Power line flicker
Encyclopedia
Power-line flicker is a visible change in brightness of a lamp due to rapid fluctuations in the voltage of the power supply. The source of this is the voltage drop generated over the source impedance
of the grid by the changing load current of an equipment or facility. These fluctuations in time generate flicker. The effects can range from disturbance to epileptic attacks
of photosensitive
persons. Flicker may also affect sensitive electronic equipment such as television
receivers or industrial processes relying on constant electrical power.
uses large electric motor
s or arc furnaces on a distribution network, or frequent starting of an elevator
motor in an office building, or if a rural residence has a large water pump starting regularly on a long feeder system. The likelihood of flicker increase as the size of the changing load becomes larger with respect to the prospective short circuit current available at the point of common connection.
From the resulting momentary value of flicker the short term flicker value Pst is calculated according to a statistical process over a predefined observation interval. Long term flicker Plt is calculated as the cubic average of several Pst values.
In the standard IEC 61000-3-3 the observation intervals and the limiting values for Pst and Plt are specified:
If the EUT is operated in a (relatively) constant fashion during the whole test, Plt = Pst will result. If this state is feasible and realistic this means Pst has to fulfill the limits for Plt (which are lower).
) instead of step fashion, this also makes flicker less perceptible.
The relationship between amplitude of load changes and Pst is linear, i.e. halving the switched load results in half the Pst. The relationship between number of load changes per time (n/ Tp) and Pst is non-linear. A halving of load changes reduces Pst by only about 20%. In order to have half the Pst, the number of load changes must be reduced by a factor of 9.
Impedance
Impedance may refer to:*Electrical impedance, the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit**Characteristic impedance of a transmission line...
of the grid by the changing load current of an equipment or facility. These fluctuations in time generate flicker. The effects can range from disturbance to epileptic attacks
Epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
of photosensitive
Light sensitivity
Light sensitivity or photosensitivity is an increase in the reactivity of the skin to sunlight. Apart from vision, human beings have many physiological and psychological responses to light. In rare individuals an atypical response may result in serious discomfort, disease, or injury. Some drugs...
persons. Flicker may also affect sensitive electronic equipment such as television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
receivers or industrial processes relying on constant electrical power.
Causes
Flicker may be produced, for example, if a steel millSteel mill
A steel mill or steelworks is an industrial plant for the manufacture of steel.Steel is an alloy of iron and carbon. It is produced in a two-stage process. First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or...
uses large electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s or arc furnaces on a distribution network, or frequent starting of an elevator
Elevator
An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...
motor in an office building, or if a rural residence has a large water pump starting regularly on a long feeder system. The likelihood of flicker increase as the size of the changing load becomes larger with respect to the prospective short circuit current available at the point of common connection.
Measurement of flicker
The requirements of a flicker measurement equipment are defined in the standard . The flicker meter is subdivided in several function blocks which simulate a 230 V/60 W incandescent lamp (reference lamp) and the human perception system (eye-brain model).From the resulting momentary value of flicker the short term flicker value Pst is calculated according to a statistical process over a predefined observation interval. Long term flicker Plt is calculated as the cubic average of several Pst values.
In the standard IEC 61000-3-3 the observation intervals and the limiting values for Pst and Plt are specified:
| Value | Observation Interval | Limiting Value |
|---|---|---|
| Pst | 10 min | 1.0 |
| Plt | 2 h | 0.65 |
Operating condition of the EUT
The IEC-flicker standard states that the EUT (Equipment Under Test) has to be operated during the test in a way which is the worst case state with respect to flicker.If the EUT is operated in a (relatively) constant fashion during the whole test, Plt = Pst will result. If this state is feasible and realistic this means Pst has to fulfill the limits for Plt (which are lower).
Estimation
A purely analytical calculation of Pst is almost impossible. In the standard there are formulas which allow the estimation of the Pst values to be expected.Flicker mitigation
Flicker is generated by load changes. Only the amplitude of the load change is relevant, not the absolute value. A reduction in flicker can be attained through making less frequent load changes, or smaller load changes. If the load is changed gradually (for example, by the help of power electronicsPower electronics
Power electronics is the application of solid-state electronics for the control and conversion of electric power.-Introduction:Power electronic converters can be found wherever there is a need to modify a form of electrical energy...
) instead of step fashion, this also makes flicker less perceptible.
The relationship between amplitude of load changes and Pst is linear, i.e. halving the switched load results in half the Pst. The relationship between number of load changes per time (n/ Tp) and Pst is non-linear. A halving of load changes reduces Pst by only about 20%. In order to have half the Pst, the number of load changes must be reduced by a factor of 9.