Specifying medium-voltage distribution

Engineers and contractors should be aware of the requirements associated with medium voltage to ensure a proper design and safe installation.

09/19/2012


In the past, when people thought about electrical power distribution at voltages above 600 V, “Danger—high voltage” was the phrase that generally came to mind. This perception of danger and the skill levels required when working with high-voltage equipment often drove design engineers, installation contractors, and owners to use low-voltage distribution systems. The higher voltage distribution systems were reserved for utility systems and large industrial plants that required large amounts of power.

Today, much of that perception has changed along with the required skill level. Standards and installation requirements make working with high- and medium-voltage equipment safer. Products, such as splicing and termination kits eliminate the specialized skill level required for working with high-voltage equipment. Smaller facilities are taking advantage of lower utility rates by using primary metering, and larger facilities are using higher voltages to economically distribute power around the facility.

When faced with decisions about implementing or planning a medium-voltage distribution system, consider the following questions:

  • Is a medium-voltage system right for the application?
  • What are the advantages of using medium-voltage distribution?
  • How will the system design and operation change if medium-voltage distribution is used?  

Voltage class definition

Both ANSI C84.1 and IEEE 141 divide system voltages into classes. Although the extra-high and ultra-high voltage ranges are not specifically defined in IEEE 141, the standard nominal voltages used in the U.S. and throughout the world fall within the same voltage classes. This article will focus on medium voltage.

Advantages of medium voltage

Over the past 10 years, the cost of copper has increased from $0.77/lb to $4/lb—more than a 400% increase. As this cost continues to increase, contractors, design engineers, and facility owners are trying to find ways to reduce the amount of copper in the distribution system. One method is implementing medium-voltage distribution systems. In this case, the goal of using medium-voltage distribution is to move the transformation to utilization voltage closer to the load to take advantage of the reduced current at higher voltages. Lower current means smaller or fewer conductors to distribute power. Using smaller or fewer conductors decreases the amount of copper and therefore reduces cost. Conduit and installation costs are also lower. 

The following example compares the cost of running 2,500 kVA for 100 ft at 480 V with 600 V cable and running the same amount of power for the same distance at 13.2 kV with 15 kV EPR cable:

  • Running 480 V, 3,000 A for 100 ft requires eight sets of 3.5-in. conduit, each with three 500 kcmil phase conductors and one 500 kcmil ground conductor, and costs around $100,000
  • Running 13.2 kV, 110 A for 100 ft requires one 4-in. conduit with three 2/0 phase conductors and one 2/0 ground conductor, and costs around $12,000.  

Note that for this installation, feeders were routed overhead using rigid metal conduit. Costs were determined using RS Means and project data. 

Medium-voltage distribution also helps to minimize voltage drop. The National Electrical Code (NEC) Article 215.2 recommends a voltage drop of 5% or less from the utility service to the load. Ohm’s law (V=IR) defines the basic relationship between voltage drop and load current. The larger the load current and the larger the conductor impedance, the larger the voltage drop. In low-voltage facilities where the distribution system has long feeder runs, the feeders must be oversized to compensate for the conductor impedance. In medium-voltage distribution systems, the load current is smaller, resulting in less voltage drop. This smaller voltage drop eliminates the need to oversize conductors and makes the overall system more efficient. 

Another way medium voltage can help reduce cable sizes is by reducing or eliminating ampacity adjustment factors when installing conductors underground. Ampacity values stated by the cable manufacturers and the NEC are based on very specific conditions. In practice, the surrounding environment in which the cables are installed rarely matches those conditions. This difference has a direct effect on the operating temperature of the conductors and is the basis for adjustment factors. These adjustment factors modify the base ampacity to accommodate the cable ampacity in that specific environment.

With large low-voltage systems, conductors must be run in groups, which operate at a higher temperature than isolated cables because they act as heat sources. To account for this additional heat source, adjustment factors must be applied to the ampacity rating of the conductor (IEEE Std 399-1997). A 2x4 (2 rows, 4 columns) duct bank arrangement has an adjustment factor that reduces the capacity of the conductor by roughly 60% (0.608). The following example makes the same comparison as the previous example, except the costs are based on routing the conductors underground in PVC conduit:

  • Running 480 V, 3,000 A for 100 ft requires 15 sets of 3.5-in. conduit, each with three 500 kcmil phase conductors and one 500 kcmil ground conductor, and costs around $160,000
  • Running 13.2 kV, 110 A for 100 ft requires one 4-in. conduit with three 2/0 phase conductors and one 2/0 ground conductor, and costs around $14,000.  

Another advantage of receiving medium voltage from the utility instead of low voltage is primary metering (metering on the primary side of the transformation). On average, primary metering can save up to 5% on kWh charges. However, the potential energy rate savings must be compared to the initial cost and maintenance cost of owning the transformation equipment to determine if primary metering works for the application. 

In addition to primary metering, the specification of the distribution transformer can be an advantage of receiving medium voltage from the utility instead of low voltage. ANSI C84.1 specifies voltage tolerance limits for service voltages provided by the supplying utility. In some facilities with critical equipment, that tolerance may be too great. Specifying a power distribution transformer with taps on the primary windings allows the user to change the transformer ratio to raise or lower the secondary voltage, providing a closer fit to the equipment using the power. High-impedance transformers reduce the available short-circuit current and allows users to purchase less-expensive 480 V distribution equipment with lower short-circuit ratings. Higher impedance transformers also lower the arc flash incident energy level, decreasing personnel exposure to hazards. 

Medium-voltage applications

Figure 1: The dual 25 MW, 120 kV-to-13.8 kV substations shown in the photo serve a 200,000-sq-ft data center. Courtesy: KlingStubbinsAny facility where the operation demands large amounts of power, such as data centers and industrial plants, is a great candidate for using medium voltage (see Figure 1). In data centers, the distribution system includes backup generators to keep the facility operational during outages and redundancy to eliminate single points of failure. This backup power and redundancy can double or triple the overall size of the electrical distribution system. It is with these types of systems that it is important to move the transformation to utilization voltage (substations) as close as possible to the load to maximize efficiency, reduce cost, and meet the requirements of the NEC to limit the number of services. 

Figure 2: The 38 kV outdoor switchgear shown in the photo includes a primary utility metering cabinet, vacuum circuit breaker, mains, and load break switches for the distribution feeders.Other key aspects of data centers that make medium voltage attractive are modularity, scalability, and adaptability. Data centers are going to grow and change with technology. The facility must be designed to allow the data center to grow and change over time without interruption. Using multiple smaller power trains (grouped substation, UPS, and generators) allows for that growth and minimizes up-front capital costs. Using medium-voltage distribution allows users to cost-effectively connect additional power trains as the facility grows (see Figure 2). 

Large facilities and campus-type facilities often contain centralized chiller plants and/or a generating plant. Both of these systems can be operated at medium voltage, eliminating transformation equipment and their associated losses. It should be noted that chillers are available only at 480 V, 2,300 V, and 4,160 V. 

Facilities that have the potential for long feeder runs, such as large footprint warehouses, campuses, and high-rise buildings, are also good candidates for medium-voltage distribution. Again, medium voltage reduces the feeder conductor sizes and eliminates voltage drop. In the case of a high-rise building, it can also reduce shaft space throughout the building, which increases the usable square footage. 

Figure 2: This BIM model rendering is for a state-of-the-art modular data center that allows for flexible Tier levels and various distribution voltages: 208 Vac, 415 Vac, and 380 Vdc. The electrical system uses medium-voltage generation and distribution tOf course, all facilities large and small can take advantage of the primary metering savings obtained when receiving medium-voltage service from the utility (see Figure 3). As stated previously, the potential savings must be compared to the initial, operational, and maintenance costs of primary metering. 

Differences between low and medium voltage

Some of the key aspects that differ between low- and medium-voltage systems include working space, overcurrent protection, and equipment ratings. These key aspects must be considered when designing systems above 600 V. 

Working space: Article 110 of the NEC requires the depth of the working space in front of low-voltage equipment to be 3 to 4 ft depending on the nominal voltage to ground and whether the equipment on the opposite side of the working space is ungrounded, grounded, or energized. For medium-voltage equipment, that depth of working space increases to a range of 3 to 12 ft. Table 1 defines the minimum depth of working space at electrical equipment. 

NEC Article 450 has specific vault and fire resistance requirements for voltages above 35,000 V. To avoid the cost of those requirements, voltages greater than 35,000V are generally not run into the building. 

In addition to work space, the actual footprint of medium-voltage equipment is larger than that of low-voltage equipment. The average cubicle size for a 600-V switchboard is about 30 in. by 36 in. The average cubicle size for 600-V switchgear is about 30 in. by 72 in. However, the average cubicle size for 15-kV switchgear is 36 in. by 96 in. The additional working and footprint space requirements must be considered when designing the space for medium-voltage equipment. 

Protection schemes: Another difference between low-voltage and medium-voltage is overcurrent protection. Low-voltage circuit breakers contain both the switching mechanism and the overcurrent protection in one device. Medium-voltage circuit breakers contain only the switching mechanism to open/close and do not have an integral overcurrent protection device. 

Medium-voltage breakers rely on separate devices, such as current transformers and protective relays, to provide the overcurrent protection. The code-required overcurrent protection is established by selecting the proper current transformer (CT) ratio and protective relay pickup value. For example, a CT ratio of 300:5 and a relay setting of 5 A equates to a circuit protection setting of 300 A. Another difference between low- and medium-voltage circuit breakers is the interrupting medium. Low-voltage breakers use an air interrupting medium, while medium-voltage breakers use vacuum or an SF6 gas medium. 

Two advantages of using protective relays in medium-voltage applications are better coordination and the ability to easily enable differential protection. In general, protective relays have higher accuracy and a wider range of settings, which allows the engineer to reduce the tolerances and eliminate zones of overlap between protection-device time-current curves. Differential protection is considered superior to directional or phase protection due to its selectivity, sensitivity, and speed of operation. It uses the sum-of-all-currents in the zone of protection. Ideally, this sum equals zero until there is a fault within that zone. 

Fuses can also vary between low voltage and medium voltage. Low-voltage fuses are typically sealed, nonexpulsive type fuses. Medium-voltage fuses can be either expulsive or nonexpulsive (current limiting). Expulsive fuses are vented and intentionally expel gases during fault interruption. When using expulsive-type fuses, the installer must ensure that the area where the gas is discharged is clear of other equipment (see Figure 4). 

Figure 4: This diagram illustrates the common terms that describe the properties of medium-voltage fuses: type, class, application, and amp rating. General-purpose current-limiting fuses are commonly used to protect feeders and transformers. The “E” ratin

Equipment ratings: Low-voltage equipment has a large range of available amperage ratings. Power circuit breaker ratings include 800 A, 1,200 A, 1,600 A, 2,000 A, 2,500 A, 3,000 A, 4,000 A, and 5,000 A. Medium-voltage equipment is limited to the standard rating sizes of 1,200 A, 2,000 A, 3,000 A, and 4,000 A. 

Low-voltage distribution systems use fuses rated from 20 A through 6,000 A. Medium-voltage distribution systems are limited to 720-A fuses (twin 400 E fuses derated to 720 E). Above 720 A, the engineer must specify breakers instead of fuses. 

Cost differences between low-voltage and medium-voltage equipment can vary depending on the type and ratings of equipment. The following is a comparison of similar type equipment at 600 V and 15 kV:

  • 4,000 A, 85 kA IC at 480 V EO ANSI power breaker LSIG = $27,000
  • 4,000 A switchgear section with CTs and meter = $13,000
  • 1,200 A 15 kV switchgear with PTs, CTs, meter, and OC relay = $31,000
  • 1,200 A 15 kV 40 kA breaker = $19,500  

Terminations

Medium-voltage cables often require special terminations, commonly referred to as stress cones. The purpose of the stress cone in the cable termination is to control the electric field distribution beyond the end of the semiconducting insulation. It minimizes the field into the cable insulation and thus significantly reduces both the radial and longitudinal components of the stress at the end of the insulation shield. 

Safety

Although working with medium-voltage equipment has become less specialized, it still requires someone who is qualified to work with medium-voltage equipment. As defined by the NEC, a qualified person is someone who “has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved.” 

Therefore, anyone working on or around medium-voltage equipment must fully understand the hazards involved and must be aware of the precautions, safety procedures, and levels of personal protective equipment required when exposed to energized parts at these voltages. 

Even though medium-voltage systems do have an inherent safety risk over low-voltage systems (severity of tissue damage due to the energy at higher voltages), they are considered more reliable and can be very cost-effective. 

References

Refer to the following list for more information about working with medium-voltage distribution systems:

  • IEEE 48: Test Procedures and Requirements for AC cable terminations 2.5 kV through 765 kV
  • IEEE 141: Recommended Practice for Electric Power Distribution for Industrial Plants
  • IEEE C37.06: AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis-Preferred Ratings and Related Required Capabilities for Voltages Above 1,000 V
  • IEEE C37.010: Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis
  • IEEE C37.100.1: Standard of Common Requirements for High Voltage Power Switchgear Rated Above 1,000 V
  • ANSI C84.1-2011: American National Standard for Preferred Voltage Ratings for Electric Power Systems and Equipment
  • NEMA SG 10-2008: Guide to OSHA and NFPA 70E Safety Regulations when Servicing and Maintaining Medium-Voltage Switchgear and Circuit Breakers Rated Above 1,000 V
  • NFPA 70: National Electrical Code
  • NFPA 70E: Standards for Electrical Safety in the Workplace

 


Ken Kutsmeda is an engineering design principal at KlingStubbins in Philadelphia. For more than 18 years, he has been responsible for engineering, designing, and commissioning power distribution systems for mission critical facilities. His project experience includes data centers, specialized research and development buildings, and large-scale technology facilities containing medium-voltage distribution.



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