When people talk about power quality, most immediately think of harmonics — which were covered in detail in a previous article. But harmonics are just the tip of the iceberg. In real distribution systems, there are four equally common, equally damaging, but frequently overlooked power quality issues: voltage deviation, three‑phase unbalance, frequency deviation, and voltage flicker. They don’t sound as technical as harmonics, but their impact on production equipment is just as severe — motor burnouts, product rejects, mysterious equipment stoppages — often the culprit is one of these seemingly minor parameters exceeding its limit. This article explains these four indicators in plain language, tells you what symptoms appear when they exceed limits, and what you can do about them. Voltage deviation is the difference between the actual operating voltage and the rated voltage, expressed as a percentage. Chinese standard GB/T 12325 specifies: for 35kV and above supply systems, the allowable voltage deviation is ±10% of rated voltage; for 10kV and below three‑phase systems, ±7%; and for 220V single‑phase systems, +7% and -10%. Excessively high voltage: for motors, a 10% voltage increase reduces current by only about 5%, owers power factor, and increases torque — but more dangerously, elevated voltage significantly shortens the life of electronic equipment; the DC‑bus capacitors in switch‑mode power supplies and VFDs age faster under overvoltage, and for every 10°C rise, capacitor life roughly halves; filament lighting (such as halogen lamps) suffers a dramatic lifespan reduction — a 5% voltage increase may halve bulb life. Excessively low voltage: asynchronous motor torque is proportional to the square of voltage — a 10% voltage drop reduces torque by 19%, meaning the motor cannot drive the load and current increases, winding heating intensifies, and in severe cases the motor burns out; VFDs may trip on undervoltage, and contactor coils may lack sufficient force to hold, causing chattering and contact damage. To improve voltage deviation: for low voltage, the most direct method is adjusting the transformer tap changer; for low voltage at the end of long transmission lines, adding reactive compensation can raise the voltage; for high voltage, you can increase line impedance or adjust transformer output settings. Three‑phase unbalance means the three‑phase voltages and currents are not equal in magnitude and not 120° apart in phase. In practice, unbalanced single‑phase loads, large single‑phase equipment (such as welders and high‑power lighting), and other factors cause significant current unbalance. Standard GB/T 15543 specifies that the allowable three‑phase voltage unbalance factor at the point of common coupling is 2%, and must not exceed 4% for short durations. Unbalance causes additional motor heating: negative‑sequence currents create a reverse rotating magnetic field, inducing double‑frequency currents in the rotor that cause extra heating; at 3.5% unbalance, motor temperature rise can be over 20% higher than under balanced conditions, significantly shortening life. It also causes neutral overloading — neutral current becomes the vector sum of the three phases, and if one phase is heavily loaded, neutral current may approach or even exceed phase current, causing overheating and insulation ageing. Transformer derating occurs because one phase winding overheats, limiting overall output — a 1000kVA transformer under severe unbalance may only deliver 800kVA or less. And rectifier equipment is sensitive to unbalance, with increased input current distortion and DC‑bus ripple that can cause failures. To improve unbalance: distribute single‑phase loads evenly; install automatic unbalance correction devices such as SVG/STATCOM; and consider load balancing at the design stage. Frequency deviation — the power system’s nominal frequency is 50Hz in China (60Hz in North America) — reflects the balance between generation and load: if generation exceeds load, frequency rises; if load exceeds generation, frequency falls. Standard GB/T 15945 specifies a normal frequency deviation of ±0.2Hz, extendable to ±0.5Hz for smaller systems. Frequency deviation causes motor speed changes — asynchronous motor speed is directly related to frequency, so textile, paper, and conveyor machinery experience speed instability and product quality variations. It also affects electronic clocks that use mains frequency as a timing reference, and can trip power electronics such as VFDs and UPS. Frequency deviation is a system‑level issue that individual users cannot directly adjust; user‑level measures include choosing wide‑input power supplies for sensitive equipment, configuring UPS or converter supplies, and factoring frequency fluctuations into equipment selection. Voltage flicker refers to rapid voltage fluctuations, typically caused by frequent starting and stopping of high‑power equipment — large welders, cranes, rolling mills, and lifts. When these start, current surges cause instantaneous voltage drops; when they stop, voltage recovers. This “up‑and‑down” variation is flicker. Standard GB/T 12326 sets strict limits measured by short‑term flicker severity (Pst) and long‑term flicker severity (Plt). Flicker causes visible lighting flicker — incandescent and fluorescent lamps show brightness changes that can cause visual fatigue and headaches in prolonged exposure. It also affects precision equipment — PLCs and DCS may experience false signals, data loss, or program jumps; medical equipment such as CT and MRI may interrupt scans or produce image artefacts. Contactors may release due to insufficient coil voltage, stopping motors, then re‑close when voltage recovers — this repeated starting/stopping imposes severe mechanical stress. VFDs may trip on bus undervoltage or input phase loss. To improve flicker: install reactive compensation near flicker sources to reduce voltage dip magnitude; replace direct‑on‑line starting with soft‑starters or VFDs to reduce starting current from 6‑8 times rated to 1.5‑2 times, significantly reducing bus voltage impact; isolate sensitive equipment on separate supply buses; and in severe cases, provide UPS or regulated power supplies for sensitive equipment. These four indicators are not isolated. Unbalance can amplify voltage deviation — when one phase is heavily loaded, its voltage is lower and current higher, further worsening unbalance. Flicker and unbalance often occur together — high‑power single‑phase loads cause both flicker and voltage dip on that phase. Addressing power quality requires comprehensive diagnosis — fixing one issue in isolation may miss deeper causes. The most reliable approach is to install power quality monitoring at the incoming supply, record all four parameters continuously, and establish a baseline. When problems arise, comparing against historical data quickly identifies which indicator is at fault, enabling targeted correction.