Energy savings in electrical distribution systems

Learning objectives:

Engineers and building owners often focus on payback periods and return on investment as economic decision-making thresholds for energy efficiency investments. These energy efficiency improvements typically revolve around energy-efficient lighting (i.e., LEDs), using variable frequency drives (VFDs), or using National Electrical Manufacturers Association (NEMA) Premium motors, for example. However, there are other parameters that are more accurate and effective in making investment decisions for building systems, specifically electrical distribution systems.

The codes and standards associated with energy efficiency establish the minimum energy efficiency requirements needed for the design of new buildings and renovations to existing buildings. The codes, however, are geared toward the efficiency of mechanical and lighting systems. Not a lot of information is provided within these codes to establish energy efficiency measures for the design of the power distribution systems, just the systems that the power distribution systems serve. These codes and standards include the following:

The adoption of energy codes and the energy code used in each state varies greatly.

ASHRAE 90.1 includes a chapter on power (Chapter 8). Although the standard includes the requirement for transformers to meet the Energy Policy Act (EPAct) of 2005, it does not discuss other aspects of the power distribution system.

The standard also establishes that the voltage drop shall not exceed 2% for feeders and 3% for branch circuits (Chapter 8.4.1). Although ASHRAE 90.1 is not more stringent than NFPA 70: National Electrical Code (NEC) voltage-drop recommendations outlined in the fine point notes (FPNs) section included in Article 210.19, ASHRAE 90.1 does establish voltage drop as a requirement for meeting the standard. An NEC FPN recommends a maximum voltage drop of 5%, with the feeders to not exceed 3%; because this is an FPN, it is not a code requirement. However, accounting for voltage drop is important in designing an energy-efficient power distribution system. Voltage drop can cause overheating and shorter lifespan of equipment due to undervoltage conditions for inductive loads. The excess heat will require additional HVAC requirements for cooling spaces. Additionally, voltage drop causes inefficiency in lighting loads by causing losses in the cables rather than the power being fully applied to the lights. The increase in cable size will permit higher output of lights without applying additional power from the source.

The remainder of Chapter 8 discusses submittals (shop drawings) and controls for receptacles. The remainder of the standard primarily pertains to mechanical equipment and building measures to achieve energy efficiency. However, it does address some aspects of electrical design including lighting (Chapter 9) and motors (Chapter 10). This article does not discuss lighting or motors, except in regard to VFDs.

The automatic control of receptacles was included in ASHRAE 90.1-2010 and has been a point of contention in the standard. Although automatic receptacle control provides additional energy efficiency to plug loads, which has been difficult item to reduce energy consumption in commercial and residential applications, it also requires educating owners on the proper application and the benefits.

Similar to ASHRAE 90.1, the energy efficiency measures outlined in the IECC pertain mostly to building measures and mechanical equipment. The code discusses lighting requirements for power density and controls (Chapter 4), but unlike ASHRAE, they do not establish electrical distribution system energy efficiency design minimums. The Massachusetts Stretch Energy Code, for example, is based on the 2009 IECC, however, in order to meet the requirements the designer is required to exceed the baseline IECC standards by 20% for commercials (medium or large) buildings or providing incremental improvements of the baseline IECC standards for (medium) buildings by choosing two of the three options available in the code, which include improve HVAC efficiency, on-site power generation, or further reduction of lighting power density.

Because they are more stringent than the IECC, using ASHRAE 90.1 or a stretch energy code as a basis for design is a good starting point in any project and should be considered as a standard practice for engineering design firms.

Copper versus aluminum

Copper and aluminum are the most commonly used materials for conductors, busing in distribution equipment, and windings in transformers. There is a common misconception that because copper is more conductive than aluminum, copper distribution equipment will be more energy-efficient than aluminum. That is not the case. There are other factors to take into account including conductor size, equipment size, cost, and weight of the equipment and conductors.

Depending upon the alloy of aluminum used for the conductors or bus, the conductivity of aluminum is approximately 56% to 61% that of copper. Although the difference in conductivity is significant, this will not significantly affect the overall efficiency of the distribution equipment because the panelboards, switchboards, and transformers because, regardless of the material used, the equipment is still required to meet NEMA and UL standards for temperature rise, which would affect the efficiency of the equipment.

Similarly, although the conductors will be larger, the efficiency of the cables will not be affected. The increase in size of the conductors will require larger conduit and potentially additional feeders, depending on the rating of the conductors. As an example, per Table 310.16 of the NEC, if 380 amp is required, then a 500 MCM cable is required, the equivalent aluminum conductor is a 750 MCM cable. This is a 50% increase in the cross-sectional area of the conductors.

The cost of the materials is dependent upon the market. However, it is typical to see the following cost savings when using aluminum: 30% to 50% for dry-type transformers, 20% for substation dry-type transformers, 25% for liquid-filled pad-mounted transformers, $1,000 per vertical section for a 1,000-amp switchboard, and $1,500 per vertical section for 3,000/4,000-amp switchboards.

Additionally, in regard to aluminum cable, the voltage drop is a larger factor to be considered because it is less conductive. On average, the equivalent aluminum conductor will reduce the length a cable can be run by approximately 40% to still meet the ASHRAE-recommended 3% voltage drop.

Although the busing or windings within the equipment will increase, there typically is not a significant increase, if an increase at all, in the overall size of equipment enclosure when using aluminum versus copper. Large equipment, such as pad-mounted oil-filled transformers and switchboard/switchgear, may increase based upon the current or kVA rating of the equipment. However, panelboards and dry-type transformers will not require an increase in the enclosure size when choosing between these materials.

The more significant difference is the reduction in weight, even though the busing/windings of the equipment have increased in size. As an example, a 1,000-amp busway will be approximately 22% larger for aluminum; the copper bus will be approximately 50% heavier. This will dramatically increase as the bus ratings increase. For instance, for a 4,000-amp busway, the size increase to aluminum over copper is approximately 27%; the weight increase is approximately 73%.

Although there isn't a significant energy efficiency advantage using copper versus aluminum, the material selected for the project application should always be evaluated during design.

As noted above, the major differences between the metals are in regard to the weight and cost of the materials, although there are many other factors that should be taken into account when choosing a current-carrying material, which includes environment. Manufacturers have useful information as well as many white papers on this topic that can be reviewed for additional information. Although the efficiency difference between copper and aluminum transformers is not significantly above 15 kVA, there is enough difference that dictates copper should be used when energy efficiency is the main goal of the project. However, when cost is accounted, the initial cost savings for aluminum often outweighs the loss in energy efficiency.

Balancing electrical loads

One of the no-cost measures to establish energy efficiency in the distribution system design is to balance the single-phase loads on 3-phase distribution systems. If the loads are not properly distributed among the 3-phase buses, the result will be unequal current and unbalanced voltage at the load (unbalanced distortion). Although not a code requirement, the designer should always take balancing the loads into consideration during their design. As a good engineering practice, the unbalanced load should be designed to not exceed 2% unbalance. The unbalanced distortion will cause power loss, voltage-drop issues, and overheating of induction motors and transformers.

This energy efficiency measure is also a no-cost measure for the owner if performed during design and construction, and it does not require additional capital cost to improve energy efficiency in the design. Although the design may be balanced, it also should be emphasized to the electrical contractor during construction to balance the loads.

Even with the care that is taken during design to achieve balanced loads, it should be noted that the unbalance in the electrical system will change as the loads cycle on and off. Therefore, it should be taken into account as much as possible during design that the operation of the building's systems will vary during the actual operation of the building. Thus, metering should always be included in the design of distribution equipment to record trending in the power distribution system and identify issues.

Similarly, the addition of meters in an existing electrical distribution system to monitor trending also can help improve the energy efficiency of those buildings. Identifying areas of unbalanced loads and establishing a maintenance plan to reallocate electrical loads to achieve a maximum of 2% unbalance of phases. As part of this process, load management (load shedding) also should be taken into consideration because relocation of electrical loads can be costly and has the potential of disrupting normal facility operation.

Transformers

Additionally, unbalanced single-phase loads lead to harmonics within the electrical system.

Starting Jan. 1, 2016, the Department of Energy (DOE) established a more stringent mandate for distribution transformers. This mandate serves to establish higher minimum efficiencies of the transformers previously required to meet the NEMA TP-1 efficiency standards, essentially making transformers previously designated as NEMA Premium to now be the standard efficiency levels for 3-phase transformers. It should be noted that the efficiencies in the 2016 DOE mandate are equivalent to the NEMA TP-1 standard for single-phase transformers. But the 2016 DOE mandate does serve to eliminate the manufacturer of the 3-Phase NEMA TP-1-compliant transformers after Jan. 1, because the new DOE standard mandates that new transformers manufactured for the United States are required to meet the 2016 standards.

However, transformers manufactured prior to Dec. 31, 2015 still will be available for purchase by some manufacturers, which is reason to discuss the efficiencies and cost comparisons between TP-1 and the NEMA Premium efficient transformers that are now the minimum standard. But it should be noted that some manufacturers stopped taking orders for transformers that didn't meet the 2016 DOE standards at the end of the third quarter of 2015.

It should also be noted that all the transformers meet the requirements for high-efficiency transformers as outlined in ASHRAE 90.1 Table 8.1. However, not all the transformers meet the requirements of the new DOE 2016 transformer efficiency mandate as mentioned above. The DOE 2016 mandate applies to the following transformers:

The 2016 DOE mandate does not apply to the following transformers:

Heat loss

Heat is the byproduct of inefficiency in electrical systems. Typically, the more efficient the transformer, the less heat that is dissipated from the transformer, therefore, transformers with lower temperature rise tend to be more efficient. General-purpose dry-type transformers used for distribution within buildings come in three standard temperature rises: 80°C (176°F), 115°C (239°F), and 150°C (302°F).

The cost of the power lost to heat can be significant over the lifetime of the transformer. This cost does not take into account the cost of the additional air conditioning and ventilation required to make up for the heat lost into the space, which can also be significant.

Over the lifetime of the transformer, the cost of power lost to heat will justify the additional cost associated with purchasing a transformer with the lower temperature rise when choosing between the 150°C (302°F) and 115°C (176°F) temperature-rise transformers. However, the increase in cost between the 115°C (239°F) and 150°C (302°F) temperature-rise transformers does not always get a return on investment. Therefore, if initial cost is a factor in the decision to choose the transformer, the 239°F temperature-rise transformer provides the best combination of energy efficiency and cost savings.

For the purposes of the following discussion on high-efficiency (NEMA TP-1) and NEMA Premium efficiency (DOE 2016 and Candidate Standard Level-3) transformers, the focus will be on the 115°C (239°F) temperature-rise dry-type transformers because these provide the best combination of energy efficiency and cost savings.

Although manufacturers list efficiencies at 25%, 50%, 75%, and 100% full load, the NEMA TP-1 and 2016 DOE transformer efficiency standards are based upon A DOE study determined transformers are typically loaded at 32%. Therefore, the efficiency standards are required to be met at 35% loading of the transformer, making the transformers more efficient when they are not operating at full load.

The Premium Efficient Transformer Program established by the DOE to establish higher minimum efficiencies for transformers is referred to as Candidate Standard Level, or CSL. The term CSL is used by the DOE in the evaluation process of transformers and includes five levels. Level 1 (CSL-1) is the equivalent efficiency of NEMA TP-1 transformers. The standard additionally breaks down higher levels of efficiency based upon three categories: single-phase transformers (DL6), 3-phase transformers of 15 to 150 kVA (DL7), and 3-phase transformers of 225 to 1,000 kVA (DL8).

Of the NEMA Premium efficient transformers, the most common available by major manufacturers meets CSL-3. Transformers are required to be a minimum of 0.6% more efficient than the TP-1 standard. Although the CSL-3 efficiencies don't meet most of the 2016 DOE mandate, the CSL-3 efficiencies are close to the mandate and can still be purchased, although they are becoming difficult to purchase because they can no longer being manufactured.

Additionally, many manufacturers that met CSL-3 often exceeded the 0.6% minimum-efficiency requirement. However, transformers above this level were not widely available until the 2016 DOE mandate. As the transformers increase in size, the energy efficiency of the CSL-3 and 2016 DOE transformers provide significant annual cost savings that will justify the increased initial cost over the lower-priced TP-1 transformers.

Once again, using the example of the 75-kVA transformer, loaded at 50%, there is an annual cost savings of $256.23 and $337.37, which equates to $6,405.75 and $8,434.25, respectively, over the 25- to 30-year lifetime of the transformers. Additionally, engineers should account for the higher costs of cooling and ventilating to make up for the added heat loss. The cost premium for the 75-kVA CSL-3 transformer over the TP-1 transformer is approximately $3,000, for which the owner would see a payback on initial investment within 10 years. The 2016 DOE transformers also increase the initial cost of the transformer over the NEMA Premium efficient transformers by approximately 10%; the annual cost savings is 20% to 25%.

Because the efficiency standards have changed, the specification of transformers will have to be designed toward the 2016 DOE mandate. Transformers meeting the 2007 DOE efficiencies will still be available for purchase, but it is not known for how long. Therefore, NEMA Premium efficiency transformers meeting the 2016 DOE mandate should be used as a standard design approach for all energy efficiency designs and they, for the most part, serve to meet the requirements of the 2016 DOE mandate.

However, the NEMA TP-1 CSL transformers are tested using only linear loads and don't account for harmonic distortion, which is also a cause of energy efficiency loss in power distribution systems.

VFDs versus motor starters

The addition of VFDs to motors is one of the most commonly used methods to achieve energy efficiency in the power distribution system, because the VFD allows you to adjust the power input to the motor. This ability is useful in many applications, especially in HVAC systems that permit the owner to adjust the cooling or ventilation of a room or area based upon the actual demands on the room on a varying basis.

VFDs typically have an efficiency of 95% to 98%, depending on the type of VFD provided (6- or 18-pulse, active front-end, low harmonic, etc.), while across-the-line and soft starters (reduced-voltage solid-state) have an efficiency of 99.5% to 99.9%. Reduced-voltage solid-state starters use semiconductor devices to temporarily reduce the terminal voltage of motors, which reduces the inrush current and limits shaft torque to gradually start motors. Soft starters and across-the-line starters typically are less expensive and have a smaller footprint than VFDs, with across-the-line starters being the least expensive and requiring the smallest footprint. Therefore, in applications where there is no intent to adjust the speed of the motor, a full load, or off-control scheme, a soft starter provides a greater degree of energy efficiency in the system. It should also be noted that VFDs are most energy-efficient when operated at 50% to 100% of the full load, making it important to size the motor properly to achieve the maximum energy efficiency.

When determining whether to select across-the-line starters or soft starters, energy efficiency typically is not merely a consideration. It is a factor of limiting the inrush to reduce cost in the electrical infrastructure including cable size and circuit breaker size.

VFDs are also a source of harmonics in the electrical distribution system. Whenever VFDs are used, harmonics should be taken into consideration.

Harmonic mitigation

Harmonics are currents or voltages with frequencies that are multiples of the fundamental power frequency (60 Hz), which cause distortion in the electrical distribution system. Harmonics are generated by nonlinear loads that include switch-mode power supplies (SMPS), VFDs, copiers, computers, printers, battery chargers, medical diagnostic equipment, and uninterruptible power supplies (UPSs), to name a few. The reason for this is that electronic equipment requires dc voltage to operate, and in order to convert the ac power supplying the building to dc for the electronic equipment rectifiers and capacitors are required. During the conversion, capacitors will charge and discharge, which will draw current in pulses and at a noncontinuous rate. The noncontinuous draw of current will cause distortions in the electrical system (harmonic distortion). Harmonic currents reduce the electrical system's efficiency by increasing overheating of electrical equipment, therefore increasing air conditioning requirements.

It is a common misconception that K-rated transformers help mitigate harmonic distortion; however, the transformers are actually oversized to withstand the additional heat generated by the harmonics. This means the transformer is less efficient because it will be using higher wattage to provide power to the same electrical load.

Harmonic-mitigating transformers (HMTs) cancel the effects of harmonics being transferred from the secondary windings of the transformer to the primary windings of the transformer, to prevent the harmonics from being introduced into the rest of the electrical distribution system including the electric utilities system. This is achieved through the zigzag winding configuration of the HMTs. Additional information on how the transformers are constructed, how to properly choose the phase shifting, and how they work should be reviewed on the manufacturer's website.

Additionally, HMT transformers meet NEMA TP-1 transformers efficiency ratings. The HMT transformers are tested using nonlinear loads with 100% harmonic distortion, while the TP-1 transformers are tested using linear loads only to achieve the efficiency ratings of the standard. Therefore, HMTs should be used to improve efficiency in the electrical system whenever nonlinear loads are used. However, if only linear loads are powered from a transformer, the energy efficiency of the transformer is essentially the same, and the additional cost associated with an HMT does not justify the purchase.

It should be noted that HMTs are most effective when multiple (two or more) identical loads are being fed from the transformers. When multiple unique loads are being fed from the system that can't easily be matched, other forms of harmonic mitigation should be considered-which include harmonic filters (active and passive) and line reactors.

Cameron Bellao is the lead electrical project engineer at Fitzemeyer & Tocci Associates Inc. He has approximately 10 years of electrical engineering experience that includes energy-efficient designs meeting various energy codes and standards.

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