Danfoss VLT FC 103, Руководство по конструкции

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System
type Description
TN Mains
Systems There are 2 types of TN mains distribution systems:
TN-S and TN-C.
TN-S A 5-wire system with separate neutral (N) and
protective earth (PE) conductors. It provides the
best EMC properties and avoids transmitting
interference.
TN-C A 4-wire system with a common neutral and
protective earth (PE) conductor throughout the
system. The combined neutral and protective earth
conductor results in poor EMC characteristics.
TT Mains
Systems A 4-wire system with a grounded neutral conductor
and individual grounding of the frequency
converter units. It has good EMC characteristics
when grounded properly.
IT Mains
System An isolated 4-wire system with the neutral
conductor either not grounded or grounded via an
impedance.
Table 3.14 AC Mains System Types
3.4.2 Low-frequency Mains Interference
3.4.2.1 Non-sinusoidal Mains Supply
The mains voltage is rarely a uniform sinusoidal voltage
with constant amplitude and frequency. This is partly due
to loads that draw non-sinusoidal currents from the mains
or have non-linear characteristics, such as:
•
Computers.
•
Television sets.
•
Switching power supplies.
•
Energy-efficient lamps.
•
Frequency converters.
Deviations are unavoidable and permissible within certain
limits.
3.4.2.2 EMC Directives Compliance
In most of Europe, the basis for the objective assessment
of the quality of mains power is the Electromagnetic
Compatibility of Devices Act (EMVG). Compliance with this
regulation ensures that all devices and networks connected
to electrical distribution systems fulfil their intended
purpose without generating problems.
Standard Definition
EN 61000-2-2, EN
61000-2-4, EN 50160 Defines the mains voltage limits to
observe in public and industrial power
grids.
EN 61000-3-2,
61000-3-12 Regulates mains interference generated
by connected devices.
EN 50178 Monitors electronic equipment for use in
power installations.
Table 3.15 EN Design Standards for Mains Power Quality
3.4.2.3 Interference-free Frequency
Converters
Every frequency converter generates mains interference.
Present standards only define frequency ranges up to 2
kHz. Some frequency converters shift the mains
interference in the region above 2 kHz, which the standard
does not address, and labels them as interference-free.
Limits for this region are currently being studied.
Frequency converters do not shift mains interference.
3.4.2.4 How Mains Interference Occurs
Mains interference distortion of the sinusoidal waveform
caused by the pulsating input currents is referred to as
harmonics. Derived from Fourier analysis, it is assessed up
to 2.5 kHz, corresponding to the 50th harmonic of the
mains frequency.
The input rectifiers of frequency converters generate this
typical form of harmonic interference on the mains. When
frequency converters are connected to 50 Hz mains
systems, the 3rd harmonic (150 Hz), 5th harmonic (250 Hz)
or 7th harmonic (350 Hz) show the strongest effects. The
overall harmonic content is called the total harmonic
distortion (THD).
3.4.2.5 Effects of Mains Interference
Harmonics and voltage fluctuations are 2 forms of low-
frequency mains interference. They have a different
appearance at their origin than at any other point in the
mains system when a load is connected. So, a range of
influences must be determined collectively when assessing
the effects of mains interference. These influences include
the mains feed, structure, and loads.
Undervoltage warnings and higher functional losses can
occur as a result of mains interference.
Undervoltage warnings
•
Incorrect voltage measurements due to distortion
of the sinusoidal mains voltage.
•
Cause incorrect power measurements because
only RMS-true measuring takes harmonic content
into account.
System Integration
VLT
®
Refrigeration Drive FC 103
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