If a power plant outputs relatively clean, high-quality power, transmission and distribution can introduce quality problems due to weather, customer equipment, and human factors. After distribution, issues such as voltage surges, sags, interruptions, and common-mode noise become prominent. Power quality is often worse in industrial environments, and many older building distribution systems are not suitable for modern information systems. The grid can cause interference or damage to computers and precision instruments in the following ways.
Typical issues
1. Power surges: A power surge occurs when the output voltage RMS exceeds the nominal value by around 10% or more and lasts one or several cycles. Surges often result from large loads suddenly disconnecting from the mains (for example, when large appliances are switched off). A common everyday example is house lights dimming during a usage peak and then briefly brightening afterwards when the load drops.
2. High-voltage spikes: These are spikes with peak values up to about 6000 V and durations ranging from 1e-4 s to a half-cycle (about 10 ms at 50 Hz). They are typically caused by lightning strikes, arc discharges, static discharges, or switching operations on large electrical equipment.
3. Switching transients: Transients can reach peak voltages up to around 20,000 V but have much shorter durations, typically between 1e-6 s and 1e-4 s. Causes and potential damage are similar to high-voltage spikes, though mitigation techniques differ.
4. Power sags: A power sag is a low-voltage condition where the mains RMS voltage falls to about 80%–85% of nominal and lasts one to several cycles. Large equipment starts, motor startups, or the connection of large stabilizers can cause sags.
5. Electrical line noise: This includes radio-frequency interference (RFI), electromagnetic interference (EMI), and other high-frequency disturbances. Motor operation, relay switching, motor controllers, broadcast transmitters, microwave radiation, and electrical storms can all introduce line noise.
6. Frequency variation: Frequency variation refers to changes in mains frequency exceeding about 3 Hz. This is mainly caused by unstable operation of emergency generators or supplies with poor frequency regulation.
7. Brownout: A brownout is a prolonged reduction in mains RMS voltage below nominal levels. Causes include heavy equipment startup, switching of primary power lines, large motor startups, and line overloads. This issue exists in many regions of China.
8. Power interruptions: Mains power interruptions, or outages, occur when circuit breakers trip, utility supply is interrupted, or there is a grid fault.
Mitigation and design considerations
Protecting computers, networks, and data requires attention to power quality. Design, monitoring, and mitigation of power quality are not only quality concerns but also safety concerns. Typical user-level measures include AC voltage regulators, EMI filters, and uninterruptible power supplies to improve the power available to information equipment. At the same time, consider reducing equipment pollution of the grid, improving power factor, reducing current harmonics, and suppressing conducted and radiated interference.
