How Harmonic Mitigation Transformer (HMT) is much better option than a K rated transformer ?

It is the normal Practice for data centers, medical facilities and CNC machining centers using K rated transformer for providing Galvanic isolation, filtering common mode noise, reduction of earth leakage current and for controlling elevated neutral voltage.

K factor is a method of rating or simply de- rating a transformer to withstand non linear loads with the sole purpose of preventing transformer overheatinh and the consequent damage to the transformer.

The normal range of the K factor is from 1 to 50. A k factor of 1 indicates a purely non linear load without harmonics. The load is a pure sine wave voltage and current with perfect symmetry. Normally this is not the case in many installations.

A K factor for 5o indicates severe non-linear loads rich in harmonics of the different spectrum (3rd to 50th) thus requiring a highly K rating of Transformers, and are designed to withstand the heat generated by harmonics.

It is well-known fact that heat is the worst enemy of transformers and electronic equipment, and causes severe damage to electronic components and premature failure of Transformers.

With the advent of automation and digitalization in order to improve productivity businesses have adopted automation and digitalization to reduce costs and improve productivity. But the flip side is the introduction of Power supplies, UPS, and drives which are rich in harmonics. These Power supplies generate harmonics and consequently power quality degrades and reliability is affected severely. Voltage and current distortion contribute to failure and damage to equipment, and therefore productivity suffers due to down time of these expensive equipments.

The entire electrical network comes under severe stress due to resonance, this is a dangerous phenomenon that causes high voltages and high current which causes failures of the electrical network itself. Heating of Cables, MCBs tripping are quite common in such networks. Besides electrical losses occur due to skin effect (I2R losses)due to the higher frequency components, which have different phase angles, amplitudes, and frequencies.

The triplen Harmonic generate by non-linear loads are the most harmful, they cause additional current to flow to the neutrals conductor and consequently burning of the neutral current which in turn causes high voltages in the entire facility.

K-rated transformer neutral conductors are sized higher than the phase currents to handle the additional currents to reduce the overall impedance.

There are various other losses in transformers that cause heating, but the major contributor to heat is the eddy currents. The transformer eddy current losses are proportional to the harmonic current squared times the harmonic number squared. The ohms law (Ih x Zh) which applies to 50 Hz applies to all other frequencies and hence adds to the impedance in the electrical system.

As the transformer starts heating up, the resistance to winding will also increase due to the temperature coefficient of copper, which can aggravate the thermal problem. Heat has an adverse effect on the transformer insulation, and therefore sizing of the transformer with harmonic currents content is very critical to prevent failures and to ensure the transformer life span is extended to 25 to 30 years.

K factor Transformer involves special design like using multiple wires (to reduce skin effect) ample space for air circulation (for cooling) and the extra iron cores to avoid derating. The K factor indicates the amount of derating a specific transformer for loads with non-linearity. The higher the K factor higher is the capacity to withstand the extra heat generated.

K factor transformer is designed by the following equation.

As seen from the above equation, the value of the K factor is affected much more by the higher harmonics than by the lower ones.

Eg: If 1 ampere is the 3rd harmonic the heating effect is 3x3 9 times, at 5th harmonics the heating will be 5x5 25 times, and so on. Hence the harmonics of the 5th are more than the 3rd and contribute higher to heating, Higher the harmonics higher the K rating required.

 Levels of harmonics:

• K-1 Standard transformers for standard lightning and motors

• K-2 Induction heat, SCR, AC Drives

 • K-4 Electrical Discharge Lights, UPS w/option input filters, welders, inductive heating, PLC and solid-state controls

• K-13 Solid-state lightning and hospital use

• K-20 Data processing computer, computer rooms

 • K-30 Multi-wiring receptacles circuits in Commercial office space and small mainframe computers

• K-40 Loads producing very high amounts of harmonics.

 How Harmonic Mitigating Transformers Outperform K-Rated Transformers:

 The use of K-rated transformers has become a popular means of addressing harmonic-related overheating problems where personal computers, telecommunications equipment, broadcasting equipment, and other similar power electronics are found in high concentrations. These non-linear loads generate harmonic currents which can substantially increase transformer losses. The K-rated transformer has a more rugged design intended to prevent failure due to overheating. Unfortunately, a transformer designed simply to protect itself fails to address the other important problems associated with harmonics. Specifically, in order to prevent high voltage distortion levels from adversely affecting the equipment loads, the transformer must be capable of canceling harmonic currents and fluxes within its windings. Harmonic Mitigating Transformers (HMTs) will not only prevent transformer overheating failures but will also reduce failures in connected equipment by ensuring that IEEE Std 519 voltage distortion limits are met throughout the power distribution system.

Introduction

It is quite commonly known that harmonics generated by non-linear loads can cause serious overheating problems in standard distribution transformers. Even under much less than fully loaded conditions, transformers have been known to fail catastrophically. One of the main reasons for this is that harmonic currents will dramatically increase the eddy current losses in a transformer.

 The relationship is as follows:

Where: PEC = total eddy current losses

PEC-1 = eddy current losses at fundamental

Ih = rms current at harmonic h

h = harmonic number

This has prompted transformer manufacturers to build more robust designs which can tolerate the additional harmonic losses. In the interest of standardization, a rating scheme has been adopted known as K-factor. K-factor reflects the increase in eddy current losses and is defined as follows:

here: Ih = rms current at harmonic h, in per unit of rated rms current

A K-rated transformer then is one that can be loaded to its full load rating when servicing a non-linear load with a K-factor rating no greater than the transformer’s K-rating. Standard K-factor ratings are 4, 9, 13, and 20.

It is important to note that specifying an extremely high K-rating is not necessarily a good thing. For example, a K-20 transformer is only required if the load it services is actually a load above K=13 and if it is expected to operate at or near full load rating. Specifying too high a K-rating will result in an oversized transformer which can introduce problems such as very high inrush currents, excessive current fault levels, higher core losses and a larger footprint.

The harmonics problem, however is much more than simply the overheating of electrical distribution equipment. When servicing a high concentration of non-linear loads, power distribution systems can experience a wide variety of problems, which include:

1.  Power factor correction capacitor failures due to overloading and/or system resonance

2.  Overheating cables, transformers, and other distribution equipment reducing their life span

3.  High voltage distortion (typically in the form of flat-topping) especially when operating on weak sources, such as emergency generators or UPS systems.

4. False tripping of circuit breakers

5.  Premature failure of rotating equipment (motors, generators, etc.)

56  Mis-operation or component failure in PLCs, computers or other sensitive loads

Of all the problems listed above, those resulting from high voltage distortion are often the most severe and generally the most difficult to identify.

How Harmonic Currents Create Voltage Distortion

Voltage distortion is created as harmonic currents, generated by non-linear loads, pass through the impedance of a power distribution system. Current at any frequency flowing through an impedance will result in a voltage drop in the system at that frequency. This is a simple application of Ohms Law, Vh = Ih x Zh, where:

Vh = voltage at harmonic number h

Ih = current at harmonic number h

Zh = impedance of system to harmonic h

The cumulative effect of the voltage drops at each frequency produces voltage distortion. A common term used to indicate the amount of waveform distortion is Total Harmonic Distortion or THD. THD is expressed in percent and for power systems can be applied to both voltage and current. Voltage total harmonic distortion (Vthd) is defined as a root mean square of all the harmonic voltage drops and is expressed as follows:

Current distortion is a measure of the combined effect of the various harmonic currents present

Voltage distortion then is a function of both the system impedance and the amount of harmonic current in the system. High system impedance due to long cable runs, high impedance transformers, generators, UPS systems, etc. usually causes high voltage distortion levels.

The basic relationship between current and voltage distortion can be seen by examining the waveforms themselves. A typical non-linear load, the Switch-Mode Power Supply (SMPS) is shown in Figure 1. This device draws current only during the peaks of the voltage waveform while charging the smoothing capacitor. As the applied voltage drops during the rest of the cycle, the capacitor discharges to support the load. The pulses of current which recharge the capacitor cause voltage drops which clip-off or flat-top the voltage peaks.

The Effect of Voltage Flat-topping on the Switch-mode Power Supply

How voltage flat-topping affects the operation of an SMPS is a fairly controversial topic. Many equipment manufacturers specify < 5% Vthd in their installation manuals but readily admit that many installations exceed these recommendations. IEEE Std 519 (2014) also lists 8% Vthd as a recommended limit and states, “All users limit their harmonic current emissions to reasonable values determined in an equitable manner based on the inherent ownership stake of each user has on the supply system.

 The problem of voltage flat-topping becomes even more severe when weak sources such as standby generators or UPS systems are used. This is due to their higher source impedance. A generator’s impedance, for instance, can be several times higher than that of a distribution transformer. Therefore, if under normal utility power, voltage distortion at an emergency transfer switch is around 3% or 4%, it would very likely be over 10% when transferred to the standby generator during a power interruption. Often harmonic problems become obvious only when operating under emergency conditions.

High voltage distortion has been known to prevent closed transition returns to the utility when power is restored after an interruption. The highly distorted voltage waveform at the generator can prevent synchronization with the normal power source. Without proper synchronization, the two sources cannot be paralleled and therefore, the load must be dropped before it can be transferred back to normal power.

Using HMTs to Prevent Voltage Flat-topping - Harmonic Mitigating Transformers are specially designed to reduce system voltage distortion in addition to reducing the heating effects caused by the harmonic currents. This is accomplished by cancelling load generated harmonic fluxes and currents within the transformer’s windings:

1.  Zero sequence harmonic currents, which include the 3rd, 9th and 15th, are prevented from circulating in the primary windings by cancelling their fluxes within the secondary windings.

2.  Single output HMTs are available in 2 models with differing phase shifts which when paired will induce cancellation of 5th, 7th, 17th & 19th harmonic currents upstream.

3.  Dual output HMTs phase shift to cancel the balanced portion of 5th, 7th, 17th & 19th harmonic currents within their secondary windings.

4.  Three output HMTs phase shift to cancel the balanced portion of 5th, 7th, 11th & 13th harmonic currents within their secondary windings.

5.  Reduction of harmonic currents in the primary windings and upstream of the HMT reduces the harmonic voltage drops and voltage distortion these drops produce.

6.  Losses are also reduced because the transformer and upstream distribution equipment is subjected to less harmonic current.

This means that an HMT will produce significantly less voltage distortion than a conventional or K-rated transformer when servicing similar non-linear loads. In Table 1, the performance of various HMT models is compared with a typical K-13 transformer. Included as well, is a simple dual output phase shifting transformer (Forked Wye) which is also finding application as a harmonic cancelling transformer. This transformer has dual outputs which are phase shifted by 30o to cancel the balanced 5th and 7th harmonic currents in much the same manner as the dual output HMT. The principal difference from the HMT is in the treatment of the 3rd and other triplen harmonics. Where the HMT prevents these currents from circulating in its primary windings, the Forked Wye does not. This means that voltage distortion at the 3rd harmonic will be just as high as it would be in the K-13 and conventional transformer.

Computer modeling was used to determine the voltage distortion that would appear at each transformer’s output when servicing a K-13 non-linear load of Ithd = 88%. As can be seen, the HMT transformers produced lower voltage distortion than both the K-13 and the Forked Wye. The multiple output HMTs achieved the best results by treating 5th, 7th and 15th.

 Conclusion:

From the above it is clear these HMT is a better option in terms of cost and performance, especially of Datacentres and CNC machines.

Credits:

1. mirus international
2. Hammond solutions