Electrical Distribution Design: Low Voltage and Medium Voltage

Whether retrofitting an existing facility or selecting equipment for a new location, the components and configurations are chosen based on the client’s technical, economic, and performance goals. A major consideration for any project is planning for the electrical power required to run the facility. Coordination with the local power utility can affect many choices in the design phase, including proximity to major transmission lines to address capacity constraints.

In many cases, the amount of energy required to power a facility will require a choice between delivery at low voltage (LV: up to 1kV), medium voltage (MV: 1 to 35kV), or high voltage service. For low to moderate power consumers, LV has been the natural choice for many facilities. Facilities that include multiple large horsepower (HP) motors or other electrically-intensive process equipment may be better served using MV resources. Most real-world facilities include a mix of power consumers that are served at different voltage levels according to the needs of the equipment. There are practical and economic considerations with voltage choices.

LV Power Distribution vs. MV Power Distribution

As the amount of power consumed increases, the transmission of alternating current at higher voltage levels is more efficient, because there is significantly less loss in the lines when the current flow is kept at a minimum. Note the inverse relationship between voltage and current in the following examples, all at 2.58MW and 80% PF:

2.58mW ► 34,500V (MV) = 54A
2.58mW ► 13,800V (MV) = 135A
2.58mW  12,470V (MV) = 149A
2.58mW ► 4,160V (MV) = 448A
2.58mW ► 2,400V (MV) = 776A
2.58mW ► 480V (LV) = 3,879A

At the illustrated power level, connection and distribution at LV would require large conductors to handle the current. Distribution at MV levels requires much less copper throughout with less distribution line losses at that given power. For the typical manufacturing or process facility, the analysis identifies the appropriate voltage levels for the given equipment to configure a distribution strategy. Within a plant, some equipment voltage levels are easy to predict based on available equipment. But innovation and lower price points offer more choices than ever before.

LV and MV Equipment Gap

Historically, deciding between LV vs. MV equipment in the range of 300 to 1000 HP, has posed a challenge due to a cost gap between LV and MV equipment. The power consumption of LV solutions in this HP range can pose ongoing operating expenses due to inefficiencies in distribution and peripheral equipment for harmonic noise mitigation. MV solutions for the range require more space, more costly components, and additional training of support personnel. But innovation and increasing competition are two factors helping to bring MV options within reach for bridging this gap.



Variable Frequency Drive (VFD) Systems

Price points for MV VFD systems are falling due to manufacturing efficiency, innovative designs, and vendor desire to open more markets. While LV VFD price points can be lower, peripheral costs can negate that advantage.

Several vendors offer MV VFD solutions with harmonic noise mitigation inherent in their design. Comparable LV solutions operate at higher current levels and require more complex configurations to mitigate harmonic distortion.

MV vs. LV Motor Designs

Internal construction specifications are more rigorous for MV designs due to the voltage levels applied. Typically, MV motors have a copper bar rotor design that can be higher in efficiency and are thought to be more serviceable than LV aluminum bar motor coils. For a given HP, the MV motor is larger and more costly but may be projected to have a longer service life.

Conductors and Cabling

MV distribution conductors can be significantly smaller in gauge than LV-rated conductors due to the lower currents. Long MV cable runs do not pose significant losses. Some applications can realize a savings in copper costs for distribution runs, due to the smaller gauge required at MV. Flexibility of cables and easier handling for transportable operations can be part of the selection criteria as well.

Other Considerations

Machine-specific requirements limit some options that may be available otherwise. Additional equipment for LV drops may be necessary for machinery available only with LV motors, controls, or other constraints. Likewise, power-intensive OEM equipment may be supplied only for MV power feeds. In all cases, optimal and safe operations require a coordinated assessment of plant resources and a system analysis in the design phase to prevent costly mistakes.

electrical distribution design
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