Back to basics: How to design medium-voltage distribution systems

MV insights

• Medium-voltage (MV) electrical systems can be used in many commercial and industrial applications.
• While MV equipment can be more expensive, it requires fewer conduits and conductors.

Several factors must be considered and evaluated, by the designer, when determining whether to use a low-voltage (LV) or medium-voltage (MV) distribution system.

The owner’s satisfaction and safety are the goal of any project. Collaboration and input from the owner on their opinion, operational staff experience and comfort level with MV equipment should be considered when deciding to use a MV versus LV system for a facility. In addition, equipment availability, system capacity, cost, physical space and site constraints are the main factors when selecting the distribution system.

The availability of LV equipment becomes limited when the total electrical load approaches certain sizes. For example, for an LV system, 5,000-6,000 amperes (A) is the threshold for LV switchboards and switchgears that are commonly available by manufacturers. Exceeding this threshold increases the risk of arc flash. Equipment availability also applies to the driven equipment, such as variable frequency drive (VFD) motor starters may not be commonly available at LV for motors 700-800 horsepower (hp) or larger by all the VFD suppliers.

Early coordination with the electric utility is necessary to determine the available voltage rating near the project site. This will allow the designer to select the appropriate voltage system and equipment accordingly. Similarly, early coordination with the mechanical engineers is needed to determine the expected loads and hp sizes for their pump motors and equipment.

Another deciding factor is cost. Generally, MV equipment is more expensive than LV equipment. However, the using MVs will reduce the amount of current traveling through the conductors, which reduces the quantities and sizes of conduits, conductors and duct banks installed as well as labor costs, especially for long runs on campuses. The designer should consider evaluating the cost effectiveness between LV and MV early during the design.

Physical space considerations and site restrictions could also affect the designer’s voltage choice. MV equipment is typically larger than LV equipment as it requires higher-rated insulation and more separation between the buses within the enclosure. In addition to the equipment itself, the working space clearance in front of MV equipment required by NFPA 70: National Electrical Code (NEC) is greater than LV.

NEC Sections 110.31 and 110.34 cover the minimum distance from fences to live parts and the minimum depth of clear working space at electrical equipment, respectively, for systems more than 1,000 volts, nominal (refer to NEC Tables 110.31 and 110.34(A)). However, as discussed, MV will require less conduits and conductors installed for distribution and between equipment in comparison to LV, which is more effective for project sites with physical space constraints.

Figure 1: Field-installed neutral grounding resistor for an indoor medium-voltage generator. Courtesy: CDM Smith

MV design considerations

Compared to LV equipment, MV equipment is more expensive. Due to a higher system voltage, the amount of power generated during a fault, depending on the system impedance, may be larger and more damaging. Additionally, with the increase of the voltage, the concern of electrocution increases. Therefore, MV system requires specific considerations including, but not limited to, the following.

Protection: The most common form of protection in MV systems is the use of protective relays. Protective relays are devices that provide open and close commands to the associated circuit breakers based on input from current transformers and voltage or potential transformers. The protection can range from time overcurrent, instantaneous overcurrent, over/undervoltage, over/underfrequency, ground fault, etc.

ANSI/IEEE C37.2 assigns standard device numbers to the type of protection a certain relay performs. The following are commonly used protective relays:

• 25 – Synchronizing or Synchronism-Check Device.

• 27 – Undervoltage Relay.

• 32 – Directional Power Relay.

• 47 – Phase Sequence or Phase Balance Voltage Relay.

• 50 – Instantaneous Overcurrent Relay.

• 51 – AC Time Overcurrent Relay.

• 51G – Ground Time Overcurrent.

• 59 – Overvoltage Relay.

• 67 – AC Directional Overcurrent Relay.

• 81 – Frequency Relay.

• 86 – Locking Out Relay.

• 87 – Differential Protective Relay.

Standby generator neutral grounding resistor: For a MV standby generator, it is important to consider using a neutral grounding resistor (NGR) connected to the alternator grounding system. If a fault occurs while the standby generator is running, the NGR will limit the ground fault current that will return to the alternator and protect the generator from damage. See Figure 1 for a field installed NGR for an indoor MV generator.

MV enclosure and assembly: In general, there are two types of MV enclosures, metal-clad (MC) and metal-enclosed (ME). IEEE C37.20.2: Standard for Metal-Clad Switchgear defines MC switchgears as a switchgear that contains drawout electrically operated circuit breakers that is compartmentalized to isolate components such as instrumentation, main bus and both incoming and outgoing connections with grounded metal barriers.

ME is defined by IEEE C37.20.3: Standard for Metal-Enclosed Interrupter Switchgear as a switchgear assembly completely enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) containing primary power circuit switching or interrupting devices or both, with buses and connections and possibly including control and auxiliary devices. Access to the interior of the enclosure is provided by doors or removable covers.

Both MC and ME switchgears contain interrupting devices, such as a circuit breaker, fuses (ME only) or load break interrupter switches. The MV circuit breaker switching mechanisms are electrically operated and have no integral protection and require external control power, unlike LV circuit breakers that have integral protection.

The MC switchgear has a higher initial cost in comparison to ME,however, the MC assembly is more robust as it contains insulated buses, does not have exposed parts and the MV and LV sections are completely segregated. Another difference is that MC assemblies require front and rear access, while ME typically only requires access from the front. Therefore, physical space constraints, accessibility and working space clearance requirements should be considered when selecting the type of MV enclosure and assembly during the design phase.

DC control system: As mentioned above, MV operation and protection requires a reliable source of power. Unlike LV, MV uses protective relays to open and close the circuit breakers. The reliability of the control power for protective relays is extremely critical for the system to be always functional and safe. In general, direct current (dc) control power is used for MV protection systems and circuit breaker operations, in lieu of alternating current (ac) control power. DC systems are more reliable, less impacted by the inrush of the circuit breaker coils, require less physical space and can store more energy.

Typical DC control systems require batteries, such as flooded lead acid, valve regulated lead acid, nickel cadmium, etc. and 48 V or 120 Vdc control voltages are commonly used. Based on the type and quantity of the batteries, additional ventilation or special storage, such as a dedicated room, may be required. IEEE 485: Recommended Practice for Sizing Lead-Acid Batteries for Station Applications and IEEE 1115: Recommended Practice for Sizing Nickel-Cadmium Batteries for Station Applications are standards that can be used to size the battery system, which is measured in amp-hours (Ah). The Ah is based on the MV protection system components power usage, such as circuit breaker coil trips, spring charging and closing, steady state loads and the number of circuit breaker operations within a certain period of time, which should be determined during the design.

NEC Article 480, Stationary Standby Batteries, shall be complied with for the battery system, including, but not limited to, protection, accessibility, wiring, illumination and ventilation.

Redundancy: To provide a reliable electrical system, a double-ended (main-tie-main circuit breakers) switchgear assembly should be considered to provide flexibility during maintenance and redundancy to maintain operations in the event of failure from one source. Redundancy should also be considered with respect to the dc control system such as including a second dc control system (battery system).

Figure 2: Surge arrestor connected to the medium-voltage side of an outdoor pad-mounted transformer. Courtesy: CDM Smith

Surge arresters in MV systems

Surge arresters, also known as lightning arresters, are used to protect MV equipment from high-voltage surges and are typically installed to protect transformers and distribution equipment by connecting the line terminal to ground (see Figure 2). When a surge arrester experiences a voltage higher than its maximum continuous operating voltage, it becomes conductive and shunts to ground.

There are three classes of surge arresters commonly used in MV protection: distribution class, intermediate class and station class. NEC Article 242 Part III has requirements for surge arresters rated more than 1,000 V.

Additional information and applications for commonly used surge arresters can be found in IEEE C62.11: IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV) and IEEE C62.22: IEEE Guide for the Application of Metal-Oxide Arresters for Alternating-Current Systems. Figure 2 shows a surge arrester connected to the MV side of an outdoor pad-mounted transformer.

Cable and conduit size

NEC Tables 315.60(C)(1) through 315.60(C)(20) provide the ampacities of conductors for the various installation configurations and methods for the different cable temperature ratings (Type MV-90 for 90°C and MV-105 for 105°C).

In addition to properly sizing MV conductors based on full load requirements, MV conductors and terminations are required to have insulation that can withstand the stresses experienced during normal and abnormal operating conditions, including fault events and rated for the environment that they will be installed. NEC Article 315 defines the requirements for MV conductors sizing, applications, installation and other requirements.

Normally, there are two types of MV cables that are used in underground applications, ethylene propylene rubber and cross-linked polyethylene. MV cables have three insulation thickness levels (100%, 133% and 173%) depending on the amount of time it takes to clear a ground fault. According to NEC Article 315, 100% insulation level cables are used with relay protection to clear ground faults as rapidly as possible but within a minute. Cables with 133% insulation levels are used when the ground fault clearing of less than one minute cannot be met but can clear in one hour and 173% insulation class is used when one hour cannot be met and an orderly shutdown is required.

The ampacity of MV cables is dependent on the cable size, conductor material (aluminum or copper), based on the configuration of the cables (single conductor or three conductor) and how they are arranged in a duct bank. NEC Figure 315.60(D)(3) displays different configurations for cables installed underground.

Electrical safety for MV systems

The higher the voltage, the increased concern of electrocution or electric shock (see NFPA 70E: Standard for Electrical Safety in the Workplace). The safety of the end user is of paramount importance and should be incorporated into every design. Operating MV switches and circuit breakers while standing in front of the MV switchgear could potentially cause significant harm to personnel.

One way to mitigate the risk of electrocution and arc flash is to provide a remote racking mechanism or a LV mimic panel to operate the MV switchgear remotely. A mimic panel will have a copy of the MV switches and controls, but at a lower and safer voltage, which allows personnel to operate the MV switchgear operations remotely without having to stand in front of the MV switchgear.

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