Short-Duration Voltage Variations
This category encompasses the IEC category of voltage dips and short interruptions. Each type of variation can be designated as instantaneous, momentary, or temporary, depending on its duration as defined in Table 2.2.
Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either tempo rary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions). The fault condition can be close to or remote from the point of interest. In either case, the impact on the voltage during the actual fault condition is of the short-duration variation until protective devices operate to clear the fault.
Depending on the fault location and the system conditions, the fault can cause either
- Temporary voltage drops (sags),
- voltage rises (swells),
- a complete loss of voltage (interruptions
- An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min.
- Interruptions can be the result of power system faults, equipment failures, and control malfunctions.
- The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal.
- The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices.
- Instantaneous reclosing generally will limit the interruption caused by a non permanent fault to less than 30 cycles.
- Delayed reclosing of the protective device may cause a momentary or temporary interruption.
- A sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.
- A short-duration voltage decrease is called as “sag”.
- The IEC definition for this phenomenon is “dip”.
- Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors.
- Figure below shows a voltage sag waveform.
- As shown in the Fig 2.6(b) An 80 percent sag exists for about 3 cycles until the substation breaker is able to interrupt the fault current. Typical fault clearing times range from 3 to 30 cycles, depending on the fault current magnitude and the type of overcurrent protection.
- The above Fig illustrates the effect of a large motor starting. An induction motor will draw 6 to 10 times its full load current during start-up.
- If the current magnitude is large relative to the available fault current in the system at that point, the resulting voltage sag can be significant.
- In this case, the voltage sags immediately to 80 percent and then gradually returns to normal in about 3 s.
- A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.
- Swells can occur due to temporary voltage rise on the unfaulted phases during an SLG fault.
- Swells can also be caused by switching off a large load or energizing a large capacitor bank.
- Fig 2.8 illustrates a voltage swell caused by an SLG fault.
- Swells are characterized by their magnitude (rms value) and duration.
- The severity of a voltage swell during a fault condition is a function of the fault location, system impedance, and grounding.
- A swell is commonly defined as “momentary over voltage”.
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