Solving power quality problems depends on acquiring meaningful data at the optimum location or locations and within an expedient time frame. In order to acquire useful and relevant data, instruments most suited for a particular application should be utilized.
Most power quality problems that go unrecognized are due to use of instruments not ideally suited for that application. One also needs to have a sense about the location or locations where data need to be collected and for how long.
After the data is acquired, sort it to determine what information is pertinent to the problem on hand and what is not. This process requires knowledge of the power system and knowledge of the affected equipment.
Initially, all data not determined to be directly useful should be set aside for later use. All data deemed to be relevant should be prioritized and analyzed to obtain a solution to the problem. It should be stressed once again that some power quality problems require not a single solution but a combination of solutions to obtain the desired end results.
Power Quality Measurement Devices
Harmonic Analysers
A Harmonic Analysers or harmonic meters are relatively simple instruments for measuring and recording harmonic distortion data. Typically, harmonic Analysers contain a meter with a waveform display screen, voltage leads, and current probes.
Some instruments provide a snapshot of the waveform and harmonic distortion pertaining to the instant during which the measurement is made. Other instruments are capable of recording snapshots as well as a continuous record of harmonic distortion over time.
Depending on the power quality issue, snapshots of the harmonic distortion might suffice. Other problems, however, might require knowledge of how the harmonic distortion characteristics change with plant loading and time.
Harmonic Analysers from various manufacturers tend to have different, upper-harmonic frequency measurement capability.
In order to accurately determine the frequency content, the sampling frequency of the measuring instrument must be greater than twice the frequency of the highest harmonic of interest. This rule is called the Nyquist frequency criteria.
According to Nyquist criteria, to accurately determine the frequency content of a 50-Hz fundamental frequency waveform up to the 25th harmonic number, the harmonic measuring instrument must have a minimum sampling rate of 2500 (25 × 50 × 2) samples per second. Of course, higher sampling rates more accurately reflect the actual waveform.
Measurement of voltage harmonic data requires leads that can be attached to the points at which the distortion measurements are needed. Typical voltage leads are 4 to 6 ft long. At these lengths, cable inductance and capacitance are not a concern, as the highest frequency of interest is in the range of 2500 to 5000 Hz (25th to 50th harmonic); therefore, no significant attenuation or distortion should be introduced by the leads in the voltage distortion data.
Measuring current harmonic distortion data requires some special considerations. Most current probes use an iron core transformer designed to fit around the conductors in which harmonic measurements are needed.
Iron-core current probes are susceptible to increased error at high frequencies and saturation at currents higher than the rated values. Prior to installing current probes for harmonic distortion tests, it is necessary to ensure that the probe is suitable for use at high frequencies without a significant loss in accuracy.
At higher frequencies, currents and distortions normally looked at are considerably lower than at the lower frequencies, and some loss of accuracy at higher frequencies might not be all that important. Typically, a 5.0% loss in accuracy might be expected, if the waveform contains significant levels of higher order harmonics.
Transient Disturbance Analysers
Transient-disturbance analyzers are advanced data acquisition devices for capturing, storing, and presenting short-duration, subcycle power system disturbances. As one might expect, the sampling rates for these instruments are high.
It is not untypical for transient-disturbance recorders to have sampling rates in the range of 2 to 4 million samples per second. Higher sampling rates provide greater accuracy in describing transient events in terms of their amplitude and frequency content.
Both these attributes are essential for performing transient analysis. The amplitude of the waveform provides information about the potential for damage to the affected equipment. The frequency content informs us as to how the events may couple to other circuits and how they might be mitigated.
Equipment that contains power supplies or capacitor filter circuits is especially susceptible to fast rise-time transients with high-frequency content.
The use of longer cable lengths in transient measurements results in higher inductance and capacitance and greater attenuation of the transient waveform. Also, in order to minimize noise pickup from external sources, the voltage leads should be kept away from high-voltage and high-current conductors, welding equipment, motors, and transformers.
Current transformers used in transient current measurements must have a peak current rating at least equal to the maximum expected currents; otherwise, current peaks are lost in the data due to saturation of the current probe.
Oscilloscopes
Oscilloscopes are useful for measuring repetitive high-frequency waveforms or waveforms containing superimposed high-frequency noise on power and control circuits. Oscilloscopes have sampling rates far higher than transient-disturbance analyzer. Oscilloscopes with sampling rates of several hundred million samples per second are common. This allows the instrument to accurately record recurring noise and highfrequency waveforms.
The pulse-width-modulated waveform of the voltage input to an adjustable speed AC motor, data are not within the capabilities of harmonic analyser and transient disturbance recorders. Digital storage oscilloscopes have the ability to capture and storewaveform data for later use. Using multiple-channel, digital storage oscilloscopes, more than one electrical parameter may be viewed and stored.
Selection of voltage probes is essential for proper use of oscilloscopes. Voltage probes for oscilloscopes are broadly classified into passive probes and active probes. Passive probes use passive components (resistance and capacitance) to provide the necessary filtering and scale factors necessary. Passive probes are typically for use in circuits up to 300 VAC. Higher voltage passive probes can be used in circuits of up to 1000 VAC.
Most passive probes are designed to measure voltages with respect to ground. Passive probes, where the probe is isolated from the ground, are useful for making measurements when connection to the ground is to be avoided. Active probes use active components such as field effect transistors to provide high input impedance to the measurements.
High input impedance is essential for measuring low-level signals to minimize the possibility of signal attenuation due to loading by the probe itself. Active probes are more expensive than passive probes. The high frequency current probe is an important accessory for troubleshooting problems using an oscilloscope. By using the current probe, stray noise and ground loop currents in the ground grid can be detected.
Data Loggers and Chart Recorders
Data loggers and chart recorders are sometimes used to record voltage, current, demand, and temperature data in electrical power systems. Data loggers and chart recorders are slow-response devices that are useful for measuring steady-state data over a long period of time.
These devices record one sample of the event at predetermined duration, such as 1 sec, 2 sec, 5 sec, etc. The data are normally stored within the loggers until they are retrieved for analysis.
The data from data loggers and chart recorders are sufficient for determining variation of the voltage or current at a particular location over an extended period and if there is no need to determine instantaneous changes in the values. In some applications, this information is all that is needed.
But, in power quality assessments involving transient conditions, these devices are not suitable. The advantage of data loggers is that they are relatively inexpensive compared to power quality recording instrumentation. They are also easier to set up and easier to use. The data from the device may be presented in a text format or in a graphical format.
True RMS Meters
The term true RMS is commonly used in power quality applications. What are true RMS meters? As we saw in previous chapters, the RMS value of the current or voltage can be substantially different from the fundamental component of the voltage or current. Using a meter that measures average or peak value of a quantity can produce erroneous results if we need the RMS value of the waveform.
For waveforms rich in harmonics, the average and peak values would be considerably different than waveforms that are purely sinusoidal or close to sinusoidal. Measuring the average or peak value of a signal and scaling the values to derive a RMS value would lead to errors.
Analog panel meters give erroneous readings when measuring nonsinusoidal currents. Due to higher frequency components, analog meters tend to indicate values that are ower than the actual values. The presence of voltage and current transformers in the metering circuit also introduces additional errors in the measurements.
True RMS meters overcome these problems by deriving the heating effect of the waveform to produce an accurate RMS value indication. After all, RMS value represents the heating effect of a voltage or current signal. Most true RMS meters do not provide any indication of the waveform of the quantity being measured.
To accomplish this, the meters require high-frequency signal sampling capability. The sampling rate should satisfy Nyquist criteria in order to produce reasonably accurate results. Some bench top RMS meters do have the sampling capability and ports to send the information to a computer for waveform display.