Modern power quality issues

October 9th, 2014, Published in Articles: Vector

 

As we connect more electronic devices to our power systems, the quality of the power becomes more complex. Grounding affects voltage stability and, more importantly, is critical to personal safety.

Numerous articles have been written on the subject of harmonics. However, these articles do not always address our basic problems in an understandable way:

  • What are harmonics?
  • How do they affect my building or my product?
  • What are the symptoms of harmonics?
  • How do I address these systems?
  • How do I solve the problem?
Fig. 1: The voltage applied in Australia operates at 50 Hz.

Fig. 1: The voltage applied in Australia operates at 50 Hz.

Harmonics are a mathematical model of the real world. Harmonics are simply a technique to analyse the current drawn by computers, electronic ballast, variable frequency drives and other equipment which has modem “transformer-less” power supplies. Ohm’s Law states that when a voltage is applied across a resistance, current will flow. This is how all electrical equipment operates. In the United States, the voltage we apply across our equipment is a sinewave which operates 50 Hz (cycles per second). In Australia, the applied voltage operates at 50 Hz (see Fig. 1).

Utilities constantly do a wonderful job of generating this voltage sinewave. It has (relatively) constant amplitude and constant frequency.

Once this voltage is applied to a device, Ohm’s Law kicks in. This law states that current equals voltage divided by resistance, expressed mathematically as:

I = V/R

Expressed graphically, the current ends up being another sinewave, since the resistance is a constant number. Ohm’s Law dictates that the frequency of the current wave is also 50 Hz. Although the two sinewaves may not align perfectly, the current wave will indeed be a 50 Hz sinewave (see Fig. 2).

Fig. 2: A 50 Hz sinewave.

Fig. 2: A 50 Hz sinewave.

Since an applied voltage sinewave will cause a sinusoidal current to be drawn, systems which exhibit this behavior are called linear systems. Incandescent lamps, heaters and, to a great extent, motors are linear systems. Some of our modem equipment, however, does not fit this category. Computers, variable frequency drives, electronic ballast and uninterruptable power supply systems are non-linear systems. In these systems, the resistance is not a constant and, in fact, varies during each sinewave. This occurs because the resistance of the device is not constant. The resistance, in fact, changes during each sinewave.

The “front end” or power supply of these systems contain solid state devices such as power transistors, thyristors or silicon controlled rectifiers (SCRs). These devices draw current in pulses.

Examine Fig. 3. As we apply a voltage to a solid state power supply, the current drawn is approximately zero until a critical “firing voltage” is reached on the sinewave. At this firing voltage, the transistor or other device gates or allows current to be conducted. This current typically increases over time until the peak of the sinewave and decreases until the critical firing voltage is reached on the “downward side” of the sinewave. The device then shuts off and current goes to zero. The same thing occurs on the negative side of the sinewave with a second negative pulse of current being drawn.

The current drawn then is a series of positive and negative pulses, and not the sinewave drawn by linear systems. Some systems have different shaped waveforms such as square waves. These types of system are often called non-linear systems. The power supplies which draw this type of current are called switched mode power supplies. Once these pulse currents are formed, we have a difficult time analysing their effect.

Power engineers are taught to analyse the effects of sinewaves on power systems. Analysing the effects of these pulses is much more difficult (see Fig. 3).

Fig. 3: Analysing the effects of sinewaves on power systems.

Fig. 3: Analysing the effects of sinewaves on power systems.

To solve this problem, we turn to mathematics and, specifically, Fourier Analysis. Simply stated, using Fourier Analysis, we can prove that any periodic waveform can be expressed as a series of sinewaves with varying frequencies and amplitudes.

That is, we can create a series of sinewaves of varying frequencies and amplitudes to model this series of pulses mathematically. The frequencies we use are multiples of the fundamental frequency, 50 Hz. We call these multiple frequencies harmonics. The second harmonic is two times 50 Hz (100 Hz).

The third harmonic is 150 Hz and so on. In our 3-phase power systems, the “even” harmonics (second, fourth, sixth, etc.) cancel, so we only have to deal with the “odd” harmonics. Fig. 4 illustrates these harmonics. This figure shows the fundamental the third harmonics. There are three cycles of the third harmonic for each single cycle of the fundamental. If we add these two waveforms, we get a non-sinusoidal waveform (see Fig. 4).

This now starts to form the peaks that are indicative of the pulses drawn by switch mode power supplies.

Fig. 4: "Old" harmonics.

Fig. 4: “Old” harmonics.

Distorted periodic waveform

This resultant waveform is very similar to that shown in Fig. 3. If we add other harmonics, we can model any distorted periodic waveform, such as square waves generated by UPSes of VFD systems.

It is important to remember that these harmonics are simply a mathematical model. The pulses, square waves or other distorted waveforms are what we would actually see if we were to put an oscilloscope on the building’s wiring systems.

Because of Ohm’s Law, these current pulses will also begin to distort the voltage waveforms in the building. This voltage distortion can cause premature failure of electronic devices.

On 3-phase systems, the three phases of the power system are 120’ out of phase. The current on phase B occurs 120 degrees (1/3 cycle) after the current on A. Likewise, the current on phase C occurs 120’ after the current on phase B. Because of this, our 50 Hz (fundamental) currents actually cancel on the neutral. If we have balanced  50 Hz currents on our 3-phase conductors, our neutral current will be zero. It can be shown mathematically that the neutral current (assuming only 50 Hz is present) will never exceed the highest loaded phase conductor. So, our overcurrent protection on our phase conductors also protects the neutral conductor, even though we do not put an overcurrent protective device in the neutral conductor. We protect the neutral by the mathematics!

When harmonic currents are present, this math breaks down. The third harmonic of each of the 3-phase conductors is exactly in phase.

The neutral conductor

When these harmonic currents come together on the neutral, rather than cancel, they actually add and we can have more current on the neutral conductor than on phase conductors. Our neutral conductors are no longer protected by mathematics.

These harmonic currents create heat. In time, this heat will raise the temperature of the neutral conductor. This rise in temperature can overheat the surrounding conductors and cause insulation failure.

These currents will also overheat the transformer sources which supply the power system. This is the most obvious symptom of harmonics problems; overheating neutral conductors and transformers. Other symptoms include:

  • Nuisance tripping of circuit breakers.
  • Malfunction of UPS systems and generator systems.
  • Metering problems.
  • Computer malfunctions.
  • Overvoltage problems.

Oversizing neutral conductors

In 3-phase circuits with shared neutrals, it is common to oversize the neutral conductor up to 200% when the load served consists of non-linear loads. For example, most manufacturers of system furniture provide a #10 AWG conductor with 35 A terminations for a neutral shared with the three #12 AWG phase conductors. In feeders with a large amount of non-linear load, the feeder neutral conductor and panelboard busbar should also be oversized.

Separate neutral conductors

On 3-phase branch circuits, another philosophy is to not combine neutrals, but to run separate neutral conductors for each phase conductor. This increases the copper use by 33%. While this eliminates the addition of the harmonic currents on the branch circuit neutrals, the panelboard neutral bus and feeder neutral conductor must still be oversized.

Oversizing transformers and generators

The oversizing of equipment for increased thermal capacity should also be used for transformers and generators which serve harmonics-producing loads. The larger equipment contains more copper.

K-Rated transformers

Special transformers have been developed to accommodate the additional heating caused by these harmonic currents. These types of transformer are now commonly specified for new computer rooms and computer lab facilities.

Special transformers

There are several special types of transformer connection which can cancel harmonics. For example, the traditional delta-wye transformer connection will trap all the triplen harmonics (third, ninth, fifteenth, twenty-first, etc.) in the delta. Additional special winding connections can be used to cancel other harmonics on balanced loads. These systems also use more copper. These special transformers are often specified in computer rooms with well-balanced harmonic producing loads such as multiple input mainframes or matched DASD peripherals.

Filtering

While many filters do not work particularly well at this frequency range, special electronic tracking filters can work very well to eliminate harmonics. These filters are relatively expensive at present, but should be considered for thorough harmonic elimination.

Special metering

Standard clamp-on ammeters are only sensitive to 50 Hz current, so they only tell part of the story. New “true RMS” meters will sense current up to the kilo-Hertz range. These meters should be used to detect harmonic currents.

The difference between a reading on an old-style clamp-on ammeter and a true RMS ammeter will give you an indication of the amount of harmonic current present. The measures described here only solve the symptoms of the problem. To solve the problem, we must specify low harmonic equipment. This is done most easily when specifying electronic ballast. Several manufacturers make electronic ballast which produce less than 15% harmonics. These ballast should be considered for any ballast retrofit or any new project. Until low harmonics computers are available, segregating these harmonic loads on different circuits, different panelboards or the use of transformers should be considered.

This segregation of “dirty” and “clean” loads is fundamental to electrical design today. This equates to more branch circuits and more panelboards, and therefore more copper usage.

Grounding conductors

Grounding conductors are required by the United States National Electrical Code and by most other major electrical codes in the world. In the British IEE Wiring Regulations they are referred to as earthing conductors. No matter what they are called, these conductors serve the same purpose. Grounding conductors connect all of the non-current carrying parts of the electrical system, or any metallic parts in the vicinity of the electrical system together.

This part includes conduits, enclosures, supports and other metallic objects. This grounding system has two purposes:

  • Safety: The grounding conductor system provides a low impedance path for fault currents to flow. This allows the full current to be detected by overcurrent protective devices (fuses and circuit breakers), clearing the fault quickly and safely.
  • Power quality: The grounding system allows all equipment to have the same reference voltage. This helps the facility’s electronic equipment operation and helps prevent the flowing of objectionable currents on communication lines, seals and other connections.

To examine the safety issue more closely, consider a power system consisting of a voltage source (transformer or generator) connected to a disconnect and a panelboard. An appliance is fed from this panelboard. When the circuit is formed, current flows in the circuit, allowing the appliance to operate. The grounding conductor connects the frame of the appliance to be panelboard enclosure and to the service enclosure. This enclosure is connected to the grounded conductor (often the neutral conductor) which, in turn, is connected to the grounded terminal of the transformer. If a fault (or short circuit) occurs, the grounding conductor connection allows current to flow. This current will be much greater than the normal load current and will cause the circuit breaker to open quickly. This clears the fault safely and minimises any safety hazard to personnel.

Suppose the grounding conductor is interrupted. If a fault occurs, no current will flow in the grounding conductor since the circuit is interrupted. This opened grounding conductor could be caused by a grounding prong cut off a plug illegally, a loose connection, a conduit which is not connected properly or many other causes. This fault leaves the frame of the appliance energised.

When someone touches the appliance, the building steel, another piping system, and possibly even a wet concrete floor, the circuit will be completed in current flow through the person’s body, injuring or killing them.

Conclusion

Many codes allow the use of metallic conduits to be used as grounding conductors. Many designers today do not believe that using steel conduits is adequate for this use. The conduit has connections every 3 m and low grade, cast metal couplings and connectors are often are used. For long branch circuits, the impedance of the steel conduit may limit the fault current so that the overcurrent protective device will not operate correctly. For this reason, copper grounding conductors should be specified in every power circuit.

The secondary benefit of this copper grounding conductor is that it will provide an equipotential plane for all equipment connected to it. This often makes the so-called isolated grounding conductors specified by computer and other manufacturers unnecessary.

The importance of grounding to protect motor bearings has been underestimated for too long. To minimise harmful currents and to realise the full “green” potential of VFDs, an economical, long-term method of shaft grounding is a must. Until all OEM motors marketed for use with VFDs are truly “inverter-ready,” retrofitting them with shaft grounding is the best approach.

Acknowledgement

This article was originally published by the Copper Development Association, New York, and in Industrial Electrix. It is republished here with permission.

Contact the Copper Development Association, New York, www.copper.org

Related Articles

  • New plug and connector range launched
  • Contact ball bearings
  • Flanged enclosure range expanded
  • Sealing solutions for wheel-end maintenance
  • Split elastic couplings for high torque