it is typically located at the point where the power enters the switchgear. This allows the arrester to protect not only the switchgear itself, but also the equipment connected to it. The surge arrester is connected in parallel with the equipment to be protected. This means that the arrester and the equipment share the same voltage level.
The most common type of surge arrester used in medium voltage metal-enclosed switchgear is the metal oxide varistor (MOV) based arrester. MOVs are non-linear voltage-dependent devices that have a very high non-linearity, which allows them to effectively divert overvoltage transients. MOVs are relatively inexpensive, have a long service life, and are relatively easy to install. The key parameters of the arrester are its voltage rating, energy rating and discharge current rating. The voltage rating is the highest continuous operating voltage at which the arrester can operate without suffering damage. The energy rating is the amount of energy that the arrester can safely dissipate without suffering damage. The discharge current rating is the highest current that the arrester can safely interrupt without suffering damage.
Another important aspect of surge arresters used in medium voltage metal-enclosed switchgear is the need for proper maintenance. MOV-based arresters can degrade over time, which can lead to a reduction in their protective capabilities. To ensure that the arresters are functioning properly, it is important to regularly perform visual inspections and to test the arresters to verify their performance.
The ground bus, also known as a grounding busbar, is a conductive metal bar that is used to connect all of the grounding points within the switchgear to a common grounding point. This allows for the safe and efficient distribution of electrical energy and helps to prevent electrical hazards. The ground bus is typically made from copper or aluminum and is connected to the switchgear's frame, which acts as the ground reference. This allows for the effective dissipation of any electrical current that may be present in the system, preventing the buildup of dangerous voltages.
The ground bus is also used to connect the switchgear to an external grounding system, such as a grounding rod or grid. This provides an additional layer of protection against electrical hazards and helps to ensure that the switchgear is properly grounded. In addition to its safety functions, the ground bus also plays an important role in the performance of the switchgear. It helps to ensure that the electrical system is operating at the correct voltage and helps to prevent equipment damage.
A medium voltage metal-enclosed switchgear typically includes several sections, including:
1. Incoming feeder section: This section contains the main switchgear and protection devices that control and protect the incoming power supply to the switchgear. This can include circuit breakers, fuses, and other protective devices.
2. Utility metering section: This section contains the metering equipment that is used to measure and record the amount of power being supplied by the utility company. This can include energy meters, power factor meters, and other metering devices.
3. Customer metering section: This section contains the metering equipment that is used to measure and record the amount of power being consumed by the customer. This can include energy meters, power factor meters, and other metering devices.
4. Outgoing sections: This section contains the main switchgear and protection devices that control and protect the power supply to the load. This can include circuit breakers, fuses, and other protective devices.
Each section of the switchgear is designed to perform a specific function and is typically isolated from the other sections to prevent damage or malfunction in one section from affecting the operation of other sections. The main switchgear and protection devices in each section are typically designed to handle the specific voltage and current levels that are present in that section.
Metal-enclosed switchgear (MES) can operate both manually and automatically. The specific mode of operation depends on the design and configuration of the equipment.
Manual operation of MES typically involves the use of manual switches or breakers to open or close the electrical circuits. This is done manually by a operator using a handle or lever to physically open or close the switch or breaker. This is typically used for switching on and off the power to a building or facility and also as a means of isolating the power supply for maintenance or repair work.
Automatic operation of MES, on the other hand, uses protection relays, microprocessors or other electronic devices to automatically open or close the electrical circuits in response to certain conditions or signals. For example, protection relays can be used to automatically open a circuit breaker in case of an overcurrent or short-circuit, protecting the electrical equipment and wiring from damage. Additionally, a microprocessor-based control system can be used to automatically control and monitor the operation of the switchgear, such as automatic reconnection to the grid and automatic load shedding during abnormal conditions.
The choice of manual or automatic operation depends on the specific application and requirements of the system. Automatic operation is typically used for protection and control functions, while manual operation is typically used for switching on and off the power. In most cases, MES include both manual and automatic operation to provide flexibility and reliability.
Utility companies may require a variety of documents in order to review and approve a medium voltage metal-enclosed switchgear installation. These documents may include:
1. Equipment submittals: This may include detailed technical specifications, manufacturer shop drawings, and test reports for the switchgear, as well as any associated components such as breakers, busbars, and control systems.
2. Engineering analysis: This may include load flow studies, short-circuit calculations, and coordination studies to ensure that the switchgear will function properly and safely within the existing electrical system.
3. Safety and compliance documentation: This may include certifications and test reports demonstrating that the switchgear meets relevant safety and performance standards, such as UL 891, ANSI C37.20.3, and IEEE C37.20.2.
4. Installation and maintenance manuals: These manuals should provide detailed instructions on how to properly install, operate, and maintain the switchgear, and should be provided to the utility company for review.
5. System one-line diagrams and schematics: These diagrams should provide a clear overview of the switchgear and its connections to the rest of the electrical system, as well as any protective device settings.
6. Power system studies and analyses, such as Arc flash hazard analysis and coordination study.
7. Field inspection and test reports, such as megger test, hi-pot test, etc.
The specific documentation requirements may vary depending on the utility company and the jurisdiction in which the switchgear is being installed. It is important to consult with the utility company and a qualified electrical engineer to ensure that all necessary documentation is provided and that it meets the utility company's requirements.
Testing and maintenance of medium voltage (MV) switchgear is crucial for ensuring the safe and reliable operation of electrical power systems. Periodic testing and maintenance of MV switchgear is necessary to identify and correct any issues before they lead to equipment failure or even a power outage. This can include visual inspections, electrical tests, and functional tests to check for proper operation and compliance with safety standards.
One of the most important tests for MV switchgear is the insulation resistance test, which measures the resistance between the different parts of the equipment, such as the live parts and the earth. This test can identify any issues with the insulation of the equipment, such as cracks or damage, that could lead to electrical leakage or even a power outage.
Another important test is the continuity test, which checks the connections and wiring of the equipment to ensure that they are properly connected and functioning correctly. This test can identify issues such as loose connections or broken wires, which could lead to equipment failure or poor performance.
Functional testing is also an important aspect of MV switchgear maintenance. This includes checking that all of the switchgear's functions are working correctly, such as the operation of the breaker, protection relay and other control devices. In addition to these electrical tests, visual inspections of MV switchgear are also important. This includes looking for signs of wear and tear, such as rust or corrosion, as well as checking for any physical damage to the equipment.
Regular cleaning of MV switchgear is also necessary to ensure proper operation. Dust and debris can accumulate on the equipment over time, which can cause overheating and equipment failure. Cleaning the equipment can also help to prevent the buildup of harmful gases and chemicals that can corrode the equipment. It is also important to check all the safety devices, such as the safety switch, earthing system and any other protection devices that are installed to protect the switchgear. These should be tested regularly to ensure that they are functioning correctly and can activate in case of an emergency.
In addition to these regular testing and maintenance tasks, it is also important to have a preventative maintenance program in place. This can include scheduled maintenance tasks, such as oil changes and component replacements, to ensure that the equipment is always in good working order.
Overall, testing and maintenance of medium voltage switchgear is essential for ensuring the safe and reliable operation of electrical power systems. Regular testing and maintenance can help to identify and correct any issues before they lead to equipment failure or power outages, and can help to prolong the lifespan of the equipment.
It is highly recommended to have the maintenance and testing done by a qualified and certified personnel, who are trained and experienced in working with MV switchgear. This will ensure that the equipment is being properly maintained and that any issues are identified and corrected in a timely manner
(1) DER
DER, or Distributed Energy Resources, refer to decentralized energy generation and storage systems that are connected to the electric grid. Examples of DER include solar panels, wind turbines, and batteries. These resources can be owned and operated by utilities, businesses, or individuals, and they can be used to generate electricity, store it, or both.
DER is becoming increasingly popular as a way to reduce dependence on fossil fuels and increase the amount of renewable energy on the grid. By generating and storing energy locally, DER can also help to reduce transmission and distribution losses, and improve power quality and reliability. In addition, DER can also help to reduce the need for expensive transmission and distribution upgrades, and it can provide a way to increase the amount of renewable energy on the grid.
DER systems can also be used in conjunction with demand response programs, where customers are incentivized to reduce their electricity use during periods of high demand. This can help to reduce the need for expensive peaker power plants, and it can also help to improve the overall efficiency of the electric grid.
(2) CT and PT
In medium voltage switchgear, the CT (Current Transformer) and PT (Potential Transformer) are used for measuring and protecting electrical equipment. The CT is used to measure current by reducing the current to a more manageable level, while the PT is used to measure voltage by reducing the voltage to a standard level. Both the CT and PT are used to provide accurate current and voltage measurements to protection relays and metering equipment, allowing for safe and efficient operation of the electrical system.