Phase Shifting with Transformers – New Approaches for Harmonic Mitigation & Power Factor Correction Abstract Improved designs for electrically powered equipment such as solid-state motor drives, electronic lighting ballasts, DC power supplies and computers offer the promise of dramatically improving energy efficiency. These new technologies can have power quality side effects, which must be considered along with the energy benefits. Current demand characteristics, harmonic production and power susceptibility requirements create new issues of compatibility with other devices in the electrical environment. Contrary to this new energy saving product design, the dry type transformer efficiency in the last 20 years has been decreasing to meet market price competition. Under these modern loads, the standard distribution transformer became much less efficient and in some cases obsolete. In view of the increasing cost of energy we are seeing improvements in the design of distribution transformers. C802.2 and TP1 became legislation in the U.S. on January 1st, 2007 and became law in Canada on January 1st, 2005. We will review the reasons and the new style of transformers being designed for these applications. The positive side is that the new energy saving designs are converging with the power quality needs for the new electronic rich environments of our modern facilities. Introduction At the turn of the century, electricity invaded homes all over North America. The power grid that evolved from the beginning has remained essentially the same to this day. Important power transmission over long distances has led to the use of alternating current technology to simplify power transfer. This has served us well for the needs of namely light bulbs and motors. Is this still adequate? We must however take into account other technological developments and energy saving devices that have been developed the last 30 years. Today, electricity is used quite differently than at the beginning of the century. The new generation of computers and peripherals, which use very large-scale integrated circuitry, operate at low voltages and currents. Non-linear AC-DC converters, rather than the linear AC-DC converters, power local area network (LAN) components and workstations. CSA C802.2 and TP1 In 1995 the Department of Energy of the United States requested a study on the true loading of dry-type distribution transformers and subsequently after the first result, ordered more studies to discover that the loading of a transformer was actually much less of that the name plate rating. The result was not much of a surprise, but the problem was that these transformers were designed to be at their maximum efficiency at 100% of their name plate rating. This clearly demonstrated that the standard transformer was a huge energy burden on the electrical power grid. Phase Shifting With Transformers – New Approaches For Harmonic Mitigation & Power Factor Correction _________________________________________________________________________________________________________________ The first standard that was developed from the study was the NEMA TP1, which defined the construction of dry-type low voltage distribution transformer efficiency to be at its best at 35% load and the medium distribution dry-type transformer efficiency to be at 50%. The standard also defined the level of efficiencies. The Department of Energy included this standard in their energy saving program and introduced it as law, on January 1st, 2007. Canada made it law on January 1st, 2005. What does all of this mean? This means a huge Energy Savings for one, but also that most manufacturers had to redesign their line of products to meet these efficiencies. It was all about core loss. The consequences resulted in better efficiency at lower actual loading of the transformer and this meant energy savings due to lower loss and less need to cool the heat losses. The problem is that these loss calculations are made under non-linear loads and do not take into consideration the actual load profiles of the modern installations and their energy saving non-linear loads. Modern non-linear loads In the beginning of the computer era, manufacturers built linear power supplies to power computers, taking continuous sine wave AC power and converting it to DC voltage. This proved to be heavy, large and expensive. In addition, these power supplies were sensitive to power variations, thus creating problems when the power supplied varied more than a few percent from the required voltage (110-120 VAC). To eliminate the drawbacks of the linear supply, non-linear switching power supplies were developed. These replaced the low frequency transformer (60Hz) with a high frequency one, chopping a DC voltage into high frequency bursts of between 20 and 100,000 cycles per second (20-100KHz). Switching units take power in large bursts every half cycle of the sinusoidal waveform. By not being susceptible to voltage variations, they regulate voltage fluctuations by varying the current draws. _________________________________________________________________________________________________________________ Hammond Power Solutions Inc. Page 2 of 8 Literature Code: HPS-TA13 Phase Shifting With Transformers – New Approaches For Harmonic Mitigation & Power Factor Correction _________________________________________________________________________________________________________________ And Power Quality? Due to their design, switching power supplies are more susceptible to many types of problems caused by poor utility power quality and from interference from other nearby equipment (motors, ballast, etc.). They also cause distortion of the sine wave, which creates a set of problems called harmonics. The following illustrates maximum values as per IEEE Std 519-1992 11.5: Bus Voltage at PCC Individual Voltage Distortion (%) Total Voltage Distortion THD (%) 69KV and below 3 5 69.001KV-161KV 1.5 2.5 161.001KV and above 1 1.5 Personal computers, workstations and networks using a switching power supply are sensitive to problems caused by transformers, motors etc. This computer equipment may also feed back problems to those same transformers and motors. Noise comes from anywhere and everywhere. The motor running the elevator in your building or even the one in your photocopier can be the cause of electrical noise. In addition, common mode noise appears as a voltage between the line and the neutral to ground wires of utility Current Distortion vs Voltage and building wiring. This can be more damaging because Distortion integrated circuits are sensitive to the voltage difference In the future, more current distortion will with respect to ground. Several other problems associated with AC power, which users should be aware of are voltage occur due to the growing use of non- spikes, surges, sags or dips, brownouts and blackouts. linear loads. So why is there a concern? The RMS values and voltage distortion! Non-Linear Loads Versus Transformers and Resistance The Ohms law (E=IZ) that we all know Loads still applies and the relationship between voltage, current and impedance is very Over 80% of all DC power supplies installed in electronic important. equipment today are switch-mode power supplies. A non- linear load, such as a computer containing a switch-mode Because of its design, if we look at a power supply, draws short pulses of current within each normal system, a non-linear load will cycle. That results in a distorted or non-sinusoidal current produce, harmonic current which is fed that is made up of many multiples of the line fundamental back to the transformer, the length, the frequency (60Hz). size of the line and the transformer are Because of the skin effect those higher frequencies, travel impedances that will transform current on the outside circumference of the cables increases its harmonic distortion into voltage distortion impedance at those frequencies and also creates more (see IEEE restrictions). losses. These non-linear loads, such as computers, electronic ballasts and variable speed drives, could create safety hazards in buildings, but also will create more losses that will subtract from the original energy saving targets. The same applies for transformers losses. At first there was the “K” factor that was designed to sustain the maximum capacity of the name plate Harmonic Rotation with those types of loads. The second was a new design such as the 60Hz 120Hz 180Hz 240Hz 300Hz harmonic mitigating transformer that was designed to cancel the triplen 360Hz harmonics at very low impedance in the secondary flux of the transformer. F23456 +- 0 +- 0 With the exception of the single phase transformer, it could be classified in two main designs: Delta Wye and Delta Zigzag. The Delta Wye can be Rotation Speed and Resistance to Metal Increase made TP1 and C802.2 compliant or better, but the Delta Zigzag can be better under non-linear loads, due to its low impedance cancellation of the triplen harmonics on the secondary flux. Also, the Harmonic Mitigating transformer’s Delta Zigzag can be used for phase shifting to cancel other harmonics at low impedance improving the total power factor of a building (see IEEE519). _________________________________________________________________________________________________________________ Hammond Power Solutions Inc. Page 3 of 8 Literature Code: HPS-TA13 Phase Shifting With Transformers – New Approaches For Harmonic Mitigation & Power Factor Correction _________________________________________________________________________________________________________________