Measurement of Power Quality: 7 Devices | Electrical Engineering

The following points highlight the top seven devices used for measuring power quality. The devices are: 1. Multimeters 2. Digital Cameras 3. Oscilloscopes 4. Disturbance Analyzers 5. Spectrum Analyzers and Harmonic Analyzers 6. Combination Disturbance and Harmonic Analyzers 7. Flicker Meters.

Device # 1. Multimeters:

After initial tests of wiring integrity, it may also be necessary to make quick checks of the voltage and/or current levels within a facility. Overloading of circuits, under-voltage and overvoltage problems, and unbalances between circuits can be detected in this manner. These measurements just require a simple multimeter.

Signals used to check for these include:

i. Phase-to-ground voltages

ii. Phase-to-neutral voltages

iii. Neutral-to-ground voltages

iv. Phase-to-phase voltages (three-phase system)

v. Phase currents

vi. Neutral currents

The most important factor to consider when selecting and using a multimeter is the method of calculation used in the meter. All the commonly used meters are calibrated to give an rms indication for the measured signal. However, a number of different methods are used to calculate the rms value.

The three most common methods are:

i. Peak Method:

Assuming the signal to be a sinusoid, the meter reads the peak of the signal and divides the result by 1.414 to obtain the rms.

ii. Averaging Method:

The meter determines the average value of a rectified signal. For a clean sinusoidal signal (signal containing only one frequency), this average value is related to the rms value by a constant.

iii. True rms:

The rms value of a signal is a measure of the heating that will result if the voltage is impressed across a resistive load. One method of detecting the true rms value is to actually use a thermal detector to measure a heating value. More modern digital meters use a digital calculation of the rms value by squaring the signal on a sample-by-sample basis, averaging over the period, and then taking the square root of the result.

These different methods all give the same result for a clean, sinusoidal signal but can give significantly different answers for distorted signals. This is very important because significant distortion levels are quite common, especially for the phase and neutral currents within the facility. Table 5.1 can be used to better illustrate this point. Each waveform in Table 5.1 has an rms value of 1.0 pu (100.0 percent).

The corresponding measured values for each type of meter are displayed under the associated waveforms, normalized to the true rms value.

Device # 2. Digital Cameras:

Photographs are extremely useful for documentation purposes. Those conducting the measurements often get distracted trying to get instruments to function properly and tests coordinated. They are rushed and fail to write down certain key data that later turn out to be important. Unfortunately, human memory is unreliable when there are dozens of measurement details to remember. The modern digital camera has become an indispensable tool when taking field measurements. It is a simple matter to take photographs to document the tests. The photographer can immediately tell if the shot failed and retake it with a different exposure.

Typical items to record photographically during field measurements include:

i. Nameplates of transformers, motors, etc.

ii. Instrumentation setups

iii. Transducer and probe connections

iv. Key waveform displays from instruments

v. Substations, switchgear arrangements, arrester positions, etc.

vi. Dimensions of key electrical components such as cable lengths

Video cameras are similarly useful when there is moving action or random events. For example, they may be used to help identify the locations of flashovers. Many industrial facilities will require special permission to take photographs and may place stringent limitations on the distribution of any photographs.

Device # 3. Oscilloscopes:

An oscilloscope is valuable when performing real-time tests. Looking at the voltage and current waveforms can provide much information about what is happening, even without performing detailed harmonic analysis on the waveforms. One can get the magnitudes of the voltages and currents, look for obvious distortion, and detect any major variations in the signals. There are numerous makes and models of oscilloscopes to choose from.

A digital oscilloscope with data storage is valuable because the waveform can be saved and analyzed. Oscilloscopes in this category often also have waveform analysis capability (energy calculation, spectrum analysis). In addition, the digital oscilloscopes can usually be obtained with communications so that waveform data can be uploaded to a personal computer for additional analysis with a software package.

The latest developments in oscilloscopes are hand-held instruments with the capability to display waveforms as well as performing some signal processing. These are quite useful for power quality investigations because they are very portable and can be operated like a volt ohm meter (VOM), but yield much more information. These are ideal for initial plant surveys.

Device # 4. Disturbance Analyzers:

Disturbance analyzers and disturbance monitors form a category of instruments that have been developed specifically for power quality measurements. They typically can measure a wide variety of system disturbances from very short duration transient voltages to long-duration outages or under-voltages. Thresholds can be set and the instruments left unattended to record disturbances over a period of time. The information is most commonly recorded on a paper tape, but many devices have attachments so that it can be recorded on disk as well.

There are basically two categories of these devices:

i. Conventional Analyzers:

Conventional analyzers that summarize events with specific information such as overvoltage and under-voltage magnitudes, sags and surge magnitude and duration, transient magnitude and duration, etc.

ii. Graphics-Based Analyzers:

Graphics-based analyzers that save and print the actual waveform along with the descriptive information which would be generated by one of the conventional analyzers. It is often difficult to determine the characteristics of a disturbance or a transient from the summary information available from conventional disturbance analyzers.

For instance, an oscillatory transient cannot be effectively described by a peak and a duration. Therefore, it is almost imperative to have the waveform capture capability of a graphics-based disturbance analyzer for detailed analysis of a power quality problem. However, a simple conventional disturbance monitor can be valuable for initial checks at a problem location.

Device # 5. Spectrum Analyzers and Harmonic Analyzers:

Instruments in the disturbance analyzer category have very limited harmonic analysis capabilities. Some of the more powerful analyzers have add-on modules that can be used for computing fast Fourier transform (FFT) calculations to determine the lower- order harmonics. However, any significant harmonic measurement requirements will demand an instrument that is designed for spectral analysis or harmonic analysis.

Important capabilities for useful harmonic measurements include:

i. Capability to measure both voltage and current simultaneously so that harmonic power flow information can be obtained.

ii. Capability to measure both magnitude and phase angle of individual harmonic components (also needed for power flow calculations).

iii. Synchronization and a sampling rate fast enough to obtain accurate measurement of harmonic components up to at least the 37th harmonic (this requirement is a combination of a high sampling rate and a sampling interval based on the 60-Hz fundamental).

iv. Capability to characterize the statistical nature of harmonic distortion levels (harmonics levels change with changing load conditions and changing system conditions).

There are basically three categories of instruments to consider for harmonic analysis:

i. Simple Meters:

It may sometimes be necessary to make a quick check of harmonic levels at a problem location. A simple, portable meter for this purpose is ideal. There are now several hand-held instruments of this type on the market. Each instrument has advantages and disadvantages in its operation and design.

These devices generally use microprocessor- based circuitry to perform the necessary calculations to determine:

1. Individual harmonics up to the 50^th harmonic, as well as the rms

2. The THD, and

3. The telephone influence factor (TIF).

Some of these devices can calculate harmonic powers (magnitudes and angles) and can upload stored waveforms and calculated data to a personal computer.

ii. General-Purpose Spectrum Analyzers:

Instruments in this category are designed to perform spectrum analysis on waveforms for a wide variety of applications. They are general signal analysis instruments. The advantage of these instruments is that they have very powerful capabilities for a reasonable price since they are designed for a broader market than just power system applications. The disadvantage is that they are not designed specifically for sampling power frequency waveforms and, therefore, must be used carefully to assure accurate harmonic analysis. There are a wide variety of instruments in this category.

iii. Special-Purpose Power System Harmonic Analyzers:

Besides the general-purpose spectrum analyzers just described, there are also a number of instruments and devices that have been designed specifically for power system harmonic analysis. These are based on the FFT with sampling rates specifically designed for determining harmonic components in power signals. They can generally be left in the field and include communications capability for remote monitoring.

Device # 6. Combination Disturbance and Harmonic Analyzers:

The most recent instruments combine harmonic sampling and energy monitoring functions with complete disturbance monitoring functions as well. The output is graphically based, and the data are remotely gathered over phone lines into a central database. Statistical analysis can then be performed on the data. The data are also available for input and manipulation into other programs such as spreadsheets and other graphical output processors.

One example of such an instrument is shown in Fig. 5.2. This instrument is designed for both utility and end-user applications, being mounted in a suitable enclosure for installation outdoors on utility poles. It monitors three-phase voltages and currents (plus neutrals) simultaneously, which is very important for diagnosing power quality problems. The instrument captures the raw data and saves the data in internal storage for remote downloading. Off-line analysis is performed with powerful software that can produce a variety of outputs such as that shown in Fig. 5.3.

The top chart shows a typical result for a voltage sag. Both the rms variation for the first 0.8 s and the actual waveform for the first 175 ms are shown. The middle chart shows a typical wave fault capture from a capacitor-switching operation. The bottom chart demonstrates the capability to report harmonics of a distorted waveform. Both the actual waveform and the harmonic spectrum can be obtained.

This is a power quality monitoring system designed for key utility accounts. It monitors three-phase voltages and has the capability to capture disturbances and page power quality engineers. The engineers can then call in and hear a voice message describing the event. It has memory for more than 30 events. Thus, while only a few short years ago power quality monitoring was a rare feature to be found in instruments, it is becoming much more commonplace in commercially available equipment.

Device # 7. Flicker Meters:

Over the years, many different methods for measuring flicker have been developed. These methods range from using very simple rms meters with flicker curves to elaborate flicker meters that use exactly tuned filters and statistical analysis to evaluate the level of voltage flicker.  

Flicker Standards:

Although the United States does not currently have a standard for flicker measurement, there are IEEE standards that address flicker. IEEE Standards 141-19936 and 519-19927 both contain flicker curves that have been used as guides for utilities to evaluate the severity of flicker within their system. Both flicker curves, from Standards 141 and 519, are shown in Fig. 5.5.

In other countries, a standard methodology for measuring flicker has been established. The IEC flicker meter is the standard for measuring flicker in Europe and other countries currently adopting IEC standards. The IEC method for flicker measurement, defined in IEC.