HV, MV AND LV SWITCHGEAR OF POWER SYSTEM

Power System dispatchers have one of the most challenging jobs in the electric power industry. They must have a basic understanding of various types of electric power generation, plant control and systems, protective relaying schemes, transmission switching procedures, automatic generation control, and the fundamentals of economic operation. In addition, they must also have the ability to quickly analyze the emergency situations and take appropriate corrective actions promptly. The control of the power system involves many elements and is one of the major responsibilities of system operators. Included in the elements to be controlled are system frequency, tie-line flows, line currents, equipment loading, and voltage. All must be kept within limits determined to be safe in order to provide satisfactory service to the power system customers and to ensure that equipment is not damaged by overloading or other improper operations. When electricity is generated and fed to the power system, all these factors are controlled by the high voltage, medium voltage and low voltage power  supply to the customers. Switchgear is a system comprised of electrical and control equipment, measurement devices, protection and monitoring components, and communication devices. Switchgears are mainly used in electrical systems used in Industrial facilities, power plants, oil and gas, petrochemical industries to distribute and supply loads within such industries. The load could be motors, ovens, heating and cooling systems and power to the motors, welding machinery, lighting equipment, and etc.

Loads connected to the distribution system in terms of importance are divided into the followings:

Critical loads that any disruption and difficulty in supplying them would might be life threatening

Semi-critical loads that any disruption in feeding them impair the production line, product waste and loss in time and money.

Insignificant loads that supply interruption is not critical.

Therefore, it becomes clear that the design, engineering and construction of switchgears would bring a lot of advantages such as reduced repair costs and less time needed to repair and troubleshooting. Different types and Classification of Switchgear is normally as follows.

1. Low voltage switchgears up to 1 kV voltage.

2. Medium voltage switchgear systems up to 52kV.

3. High voltage switchgear systems from 52 kV to 750 kV voltages (used in high-voltage substation).

A basic function of switchgear power systems is protection of short circuits and overload fault currents while simultaneously providing service continuously to unaffected circuits while avoiding the creation of an electrical hazard. Switchgear of power system also provides important isolation of various circuits from different power supplies for safety issues. Different types and classifications of switchgear of power systems to meet a variety of different needs are explained as under:

All electrical energy whether it be coal, nuclear or renewable, which is utilized globally for residential, industrial, commercial and infrastructural purposes is generated at power stations. The transfer or distribution of electrical energy from a power station to the consumer is an integral part of this chain. This is due to the fact that, without a reliable energy distribution system many residential, industrial and commercial entities would no longer efficiently operate.

The same is also true on a smaller scale, regarding the distribution of energy within an installation. Two crucial components of a power distribution system within a facility are ‘switchboards‘ and ‘switchgear’ devices. Facilities that require heavy industrial applications generally utilize low voltage ‘switchgear’ devices, or ‘switchgear’ power systems, as the heavier electrical load requires more a robust structure.

Low voltage switchgear power systems which comprise both passive components, which refer to the mechanical structure of the device, and active components such as: fuses, circuit breakers and electrical relays, are utilized to make or break electrical connections. Systems such as these have been in existence since the 1930′s.

While not many changes have occurred regarding fuse technology, there have been numerous advances in the technology surrounding Circuit Breakers and electrical relays. Such as the conception of new tripping devices for circuit breakers and more advanced detection devices for electrical relays. Contemporary circuit breakers are consequently: manufactured from much more advanced materials, include digital trip devices and require far less maintenance than their forbearers. Electrical relays in comparison are now fitted with highly sophisticated microcontrollers, which use complex software programs to detect faults.

LOW VOLTAGE SWITCHGEAR

Low voltage switchgear power systems are therefore, contemporarily much more complex entailing not only distribution and protection regarding the electrical load, but additional measurement, and regulation of the system.

Low voltage switchgear power systems in an industrial setting provide continual power to as much of the facility as possible and protect personnel from electrical hazards such as arc blasts. This is highly beneficial as arcing faults have been reported as one of the biggest issues facing the manufacturing industry. Arcing faults may not seem to be that significant however, in 2008 alone; over 2000 people were treated for severe burns steaming from workplace arc blasts.

Protection from arcing faults is mainly undertaken via various preventive measures such as remote real-time monitoring and diagnostics that allow workers to remain distanced from equipment. Recent technological advances have however, also reduced the amount of time taken for a system to register a fault and deny energy to the faulting component.

Whilst these types of power systems have advanced greatly since their inception, these advances don’t compare to the huge developments that have been seen in both high and medium voltage systems. Many experts consequently propose that the technology involved in low voltage switchgear systems will soon undergo further improvement, especially regarding faster interruption of faults.

MEDIUM VOLTAGE SWITCHGEAR:

The electricity industry is conservative. Among the reasons for this is the fact that the lifetime of medium voltage and high voltage switchgear is around 40 years. Transmission system operators (TSOs) and distribution network operators (DNOs) need stability. Maintenance and repair of such long-life devices needs to be ensured. And of course, work is easier for service crews if there is no change in technology.

All elements of a medium voltage installation are subject to evolution

In a substation are found all three categories of components of protection chains: sensors, protection relays and circuit breakers (CBs). Traditionally, the design of these components has evolved independently, but with some constraints at interfaces to ensure interoperability. Protection relays are particularly sensitive to the type of signal coming from current transformers. Some associations are possible; others are not. For example, you may connect old technology 5A CTs to most modern protection relays, but the opposite — connecting an LPCT to an old electromechanical relay — is impossible. Electrical switchgear needs an insulation medium for two different functions: current breaking and isolation between conductors or between conductors and earth. For current breaking, the available technologies are air, oil, SF6, and vacuum. To isolate conductors, the same technologies may be used plus solid insulation. All elements of a medium voltage installation are subject to evolution Available technologies for electrical switchgear

EVOLUTION OF CIRCUIT-BREAKER TECHNOLOGIES

The first technology used for breaking in CBs was air. These CBs were big because the principle of breaking was a large expansion of the arc and noisy because of the breaking in the air. They needed much maintenance and, for that reason, were withdrawable. In an effort to reduce the footprint, oil CBs came next. However, they also needed much maintenance, for example to change oil after some operations. Additionally, oil breakers are not safe to operate because of the fire risk. Oil CB failures can easily result in a fatal accident among operators and the public. In the late sixties came SF6 and vacuum circuit-breakers. Both technologies brought many similar advantages. They are compact, thanks to vacuum or SF6 insulation. They are much safer, drastically reducing fire risk. They became more and more reliable. Electrical endurance has been increased, thus CBs were able to perform a much higher number of fault and load breakings. As a consequence of the improved reliability, maintenance is less and less required and we can consider that state-of-the-art CBs are now almost maintenance-free. Often, they remain withdrawable because of installation in traditional metal enclosed panels. From 1930 to 1950, most of the MV switchboards were in fact an assembly of fixed components in an electrical room connected to visible busbars. Only simple wire fencing prevented to access the live parts. Then, because of more safety awareness, switching components and busbars were integrated in metal-enclosed cubicles. Doors and sheet plates and frames were earthed to avoid any accident from direct or indirect contact with live parts. Busbars and connections were air insulated. There were several generations of metal-enclosed air-insulated switchgear (AIS) cubicles. The first generation, from 1950 to 1970, integrated withdrawable air or oil CBs. The second generation, from 1970 to 1990, integrated withdrawable SF6 and vacuum CBs. Another step in safety was introduced in the current, third generation of metal-enclosed cubicles, which began in 1990. This new generation introduced internal arc withstands capability to protect people standing in front of the switchboard in case of an extremely rare internal fault. Generally, CBs are withdrawable and installed in cassette to allow wall mounting and front access cables. But more recently, in the 1990s, fixed CBs were also used. This change was possible with modern highly reliable CBs and new testing facilities of the protection relays.

 EVOLUTION OF PRIMARY DISTRIBUTION SWITCHGEARS

There are some recent variants in metal-enclosed cubicles with fixed CBs, where the insulation of busbars and all components, including CBs and connections, are made with epoxy or some other resin. These panels are generally called a solid insulation system (SIS). However, always looking for better electricity availability, utilities started to require more and more insensitivity to ambient environmental conditions. And, all AIS and SIS panels are still sensitive to environmental conditions if not properly installed in protected rooms. That was the reason for the arrival of metal-enclosed gas-insulated switchgear (GIS) in the 1990s. All components, busbars, and connections are fitted in one or several hermetically sealed tanks filled with SF6. Thanks to SF6 insulation, this type of equipment is very compact. Both AIS and GIS panels coexist today. The final choice may differ for each application, depending on the importance given to many criteria such as compactness, insensitivity to the environment, the availability of high performance, criticality of the application, power restoration mode in case of failure, ergonomics of operation, and/or ergonomics of cable testing.                                                                          

CONCLUSION

The development of smart grids will result in the inclusion of more intelligence in MV equipment. This network evolution may be the opportunity to introduce new criteria for the choice of products, such as flexibility, insensitivity to harsh environments, compactness, optimization of remote control, etc. In conclusion, the physics are the same but some technological points are changing as well as the way to optimize them. For all these reasons, there is a great confidence that the 2SIS modular architecture using the three-position scheme and vacuum interrupters is very well adapted for the coming deployment of smart grids. This architecture can address a large number of applications in secondary distribution but, thanks to its modularity, can also challenge some low-end applications where, traditionally, primary equipment is used. In this respect, this architecture is able to bridge the gap between secondary and primary specialized equipment.

HIGH VOLTAGE SWITCHGEAR

In recent years there here has been a great deal of activity in the power industry in construction of extra high voltage (EHV) lines and stations. EHV is usually considered to be lines and equipment operating above 230 kv. A considerable amount of 345-kv is in existence and 500-750 kv have been built. Research is in progress for transmission of power at voltages up to 1000 kv. As the amount of power to be transmitted increase and as the distances between the point of generation and load become greater, it becomes necessary to increase the voltages. Factors limiting transmission voltage are the availability of transformation equipment, line insulation, and switching devices capable of operating at extremely high voltages.

When it became apparent that higher transmission voltages would be required, electrical equipment manufacturers and power utilities initiated research projects to develop Switchgear, transmission equipments and methods of insulating and constructing lines to operate in the 500-to 750 kv range. For many years, automatic circuit breakers used in power transmission systems were oil-filled. At so called EHV levels, oil- filled circuit breakers became impractical, and interrupters were developed that operate at line potential on insulated columns. In order to interrupt voltages of 500 kv and higher, several interrupters are usually connected in series. For quenching the arcs resulting when a power circuit is interrupted in these applications, high pressure dry air or a gas such as sulphur hexafluoride (SF6) in place of insulating oil.

MANUFACTURERS OF SWITCHGEAR

Many manufactures of different countries of the world are providing the widest range of Medium to Ultra High Voltage (UHV) switchgear products, including Gas Insulated Switchgear (GIS) and Dead Tank Circuit Breaker (DTB) to meet the requirements of power generation and distribution substations in utilities and industries. These switchgears are in service for several years in many countries, as

·        Gas Circuit Breakers

·        Ring main units

·        Disconnectors

·        Surge Arrestor

·        Gas Insulated Swithchgears

The reliability of these switchgear depends upon the routine scheduled and preventive maintenance carried out by the Grid System Operators .After recommended Operations, switching, both in normal and Tripping, In severe fault conditions, the maintenance and routine inspection and replacement of parts becomes necessary to avoid total breakdown of the power system. Usually breakdown maintenance is carried out, which means there is no care for the delicate equipments which may damage in this case. Now there are vast concepts of maintenance, as preventive and perceptive maintenance, which can’t be ignored for the reliability of the power system, therefore this article will remain incomplete without the mention of maintenance of the switchgears.  

MAINTENANCE OF SWITGGEAR

HV and LV switchgear is an essential, yet potentially hazardous, element in the provision of power to your business and involves extensive levels of knowledge and expertise. Annual switchgear inspections and switchgear maintenance schedules can be performed on all types of HV and LV equipment, including HV pressure testing, primary and secondary injection, infra-red thermographic surveys and insulating oil sample testing. A 24-hour call-out service, consultancy, technical advice and facilities management of HV rings, controls and associated LV equipment is required for the switchgear of power system. Other switchgear services include on-site investigation, emergency repair and site surveys.

The safety and reliability of electrical switchgear equipment installed in public buildings throughout the world is often overlooked with circuit breakers, for instance, providing the backbone of power distribution infrastructure. The replacement of digital solid state trip units designed to extend the life of power circuit breakers is very important for reliability of the power system.

 Reliable performance is essential for your all your EHV, HV, MV and LV Switchgears installed in the power System. High Voltage preventive maintenance of switchgear is essential to help keep your electrical power operating without interruption in emergency situations.

On-site at facilities, High Voltage Switchgear preventive maintenance ensures the reliability and integrity of electrical equipments. High Voltage switchgear testing and repairs can save time and money. Don't defer or delay maintenance, before another failure occurs. I shall tell you a great secret my friends. Do not wait for the last judgment , it takes place every day.

In general, HV switchgear has a proven record of reliability and performance. Failures are rare but, where they occur, the results may be catastrophic. Tanks may rupture and, with oil-filled switchgear, this can result in burning oil and gas clouds, causing death or serious injury and major damage to plant and buildings in the vicinity. Failures of switchgear can also result in serious financial losses.

Retrofitting old oil insulated switchgear is a proven highly cost-effective and reliable process which once installed reduces HV maintenance and the overall operational costs of all switchgears by means of reducing shut down times and ultimately improving existing system reliability and operational safety.

SULTAN ZAFAR