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We are accustomed to viewing electricity, just like any other utility delivered to our home or business. This is the correct way to look at it. Just like water and natural gas, electricity is transmitted and distributed for general purposes. Just as pressure (or the pressure difference between two points) moves water and gas, voltage "moves" electric current. In order to provide power to the end user, it must go through several iterations.
Power supply, power distribution
Electricity is produced by using magnetic energy and kinetic energy. When the magnetic field generated by the permanent magnet is interrupted by the moving coil, a current is induced in the coil. This process is how most electrical energy is generated today. For example, nuclear power plants use nuclear energy to produce high-pressure steam that moves turbine blades. This movement is then transmitted to the rotor of the turbine. The magnetic field of the generator connected to the turbine shaft is used by the moving rotor to generate current in the armature windings. Coal-fired power plants also use the heat of burning coal to generate steam and generate energy through steam turbines, but the efficiency is much lower than that of nuclear power plants. Hydroelectric power plants use the potential energy of falling water to move turbine blades. Similarly, wind turbines use the kinetic energy of the wind to rotate the blades. Solar cell plants do not use turbines, but they use the energy of the sun to stimulate the electrons of special photovoltaic modules to generate direct current (dc). This direct current is then converted to alternating current (ac) by an inverter.
Although there is so much energy that can be converted into electricity, it is impractical and often impossible to build a power plant wherever electricity is needed. To overcome this challenge, power is transmitted from the power source to where it is needed. In order to transition from public transmission lines to end users, utility companies use substations. These substations reduce the transmission level voltage to the distribution level voltage. From these substations (called utility substations), electricity is delivered (distributed) to residential, commercial, and industrial users.
Electricity can be transmitted by DC or AC. The first power station was the Pearl Street Station in New York City (built by the Edison Lighting Company led by Thomas Edison). The power station provides DC power to customers near the power station. However, the problem with DC is that it cannot be transmitted over long distances because it cannot be converted to a higher voltage. Nikola Tesla was convinced that the way to overcome the distance barrier was to alternately use transformers to transmit electricity at a higher voltage. Westinghouse Electric applied for a patent for Tesla's idea and built the first AC transmission line in New York State to transport electricity from Niagara Falls to Buffalo. Today's technological progress makes it possible to transmit HVDC in an economical way-this is likely to be the future direction of development.
The first AC transmission line was built in Serchi, Italy in 1886, with a voltage of 2,000 V for 17 miles. In order to avoid the high cost of conductors required to transmit large currents and the losses associated with large currents, higher voltage transmission lines have been developed. In 1936, the United States laid a 287 kV transmission line: the Hoover Dam to Los Angeles line. Currently in the United States, voltages up to 345 kV are commonly used to transmit power. It is possible to use higher voltages, but the economics must be carefully analyzed, because when the higher voltage levels are reached, the price of the equipment will increase substantially.
There are many reasons for choosing one voltage level over another for power transmission. One main reason is cost. The higher the voltage, the less copper is used for wiring, but the more money is for electrical equipment-this is a balance. Another reason is the length of the lines. For longer power cords, it makes sense to use higher voltages, but this will result in greater wire spacing. Often, the decision will be influenced by the existing transmission lines in a particular area. Using the same voltage system makes it easier to interconnect different lines to the grid, and even if the direct cost is higher, a certain voltage level can be very attractive.
Voltage levels have been standardized, so manufacturers can focus on developing certain types of equipment. ANSI C84.1 defines medium voltage (MV) as "a type of nominal system voltage greater than 1,000 V and less than 100 kV". IEEE 141 (Red Book) refers to ANSI C84.1 when identifying the same voltage level relative to the MV range. Among all the possibilities of voltage levels between 1 kV and 100 kV, the most commonly used standard voltages in the United States are 4,160 V, 12,470 V, 13,200 V, 13,800 V, 24,940 V and 34,500 V (for four-wire systems) and 69,000 VV is used for three-wire systems. Other voltage systems can also be used, such as 2,400 V, 4,800 V, 6,900 V, 8,320 V, 12,000 V, 20,780 V, 22,860 V, 23,000 V, and 46,000 V. Certain voltages, such as 8,4.9 kV and 3.9 kV. kV, which is consistent with the standard motor voltage, so they are the first choice.
Depending on the size of the park, end users will have to choose the voltage level of the distributed power. When choosing a voltage level, several decisions must be made. In addition to the cost of the project, one of the most important aspects is safety. Years ago, electricians often worked on live equipment, not only low-voltage (LV; 1,000 V or lower) equipment, but also medium-voltage equipment. This approach is very limited because it is very dangerous. In the case of still carrying out maintenance activities on energized equipment, safety is the primary issue. In order to solve safety issues, NFPA: National Electrical Code (NEC) Article 110: Electrical installation requirements require a certain working space clearance around electrical equipment-the higher the nominal voltage, the greater the clearance required. Equipment maintenance is another factor that determines the voltage level of an electrical system. If the maintenance team has been trained on certain voltage types of equipment, it makes sense to continue to use the same voltage level. Otherwise, additional training will be required.
Compared with low-voltage power distribution, the use of medium-voltage power distribution system has several advantages. Voltage and current are inversely proportional. Given a certain power demand, the higher the voltage, the lower the current, based on the following formula:
Where P = power, V = voltage, and I = current.
Sometimes distance is not an issue, but the amount of power to be distributed. The demand for electricity in residential buildings is not very large, so the use of low voltage is very suitable. But commercial customers usually require a lot of electricity. Suppose a customer needs 10 MW of electricity (or 12 MVA). If this power were distributed in LV (eg 480 V), the facility would need to accommodate nearly 14,450 amps. This is a huge current and requires a lot of wiring. In contrast, the same 12 MVA can only produce about 500 amperes at 13.8 kV. This low-current solution allows the owner the flexibility to transmit power through the building as close as possible to the load, and then reduce the power to LV for consumption. Choosing to distribute power through MV also helps to minimize power loss, thereby increasing operating costs. The reverse is also true: the lower the voltage, the greater the current. Compared with low-voltage systems, medium-voltage systems provide the same amount of power through a smaller current. The lower amount of current provides smaller conductors and/or fewer conductor sets to distribute power, which results in significant savings. Lower current levels also result in lower power losses, which in turn leads to lower voltage drops. The lower voltage drop allows the power distribution to be transmitted to greater distances. For campus layout, it is very common to have a 13.8 kV power distribution system. The voltage of the building is reduced to 480 V, and the voltage of the central utility building is reduced to 4,160 V and 480 V. If the main public substation of the campus is far away from the buildings, higher voltages can be used, but the 13.8 kV power distribution system is very common. Other commonly used voltages are 12.47 kV, 24 kV and 24.9 kV (nominal 25 kV).
When designing a medium-voltage power distribution system, special attention must be paid to the size, rating and clearance of the equipment. Compared with low-voltage systems, medium-voltage systems have larger equipment sizes. Therefore, equipment dedicated space becomes very important and should be allocated early in the design process. Table 1 shows a comparison of electrical equipment using two very common voltage systems (480 V and 13.8 kV) from the same equipment manufacturer.
The working gap around medium-voltage equipment is also larger than that of low-voltage equipment. NEC Article 110 describes the minimum working clearance around electrical equipment. Table 2 compares the working gaps of the two distribution systems that are the same as those listed in Table 1.
When there are bare live parts on one side of the work space and no live or grounded parts on the other side, condition 1 is met. If there are live parts on both sides, condition 1 can only be met by protecting the parts with insulating materials. When there are exposed live parts on one side of the workspace and grounded parts on the other side, condition 2 applies, and concrete, bricks, and tiles are considered to be grounded. Condition 3 is the worst case of live parts exposed on both sides of the working space.
If the MV equipment is outdoors, it should be at least restricted by a fence, which should be at least 10 feet away from live parts or enclosures, depending on the voltage level. For a nominal 13.8 kV system, the gap should be 15 feet. For details, please refer to Article 110.31 of the NEC.
Medium-voltage equipment is not as flexible as low-voltage equipment. For low-voltage systems, there are circuit breakers of various sizes, and larger circuit breakers are equipped with easy-to-adjust releases. For simple medium voltage systems, fuse switches can be used for protection, and these fuses are also available in a variety of sizes. However, in complex medium-voltage power distribution systems, such as in mission-critical facilities, the use of medium-voltage circuit breakers becomes necessary. The rated current of the smallest circuit breaker for a nominal 13.8 kV system (15 kV switchgear) is 1,200 amperes. The next size is 2,000 amps, and then 3,000 amps. As mentioned earlier, the great advantage of medium voltage systems is low current, but there are currently no circuit breakers small enough for these systems. However, there are 630 amp circuit breakers that can be used in the International Electrotechnical Commission (IEC) system. This circuit breaker is what we call a "dumb" circuit breaker. It is dumb because it is not equipped with any intelligence and does not know when to clear the fault. Therefore, a relay is used. Relays provide excellent protection capabilities and solutions, but this does not eliminate the fact that the smallest medium voltage circuit breaker of 1,200 amperes is usually too large for the amount of current passing. Inflexibility will have financial implications that must be considered.
Due to the impact of protection failures, the failure protection of the medium voltage system becomes very important. A 1,200-amp circuit breaker with a rated voltage of 480 V can carry close to 1 MVA (if rated at 100 and loaded). In contrast, a 1,200-amp circuit breaker with 13.8 kV can carry more than 28 MVA. As we have seen, the load provided by medium voltage circuit breakers is much larger, so it is essential to provide protection. Due to the huge impact of a single fault on the power distribution system, the reliability of the system has become an important part of the design work. IEEE 493-2007: Industrial Power System Design (Gold Book) is a good resource for reliability analysis. Based on these analyses and the needs of the owner, redundancy can be built into the system. Redundancy can be N + x (where x can be 1, 2 or any number) or 2N. The 2N system needs to provide two power supplies for each device, and each power supply is fully capable of carrying the entire load (see Figure 1). If the "A" side fails, power can still be obtained through the "B" side. When the A side is unavailable, the system is not 2N until the A side is put into use again. It is important to consider redundancy at any voltage level, but it becomes especially important in MV systems because a large amount of power is provided and may be lost. Using the same example, a 480 V 1,200 amp circuit breaker can carry close to 1 MVA, while 13.8 kV can carry more than 28 MVA. The impact of a loss of 28 MVA may be much greater than a loss of 1 MVA.
For large and complex electrical systems, relays can be used to easily design the protection of medium voltage systems, but it may become complicated and must be carefully considered. There are many protection schemes, and a robust system usually has many different relay types and functions. Each relay type has a dedicated number, as is each protection device, described in ANSI/IEEE C37.2, which makes design easier and easier to understand other people’s designs. The differential protection relay (87) adds the input currents and compares them with the sum of the output currents. This kind of protection is the most common because it works quickly. Differential protection is applied to the main bus of the equipment and the area containing all circuit breakers. It can also provide differential protection for longer medium voltage transformers and feeders. Other common types of protection include overcurrent (51), instantaneous (50), overvoltage (59), undervoltage (27), reverse power (32) and so on. IEEE 242-2001: The protection and coordination of industrial and commercial power systems is a good source of electrical system protection.
In the past decade, efforts have been focused on protecting medium-voltage gears. A separate relay sends a signal to the central device, and the central device processes the information and decides what measures should be taken to avoid false trips. The signal can be sent back and forth via optical fiber or wirelessly. This technology was first developed in Europe and the standard covering it is IEC 61850: Automation of Power Utilities. This technology is promising, but it has not yet been widely used.
Medium-voltage distribution transformers also tend to be larger and more expensive than low-voltage transformers. Since transformer failure may have an impact on the entire system, more attention is paid to the protection of medium voltage transformers. In addition to the usual overcurrent protection that low-voltage transformers can accept, medium-voltage transformers also accept a thermal relay (49) to monitor the oil temperature, a pressure switch (63) to monitor the pressure in the tank, and a liquid level switch (71) To monitor the oil level (see Figure 2). For preset values of various parameters outside the acceptable range, all these relays will trip the circuit breaker. To clarify, some low-voltage transformers can get all these levels of protection-but in medium-voltage transformers, this protection is conventional.
The medium voltage power distribution system also has the same advantages on the standby (or standby) power supply side as the utility side. For example, a 13.8 kV system can be supported by a 13.8 kV generator. The backup power generated by the generator can be easily distributed as close to the load as possible as on the utility side. Depending on the type of power distribution system design, these medium voltage generators usually need to be connected in parallel to support the entire system.
There are several issues to consider when connecting medium voltage generators in parallel. One is reliability. For example, if four generators are needed to support the power system in the event of a total utility failure, the next decision is the scale of redundancy. If N + 1 is needed, we will need to use five generators, four out of five at any given time. The reliability (availability) of this system is 0.999. If lower reliability is acceptable, then for an availability of 0.96, only four generators can be used. For details on how to calculate the reliability of the power system, please refer to the IEEE Gold Book.
Another decision is how the backup system interacts with the utility. In many cases, open conversion—disconnecting the backup system from the load before the utility power is reconnected to the load—is sufficient. Open conversion is easier to implement. In some cases, a closed transition is required-the standby power system and the utility power system are connected in parallel in a short period of time, usually several cycles.
Examples of closed conversion systems can be found in hospitals and data centers. Closed conversion increases the complexity of the power distribution system, because the control and relay must include more control areas.
In addition, due to the parallel connection between the mains and the backup power supply, the availability of short-circuit current increases the worst-case scenario of failure during closed switching. The increase in fault load may push the switchgear rating to the next higher standard rating, which may significantly affect the cost (see Figure 3).
MV systems supported by backup generators also require close attention to grounding and ground fault protection. The grounding of electrical systems is a broad topic and will not be discussed here except to direct the reader to two good resources: IEEE 142-1991: Grounding of Industrial and Commercial Power Systems, and IEEE C37.101: Grounding of Generators Protection guidelines.
Medium-voltage power distribution systems have many advantages over low-voltage power distribution, but they also have some disadvantages. The choice must be the result of careful analysis, in which cost and safety are the main factors. The advantages of the MV system include the use of less copper, in the form of smaller conductors and fewer conductor sets, less power loss, lower voltage drop, and therefore more power capacity to the load. The disadvantages of the MV system include larger equipment size, greater working clearance around electrical equipment, more investment in training, and longer maintenance time to repair the equipment.
Regardless of these advantages and disadvantages, sometimes LV allocation is impossible. In this case, MV allocation is used (see Figure 4). In this case, the safety of workers should be carefully considered by developing detailed procedures on how to maintain MV equipment. The safety of non-maintenance personnel should also be carefully considered. The most effective way to ensure safety is to lock the door of the area where the MV equipment is located, so that unauthorized persons are not allowed to enter.
Eduard Pacuku is a senior electrical engineer at Concord Engineering. He spends most of his time designing electrical systems for universities, healthcare facilities, mission-critical facilities, and high-rise commercial buildings.
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