The terminology of power control, supply impedance, and voltage flicker forms the shared language between equipment designers, power-quality engineers, and the interface to the public supply. Power control describes how the power delivered to or drawn from equipment is regulated, whether by phase-angle switching, half-cycle sequencing, or repeated on/off cycling. Supply impedance describes the electrical path from the public coupling point to the appliance, and it determines how far the supply voltage moves when the load current changes. Voltage flicker covers the resulting changes in voltage magnitude and the visual disturbance they produce in lighting. This reference defines the standard terms in these three areas so that specifications, test reports, and design reviews use consistent meanings.
Highlights:
- Power control is classified by how conduction is gated: cyclic on/off switching, multicycle (half-cycle) control, synchronous multicycle and burst firing control, and generalized phase control, of which ordinary phase control is a special case.
- Supply impedance is defined as a chain of four elements: the system impedance seen from the point of common coupling, the service connection impedance, the installation wiring impedance, and the appliance impedance.
- Voltage change and flicker terms separate the physical disturbance (voltage change, dip, surge, commutation notch) from the human perception thresholds used to judge it (perceptibility, irritability, and fusion frequency).
7 Power Control and Supply Network Impedance
Input power control
Control of the input power to equipment, a machine, or a system in order to obtain the required performance.
Output power control
Control of the output power of equipment, a machine, or a system in order to obtain the required performance.
Cyclic on/off switching control
Power control in which the supply to equipment is repeatedly switched on and off.
Program (of a control system)
A set of commands and information signals required to carry out a specified operation.
Multicycle control (by half-cycles)
The process of changing the ratio of the number of conducting half-cycles to the number of non-conducting half-cycles.
Note: For example, different combinations of conduction time and non-conduction time can change the average power supplied to the load.
Synchronous multicycle control
Multicycle control in which the start and end of conduction are synchronized with the instantaneous value of the line voltage.
Burst firing control
A type of synchronous multicycle control in which the start instant is synchronized with the voltage zero crossing and the current flows for an integer number of complete half-cycles.
Note: Burst firing control is used for resistive loads.
Generalized phase control
The process of varying the conduction interval of the current once or several times within one cycle or half-cycle of the supply voltage.
Phase control
The process of varying the starting point of current conduction within one cycle or half-cycle of the supply voltage, in which conduction is terminated when the current passes through zero or near zero.
Note: Phase control is a special case of generalized phase control.
Delay angle
The phase angle by which the starting point of current conduction is delayed by phase control.
Note: The delay angle may be fixed or variable, and the delay angles of the positive and negative half-cycles need not be the same.
Symmetrical control (single phase)
Control performed by a device designed to operate in the same way during the positive and negative half-cycles of an alternating voltage or current.
Note: This is based on the positive and negative half-cycles of the input source being identical. If the current waveforms of the positive and negative half-cycles are the same, generalized phase control is symmetrical control. If the numbers of positive and negative half-cycles are equal within each conduction cycle, multicycle control is symmetrical control.
Asymmetrical control (single phase)
Control performed by a device designed to operate differently during the positive and negative half-cycles of an alternating voltage or current.
Note: (1) If the current waveforms of the positive and negative half-cycles are different, generalized phase control is asymmetrical control. (2) If the numbers of positive and negative half-cycles within each conduction cycle are unequal, multicycle control is asymmetrical control.
Cycle
The complete set of states, or the range of values, through which a phenomenon or a group of (physical) quantities passes in a given sequence and then repeats.
Cycle of operation
A sequence of operations that can be repeated manually or automatically.
Point of common coupling (PCC)
The point in the public supply network that is electrically closest to a specific user installation and at which other user installations can be connected or are already connected.
Supply system impedance
The impedance of the supply system as seen from the point of common coupling.
Service connection impedance
The connection impedance between the point of common coupling and the user side of the metering point.
Installation wiring impedance
The wiring impedance between the user side of the metering point and a specific terminal.
Appliance impedance
The output impedance of the appliance as seen from the far end of the appliance supply cord.
8 Voltage Changes and Flicker
Voltage change
A sustained variation of the rms value or peak value of the voltage between two adjacent levels over a certain but unspecified time interval.
Relative voltage change
The ratio of the magnitude of a voltage change to the rated voltage value.
Duration of a voltage change
The time interval during which the voltage increases or decreases from an initial value to a final value.
Voltage change interval
The time interval from the start of one voltage change to the start of the next voltage change.
Voltage fluctuation
A series of voltage changes, or a periodic variation of the voltage envelope.
Voltage fluctuation waveform
The peak voltage envelope expressed as a function of time.
Magnitude of a voltage fluctuation
The difference between the maximum and minimum values of the rms voltage or peak voltage during a voltage fluctuation.
Rate of occurrence of voltage changes
The number of voltage changes occurring per unit time.
Voltage unbalance / voltage imbalance
A condition in a polyphase system in which the rms phase voltages, or the phase angles between adjacent phases, are not equal.
Voltage dip
A sudden reduction in voltage at a point in an electrical system, followed by recovery to the normal value after a brief duration ranging from a few cycles to a few seconds.
Voltage surge
A transient voltage wave propagating along a line or circuit, characterized by a rapid rise followed by a slow fall in voltage.
Commutation notch
A voltage change that appears on the AC voltage because of the commutation action of a converter, with a duration much shorter than the AC cycle.
Flicker
An unstable visual sensation caused by a light stimulus whose brightness or spectral distribution varies with time.
Flickermeter
An instrument used to measure the magnitude of flicker.
Threshold of flicker perceptibility
The minimum fluctuation value of brightness or spectral distribution that causes a flicker sensation in a defined sample of the population.
Threshold of flicker irritability
The maximum fluctuation value of brightness or spectral distribution that does not cause a sensation of discomfort in a defined sample of the population.
Fusion frequency
The alternating frequency of a visual stimulus above which, under a given set of conditions, flicker is not perceived.
Note: Fusion frequency is also called the critical flicker frequency.
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