The PX5 samples high-speed transients at a sampling rate of 1Mhz for voltage and 0.5Mhz for current. Both voltage and current are digitized using a 14 bit Analog to Digital (A/D) converter. Therefore, the PX5 can detect and record virtually any high- speed transient in a power system.
High-speed (HS) transient data is stored in 80us windows with a minimum of 4 windows (320us) of data being recorded on any HS transient event. The 4 windows are comprised of 1 fault, 1 pre, and 2 post windows of data. HS triggers are detected using an energy/area under the curve method. Each 80us window is compared to the previous window for a change in area under the curve relative to the users settings. If a change exists that exceeds the thresholds the data is stored to memory. Data is continuously recorded until an 80us window goes below the threshold, or reaches a total duration of 16/20ms (60/50Hz), whichever comes first. The PX5 HS transient circuitry always works in a worst-case mode continually recording the information with data stored to memory only if thresholds are exceeded.
Case Study - Blurry X-Rays
This hospital has an older X-ray system that has a motor driven autotransformer to provide power to produce the high voltage (typically 100 kV) needed by the tube. The hospital facility engineer wanted to correct any voltage deviation, whether it is line or load induced because he is contemplating adding additional load to the circuit that feeds the X-ray system. We were brought in to determine the kind of voltage regulator that the facility engineer should buy?
A power quality analyzer was used to characterize the operation of the X-ray machine. Figure 1 shows the voltage sag that occurs during exposure due to the large amount of current needed. Figure 2 illustrates the short (100 msec) high magnitude (170 amps) current requirement that is needed for a radiographic exposure. The voltage sag does not affect the X-ray machine, but it could effect the proposed equipment to be added to the circuit.
Figure 1: Voltage sag caused by X-ray machine operation
Figure 2: Radiographic exposure current requirements
An electronic tap switching voltage regulator was brought in to regulate the voltage to the X-ray machine. The regulator has a plus/minus 3% voltage regulation, and as seen by the previous measurements, the X-ray machine can easily cause a 5-10% drop in voltage. Therefore switching of taps during and exposure is likely. Figure 3 illustrates the performance of the regulator.
Approximately one cycle into the exposure the regulator corrects the voltage back to normal. In addition, after the end of the exposure, there is a slight voltage rise followed by regulator compensation one cycle later. The consequence of the voltage compensation is an unpredictable, blurry X-ray exposure.
Figure 3 - Regulator Output Voltage during Exposure
A magnetic power synthesizer was brought in next to regulate X-ray machine voltage. The magnetic synthesizer is an electromagnetic device that takes incoming power and regenerates a clean, three- phase ac output waveform, regardless of input power quality. A block diagram of the process is shown in Figure 4.
Figure 4 - Block Diagram of Magnetic Synthesizer
The synthesizer doesn't regulate voltage in steps, but it does ring for up to 100 msec when a large step load is applied as shown in Figure 5. Because the X-ray exposure takes only 100 msec, the output voltage is unstable throughout exposure, causing blurry X-rays. Unfortunately, older X-ray systems expect to see a voltage drop and can't compensate for fast voltage changes during exposure. In this case it is better to leave the X-ray machine alone on its own dedicated circuit and feed the proposed loads with a different circuit.
Figure 5 - Power Synthesizer Output during Exposure
PQ Tip of the Month - Cap Switching Transients
Transients are very short duration disturbances, less than ¼ cycle of power frequency and more often, measured in microseconds. The most prevalent of these is the PF cap switching transient. These capacitors are used to correct for the power factor shift caused by large inductive loads, such as motors, that typically come on line each morning as businesses start up. An uncharged capacitor will absorb a large amount of energy in a very short time, producing a negative impulsive transient. Since the feeder wires are mostly inductive and resistive, this suddenly-added capacitance causes a brief resonant period, so the negative transient is followed by a oscillatory or ringing transient with a frequency typically between 400 and 2000 Hz, lasting less than ¼ cycle. The positive peak that immediately follows the initial negative peak can have a magnitude double the normal peak voltage. This type of transient can confuse overcurrent protection circuits in some equipment, and cause them to trip off line.
PQ Waveform Analysis - Voltage Unbalance
Voltage unbalance is a steady-state quantity defined as the maximum deviation from the average of the three phase voltages or currents, divided by the average of the three phase voltages or currents, expressed in percent. Unbalance can also be quantified using symmetrical components. The ratio of the negative sequence (or zero sequence) component to the positive sequence component is used to specify the percent unbalance. The negative sequence (or zero sequence) voltages in a power system generally result from unbalanced loads causing negative sequence (or zero sequence) currents to flow. Unbalance can be characterized by a trend of the unbalance magnitude in percent vs. time or can be summarized using statistics of the unbalance magnitude over some period.
The primary source of voltage unbalance less than two percent is unbalanced single-phase loads on a three-phase circuit. Voltage unbalance can also be the result of capacitor bank anomalies, such as a blown fuse on one phase of a three-phase bank. Severe voltage unbalance (greater than 5%) can result from single-phasing conditions. Voltage unbalance is most important for three phase motor loads. ANSI Std. C84.1-1989 recommends that the maximum voltage unbalance measured at the service entrance under no load conditions should be 3%. Unbalance greater than this can result in significant motor heating & failure if there are not unbalance protection circuits to protect the motor
The above waveform, measured using a Signature System 5530 DataNodẻ shows a trend of the the voltage unbalance on a 12kV feeder during a three month period.
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