Lowest cost of ownership First class solutions Standards assured. Acrastyle has provided hundreds of power system protection systems for companies in the transport, petro-chemical, process, steel, water, gas and other industrial sectors Many industrial installations have their electrical power systems protected and controlled by designs manufactured by Acrastyle.
Leave a Reply Cancel reply Your email address will not be published. Relevant information Utilities Renewables Transport. An overload occurs when an electrical circuit, whether by the original design of a new circuit or by modification of an existing circuit, is required to convey load current in excess of the rated-load ampacity of the circuit conductors.
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For example, a amp branch circuit is modified with an additional lamp, which increases the load current to 22 amps: this would be a circuit overload. Overload conditions can occur at the service, feeder, or branch-circuit level of a building's electrical-power distribution system. An electrical overload overcurrent also occurs when a motor is mechanically overloaded. Overload is a controlled overcurrent situation, normally of low magnitude.
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Short-circuit overcurrent normally involves an accidental cross-connection of at least two circuit conductors supply and return. A single primary winding in the transformer supplies by induction two volt windings wired in series in the secondary. A utilization-equipment load will operate at volts when connected between the two ends of the two series-connected volt windings.
A utilization-equipment load will operate at volts when connected between either end of the two series-connected volt windings and the third wire shared by the two windings see Figure 1. Ground-fault overcurrent is also a short-circuit condition that normally affects only one of the circuit conductors and the grounded metal raceway or electrical distribution or utilization equipment enclosure.
Ground-fault overcurrent can occur only if the electrical power distribution system of the building or structure is referenced to earth ground. The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. The short circuit can be across two or more transformer single-phase AC windings. The ground-fault overcurrent normally affects only one single-phase AC winding in the transformer supplying power to the faulted condition.
When choosing the level of voltage, several decisions must be made. Besides the cost of the project, one of the most important aspects is safety. Years ago, electricians routinely worked on energized equipment, and not only low-voltage LV; 1, V or less equipment but also on MV equipment. This practice has been very much limited because it is very dangerous. Where maintenance activity is still performed on energized gear, safety is the primary concern.
Designing medium-voltage electrical systems
To address safety, NFPA: National Electrical Code NEC Article Requirements for Electrical Installations, requires certain working space clearances around the electrical equipment—the higher the nominal voltage, the greater the required clearance. Equipment maintenance is another factor when deciding the electrical system voltage level. If the maintenance team is already trained in certain voltage-type equipment, it makes sense to continue using that same voltage level. Otherwise, additional training will be necessary.
Using an MV distribution system has several advantages as compared with LV distribution. Voltage and current have an inverse relationship. Given a certain demand for power, the higher the voltage, the lower the current, based on the equation:.
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Sometimes the distance is not the issue, but rather, the amount of power to be distributed. Residential buildings do not have a great need for power, so the use of LV suits them well. But commercial clients routinely ask for great amounts of power. If distributing this power at LV V, for example , the facility would need to accommodate almost 14, amps. That's an enormous amount of current, which requires an enormous amount of wiring. In comparison, the same 12 MVA would only produce about amps at This lower-current solution gives the owner the flexibility of delivering the power through the building as close to the load as possible and then stepping the power down to LV for consumption.
Choosing to distribute the electrical power via MV also helps minimize power losses, which adds to operation savings.
The inverse is true as well: the lower the voltage, the higher the current. The MV system delivers the same power quantity via a smaller amount of current in comparison with LV.
Lower current levels also result in lower power losses, and as a consequence, lower voltage drop. Lower voltage drop makes power distribution to greater distances possible. It is very common for a campus arrangement to have a If the distances from the main utility substation of the campus to the individual buildings are great, higher voltages could be used, but the Other common voltages are When designing an MV distribution system, special attention must be given to equipment dimensions, ratings, and their clearances. The equipment dimensions are greater for MV systems as compared with LV systems.
Therefore, space dedicated to equipment becomes very important and should be allocated early in the design process. Table 1 shows a comparison of the electrical equipment for two very common voltage systems, V and The working clearances around the MV equipment are also greater than the clearances of the LV equipment. NEC Article describes the minimum working clearances around the electrical equipment.
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Table 2 compares the working clearances for the same two distribution systems as those listed in Table 1. Condition 1 is satisfied when there is an exposed live part on one side but no live or grounded parts on the opposite side of the working space. If there are live parts on both sides, Condition 1 is satisfied only if the parts are guarded by insulating materials. Condition 2 applies when there are exposed live parts on one side of the working space and grounded parts on the other, with concrete, brick, and tile considered to be grounded.
Condition 3 is the worst-case scenario with exposed live parts on both sides of the working space. If the MV equipment is outdoors, it should at least be confined by a fence, which, depending on the voltage level, should be a minimum of 10 feet away from live parts or the enclosure. For a nominal Refer to NEC Article MV equipment does not have the same flexibility as LV equipment. For LV systems, there are circuit breakers of all sizes, and larger breakers are equipped with easily adjustable trip units.
For simple MV systems, fused switches can be used for protection, and these fuses come in many sizes as well.
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However, in complex MV distribution systems, such as in mission critical facilities, using MV breakers becomes a necessity. The smallest circuit breaker for a nominal The next size up is 2, amps, then 3, amps. As previously mentioned, the great advantage of MV systems is that the current is low, but there is currently no breaker small enough for these systems.
This circuit breaker is what we call a "dumb" breaker. It is called dumb because it is not equipped with any intelligence, and it does not know when to clear a fault. For that reason, relays are used. Relays offer excellent protection capabilities and schemes, but that does not take away the fact that the smallest MV circuit breaker, at 1, amp, is very often too large for the amount of current that goes through. This lack of flexibility has financial implications that must be considered.
Fault protection for MV systems becomes important because of the implications of a protection failure. A 1,amp circuit breaker rated at V could carry close to 1 MVA worth of power if rated and loaded.