While voltage sags and momentary interruptions cause the most widespread power quality problems, several other power quality disturbances can damage equipment, overheat equipment, disrupt processes, cause data loss, and annoy and upset customers.
Often, customers complain of equipment failures, especially following power interruptions. Is it lightning? Voltage swells during faults? Some sort of switching transient?
Before going into deep in this subject, note that we are not discussing here whose fault is failure of electrical equipment (designer, manufacturer, customer etc.). Overvoltages are analysed only.
However, some possibilities of what happened are:
Overvoltages – Lightning and other system primary-side overvoltages can enter the facility and damage equipment.
Grounding – Poor facility grounding practices can introduce overvoltages at equipment from fault current.
Capacitive coupling – Reclose operations and other switching transients can create fast-rising voltage on the primary that capacitively couples through the transformer, causing a short pulse on the secondary.
Inrush current – While recovering from a voltage sag or momentary interruption, the inrush current into some electronic equipment can blow fuses or fail semiconductor devices.
Unbalanced sags – Three-phase electronic equipment like adjustable- speed drives can draw excessive current during a single-phase sag
or other unbalanced sag. The current can blow fuses or fail the front-end power electronics.
Equipment aging – Some equipment is prone to failure during turn on, even without a voltage transient. The most obvious example is an incandescent light bulb. Over time, the filament weakens, and the bulb eventually fails, usually when turned on. At turn on, the rapid temperature rise and mechanical stress from the inrush can break the filament.
Damaging surges can enter from strikes to the primary, strikes to the secondary, strikes to the facility, strikes to plumbing, and strikes to cable-television or telephone wires. Poor grounding practices can make lightning-caused failures more likely.
Another source of severe overvoltages is primary or secondary conductors contacting higher voltage lines. Other overvoltages are possible; normally these are not severe enough to damage most equipment, except for sensitive electronics:
- Voltage swells – Peaks at about 1.3 per unit on most distribution circuits. (READ MORE)
- Switching surges – Normally peaks at less than 2 per unit and decays quickly.
- Ferroresonance – Normally peaks at less than 2 per unit.
Just as arresters on distribution lines are sensitive to overvoltages, arresters inside of electronic equipment often are the first thing to fail. The power supply on most computers and other electronics contains small surge arresters (surge suppressors) that can fail quickly while trying to clamp down on overvoltages, especially longer-duration overvoltages. These small suppressors have limited energy absorption capability.
In addition to proper grounding, surge arresters are the primary defense against lightning and other transients. For best protection, use surge protection at the service entrance and surge protection at each sensitive load.
Surge arresters work well against short-duration overvoltages – lightning and switching transients. But arresters have trouble conducting temporary power-frequency overvoltages; they absorb considerable energy trying to clamp the overvoltage and can fail.
Small arresters often are the first component to fail in equipment. Using a higher voltage rating helps give more protection to the surge arrester during temporary overvoltages (for example, end users should not use arresters with a maximum continuous operating voltage below 150 V).
So, we want the arrester with the most energy capability to absorb most of the energy.
Surge arresters should be coordinated. The large surge arrester at the service entrance should have the lowest protective level of all of the arresters within the facility. Because arresters are so nonlinear, the unit with the lowest protective level will conduct almost all of the current.
Reference: Electric power distribution handbook – Tom Short (Buy hardcopy from Amazon)