Why are the lights suddenly flickering?
What is flicker?
Flicker is the sense of instability of the human eye caused by changes in the brightness of the light, and the discomfort of people with changes in brightness. Since the general use of electrical equipment is much less sensitive to voltage fluctuations than incandescent lamps, the subjective visual perception of the illuminance fluctuation of incandescent lamps is selected as the "flicker" as an evaluation index for measuring the hazard of voltage fluctuations.
The degree of flicker annoyance is determined by two key indicators: the magnitude of the fluctuation in the supply voltage and the repetition frequency of the variation.
Where does the flicker come from?
In daily life, when we turn on high-power household appliances such as air conditioners, high-power washing machines, or high-power equipment such as factories to start motors, rolling mills, etc., the lights suddenly become dark and can return to normal in a short time.
Flicker is caused by voltage fluctuations. Usually, the start of a large load will cause the voltage to drop for a short time and return to normal in a short time. When the voltage is reduced by 10%, the brightness of the 60W incandescent lamp will be reduced by 34%.
What is the danger of flicker?
The flicker reflects the fluctuation of the voltage, and the voltage fluctuation can cause the following problems:
1. The brightness of fluorescent lamps and TVs varies with voltage fluctuations, and there is a flickering feeling when the voltage continues to fluctuate within a large range;
2. When the incandescent lamp voltage is higher than the rated value of 10%, the life will be shortened by 70%. When the voltage is lower than the rated value, the luminous efficiency will drop sharply;
3. Test equipment requires high output accuracy, and its output accuracy will be affected when the input voltage fluctuates;
4. When the power supply voltage of the electric heating equipment is lower than the rated value of 10%, the heat supply will be reduced by more than 20%, causing the heating time to be prolonged. If the voltage is higher than the rated value, the service life of the heating element will be shortened;
5. Voltage fluctuations will affect the efficiency and service life of induction motors. If the voltage is lower than 10% of the rated value, the electromagnetic torque of the motor will decrease to about 81% of the rated torque, resulting in increased current, coil heating, and increased loss. Reduced efficiency and shortened service life;
Lamp-eye-brain
The severity of the flicker reflects the annoyance of voltage fluctuations on human vision. Through experiments on 1,200 volunteers in an incandescent lighting environment of 230V/60W, the following conclusions are drawn: 50% of volunteers, per minute The voltage variation of 1055 times is the most sensitive, the fluctuating frequency is 17.6 times/second, and the amplitude is 0.28%. The definition of the flicker severity (Pst) is 1.
IEC 61000-4-15 specifies the calculation method of flicker, namely the "lamp-eye-brain" model, which simulates the different degrees of annoyance of the human brain by simulating the flicker of different degrees of incandescent lamps. The severity of the change.
Chinese standard
In 2008, China promulgated "GB/T 12326-2008 Voltage Fluctuation and Flicker", which clearly defines the "light-eye-brain" calculation method and short-time flicker (Pst) calculation period of 10 minutes, long time flash The calculation period of the variable (Plt) is 2 hours, and the limit values ​​of the long-term flicker (Plt) for different voltage levels are specified, as shown in the following table:
| Plt | |
| ≤110kV | >110kV |
| 1 | 0.8 |
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