Voltage Classification of LV, MV and HV
In this blog, we present the definition of LV, MV and HV, the differences between voltages and their usage areas and more for you.
What is the definition of Low Voltage, Medium Voltage and High Voltage?
Electrical power systems can be divided into three main categories: generation, transmission and distribution. Because efficiency considerations each of these categories are operated in different voltage levels. Electricity generation is performed for the most efficient generation voltage for that generation type. Then usually, stepped-up to Extra-High Voltage (EHV) for transmission and this level stepped-down to High-Voltage (HV) level at the sub-transmission. As the energy arrived at the distribution area (where the energy is distributed industrial, commercial, or household consumers) the High-Voltage level is then stepped-down again to Medium-Voltage (MV) level for the distribution and finally if the load point is a household or a commercial consumer, Medium-Voltage is stepped-down to Low-Voltage (LV) level and delivered to the consumers [1, 2]. Medium-Voltage systems can be thought as circulatory system of the whole electrical grid as they are usually the linkage between transmission (HV) and Low-Voltage stages.
Voltage Classification of LV, MV and HV
As it can be understood from the above section Low-Voltage, Medium-Voltage and High-Voltage systems are classified using the voltage levels they are being operated on. There are two systems of Medium-Voltage and Low-Voltage levels: North American (mainly USA) and European , these ratings are:
For North America:
· MV: 4kV to 35kV
· LV: 120 V
· MV: 6kV to 33kV
· LV: 230V
Difference Between High, Medium and Low Voltage
Although it seems like the only difference between these systems are the voltage rating, they are operated together but design considerations and topologies of the systems are significantly different . Because of the voltage and current levels that they are being operated on; while equipment and material that are used in Medium Voltage systems are significantly different from the ones that are used in Low Voltage, they can be like the ones that are being used in High-Voltage. Medium-Voltage circuit breakers that are used in past are oil-type whereas the ones that are currently used are SF6 gas-type or vacuum type. The main reason for this is that the oil-types have a high risk of fire and fault and thus require more maintenance. Generally, Medium Voltage switchgear are placed in the same area with the other switchgear. While this area is usually have air-cooling in some systems, again, SF6 gas might be used.
Medium Voltage and Industry Usage
Medium Voltage systems, which you may have heard of as distribution systems, can be used in a variety of areas ranging from local distribution substations to industrial plants. Although, Medium Voltage substations can be built in indoor areas, they may be built in outdoors where the space is not a problem .
According to the different consumers, loads and their needs; Medium Voltage systems can have different designs. When designing a Medium Voltage system; economics, present load, present load trend, future load predictions and uninterrupted operation in fault situation are the primary considerations. In general, when the system is going to be used for an urban application, the system is designed in radial organization. This way, if there is an interruption in one of the lines (or branches), system maneuvers are performed and the consumers in the interrupted branch are continued to be supplied . In some industrial plants, because of the total load size and because of supplying some loads with Medium Voltage is more efficient; the transmission line coming from the transmission station is stepped-down with the plants own Medium Voltage substation and then distributed to the plant. Apart from this scenario, Medium Voltage lines that are coming out from Medium Voltage distribution stations, are stepped-down at the substations that are near to the consumers and further distributed from there. Medium Voltage transformers can be house type or pole mounted type. In urban areas, voltage is stepped-down first in house type primary transformers which are installed in transformer housings that can be easily seen by everyone and then, stepped-down again using pole-mounted secondary transformers and fed to the households. Medium Voltage transformers have the power ratings between 16kVA to 2500kVA and are either single-phase or three-phase. These transformers, generally have two windings per phase and are oil-insulated, natural-cooling type . To prevent any interruption or hazardous accidents, only authorized personnel can enter the enclosed areas of the Medium-Voltage transformers. While in urban areas, the authorization and the access are only on the distribution companies, in industrial plants, personnel who are only authorized and certificated by the internationally recognized institutions can access to and work on the Medium-Voltage transformers.
How Do You Measure Medium Voltage?
In consequence of the high safety risks there must be no interaction with a live Medium Voltage system for performing measurements. But most of the times, online and continuous measurements are required. Therefore, voltage level on the Medium Voltage lines is stepped-down to Low Voltage levels using Current-Transformers (CT) or Measurement-Transformers (MT). As a result, measurement devices can be connected to the output terminals of these transformers where the voltage levels are safer to work with. Also, the output of these CTs and MTs are used to supply permanently installed energy analyzers and control relays.
Medium Voltage Safety Tips
Medium Voltage equipment is generally placed in metal cubicles. In some cubicle’s busbars, circuit-breakers and other switchgear are separated by sheet metal whereas in some they are separated by non-metal materials. In specific type cubicles, equipment is placed in unmountable divisions separately. Due to the high safety risks in Medium-Voltage, the cubicle must not be opened before the system is completely de-energized. Energy presence in these cubicles can be monitored with voltage indicators like ORION EE’s OVI+SR without opening the cubicle. Apart from the fundamental safety measures for working on electrical systems, in Medium-Voltage systems, earthing should be carefully performed to completely de-energize since especially the passive components of the system (such as capacitors and inductors) are able to store energy for short periods of time and only after being sure that the system is completely de-energized and properly connected to the earth, work can start on the system.
Another important safety consideration when working on Medium-Voltage systems is that the Personal Protective Equipment (PPE) that is used for Low-Voltage systems are not suitable for working on Medium-Voltage systems. Therefore, PPEs that are tested on and certified for appropriate voltage levels for Medium-Voltage according to the international standards should be used.
In conclusion, voltage ratings are not the only difference between LV, MV and HV systems. Each have its own purposes for their own application areas and thus, each of them is a specialty area on its own which requires specialized people to design and work on them.
 Weedy, B. M., Cory, B. J., Jenkins, N., Ekanayake, J. B., & Strbac, G. (2012). Electric power systems. John Wiley & Sons.
 Sallam, A. A., & Malik, O. P. (2018). Electric distribution systems.
 Warne, D. F. (Ed.). (2005). Newnes electrical power engineer's handbook. Elsevier.
 Laughton, M. A., & Say, M. G. (Eds.). (2013). Electrical engineer's reference book. Elsevier.