How do Harmonic Mitigating Transformers save energy?

Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.

Harmonic Mitigating Transformers save energy by reducing losses in the following ways:

1. Zero phase sequence harmonic fluxes are canceled by the transformers secondary windings. This prevents triplen harmonic currents from being induced into the primary windings where they would circulate. Consequently, primary side I2R and eddy current losses are reduced.

2. Multiple output HMT’s cancel the balanced portion of the 5th, 7th and other harmonics within their secondary windings. Only residual, unbalanced portions of these harmonics will flow through to the primary windings. Again I2R and eddy current losses are reduced.

3. Many HMT designs are highly efficient at 60Hz as well as at harmonic frequencies. Energy Star compliant models meet NEMA TP-1 energy efficiency minimums at 35% loading. This is typically achieved by reducing core losses to further improve efficiencies under lightly loaded conditions. For optimum energy efficiency performance, Mirus’ Energy Star compliant Harmony Series HMT’s are designed to meet NEMA TP-1 minimum efficiencies not only at 35% but in the entire operating range from 35% to 65%.

Figure 14-1: 75 kVA Transformer losses at various loading conditions with non-linear K-9 load profile.

Figure 14-1 provides an example of the energy savings that can be realized when HMT’s are used in lieu of conventional or K-rated transformers. A K-9 load profile, typical of a high concentration of computer equipment (Ithd = 83%), was selected for the analysis. Losses were calculated for various types of 75 kVA transformers at varying load conditions. In the graph, Conv is a conventional delta-wye transformer, K-13 is a K-13 rated delta-wye and H1E is a Harmony-1E single output Energy Star compliant HMT.

Figure 14-2: Energy Efficiencies for various types of 75 kVA transformers supplying linear (K-1) loads and non-linear (K-9) loads under varying load conditions.

The chart shows how energy savings become more and more substantial as a transformer’s load increases. This is logical since it is the load losses which are most affected by the harmonic currents and these are proportional to the square of the current (I2R and I2h2). Figure 14-2 further emphasizes how transformer efficiencies are affected by non-linear loading. It compares the performance of various types of transformers with linear loading (K-1) and non-linear loading (K-9). The efficiencies of the conventional and K-13 transformer are much lower when they are subjected to a load with a K-9 profile, especially under the heavier loading conditions.

Determining the amount of energy savings associated with a reduction in harmonic losses requires information on the Electric Utility rate and the load’s operating profile. These parameters can vary quite substantially depending upon the location of the facility and the specific application. Table 14-1 shows the energy savings that can be realized when a Harmony-1E HMT is compared with a typical K-13 transformer. As in the previous examples, the transformers are 75 kVA and the non-linear load profile is that of a typical K-9 load.

Table 14-1

Table 14-1: HMT energy savings and payback estimate comparing a 75 kVA HMT to a K-13 transformer in a typical office environment with a high concentration of computer equipment

The monetary savings are based on the equipment operating 12 hours per day, 260 days per year at an average Utility rate of $0.07 per kWhr and assumes that additional cooling energy is required by the building’s air conditioning system to remove the heat produced by the transformer losses. The calculation is as follows:

Annual Consumption = (Total losses in kW) x (hrs/day) x (days/yr) + (NL loss in kW) x (24 – hrs/day) x (365 – days/yr)) $/yr Savings = (H1E Annual Consumption – K13 Annual Consumption) x 1.35 x (rate in $/kWhr)

This previous example could be typical of an office environment with a high concentration of computer loads and with the transformer located in air conditioned space. The requirement to cool the heat produced by the transformer’s losses is typically 30% to 40% of the power in the losses (thus the 1.35 multiplier in calculation of $/yr Savings). Paybacks were calculated based on estimated transformer costs and would result in recovering the Harmony-1E premium many times over based on the transformer’s life expectancy of 30 to 40 years.

Table 14.2 provides another example. In this case, a lower harmonic content K4 load profile was used with the equipment operating 24 hrs/day, 365 days a year and the transformer located in air conditioned space. An example of such a location might be a Broadcasting Facility or Data Center. As can be seen, paybacks are even more attractive.

Table 14.2

Table 14-2: HMT energy savings and payback estimate comparing a 75 kVA HMT to a K-13 transformer in a typical Broadcasting Facility or Data Center

In summary, the inherent ability of Harmonic Mitigating Transformers to cancel harmonic currents within their windings can result in quantifiable energy savings when compared with the losses that would exist if conventional or K-rated transformers were used. If we consider the average premium cost of an HMT over a K-13 transformer, the typical payback in energy savings is 1 to 4 years when loading is expected to be in the 50% to 65% range.

Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers

This document has been written to provide answers to the more frequently asked questions we have received regarding harmonics and the Harmonic Mitigating Transformer technology used to address them. This information will be of interest to both those experienced in harmonic mitigation techniques and those new to the problem of harmonics. For additional information visit our Website at

Published by PQBlog

Electrical Engineer

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