MEASUREMENT OF POWER QUALITY PROBLEMS CAUSED BY COMMON LOADS - PQ CASE STUDY

PQ CASE STUDY # 2
Measurement of Power Quality Problems Caused by Common New Loads


1. Introduction
• 2. Theoretical background of power quality and loads
• 3. Description of practical case measurement study
• 4. Results of practical case study
• 4.1 Wood splitter in rural area network
• 4.2 Heat pump in urban area network
• 5. Summary

The use of energy has faced changes during recent years
• Three main issues have driven this change
• Green house effect
• Energy price (which has risen)
• Reduced manufacturing costs of devices
• These issues have brought new devices available to every  consumer
• For example: compact fluorescent lamps, heat pumps,  welding devices, wood splitter etc.

•In this case we decided to  study power quality problems  caused by a wood splitter and  a ground source heat pump
• The decision was made by  power quality complaints  received by a distribution  network company in Finland
• Also the increasing number  of these loads has affected  to decision
•We consider power quality on the basis  of standard EN 50160
•In this study we are focusing on flicker  caused by rapid voltage changes because  it is the biggest reason for power quality  complaints (at least in rural area network)
• Changes in load/device currents produce  voltage changes over network  impedance
• Voltage changes are different depending  on connection of device
• In three phase symmetrical connection all  phases will experience same kind of  change
• In single phase connection all phases will  experience different kind of voltage  change due to star point displacement
• Two things affect to flicker index:  magnitude and repetition frequency of  voltage change.

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POWER QUALITY BUSINESS CASE FOR POWER CONDITIONING - PQ CASE STUDY #1

PQ CASE STUDY # 1
Power Quality - The Business Case for Power Conditioning

In a two-year study involving customer projects around the world, Powervar, a specialist in power management systems, examined how power quality technology can significantly reduce the service burden rate, delivering cost savings and a measurable return on investment (ROI).  Here, Powervar UK Country Manager, Rob Morris, talks about the study, and outlines the business case for power conditioning

Most organisations today recognise power quality is an issue when it comes to the detrimental effect of power disturbances, such as electrical noise and voltage impulses, on sensitive electronic systems. The problem, however, is in accurately calculating figures that demonstrate the financial downside of dealing with bad power. Or simply put, how can you establish the ROI when selecting from different power protection systems?

For many organisations, power quality problems tend to be ‘out of sight, out of mind’. While the frequency of spikes, surges and other phenomena in power distribution is generally understood and accepted, many fail to make the connection between these irregularities and the impact to the bottom line.

Over the last two years, we have worked closely with industry leading customers to identify a technique to help quantify and educate the industry about calculating real return on investment. A major focus has been about understanding the so-called ‘service burden rate’ – this is the proportion of the price of a product allocated to cover ongoing maintenance and repairs during the warranty period.

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HIOKE 3198 POWER QUALITY ANALYZER - VIDEO

This is not an endorsement post but a demonstration of one of the many PQ analyzers in the market. This is Hioke's Model 3198 PQ Analyzer.

It has the following features:


Verify power problems in accordance with the IEC61000-4-30 Class A standard
High Accuracy and continuous gapless recording (V:±0.1% of nominal voltage, A and W:±0.2% rdg. ±0.1%f.s.)
CAT IV 600V - safe enough for incoming power lines
Broadband voltage range lets you measure even high-order harmonic components of up to 80 kHz
Wide dynamic range from low voltages up to 1300V (3P4W line-to-line voltage)
Maximum 6000V transient overvoltage up to 700kHz
LAN, USB and SD card interfaces
Optional GPS BOX for synchronizing multiple devices

Stay Tuned To whatispowerquality.blogspot.com for more features.

POWER INTERRUPTION : POWER QUALITY PROBLEM?


INTERRUPTION POWER QUALITY PROBLEMS

An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment
failures, and control malfunctions.

The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices.

Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption.

The duration of an interruption due to equipment malfunctions or loose connections can be irregular. Some interruptions may be preceded by a voltage sag when these interruptions are due to faults on the source system.

The voltage sag occurs between the time a fault initiates and the protective device operates. Figure 2.5 shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.



The difference between long (or sustained) interruption and interruption is that in the former the supply is restored manually, but during the latter the supply is restored automatically. Interruption is usually measured by its duration.

For example, according to the European standard EN-50160 [24]:
- A momentary interruption is between 30 cycles and 2 seconds;
- A temporary interruption is between 2 seconds and 2 minutes; and
- A sustained interruption is longer than 2 minutes.

VOLTAGE SWELLS : POWER QUALITY PROBLEM


WHAT ARE VOLTAGE SWELLS?

A“swell” is the converse of the sag, and is a brief increase in the rms line voltage. A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.

As with sags, swells are usually associated with system fault conditions, but they are not as common as voltage sags. One way that a swell can occur is from the temporary voltage rise on the unfaulted phases during an SLG fault.


Figure 2.8 illustrates a voltage swell caused by an SLG fault. Swells can also be caused by switching off a large load or energizing a large capacitor bank.


Swells are characterized by their magnitude (rms value) and duration. The severity of a voltage swell during a fault condition is a function of the fault location, system impedance, and grounding. On an ungrounded system, with an infinite zero-sequence impedance, the line-to-ground voltages on the ungrounded phases will be 1.73 pu during an SLG fault condition.

Close to the substation on a grounded system, there will be little or no voltage rise on the unfaulted phases because the substation transformer is usually connected delta-wye, providing a low-impedance zero-sequence path for the fault current.

Faults at different points along four-wire, multigrounded feeders will have varying degrees of voltage swells on the unfaulted phases. A 15 percent swell, like that shown in Fig. 2.8, is common on U.S. Utility feeders.

Swells are not as common as sags and their main causes are

- switching off of a large load,
- energizing a capacitor bank, or
- voltage increase of the unfaulted phases during a single line-to-ground fault [10].

In some cases the term "momentary overvoltage" is used as a synonym for the term swell. As in the case of sags, UPS or power conditioners are typical solutions to limit the effect of swell.


VOLTAGE SAGS (DIPS) : POWER QUALITY PROBLEM


WHAT ARE VOLTAGE SAGS?

A sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. The power quality community has used the term sag for many years
to describe a short-duration voltage decrease.

Although the term has not been formally defined, it has been increasingly accepted and used by utilities, manufacturers, and end users. The IEC definition for this phenomenon is dip. The two terms are considered interchangeable, with sag being the preferred synonym in the U.S. power quality community.

Terminology used to describe the magnitude of a voltage sag is often confusing. A “20 percent sag” can refer to a sag which results in a voltage of 0.8 or 0.2 pu. The preferred terminology would be one that leaves no doubt as to the resulting voltage level: “a sag to 0.8 pu” or “a sag whose magnitude was 20 percent.”

When not specified otherwise, a 20 percent sag will be considered an event during which the rms voltage decreased by 20 percent to 0.8 pu. The nominal, or base, voltage level should also be specified.

Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. Figure 2.6 shows a typical voltage sag that can be associated with a single-line-to-ground (SLG) fault on another feeder from the same substation.


An 80 percent sag exists for about 3 cycles until the substation breaker is able to interrupt the fault current. Typical fault clearing times range from 3 to 30 cycles, depending on the fault current magnitude and the type of overcurrent protection.

 Figure 2.7 illustrates the effect of a large motor starting. An induction motor will draw 6 to 10 times its full load current during start-up.


If the current magnitude is large relative to the available fault current in the system at that point, the resulting voltage sag can be significant. In this case, the voltage sags immediately to 80 percent and then gradually returns to normal in about 3 s. Note the difference in time frame between this and sags due to utility system faults.

Until recent efforts, the duration of sag events has not been clearly defined. Typical sag duration is defined in some publications as ranging from 2 ms (about one-tenth of a cycle) to a couple of minutes.

Undervoltages that last less than one-half cycle cannot be characterized effectively by a change in the rms value of the fundamental frequency value. Therefore, these events are considered transients.

Undervoltages that last longer than 1 min can typically be controlled by voltage regulation equipment and may be associated with causes other than system faults. Therefore, these are classified as long-duration variations.

Sag durations are subdivided here into three categories—instantaneous, momentary, and temporary—which coincide with the three categories of interruptions and swells. These durations are intended to correspond to typical utility protective device operation times as well as duration divisions recommended by international technical organizations.


SHORT DURATION VOLTAGE VARIATION : POWER QUALITY ISSUES


A voltage sag (dip) is defined as a decrease in the root-mean-square (rms) voltage at the power frequency for periods ranging from a half cycle to a minute.11 It is caused by voltage drops due to fault currents or starting of large motors. Sags may trigger shutdown of process controllers or computer system crashes.

A voltage swell is defined as an increase up to a level between 1.1 and 1.8 pu in rms voltage at the power frequency for periods ranging from a half cycle to a minute.

An interruption occurs when the supply voltage decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be caused by faults, control malfunctions, or equipment failures.

All these types of disturbances, such as voltage sags, voltage swells, and interruptions, can be classified into three types, depending on their duration.

a. Instantaneous: 0.5–30 cycles
b. Momentary: 30 cycles–3 s
c. Temporary: 3 s–1 min

It is helpful to distinguish the term outage used in reliability terminology from sustained interruption when the supply voltage is zero for longer than 1 min.

Outage refers to the state of a component in a system that has failed to function as expected and is used to quantify reliability statistics regarding continuity of service, whereas sustained interruptions as used in monitoring power quality to indicate the absence of voltage for long periods of time.

THE IDEAL VOLTAGE WAVEFORM


WHAT IS THE IDEAL VOLTAGE WAVEFORM?

Ideal power quality for the source of energy to an electrical load is represented by the single-phase waveform of voltage shown in Figure 1.1 and the three-phase waveforms of voltage shown in Figure 1.2.


The amplitude, frequency, and any distortion of the waveforms would remain within prescribed limits.

When the voltages shown in Figure 1.1 and Figure 1.2 are applied to electrical loads, the load currents will have frequency and amplitudes dependent on the impedance or other characteristics of the load.



 If the waveform of the load current is also sinusoidal, the load is termed “linear.” If the waveform of the load current is distorted, the load is termed “nonlinear.”

The load current with distorted waveform can produce distortion of the voltage in the supply system, which is an indication of poor power quality.

THE IMPORTANCE OF POWER QUALITY

Before the widespread use of power electronic equipment, microprocessors for industrial control, and automation in factories and offices, minor variations in power did not seriously affect the operation of conventional equipment such as lights and induction motors. 

If the supply voltage dipped because of a fault (i.e., a sag in voltage occurred), the lights just dimmed, and the induction motor produced a lower output.

These days the effects of power interruptions are rather costly. Reference 10 lists the following cases to illustrate the cost of short-duration power interruptions:

a. One glass plant estimates that a five-cycle interruption, a momentary interruption less than a tenth of second, can cost about $200,000.

b. A major computer center reports that a 2-s interruption can cost some $600,000.

c. In some factories, following a voltage sag, the restarting of assembly lines may require clearing the lines of damaged work, restarting of boilers, and reprogramming automatic controls at a typical cost of $50,000 per incident.

d. One automaker estimated that total losses from momentary power interruptions at all its plants run to about $10 million a year.

Power disturbance and power quality issue is costly. Learn more about power quality on this site.

POWER QUALITY = VOLTAGE QUALITY (IS IT?)

The common term for describing the subject of this site is power quality; however, it is actually the quality of the voltage that is being addressed in most cases.

Technically, in engineering terms, power is the rate of energy delivery and is proportional to the product of the voltage and current.

It would be difficult to define the quality of this quantity in any meaningful manner. The power supply system can only control the quality of the voltage; it has no control over the currents that particular loads might draw.

Therefore, the standards in the power quality area are devoted to maintaining the supply voltage within certain limits.

AC power systems are designed to operate at a sinusoidal voltage of a given frequency [typically 50 or 60 hertz (Hz)] and magnitude. Any significant deviation in the waveform magnitude, frequency, or purity
is a potential power quality problem.

Of course, there is always a close relationship between voltage and current in any practical power system. Although the generators may provide a near-perfect sine-wave voltage, the current passing through the impedance of the system can cause a variety of disturbances to the voltage.

For example,
1. The current resulting from a short circuit causes the voltage to sag or disappear completely, as the case may be.

2. Currents from lightning strokes passing through the power system cause high-impulse voltages that frequently flash over insulation and lead to other phenomena, such as short circuits.

3. Distorted currents from harmonic-producing loads also distort the voltage as they pass through the system impedance. Thus a distorted voltage is presented to other end users.

Therefore, while it is the voltage with which we are ultimately concerned, we must also address phenomena in the current to understand the basis of many power quality problems.