* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download open file - PowerLogic
Power factor wikipedia , lookup
Spark-gap transmitter wikipedia , lookup
Brushed DC electric motor wikipedia , lookup
Transformer wikipedia , lookup
Induction motor wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Electrification wikipedia , lookup
Electric power system wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Fault tolerance wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Ground (electricity) wikipedia , lookup
Electrical ballast wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Transformer types wikipedia , lookup
Power inverter wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Amtrak's 25 Hz traction power system wikipedia , lookup
Power engineering wikipedia , lookup
Stepper motor wikipedia , lookup
Current source wikipedia , lookup
Voltage regulator wikipedia , lookup
Power MOSFET wikipedia , lookup
Opto-isolator wikipedia , lookup
History of electric power transmission wikipedia , lookup
Earthing system wikipedia , lookup
Buck converter wikipedia , lookup
Distribution management system wikipedia , lookup
Electrical substation wikipedia , lookup
Power electronics wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Surge protector wikipedia , lookup
Stray voltage wikipedia , lookup
Voltage optimisation wikipedia , lookup
Three-phase electric power wikipedia , lookup
PowerLogic Solutions Volume 5, Issue 2 Waveform Captures: The Key to PQ Solutions In This Issue we discuss how you can use waveform captures to diagnose problems in your power system. The Problem Like the lines on the EKG at your last checkup, those squiggly lines recorded by your POWERLOGIC ® Circuit Monitor mean something. We call those lines waveform capture, and, in the hands of an expert, they provide important information about the health of your power system. This issue of PowerLogic Solutions shows some typical waveform captures associated with disturbance events and provides their interpretation. By following a few guidelines, you can learn to interpret more information from your circuit monitors; using this information, you can decrease power system costs in your facility. The customer who captured the waveform in figure 1 was trying to diagnose a problem associated with an air compressor. The compressor would periodically shut down, often with catastrophic results to the plant, when nothing else in the plant was affected. The facilities engineer configured the POWERLOGIC CM-2350 Circuit Monitor on the circuit to capture waveforms during voltage sag events. The problem soon recurred, and, sure enough, the circuit monitor recorded the event in a 24-cycle waveform capture. The waveform proved that the contactor serving the motor was overly sensitive to voltage sag events. The sag shown in figure 1 is mild; it has only about 10% reduction in voltage for about five cycles on phase C. The current confirmed, however, that the contactor opened and the motor stopped. Note also that the contactor poles did not separate at the same time, as one might expect them to do. Phase A pole separated about four cycles before phases B and C. The plant replaced the contactor, and the nuisance compressor trips ceased. Phase A Current Phase A-N Voltage 386 193 0 -193 -386 735 0 -735 -1469 Phase B-N Voltage 386 193 0 -193 -386 Phase B Current 1078 0 -1078 -2157 Phase C-N Voltage Figure 1: This waveform capture helped prove that a motor contactor needed to be replaced. 387 193 0 -193 -387 Phase C Current 2526 1263 0 -1263 -2526 PowerLogic Solutions Coordination Error on Electric Utility System: Waveform Capture Nirvana Another example of the power of the POWERLOGIC system is shown in figure 2. This event was captured by a CM-2350 at the 600V service entrance of a manufacturing facility served from an electric utility overhead distribution circuit. The event lasted less than one-quarter of a second, (about 15 cycles). It damaged silicon-controlled rectifiers in the plant’s plating process, shutting down the facility for four hours. Phase A-N Voltage Phase A Current 5000 4000 3000 2000 1000 0 -1000 -2000 -3000 750 500 250 0 -250 -500 -750 Phase B Current Phase B-N Voltage 4000 3000 2000 1000 0 -1000 -2000 -3000 -4000 -5000 750 500 250 0 -250 -500 -750 Phase C Current Phase C-N Voltage 750 500 250 0 -250 -500 -750 2 PowerLogic Solutions is a publication of Square D Company’s Power Management Operation. Each issue presents a common power system problem, and offers guidance on how to solve it. 2500 caused by a fault on the 23-kV overhead circuit, but the circuit monitor is connected at the 600V circuit. Between the fault and monitoring point is a step-down power transformer connected delta-delta. For delta-delta or wye-wye connected transformers, a single-phase event on the primary side affects only one phase voltage. Figure 3 shows an extreme example of a single lineto-ground fault—single-phasing —seen through a wye-wye transformer connection. But most service-entrance transformers are connected delta-wye. For these transformers, a single line-to-ground fault on the primary side causes a drop in voltage on two phases on the secondary. Transformer connections between the circuit monitor monitoring point and the event are an important consideration when diagnosing voltage sag events. Event 2. Looking back at figure 2: the first event lasts three cycles and resembles the usual single line-toFigure 2: This waveform capture exposed a coordination ground fault on the utility system. problem on the electric utility circuit. The second event is the killer. It involves all three phases and shows that The Square D Power Quality/Energy the utility source voltage was removed Management (PQ/EM) team engineer about four cycles after the three-cycle fault proved that the event should not have been had cleared. The voltage after that fourso severe, and that it exposed a coordinacycle respite gradually decreases in tion problem on the electric utility circuit magnitude, and, if you look carefully, in serving the plant! frequency. This effect is also common. It happens when source voltage is suddenly On careful inspection, the event in figure 2 removed from three-phase induction is actually two events: motors. They instantly become induction Event 1. The first affects phase A-N more generators, supplying a gradually decreasthan the other phases, obviously the result ing voltage to the plant power system of a single-phase event on the utility while the rotors coast to a stop. system. Most sags in facility power are caused by single line-to-ground faults on the In this case, the problem is that they were electric utility overhead system. Because the not allowed to stop. The utility voltage events involve only one phase on the overreturned just as suddenly as it left. The head circuit, the waveform capture shows a voltage produced by the induction motors/ drop in voltage on one or two phases. generators was out of phase with the returning voltage. There was a brief “battle One phase affected seems straightforward, of electrons,” but the overwhelming shortbut why would two phases drop during an circuit power of the utility electrons won out event that involved only one phase? That and quickly snatched the motors back to phenomenon has to do with transformers normal speed. This struggle created a large between the monitoring point and the voltage surge, however, that damaged actual fault. The event in figure 2 was 0 -2500 -5000 60 cycles of waveform capture are available, you may decide that fewer cycles are acceptable. Phase A Current Phase A-N Voltage 681 341 0 -341 -681 460 230 0 -230 -460 Phase B Current Phase B-N Voltage 383 191 0 -191 -383 385 192 0 -192 -385 Phase C Current Phase C-N Voltage 492 246 0 -246 -492 802 401 0 -401 -802 Figure 3: This single-phasing event lasted for two hours. The resulting 24% phase-to-phase voltage imbalance caused damage to the facility’s three-phase induction motors. plant equipment and resulted in the fourhour shutdown. So what caused the twin events? The electric utility had two pieces of evidence. First, they had to dispatch a service person to replace a fuse in an overhead, single-phase circuit served by the same three-phase circuit that served the affected plant. Second, they determined that the substation circuit recloser had operated once since the last time its counter was read. Both pieces of evidence confirmed the Square D PQ/EM engineer’s suspicion: one fault had occurred, yet two overcurrent devices operated. The fuse replacement showed that a short circuit had occurred beyond the fuse, on the single-phase “branch” circuit. The substation circuit breaker had operated also — unnecessarily so—with catastrophic results in the plant. Further investigation by the utility confirmed the coordination problem, and they eventually replaced the single-phase fuse with a fuse that coordinated better with the three-phase substation circuit breaker. Setup Is Important Circuit monitors (model 2350 or above) can capture up to 60 cycles of instantaneous voltage and current on all channels simultaneously, based on a high-speed event on any channel. High-speed events need to be set up in advance, however, to ensure that the right amount of information is captured for your facility. Although Why limit the cycles? Memory, for one thing. Your circuit monitor can be equipped with over one megabyte of memory, but you may have purchased less. And you may want to share that memory between on-board data logs, fourcycle waveforms, and event logs, in addition to high-speed waveform captures. Additionally, more than 60 cycles may be unnecessary because you may rarely experience events that long. Many customers served from an electric utility transmission system experience events that last fewer than eight cycles. A 24-cycle, or even 12-cycle, waveform capture setting is enough to capture the event. Your electric utility representative may provide some guidance in determining how many cycles to select. The representative can determine the protective device settings on the electric utility distribution system serving your facility. Many utilities use 3 The Square D Power Management Operation offers complete power quality consulting services to ensure that power problems do not impact your operation. Contact our power management experts for information about the following: • Power Quality Consulting • Energy Management Consulting • Harmonic Filters • Power Factor Correction • Power Management Training and Technical Support • Digital Simulation Studies • Remote Monitoring Services • Data Collection and Analysis Our number is 1-888-PWR-MGMT. PowerLogic Solutions substation circuit breakers that are set to sense, open, and reclose in about 15 cycles. For these circuits, a 24-cycle setting is usually adequate. The magnitude of sag and swell pickup and dropout settings is important, too. For most facilities, Square D Company’s PQ/EM consulting team recommends a sag pickup (when the threshold is exceeded) setting of about 418V on a 480V system, or 241V if the nominal voltage is 277. 800 600 400 200 0 -200 -400 -600 -800 1000 750 500 250 0 -250 -500 -750 -1000 8000 6000 4000 2000 0 -2000 -4000 -6000 -8000 Phase B-N Voltage Phase A Current Phase B Current 1500 1000 500 0 -500 -1000 -1500 Phase C-N Voltage 800 600 400 200 0 -200 -400 -600 -800 8000 6000 4000 2000 0 -2000 -4000 -6000 -8000 Phase C Current Figure 4. This waveform shows a downstream fault. The fault current saturates the current transformer and results in an unusual waveform. We refer to three-wire systems as system 30 and four-wire as system 40 or 41. The dropout setting (when the voltage returns to a usable level) is typically 432V or 250 V. See PowerLogic Solutions volume 1, number 5, for additional information about sag and swell setup. Power Metering vs. Fault Recording 4 Phase A-N Voltage One customer suffered damage to a cable lug on a circuit breaker serving a power factor correction capacitor bank (figure 5). The waveform capture from the event shows a three-cycle voltage sag, and a very irregular current waveform. The voltage waveforms show how the voltage on that bus was affected before, during, and after the event. The current waveform during the brief fault, however, reflects a trade-off between accurate power metering and fault recording. conditions. A fault (or short circuit) draws much more current than normal loads, often many times full load current of the circuit. Fault current magnitudes can cause strange current waveforms, like the one in figure 4. The high fault magnitude can saturate the current transformers in the circuit monitor, thereby distorting the waveform capture for current. Whenever you see extremely high current magnitudes, especially if one or more cycles is flat-topped, a downstream fault has occurred. What About Motor Starting? Another waveform capture that sometimes resembles a downstream fault is associated with starting an induction motor. An induction motor requires many times its full-load current during starting. This event may resemble a downstream fault, but there is usually a clear distinction: current associated with a motor starting gradually Your circuit monitor system was designed to provide highly accurate readings of 200 power system parameters at normal loads. In order to do this to 0.2% accuracy, the circuit monitor usually receives its voltage and current inputs from instrument transformers sized for normal circuit loading. In particular, current transformers for circuit monitor systems are typically sized to provide a 0-5 ampere output signal under normal loads. The circuit monitor was not optimized to measure currents associated with fault Figure 5: The waveform capture in figure 4 resulted when the center leg of this cable lug failed. decreases, while a fault usually changes current magnitude almost instantaneously. This abrupt change in current is due either to the operation of a load-side or sourceside protective device. If the overcurrent device is load-side of the fault, the current returns to a nominal level quickly. If the overcurrent device is source-side of the fault (and the circuit monitor), the current drops to zero. Phase A-N Voltage Phase A Current 178 543 89 272 0 0 -89 -272 -178 -543 But there’s more. Note the sudden half-cycle drop in voltage followed by a ringing transient that begins the sag. This is the signature of a lightning strike and the subsequent response of the distribution system. In particular, the lighting strike caused a utility silicon-carbide lightning arrestor to conduct. A silicon-carbide arrestor appears as a short-circuit when a lightning strike flashes over an insulator. The short-circuit causes a sudden collapse in voltage that is restored at the next zero crossing of voltage. The ringing transient that follows the event is the typical response of the electrical system to a sudden change in voltage. Phase B Current Phase B-N Voltage 171 86 267 The sag lasts until the utility overcurrent device, probably a substation recloser, senses the fault Phase C Current Phase C-N Voltage current and opens. We also know that this lightning strike was located on an adjacent feeder sharing the same utility distribution substation as the Figure 6: Starting a 3-phase induction motor results in a high current similar to a downstream fault. plant where the circuit monitor is located. This is clear because the Figure 6 shows a typical motor starting voltage returns to nominal when the breaker event. In this case, the circuit monitor was opens. Had the fault been on the feeder located at the motor, on the load side of serving the plant, the voltage would have the motor starter. When the contactor dropped to zero, much like the second part closes, voltage is applied. The waveform of the event in figure 2. capture shows this voltage. The current to the motor quickly increases to about 490 Interpreting Waveform Captures amperes, then gradually, over about nine Saves Money! cycles, increases to a nominal full-load Learning to interpret the waveform captures value. Note that the tip about downstream from a circuit monitor may require a little time faults applies here as well. Any motor and training, but it is a skill that can save starting event that exceeds about 7.5 money for your facility. It can help you quickly amperes into the circuit monitor (after diagnose a component malfunction, identify applying the current transformer ratio) will appear flat-topped. 0 0 -86 -267 -171 -524 178 521 89 261 0 0 -89 -261 -178 -521 Phase A-N Voltage What's That Scratchy Event? The last waveform capture is a bit tougher to interpret. Sure, figure 7 looks like a voltage sag on phase C. It was probably caused by a single-phase fault on the electric utility overhead distribution circuit. And the service entrance transformer between the fault and the plant monitoring location is probably a wye-wye, since the sag shows up mostly on one phase. Phase A Current 469 314 235 157 0 0 -235 -157 -469 -314 419 Phase B-N Voltage Phase B Current 281 210 141 0 0 -210 -141 -419 402 201 -281 Phase C-N Voltage 255 Phase C Current 127 0 0 -201 -127 -402 -255 Figure 7: The phase C-N voltage shows the characteristic footprint of a silicon-carbide lightning arrestor. Note the sudden half-cycle voltage drop and ringing transient on phase C. 5 PowerLogic Solutions a problem in your distribution system, and troubleshoot a motor starting anomaly. Money savings come from: • prevention of downtime • repairing problems once, at the root cause • reducing equipment damage • negotiating price adjustments with your power supplier, for problems caused by the supplied power How can you learn more? Attend a PowerLogic University class soon. PowerLogic University constantly adds to its database of typical waveform captures. The experts at the University can teach you all the subtleties of capturing and interpreting voltage and current disturbance events. Then you, too, will be able to maintain and improve the health of your electric power system. In short, the effort spent learning the characteristics of waveform capture events is a good investment. Test Your PQ—Power Quotient The following true-or-false questions will test your knowledge of the information in this issue, as well as previous issues of POWERLOGIC Solutions. Answers are in the righthand column. 6. 5. 4. 3. 2. 1. False. Circuit monitors capture all channels of voltage and current simultaneously when a high-speed alarm is triggered. This issue. True. The 87% figure is a good trade-off value between too many and too few sag events. Most plant equipment will not be affected by sags below 87% voltage. This issue. True. Even though there is usually a transformer between your circuit monitor’s measuring point and a disturbance on the electric utility circuit, the waveform capture can provide useful information about utility events. This issue. False. Most sags are caused by faults on the electric utility distribution system. Volume 1, Number 5. False. The amount of information you can store depends on circuit monitor on-board memory and its allocation. This issue. True. Savings associated with power monitoring start at about 4% of your total power management costs (energy, downtime, and equipment utilization). Volume 5, Number 1. 6 1. POWERLOGIC Circuit Monitors capture only the voltage or current channel affected by a high-speed alarm like a voltage sag. 2. Typical voltage sag pickup setting is 418V on a 480V circuit (system 30), or 241V on a 277V circuit (system 40 or 41), which equates to a sag of about 87% of nominal. 3. Circuit monitor waveform captures can give you useful information about the quality of power on the electric utility distribution system serving your facility. 4. Voltage sags are usually caused by equipment inside your plant or facility. 5. Circuit monitors are limited to one 60-cycle waveform capture per day. 6. POWERLOGIC monitoring systems typically reduce power management costs by at least 4%. Bulletin No. 3000HO9904 10M DL January 2000 © 2000 Schneider Electric All Rights Reserved