METREL MI 3399, Инструкция с упражнениями

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Power and Voltage Quality Trainer AD 2
– MI 3399
128
2 Basic theory
There are quite a few reasons for measuring and analysing power quality nowadays.
Potential interactions between end use equipment and electric distribution system,
external electromagnetic interferences, resonant states between electrical circuits and
some other factors call for a need to be analysed in order that harmful consequences
can be omitted or prevented. Power quality analysing includes measurements of:

Phase to ground voltages;

Phase to neutral voltages;

Neutral to ground voltages;

Phase to phase voltages in three-phase systems;

Phase currents;

Current in a neutral conductor;

Frequency;

Power Factor, cos
ϕ
;

Harmonic components of current and voltage and their direction;

Waveform of current and voltage at specific circumstances (peak magnitude,
primary frequency, time of occurrence, rising rate);

Transients.
Power quality measurement is usually considered as a measurement of low frequency
conducted disturbance with the addition of transient phenomena. The following
parameters of supply voltage are influenced by disturbances:

frequency

voltage level

waveshape

symmetry of three phase system.
2.1 Power quality events
The ideal supply voltage of a single phase is a pure sinusoidal voltage with nominal
frequency and voltage amplitude.
Any variation from this is considered as a power
quality event or a disturbance.
2.2 Rapid voltage changes
Rapid voltage change is a fast change in a voltage U
rms
(1/2) between two steady
conditions. It is caused by switching on or off large loads. A typical cause of rapid
voltage change is the start of a large motor. If a rapid voltage change exceeds the
dip/swell threshold it is considered as a dip or swell.
2.3 Supply voltage dips
Supply voltage dip represents temporary reduction of a voltage below a threshold.
Duration of phenomena is limited up to 1 minute. Decreased voltage for period longer
than a minute is considered as a magnitude variation. A voltage dip is characterized
by:

dip threshold

starting time of a dip