IMPACT OF POWER QUALITY PROBLEMS

Power is simply the flow of energy and the current demanded by a load is largely uncontrollable. “Power quality” is a convenient term for many; it is the quality of the voltage, rather than power or electric current.

The performance of electronic devices is directly linked to the power quality level in a facility. The electric power industry comprises electricity generation (AC power), electric power transmission and ultimately electricity distribution to an electricity meter located at the premises of the end user of the electric power. The electricity then moves through the wiring system of the end user until it reaches the load. The complexity of the system to move electric energy from the point of production to the point of consumption combined with variations in weather, generation, demand and other factors provide many opportunities for the quality of supply to be compromised

Without the proper power, an electrical device may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power. Some of the most common power supply problems and their likely effect on sensitive equipment:

1. Voltage surges/spikes

Voltage surges/spikes are the opposite of dips – a rise that may be nearly instantaneous (spike) or takes place over a longer duration (surge). A voltage surge takes place when the voltage is 110% or more above normal. The most common cause is heavy electrical equipment being turned off. Under these conditions, computer systems and other high tech equipment can experience flickering lights, equipment shutoff, errors or memory loss. Possible Solutions are surge suppressors, voltage regulators, uninterrupted power supplies, power conditioners

2. Voltage Dips

Short duration under-voltages are called “Voltage Sags” or “Voltage Dips [IEC]”. Voltage sag [5, 6] is a reduction in the supply voltage magnitude followed by a voltage recovery after a short period of time. The major cause of voltage dips on a supply system is a fault on the system, i.e. sufficiently remote electrically that a voltage interruption does not occur. Other sources are the starting of large loads and, occasionally, the supply of large inductive loads [6]. The impact on consumers may range from the annoying (non-periodic light flicker) to the serious (tripping of sensitive loads and stalling of motors.

3. Under voltages

Excessive network loading, loss of generation, incorrectly set transformer taps and voltage regulator malfunctions, causes under voltage. Loads with a poor power factor or a general lack of reactive power support on a network also contribute. Under voltage can also indirectly lead to overloading problems as equipment takes an increased current to maintain power output (e.g. motor loads) 

4. High-Voltage Spikes

High-voltage spikes occur when there is a sudden voltage peak of up to 6,000 volts. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well. The effects on vulnerable electronic systems can include loss of data and burned circuit boards. Possible Solutions are using Surge Suppressors, Voltage Regulators, Uninterrupted Power Supplies, Power Conditioners 

5. Frequency Variation

A frequency variation involves a change in frequency from the normally stable utility frequency of 50 or 60 Hz, depending on your geographic location. This may be caused by erratic operation of emergency generators or unstable frequency power sources. For sensitive equipment, the results can be data loss, program failure, equipment lock-up or complete shutdown. Possible Solutions are using Voltage Regulators and Power Conditioners

6. Power Sag

Power sags are a common power quality problem. Despite being a short duration (10ms to 1s) event during which a reduction in the RMS voltage magnitude takes place, a small reduction in the system voltage can cause serious consequences. Sages are usually caused by system faults, and often the result of switching on loads with high demand startup currents. For more details about power sags visit our newsletter archives. Possible Solutions are using Voltage Regulators, Uninterrupted Power Supplies, and Power Conditioners

7. Electrical Line Noise

Electrical line noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) and causes unwanted effects in the circuits of computer systems. Sources of the problems include motors, relays, motor control devices, broadcast transmissions, microwave radiation, and distant electrical storms. RFI, EMI and other frequency problems can cause equipment to lock-up, and data error or loss. Possible Solutions are using Voltage Regulators, Uninterrupted Power Supplies, and Power Conditioners

8. Brownouts

A brownout is a steady lower voltage state. An example of a brownout is what happens during peak electrical demand in the summer, when utilities can’t always meet the requirements and must lower the voltage to limit maximum power. When this happens, systems can experience glitches, data loss and equipment failure. Possible Solutions are using Voltage Regulators, Uninterrupted Power Supplies, and Power Conditioners

9. Blackouts

A power failure or blackout is a zero-voltage condition that lasts for more than two cycles. It may be caused by tripping a circuit breaker, power distribution failure or utility power failure. A blackout can cause data loss or corruption and equipment damage. Possible Solutions is using Generators

10. Very short interruptions

Total interruption of electrical supply for duration from few milliseconds to one or two seconds. Mainly due to the opening and automatic re-closure of protection devices to decommission a faulty section of the network. The main fault causes are insulation failure, lightning and insulator flash-over. Consequences of these interruptions are tripping of protection devices, loss of information and malfunction of data processing equipment

11. Long interruptions

Long interruption of electrical supply for duration greater than 1 to 2 seconds. The main fault causes are Equipment failure in the power system network, storms and objects (trees, cars, etc) striking lines or poles, fire, human error, bad coordination or failure of protection devices. A consequence of these interruptions is stoppage of all equipment

12. Voltage swell

Momentary increase of the voltage, at the power frequency, outside the normal tolerances, with duration of more than one cycle and typically less than a few seconds. The main causes are Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers (mainly during off-peak hours).Consequences is data loss, flickering of lighting and screens, stoppage or damage of sensitive equipment, if the voltage values are too high

13. Harmonic distortion

Voltage or current waveforms assume non-sinusoidal shape. The waveform corresponds to the sum of different sine-waves with different magnitude and phase, having frequencies that are multiples of power-system frequency. Main Causes are Classic sources: electric machines working above the knee of the magnetization curve (magnetic saturation), arc furnaces, welding machines, rectifiers, and DC brush motors. Modern sources: all non-linear loads, such as power electronics equipment including ASDs, switched mode power supplies, data processing equipment, high efficiency lighting. Consequences are increased probability in occurrence of resonance, neutral overload in 3-phase systems, overheating of all cables and equipment, loss of efficiency in electric machines, electromagnetic interference with communication systems, and errors in measures when using average reading meters, nuisance tripping of thermal protections.

14. Voltage fluctuation

Oscillation of voltage value, amplitude modulated by a signal with frequency of 0 to 30 Hz. Causes are arc furnaces, frequent start/stop of electric motors (for instance elevators), oscillating loads. Consequences are most consequences are common to under voltages. The most perceptible consequence is the flickering of lighting and screens, giving the impression of unsteadiness of visual perception 

15. Noise

Superimposing of high frequency signals on the waveform of the power-system frequency. Main Causes are Electromagnetic interference provoked by Hertzian waves such as microwaves, television diffusion, and radiation due to welding machines, arc furnaces, and electronic equipment. Improper grounding may also be a cause. Consequences are disturbances on sensitive electronic equipment, usually not destructive. It may cause data loss and data processing errors

16. Voltage Unbalance

A voltage variation in a three-phase system in which the three voltage magnitudes or the phase angle differences between them are not equal. Causes are large single-phase loads (induction furnaces, traction loads), incorrect distribution of all single-phase loads by the three phases of the system (this may be also due to a fault). Consequences are Unbalanced systems imply the existence of a negative sequence that is harmful to all three phase loads. The most affected loads are three-phase induction machines.

HANDLING HARMONICS WITH AC DRIVES

AC motors are abundant in the most of the industry. Every fan and pump application requires an AC motor. For efficient and reliable control, adjustable-speed drives are becoming a common companion for these motors. However, as drives are boosting production, they might also be contributing to electrical noise problems, such as line current harmonics.

Pumping and mixing motors are becoming abundant in industrial wastewater facilities as systems expand to keep up with population growth and demand for more water, distributed over larger areas. To maintain water pressure and control flow, adjustable speed drives are a valuable companion for these AC motors.

Drives are an efficient alternative to throttling values, as they match power needs and motor speed to the demands of varying flow. Drives can save energy and electrical costs, increase pressure reliability, reduce cumbersome pump changes and lessen damage to mechanical equipment. However, as drives are boosting efficiency and reliability, they might also be contributing to power quality problems, such as line current and voltage harmonics.

Harmonics are deviations from the sinusoidal fundamental AC line voltage and current. Most electrical power in North America operates at a frequency of 50 hertz. A harmonic frequency is an integer multiple of this fundamental frequency. So in a 50-hertz system, the second harmonic would be 100-hertz, the third would be 150-hertz, and so on. The addition of any harmonic to the sinusoidal fundamental current or voltage will create distortion. The greater the amplitudes of the harmonics present, the greater the distortion in the electrical waveform. Very simply, whenever a voltage or current does not look like a perfect sinusoidal waveform, it contains harmonics.

Compared to DC motor drives, AC motor drives cause very few problems. However, poorly designed applications can result in power line voltage distortions. These voltage distortions can cause problems for other equipment connected to the same power lines – resulting in erratic operation of controls, dimming of lights, audible noise on telephone systems, and static on AM and FM radios. The distribution transformers and cables feeding these drives will also experience additional heating, which reduces the power utilization of those components.

What Causes Harmonics?

Unlike an AC motor operating across the power line, the current drawn from a distribution transformer feeding a typical AC drive is far from a sinusoidal waveform. This occurs because the drive is taking current from the transformer only during certain times of the cycle to convert the AC line voltage to a fixed DC voltage within the drive. The drive then pulse-width-modulates this fixed DC voltage into variable frequency, variable voltage for the motor. The AC-to-DC conversion is what causes the harmonics. Current flows only during part of the cycle and is off during other parts of the cycle, creating the odd-looking current waveform. It is this distorted current that creates the voltage distortion.

As the number of drives employed in automation systems grows, so too does the misconception that drives are the only piece of the puzzle to harmonic problems. While they may comprise a sizable portion of non-linear loads, they are only one element in the harmonic equation. That's why it's important to analyze all electrical loads that could potentially cause problems for a system before making any final conclusions.

What Affects Harmonic Current?

The harmonic current created by the drive can be affected by the presence of a DC link choke, the DC bus capacitance and the line inductance between the drive input and its source of voltage; such as transformer inductance, cable inductance and additional line reactors.

The closer you are to the drive, the greater the voltage harmonic distortion. This is why the voltage distortion is greater at the drive than at the transformer. If other equipment needs to be connected to the same transformer as the drive, connect the other equipment as close to the transformer as possible. This will minimize the effect of the current harmonics produced by the drive on the voltage waveform, which can cause problems with the other equipment.

Reducing Harmonics

There are several methods and products to reduce the line current harmonics created by drives. Cost is not the only factor that distinguishes one solution from another. Even though the addition of line reactors or passive filters can help reduce the current harmonics, they will also reduce the DC bus voltage within the drive at full speed, full load conditions. This will prevent the drive from being able to provide full power to the motor, limiting the power out of the motor to about 95% of its nameplate rating.

Why would you install a 100 hp motor only to have its capability limited to 95 hp, especially under peak demand conditions? This is why multi-pulse solutions are a better fit for most situations, since no derating is necessary and it is less expensive than other mitigation methods.

Harmonic Mitigation Methods

For a 250 hp drive, the chart above details the typical harmonic current, I(THD), and relative installed costs associated with various harmonic mitigation methods.

(For simplicity, we will say that the 250hp drive is connected to a supply with a short circuit capacity of 60,000A, will draw 260Arms of fundamental current for its load, and will have a cost of 100%.)

The goal for those specifying harmonic mitigation solutions is to combine the most cost effective solution for the particular drive and power distribution system. Since several factors come into play in determining the harmonics in the drive system, Emerich Power Quality Analysis  needs to consider all factors before arriving at a final recommendation.

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Most Frequent Power Quality Disturbances & its Impact on Industries

Widespread use of electronic equipment(for e.g. VSD, PLC, automation & data processors, inverters,computerized numerical control, etc.) in today’s industrial environment, various PQ problems like voltage dips, short / long interruptions, harmonics, transients & surges and other PQ related problems are frequently encountered. Such interruptions cause huge financial losses both – to the utility and to the end-consumer. 

Some of the key physical manifestations of the PQ issues are mentioned below:

Equipment affected due to poor PQ	External Manifestation of poor PQ	Electrical PQ problem
Relays, contactors and circuit breakers	Nuisance tripping of protective devices	Distorted voltage waveform because of voltage dip
Telecommunication system	Noise interference to telecommunication lines	Electrical noise causing interference signals
Variable speed drives, welding equipment, security, converters, access control systems, etc.	Motors or process equipment are damaged / Overheating of motors	Capacitor bank failure, shocks due to neutral voltage, Flickering of lights, noise in telecom lines
Motors and process devices	Motors or other process equipment fails or malfunctions	Presence of voltage and current harmonics in the power supply
Sensitive measurements of process control equipment	Loss of synchronization of processing equipment	Severe harmonic distortion creating additional zero-crossings within a cycle of the sine wave
Data Processing / IT equipment (that are sensitive to change in voltage signal)	Computer Screen Freeze, lock-ups / Loss of Data	Voltage Variation

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Consequences of high harmonic distortion levels

The total harmonic distortion(THD) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion factor, a closely related term, is sometimes used as a synonym.

The plant engineer’s worst fear…

Just as high blood pressure can create stress and serious problems in the human body, high levels of harmonic distortion can create stress and resultant problems for the utility’s distribution system and the plant’s distribution system, as well as all of the equipment that is serviced by that distribution system.

The result may be the plant engineer’s worst fear – the shutting down of important plant equipment ranging from a single machine to an entire line or process.

Equipment shutdown can be caused by a number of events. As an example, the higher voltage peaks that are created by harmonic distortion put extra stress on motor and wire insulation, which ultimately can result in insulation breakdown and failure. In addition, harmonics increase rms current, resulting in increased operating temperatures for many pieces of equipment, greatly reducing equipment life.

Table below summarises some of the negative consequences that harmonics can have on typical equipment found in the plant environment.

Negative Consequences of Harmonics on Plant Equipment

While these effects are categorised by problems created by current and voltage harmonics, current and voltage harmonic distortion usually exist together (current harmonic distortion causes voltage harmonic distortion) .

"Harmonic distortion disrupts plants. Of greatest importance is the loss of productivity, throughput, and, possibly, sales"

These occur because of process shutdowns due to the unexpected failure of motors, drives, power supplies, or just the spurious tripping of breakers. Plant engineers realize how costly downtime can be and pride themselves in maintaining low levels of plant downtime. In addition, maintenance and repair budgets can be severely stretched.

For example, every 10°C rise in the operating temperatures of motors or capacitors can cut equipment life by 50%.

Basics of Harmonics...

Harmonic is multiple order of the fundamental frequency and it can be voltage and current in an electric power system are a result of non-linear electric loads. Harmonic frequencies in the power grid are a frequent cause of power quality problems. Harmonics in power systems result in increased heating in the equipment and conductors, misfiring in variable speed drives, and torque pulsations in motors. Reduction of harmonics is considered desirable.

CURRENT HARMONICS:

In a normal alternating current power system, the current varies sinusoidally at a specific frequency, usually 50 or 60 hertz. When a linear electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage (though usually not in phase with the voltage)

Current harmonics are caused by non-linear loads. When a non-linear load, such as a rectifier is connected to the system, it draws a current that is not necessarily sinusoidal. The current waveform can become quite complex, depending on the type of load and its interaction with other components of the system. Regardless of how complex the current waveform becomes, as described through Fourier series analysis, it is possible to deconstruct it into a series of simple sinusoid, which start at the power system fundamental frequency and occur at integer multiples of the fundamental frequency.

Further examples of non-linear loads include common office equipment such as computers and printers, Fluorescent lighting, battery chargers and also variable-speed drives.

In power systems, harmonics are defined as positive integer multiples of the fundamental frequency. Thus, the third order harmonic is the third multiple of the fundamental frequency. This type of harmonics is generated in non-linear loads. Examples of nonlinear loads include transistors, electrical motors, and the non-ideal transformer. Non-linear loads create disturbances in the fundamental harmonic, which produce all types of harmonics. However, in this section we focus on the 3rd order harmonic due to its certain special characteristics in the context of powers systems

Power is supplied by a three phase system, where each phase is 120 degrees apart. This is done for two reasons: Firstly it is because generators/motors that use three phases are more efficient due to the constant torque the phases supply, and secondly it is because after power is supplied to a load, the three phases can theoretically be added onto a neutral wire and cancel each other out. This saves the utility from creating return wiring to the power plant. However, if the 3 phases contain 3rd order harmonics, the currents will not fully add to zero. As seen in the figure, the 3rd harmonic will add constructively with the 3rd harmonics within the other phases. This leads to an oscillating current in the neutral wire, which can be dangerous since it is designed (i.e. small-size conductors) to carry minimal current.

VOLTAGE HARMONICS:

Voltage harmonics are mostly caused by current harmonics. The voltage provided by the voltage source will be distorted by current harmonics due to source impedance. If the source impedance of the voltage source is small, current harmonics will cause only small voltage harmonics. It is typically the case that voltage harmonics are indeed small compared to current harmonics. For that reason, the voltage waveform can usually be approximated by the fundamental frequency of voltage. If this approximation is used, current harmonics produce no effect on the real power transferred to the load. An intuitive way to see this comes from sketching the voltage wave at fundamental frequency and overlaying a current harmonic with no phase shift (in order to more easily observe the following phenomenon). What can be observed is that for every period of voltage, there is equal area above the horizontal axis and below the current harmonic wave as there is below the axis and above the current harmonic wave. This means that the average real power contributed by current harmonics is equal to zero. However, if higher harmonics of voltage are considered, then current harmonics do make a contribution to the real power transferred to the load.

HARMONICS FUNDAMENTALS:

Harmonics are caused by distortions to the underlying sinusoid of any signal, be it power, audio, radio frequency, even mechanical vibrations. Harmonic behaviour is defined by the Bullard Laws Of Harmonics

1. Harmonic amplitudes are proportional to the area of the distortion.
2. The Harmonic Signature is the result of the angle where the sinusoid impacts the distortion as predicted by the Bullard Harmonic Solution.
3. Even harmonics don't appear in symmetrical distortion because they cancel each other out.
4. When a portion of a sinusoid is removed (such as in clipping), the Harmonic Signature mirrors the harmonic signature of the feature removed from the sinusoid.
5. Square waves, triangle waves and all other kinds of waveforms obey these rules and their harmonic behaviour can be predicted with simple mathematical formulas.

Harmonics are caused by steady-state distortions to current and voltage waves and repeat every cycle. They are different from transient distortions such as spikes, dips and impulses.

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