The following points highlight the seven main examples of transient waveforms of voltage. The examples are: 1. Motor Start Transient 2. Power Factor Correction Capacitor Switching Transient 3. Medium Voltage Capacitor Bank Switching Transient 4. Voltage Notch due to Uninterruptible Power Source Unit 5. Neutral Voltage Swing 6. Sudden Application of Voltage 7. Self-Produced Transients.
Example # 1. Motor Start Transient:
Figure 3.9 shows a transient produced when a 50-hp induction motor with integral power factor correction was started across the line. The notch in the voltage waveform at the instant of starring was produced by the presence of the capacitor. The quick voltage recovery was followed by ringing characteristics. The transient lasted less than half of a cycle, but it was sufficient to affect the operation of large chillers located nearby which contained solid-state starters with sensitive voltage-sensing circuitry.
Because of the severity of the motor starting transient, the chillers started to shut down. In a situation such as this, it is often prudent to apply correction to the sensitive circuitry rather than try to eliminate the problem itself or apply correction to the power system as a whole.
Example # 2. Power Factor Correction Capacitor Switching Transient:
Figure 3.10 shows the transient voltage response at the main electrical switchboard for commercial building due to capacitor bank switching by the utility. A moderate figure 3.9 Transient due to motor starting. The motor had an input capacitor for power factor correction, and the motor and capacitor were turned on simultaneously. Rise in system voltage is followed by ringing at the characteristic frequency of the utility source inductive reactance and capacitance due to the power factor correction equipment.
Example # 3. Medium Voltage Capacitor Bank Switching Transient:
The transient shown in Figure 3.11 was the result of a 5-MVAR capacitor bank switching at an industrial facility. The facility suffered from poor power factor due to plant loads which necessitated connection of the capacitor bank. The initial rise in voltage reached peak amplitude equal to 160% of the system nominal peak voltage. This practice had been going on in this facility for approximately 5 years before failures were observed in underground cables and a power transformer.
At this point, the electrical system in the facility was monitored for transient voltages by installing power quality analyzers at select locations. Once the nature of the transients and their cause were determined, corrective steps were taken to retrofit the capacitor bank with pre-insertion resistors, which helped to attenuate the amplitude of the impulse. Also, all replacement equipment at the facility was specified to be of a higher basic insulation level (BIL) than the minimum specified in standards.
Example # 4. Voltage Notch due to Uninterruptible Power Source Unit:
Typically, we associate voltage notches with adjustable speed drives. Voltage notches are also common with the outputs of uninterruptible power source (UPS) units due to power electronic switching circuitry associated with the UPS units. Unless provided with wave shaping and filtering circuitry, the output of the UPS can contain substantial notches.
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Figure 3.12 shows the output waveform of a UPS unit supplying 480-V output to a computer center at a financial institution. If the notch levels become excessive, problems can arise in the operation of sensitive communication or data-processing loads. The voltage notch phenomenon is a repetitive event. Even though we defined transients as sub-cycle events, the repetitive notching shown is included or the sake of completeness.
Example # 5. Neutral Voltage Swing:
The event shown in Figure 3.13 was observed in a computer laboratory at a university. Normally, neutral voltage should be within 0.5 V with respect to the ground. This is because in a four-wire power distribution system the neutral of the power source Figure 3.21 Voltage waveform at a 12.47-kV power system during switching in of a 5-MVAR capacitor bank.
The voltage-to-transformer ratio is 60:1. is connected to the ground at the source. This tends to hold the neutral potential close to the ground. In a typical building, neutral-to-ground voltages become higher as we move away from the source feeding the facility. In some instances, depending on the loads and the distance between the source and the load, neutral-to-ground voltage can measure 2 to 3 V.
The case illustrated by Figure 3.13 is an extreme one where neutral-to-ground voltages reached levels higher than 10 V. The computer laboratory experienced many problems with the computers locking up, in some instances during critical times when tests were being administered to the students.
Example # 6. Sudden Application of Voltage:
Fast rise time transients are produced when voltage is suddenly applied to a load. Figure 3.14 shows an example of 480 V being applied to the primary of a power distribution transformer. Typical waveform characteristics include fast rise time and ringing due to the inductance and capacitance of the load circuitry. Normally, power system should ride through such occurrences, but, if the load circuit includes capacitor banks or power supplies with capacitors, large inrush currents may be produced with possible overcurrent protection operation. In situations where the capacitors have an initial charge present, some overvoltage events may be produced. Figure 3.12 voltage notches produced at the output of an uninterruptible power source (UPS) unit.
Example # 7. Self-Produced Transients:
Some machines by nature generate transients that can affect machine operation if they contain sensitive circuits.
Figure 3.15 shows a waveform produced by a food processing machine. The transients occurred several times each day as the machine automatically turned on and off. Severe transients caused the machine to go into a lockout mode which required the operator to manually reset the machine. The problem was fixed by providing a dedicated circuit of low impedance for the machine. This reduced the transient to levels that could be lived with.
