Factors for Selecting a Low- or Medium-Voltage Electric Motor

Low-voltage motors are often a preferred choice due to familiarity with products and available services, as well as the typically lower cost of individual components. However, as horsepower (hp) increases, there can be advantages to moving to a medium-voltage motor. Low-voltage motors typically go up to 1,000 hp while medium-voltage motors can cover 250 hp and higher.

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Furthermore, in special variable frequency drive (VFD) applications, low-voltage motors can go up to or even over 5,000 hp. This high rating is preferably above the National Electrical Manufacturers Association (NEMA) low-voltage limit of 600 volts but still under International Electrotechnical Commission (IEC) low-voltage limit of 1,000 volts.

Knowing when to select the right motor for an application can save users time, space and money. Here are some areas to consider when choosing between low- and medium-voltage electric motors.

Cabling

In low-voltage motors, as the hp range increases, the size of cabling increases to handle the increase in amps. With conductors being a copper component, this increase in wire gauge can add cost, especially on longer cabling runs across a large facility or over a long distance to a remote pumping station. This increase in diameter also makes turn radii larger, which increases the difficulty in making connections within the terminal boxes. This can be time-consuming and introduce additional risk to the maintenance crew during initial setup of the motor.

A lower current in medium voltage motors allows for smaller cables (leads) even at higher hp. The use of smaller gauge leads reduces the cost per foot for those long-distance connections to remote pumping stations. Also, during the motor connection procedures, the small gauge wires are easier to work with and connect within the motor terminal box. This can reduce the maintenance crew’s time in making the connections and reduce the risk of damage to the cables.

The cost of copper as a commodity and the difference in thickness of leads sized for low-voltage machines versus medium-voltage machines can be so large that this can be the primary determining factor in what voltage service is specified. The higher cost of medium-voltage equipment can easily be offset in applications with long cable runs from distribution.

Size

When space is a consideration, more than motor size should be reviewed as the choice between a low- or medium-voltage motor that has an impact on the components in the entire system.

Low-voltage drives are smaller than medium-voltage drives when variable speed applications play a role in the motor selection. However, above 1,000 hp this ratio starts to flip, and drive size may be comparable or even smaller. Due to lower amps, medium-voltage motors also enable the use of smaller supply side switch gear, supply transformer and controls.

Windings

To prevent short circuits and preserve the longevity of medium-voltage windings, they are commonly produced using a form wound insulation system. The insulation system is sealed using a vacuum pressure impregnated (VPI) system, which fills the voids in the coils to protect from contamination. The coils are organized outside of the stator core to ensure the ideal spacing of turns, which allows for air flow around the coils to improve heat transfer. It is a more labor-intensive process but is well suited to the rigors of the voltage impulses of a medium-voltage system. Additionally, due to the smaller conductors used in the windings, there is the possibility of having more turns, so there is greater flexibility in the electrical design, making it possible to achieve specific performance characteristics.

In low-voltage motor windings with larger diameter conductors, there are more limitations to the electrical design but less need for the precisely ordered coils required to withstand medium voltage. Because of this, low-voltage machines can use a more cost-effective random or mush wound design with a thorough dip-and-bake in varnish that is often coupled with a vacuum impregnation of the winding to ensure that the insulating material fills all voids. The result is a low-voltage insulation system that is capable of exceeding industry standards for longevity while achieving the performance characteristics necessary for a broad range of applications.

Like all good questions, whether to pick a low- or medium-voltage motor for a pump system does not have an easy answer. There are several factors to weigh, including site and installation specifics that will impact what voltage service is best for a given project. When selecting a motor for an application, evaluating these three factors should provide the best all-around motor for the facility.  

Wayne Paschall is a product market specialist with ABB Inc., in the large machine and generator division. For more information, visit abb.com.

Wondering about Medium Voltage systems?

Curious of its distinction to other voltage systems and rating?

Then this blog might be of interest to you!

The following topics pertaining to medium voltage systems will be covered such as:

• What is a Medium Voltage system?
• Pro’s and Con’s?
• How do Medium Voltage systems work?
• Medium Voltage Safety and Usage

Without further ado, lets begin!

Introduction

As consumers, we visualize electrical power like any other utility service households or offices. From a general perspective, that is the correct way of looking at electrical power compared to any other service. As an example, electrical power is distributed like natural gas or a city water service for widespread use. As pressure (or pressure variation) moves water and gas transmission to the end user, electric power is viewed as voltage “moving” the electrical current.

However, for electrical power to be distributed to end users, it must go through several stages. These stages go through several types voltage systems which are defined by the ANSI (C84.1), NEC(490,2), IEEE (-) and CSA (22.1-12) mainly:

• (Extra) High Voltage Systems (HV)
  • 115,000 to 1,100,000 VAC(ANSI)
  • Anything higher than 750V (CSA)
• Medium Voltage Systems (MV):
  • 2,400 to 69,000 VAC (ANSI),
  • Greater than 600 nominal V(NEC),
  • Greater than Volts, up to Volts (IEEE -).
• Low Voltage Systems(LV):
  • 240 to 600 VAC(ANSI)
  • Less than 600V nominal (NEC)
  • Less than 750V(CSA)

However, not all generation transmits power through all three stages (e.g. Wind turbines, Distributed Generators, etc.), but almost all generation passes through MV Systems AND LV Systems. Therefore, MV systems are the most crucial as it not only can provide power directly to heavier loads, but it is also responsible for feeding power to LV systems like your home.

What are Medium Voltage Systems

As stated previously, MV Systems are crucial as they are widely utilized in high power loads like industrial machines or big office buildings while also serving as distribution functionality to LV systems. They are preferred over LV system over long transmissions since MV systems have high voltage and low current compared to the equivalent LV counterpart.

Pro’s and Cons of Medium Voltage

MV distribution systems have many advantages over LV distributions, but they also have some disadvantages. The choice must be the result of careful analysis, where cost and safety are the prevailing factors. MV advantages over LV systems include:

• Less copper required as conductors are smaller
• Lower voltage drops
• Less power losses, therefore more efficient!

MV disadvantages over LV systems:

• Larger equipment required
• Greater working clearance
• More spacing required for the conductors
• Greater requirements in safety training
• More investment and time required for maintenance.

Although MV systems have their advantages and disadvantages compared to LV systems, sometimes it is the only option to use MV systems. In such cases, it is paramount to provide adequate and detailed maintenance and training procedures for service and nonservice personnel.

How do Medium Voltage Systems Work

We have stated previously what MV systems are and what makes them advantageous and detrimental over LV systems. However, how does MV transmission and distribution really work?

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MV systems are either fed from directly from generators at distribution level (typically 12.47kV) or are fed from HVs transporting power from faraway generation like a hydroelectric plant 800 km away. Regardless of how the MV system is fed from upstream, the general rule of thumb is that there is galvanic isolation between a transformer and the HV system or generator its is fed from.

MV are required to be equipped with several types of equipment to function properly. MV systems contain similar components that are prevalent in LV systems like circuit breakers and transformers. However, MV systems tend to have other components that most LV systems tend to not incorporate such as:

• Ground grids
• Grid tie ins to other voltage systems.
• Relays to work in tandem with circuit breakers such as:
  • 50 - Instantaneous Overcurrent Relay
  • 51 - AC Time Overcurrent Relay
  • 59 - Overvoltage Relay
  • 87 - Differential Protective Relay

Note: Overvoltage relays tend to be used for distributed generators (such as wind) but not from a transmission perspective.

See Figure 1 attached for a more generalized figure for MV system

Figure 1:Illustration of an MV interconnection to other components and systems

Medium Voltage Safety and Usage

MV systems have existed for many years with electrical equipment typically being tested at the factory before being shipped. Furthermore, the equipment must be tested at site before first start-up to detect or find:

• improper installation
• damaged to equipment or materials that occurred through:
  • shipment
  • installation may be missed by a visual inspection.

Furthermore, a hi-pot test should be conducted and verified by a qualified and competent inspector. A Hipot test or dielectric strength test is a non-destructive conducted in order to evaluate the adequacy of electrical insulation.

If possible, an element of redundancy should be present when designing MV system. Compared to LV systems, more emphasis on redundancy is necessary on MV systems as MV systems can transfer much more power. IEEE 493 stated the redundancy factor should be considered as N + x (where x could be 1, 2, or any number) or 2N. However, this requires a 2N system to have two sources of power for each piece of equipment, with each source being fully capable of carrying the entire load. From a financial perspective, this will increase the capital cost of the initial system installation. However, from a purely operation perspective, redundancy opens the possibility of diverting power during a fault event, allowing the MV to be able to still transfer power downstream. From a maintenance perspective, allowing redundancy to be implemented allows for part to be safely deenergized for workers to provide upkeep and repairs on the system without the risk of having to work on energized equipment.

However, this may not eliminate the risk of potentially working near live equipment! Regardless, proper procedures should be in place/followed when working with or on MV equipment, energized or not.

Grounding rules for medium voltage systems

Proper grounding for a MV system is paramount as the system deals with substantial power flowing through it. There are many types of ground protection measures for medium voltage systems. A few are discussed below starting with the generalization of grounding rules for both DC and AC systems in CSA (22.1-12) which states that:

• Direct Current
  • Two- wire DC systems supplying interior wiring and operating above 300 V between conductors and a neutral point where the maximum difference between the neutral point does not exceed 300V, the neutral is permitted to be grounded
  • For Three wire DC systems, the neutral conductor supplying the interior is required to be grounded.
• Alternating Current
  • The AC system(s) shall be grounded if the system contains a neutral conductor or the max voltage to ground does not exceed 150V.
  • The AC system(s) shall have a grounding conductor connected each individual service on the supply side with disconnecting means while also having a grounding conductor to either the transformer or other supply with no connection between the grounding conductor and the load side service disconnect means.

IEEE 242- mentions different option for grounding electrical power systems but specifically states that states that medium system voltages utilize low resistance grounding. Low resistance grounding allows for the ground-fault current to be reduced but still be high enough to be detected by sensitive relays. These systems have rating between 2.4 and 13.8kV and have Motor Control Centers directly attached. Resistance ratings are calculated to limit fault currents to approximately 200 to 500 A with or as high as 800-A for multisource ground faults with a typical 10 second rating.

The inspector or other service personnel must ensure that all metal MV systems structures (poles, switch arms, metal support structures for racked cables, etc.) are all properly grounded and bonded using one of the conductor termination methods specified in CSA 22.1-12 or NEC 250.8, in addition to being properly sized. Grounding conductors that are placed on equipment but are not an integral part of a cable assembly cannot smaller then 6 AWG copper or 4 AWG aluminum (NEC 250.190). For switchgear servicing, a main bonding jumper must be present and properly installed with all cable shields and grounding electrode conductors and cable shield terminated in the equipment (NEC 250).

Labeling

To properly identify rating and hazards, all equipment should be labelled properly, and this includes key MV system components. Documents like IEEE 242 or NEC 110.34 indicate that labels should generally provide the following information to employees working on the equipment:

• Adequate instructions
• Diagrams and other important equipment data
• Should be properly illuminated

For removal and replacement of labels from MV systems, in particular switchgear, IEEE 242 states that if one or more switchgear is out of service, verify that particular care is taken to avoid inadvertent removal, replacement, or exchange of labels, which were previously attached to the other switchgear units removed from service.

Labelling danger and warning signs are the most important labelling requirements as they indicate and/or quantify potential hazards, in addition to, list personal protective equipment (PPE) requirements to safely work around the equipment. An example warning label is shown in Figure 2 highlighting the requirements by the CSA through 22.1-12, as well as, Z462.

Figure 2:Example Warning Label

The label above is specific for arc flash but many of the standard labelling requirements are required across all types of warning labels. Similar requirements are required by NEC, NFPA 70B, NFPA 70E, and OSHA .

Working clearance and workmanship

Working clearance and workmanship around medium voltage systems, specifically, energized overhead power lines, is described in detail in NFPA 70e.

• 1. Workers within a distance of 3 m (10 ft) for systems up to 50 kV and should be increased 100 mm (4 in.) for every 10 kV above 50 kV.
• 2. Workers should be notified and trained on the hazards and precautions when working near overhead lines.
• 3. Warning labels on items such as cranes and similar equipment should display minimum clearance of 3 m (10 ft).
• 4. Another worker should be designated to observe the equipment while the operator is working near the overhead lines. The worker should ensure there is safe working clearances between the operator and all possible overhead lines notify the operator to stay outside those zones
• 5. Warning cones should be utilized to indicate the 3 m (10 ft) safety zone when working near overhead power lines.

Figure 3:Distance requirements for overhead power lines as per NEC and OSHA

Figure 3 describes the spacing requirements of the NEC and OSHA for overhead transmission lines and highlights three scenarios. Condition 1 is met when:

• there is exposed live part on one side but no live or grounded parts on the opposite side of the working space
• there are live parts on both sides and both parts are guarded by insulating materials

Condition 2 is met when:

• the workspace has exposed live parts on one side but has grounded elements on the other side of the work area,
• Condition 2 considers surfaces such as concrete, brick, and tile to be inherently grounded.

Condition 3 is the considered to be the worst-case scenario which is when both parts of a workspace or area are considered live with an operator/worker in between. Condition 3 is prevalent in certain systems that pre-date certain NEC, ANSI, IEEE and CSA standards which were simply designed under utility supervision. However, most new electrical equipment such as switchgear are enclosing any live parts Furthermore, inspectors/operators/workmen should always verify that enclosure doors, windows, hinges, and hardware are properly installed or connected (NEC 110.12, 110.3(B), 490.38). Workers should take care and ensure barriers are installed, if needed, so that no uninsulated ungrounded service busbar or terminal is exposed to accidental contact by persons working on the equipment. Furthermore, should work be required on energized equipment for whatever reason, bare parts on doors that are energized should be guarded where the door must be opened for maintenance.

Conclusion

I hope this article has helped to better explain medium voltage systems.

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