Analytics enabled by waveform analysis by Daniel Sabin and Jon A. Bickel Executive summary Waveform analysis has long been a useful tool for troubleshooting power-quality events within a facility’s electrical system. Waveform captures, taken during anomalous power events, provide clues to an events’ causes, frequency, and their effects. Interpreting waveform captures used to be the job of experienced power systems engineers. Today, though, non-engineers often are called on to understand their meanings. Doing so successfully requires an understanding of the ways various disturbances can present themselves during a captured event. Schneider Electric White Paper 2 The ability to capture waveforms of power quality events has been available since the Introduction 1980s. It has historically been a useful method to visualize electric power disturbances, helping to troubleshoot and resolve problems. Only a subset of metering devices are capable of capturing waveforms, which often require special hardware and firmware and are typically more expensive to purchase. How exactly waveform captures are processed within the metering device is a topic for another paper; this document is focused on what to do with the waveform once it has been captured. Historically, waveforms have been gathered and analyzed by experienced power systems Waveform engineers who analyzed and provided explanations of what they saw and how it was basics impacting (or could impact) the end-user’s electrical system. Analogous to physicians interpreting X-rays or MRIs in the medical field, specialized power systems engineers have analyzed and interpreted voltage and current signals captured from unhealthy electrical systems. Regrettably, fewer businesses now employ full-time power systems engineers, requiring non-engineers to become experts in the “cryptic arts” of interpreting waveforms captured by their power monitoring devices. An example waveform measurement is exhibited in Figure 1, which shows voltage and current samples recorded during a single-phase fault. Clues that the measurement are due to a downstream fault include a drop in voltage amplitude on one phase, asymmetric rise in current magnitude on the same phase, the decay in asymmetry after one cycle, and the sudden interruption of the fault current magnitude at the same time the voltage amplitude returns to normal. Figure 1 Voltage and current waveform recorded during a single-phase fault Analytics enabled by waveform analysis Schneider Electric White Paper 3 Waveform captures taken during anomalous power events include many clues as to those events’ causes, frequency, and their effects. These clues are in the form of waveform characteristics, some which are apparent and some which are not. These can include those summarized in affected phases, peak magnitude, RMS minimum, duration, phase jump, and more. ● Affect phase(s) ● RMS min/max ● IEEE 1159 1 ● Initial polarity ● Amplitude min/max category ● Rise time ● Duration ● Imbalance ● Phase shift ● Decay rate ● Zero-sequence ● Load impact ● Periodicity ● Negative-sequence ● Frequencies ● Waveshape When evaluating waveform captures, it is important to consider external contributing influences as well. For example, a few of these influences may be load types, meter constraints, meter locations, transformer configurations, operational parameters, and more. As previously mentioned, voltage and current signals from one, two, or three phases may be captured by metering devices. Each captured signal and each phase provides insights into the cause, impact, and outcome of a power quality event. The voltage(s) provide information regarding the quality of the energy source; the current(s) provide information regarding the flow of energy from the source. Troubleshooting with waveform captures often requires the use of both voltage and current signals to fully comprehend an issue. Voltage and current are the primary quantities measured by electrical sensors. Most What sensor instruments use analog-to-digital converters that sample instantaneous values of measurements the voltage and current signals. These digital voltage and current samples are used to are available derive a number of voltage, current, power, and energy characteristics. for evaluating AC electrical power systems are designed to provide voltage that follows a sinusoidal waveshape with a frequency of 50 Hz or 60 Hz (that is, with a sinusoid that repeats 50 or power quality 60 times per second), so these instantaneous voltage and current samples are frequently disturbances? referred to as waveform samples. A “Waveform Event” is a collection of waveform samples that start and end based on assigned triggers or thresholds. Waveform samples can be summarized by computing a root mean square (RMS) value over one 50 Hz or 60 Hz cycle that will result in a “DC-like” signal. An “RMS event” is a collection of RMS voltage and/or current values. Waveforms may also be summarized using Fourier transforms that assume that the waveform is sinusoidal and periodic (that is, the waveform follows a pattern that repeats). Fourier transforms convert waveform samples into magnitudes and phasor angles known as “phasors”. 1 “IEEE Recommended Practice for Monitoring Electric Power Quality,” in IEEE Std 1159- 2019 (Revision of IEEE Std 1159-2009) , vol., no., pp.1-98, 13 Aug. 2019. Analytics enabled by waveform analysis Schneider Electric White Paper 4 Measurements created from the measured waveform, RMS, and phasor values fall into five categories, see Table 1: ● High-Speed Transient Capture Events: Voltage signals sampled at a very high rate (for example, 10 MHz) for durations of microseconds to milliseconds. Voltage disturbances captured using this type of measurement may be caused by lightning surges and circuit-switching events. Because the duration of the measurement capture is typically less than the period of a 50 Hz or 60 Hz waveform, these measurements are not usually called “waveform events,” although both types of measurements sample instantaneous voltage. More information about these types of measurements is available in another Schneider Electric white paper.2 ● Waveform Events: Voltage, current, and/or power quantities sampled at a high rate (for example 10 kHz to 60 kHz) for durations of milliseconds to seconds. These measurements are frequently used to capture voltage and/or current signals associated with motor or other load startup, capacitor switching, fuse operations, cable energizing, and other switching events. ● RMS Events: Voltage, current, and/or power quantities sampled at a slower rate than waveform logs (for example 50 to 120 Hz) for durations of 250 milliseconds to 60 seconds. In addition to RMS voltage and RMS current, these measurements may include derivations of system frequency, real power, and reactive power. These captures can be used to summarize short duration reductions in voltage known as “voltage sags” (generally due to faults) and longer events, such as a motor startup, overloaded transformers or distribution systems, or the dynamic response of a power system during and after a transmission fault. Phasor magnitudes are often stored in these logs rather than RMS values, such as when the recording is for the fundamental frequency only (e.g., 50 Hz or 60 Hz). ● Event/Alarms Logs: Text or numeric summary of electric power disturbance events that might include minimum, average, and maximum values derived from high- speed transients, RMS event logs, RMS values, and phasor magnitudes. These summaries are often the only record available from a disturbance because the monitoring system was not designed or configured to collect, store, or transmit the waveform or RMS samples. ● Data Logs: Voltage, current, power, and/or energy quantities sampled in regular intervals for over a long period of time, such as an average value every ten minutes. These may be configured to record for days, months, or years. The logs are frequently seen as a record for the electrical system during steady-state but are sometimes enhanced beyond steady-state by providing the maximum or minimum value during an interval of time. They are frequently polled by SCADA systems using periodic sampling, yet may also be more rigorously recorded by a meter following a power quality specification such as IEC 61000-4-30. 2 Jon Bickel, “An Overview of Transients in Power Systems,” Schneider Electric White Paper 998-20579579_GMA. Analytics enabled by waveform analysis Schneider Electric White Paper 5 Measurement Type Sampling Rate Measurement Duration Quantity Type Microseconds to High-Speed Transient Logs 5 to 10 MHz Waveform Samples Milliseconds Table 1 Waveform Events 10 to 120 kHz Milliseconds to Seconds Waveform Samples Summary of electric power system measurements RMS Event Logs 50 to 120 Hz Milliseconds to Minutes RMS Samples Lists of Characteristics Derived Event Summary Logs N/A N/A from Waveform or RMS Samples Trends/Time Series of RMS Values Data Logs 1 s to 2 hrs Indefinite or Phasor Magnitudes/Angles Waveform events and RMS events provide useful information to successfully understand and troubleshoot problems, as each provides its own set of clues. For example, waveform events may reveal harmonic distortion issues by direct observation; however, an RMS event capture of the same event is impractical to see the effect of harmonics. Conversely, it is inherently easier to evaluate the magnitude and duration of voltage sag events using RMS events rather than instantaneous waveform events. Transient events are more easily analyzed