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Abstract
This whitepaper describes what voltage sag is, some of the most common causes of voltage sag, the adverse effects voltage sags have on end users, and how to monitor and analyze voltage sags using PMI’s recorders along with Provision.
Causes
According to IEEE standard 1159, a voltage sag is a reduction in RMS voltage between 10 and 90 percent of nominal, that can last anywhere from 1/2 cycle up to 1 minute, but not a complete outage. Voltage sags are typically caused by loose connections, energizing a load that requires large inrush current, or power system faults. In some rare cases, voltage sags can be caused by voltage regulator failures.
Fundamentally, most sags are caused by unexpectedly high current flowing across an impedance, producing a voltage drop from the nominal system voltage. This current can be caused by a temporary fault or loads with high current spike characteristics. The system impedance also contributes to the voltage drop, and an unexpectedly high impedance due to bad connections can also produce problematic sags. Figure 1 shows a schematic of how voltage drop occurs from current flow, based on Ohms law (V=IR). Here, the customer sees voltage VL1, which is the original source voltage VS, minus the voltage drop V1 dropped across impedance Z1.
Figure 1. Current flow through an impedance results in voltage drop
Voltage sags are sometimes caused external factors such as the weather. Strong storms trigger many types of power quality problems, but voltage sags are one of the most common issues. Weather can be considered a natural source of voltage sags. Voltage sag introduce by the weather is usually short lived and does not reoccur on an hourly or daily basis as many man-made sources do. A short circuit or fault caused by fallen tree limbs from storms can drive the voltage to zero or close to zero at the fault location. The voltage at the substation and other laterals on the circuit will vary depending on the distance from the actual fault and the system’s impedance. A recloser may operate, creating brief interruptions and sags on its primary circuit as it recloses, and voltage sags on nearby circuits or laterals.
A very common man-made source of voltage sags is high motor starting current. A typical motor’s starting current can be from four to eight times the normal running current. When the motor is first energized, the rapid increase of inrush current can cause an excessive voltage drop in the building and distribution wiring lasting a few cycles. The specific current and system impedance determines the voltage drop. It would not be practical, if not impossible to size transformers and wiring to completely eliminate current-caused voltage drops. Due to the high material cost, transformers with reasonable ratings and wiring are used in most cases to make costs practical, but in some cases unexpected or high inrush currents can create unexpected sags. A heavily loaded transformer or low system voltage may exacerbate the situation.
In many cases when a local utility customer complaints of voltage problems, it can often be traced back to a neighboring electrical load change that has caused voltage dips on connected services on the same circuit. Other customers on the same distribution transformer secondary are especially common culprits. One example may be where a large electric motor is used where a hard start occurs. One solution greatly reducing the current inrush (thus the voltage sag) is setting a permanent soft start condition or possible adding a variable frequency drive (VFD) to soften the sag. If a VFD is added, which smoothes out the initial start up current, it may create other power quality issues, such as THD which may need to be dealt with. Sometimes power quality is a compromise.
Identifying Voltage Sags
Voltage sags can be very detrimental to many industrial systems, causing loss of productivity, labor costs if machinery needs to be restarted, and with some continuous factory processes such as plastic injection molding, the line machinery needs to be disassembled, completely cleaned, and reset for a new run. Sensitive equipment using microprocessor or computer control can sometimes be helped by installing ferroresonant transformers, UPS or other ride-through technologies. These devices mitigate a voltage sag by holding the voltage up during the sag period. Since they are providing energy to the load (making up the loss of energy during the sag time), this method is more difficult for very large, but sensitive loads.
It is important to identify the voltage sag source correctly, because in some situations, trying to correct voltage sag can sometimes backfire and actually make the situation worst. For example, consider a motor whose starting current is causing the voltage sag. If a filter is installed upstream from the motor with a sensitive computer on the same circuit, the filter can raise the source impedance (as seen by the computer), causing the computer to actually see a much larger voltage sag than if the filter was not installed. A filter or UPS installed inline with the computer or electronics, isolating it from the source of the voltage sag, would be a better solution.
In Figure 2, the current going to the small motor goes from its initial value of 0 to a maximum of over 30 amps, causing voltage sag of over 10 volts. The larger the motor compared to the system’s impedance, the more current it will pull and thus, the more drastic the voltage sag becomes. A waveform capture of a similar event is shown in Figure 3.
Figure 2. RMS Voltage and current cycle stripchart of a motor startup causing a voltage sag (above)
Figure 3. Raw waveforms of a motor startup causing a voltage sag (above)
In Figure 4, a longer RMS stripchart of many motor starts is shown. The 1 cycle voltage minimums are very low during the motor start, where the current spikes to almost 150A. The running current of the motor is much lower, at 25A.
Figure 4. RMS voltage and current stripchart showing multiple motor starts
Another result of voltage sags is flicker. For example, if a rural resident has a water well with an electric pump, it’s very common for the lights to flicker when the pump is energized. Sometimes this is a sign that the service wiring to the resident needs to be upgraded or maybe a connection has become slightly resistive and needs to be remade or tightened. Even under the best conditions, if the pump is a large one, some slight voltage sag may cause a noticeable flicker. Frequent motor starts such as in Figure 4 may cause objectionable flicker. The specific amount of flicker depends on the timing of the starts compared to the IEEE flicker curve.
In some situations, a resistive junction (e.g. from corrosion, or mechanical creep causing lugs to loosen over time) or connection between the power source and a fluctuating load that is also feeding other parts of the distribution system, is all it takes to cause a serious voltage sag to occur. In this case, simply finding and tightening loose hardware can sometimes reduce, if not eliminate, the voltage sag altogether.
In this white paper I touched on a few methods for preventing and correcting voltage sag, such as stifling the power source by tightening connection hardware, soft starts and VFD on large motors, using UPS and ferroresonant regulators on computers and microcontrollers, but there are many others methods currently in use today or on the horizon. To name a few that are used by industry and distribution systems are as follows; electronic voltage regulators, phase controlled regulators, motorized variacs, static voltage regulator, magnetic synthesizers, flywheels and motor-generator, compressed air energy storage (CABS), superconducting magnetic energy storage (SMES), dynamic voltage restorer (DVR) and dynamic sag correctors (DySC).
Conclusion
It may be impossible to eliminate voltage sags totally, but with a properly designed system, and good monitoring and maintenance, it is possible to find the source of the major voltage sags and take action to reduce or contain them. In some cases, simple maintenance can reduce many of the factors that cause voltage sags, such as keeping electrical joints tight and corrosion-free and aggressive tree-trimming programs. When a man-made source such a motor startup situation occurs, a soft start controller or VFD makes a big difference. When multiple motors start simultaneously, staggering the starts can reduce the instantaneous inrush current, thus the voltage sags. Increasing the wire size in the service drop and VA rating of the distribution transformer can also reduce voltage sags by reducing the system’s impedance feeding the load. Power Monitors’ recorders have the capability of making valuable measurements and can be used in different modes such as in the stripchart mode and waveform capture mode shown in this whitepaper. Having the proper tools and knowledge to use these tools properly is the first step in solving most power quality issues including voltage sags.
Cowles Andrus III
Communications Specialist
candrus@powermonitors.com
http://www.powermonitors.com
(800) 296-4120