Differences Between Emi And Harmonics; Power Harmonics And Their Reduction Techniques - Mitsubishi Electric FR-A700 Technical Manual

3.5 Inverter-generated harmonics, EMI and leakage
current
3.5.1

Differences between EMI and harmonics

Sometimes inverter affects power supply or other
peripherals. Confusion of EMI and harmonics may cause a
malfunction of the peripheral devices. Countermeasures
against EMI and harmonics are different, so different
countermeasures for each causes are required. For
example, electromagnetic interference produced by a
personal computer are distinctly different in their sources,
adverse effects etc. from harmonics in a power circuit, and
their countermeasures differ greatly.
Their differences will be described below.
Harmonics are defined to have a frequency that is an
integral multiple of the fundamental wave and is different
from high frequencies. The composition of a single
Differences between EMI and harmonics of an inverter and leakage current
Item
High frequency (several 10kHz to
Frequency band
1GHz order)
Source Inverter circuit
Cause Transistor switching
Depends on voltage variation ratio
Generated amount
and switching frequency
Propagation path Electric channel, space, induction
Transmission amount Distance, wiring route
Sensor, etc: Mis-detection
Affected equipment
Radio, wireless equipment : Acoustic
and influence
noise
Main countermeasure Change the wiring route
examples Install a EMI filter
3.5.2

Power harmonics and their reduction techniques

Constituted by a power rectifier, the converter circuit of the
inverter generates harmonics, distorting the voltage and
current waveforms of the input power supply. It is
necessary to have a correct understanding of harmonics
and take appropriate measures against them when using
equipment having a converter circuit in the power supply
circuit, e.g. a large-capacity inverter, many inverters,
thyristor leonard, thyristor motor or CVCF.
The Japanese harmonic reduction guidelines issued by the
Ministry of Economy, Trade and Industry (formerly Ministry
of International Trade and Industry) in September 1994
require that the following two main points should be
considered as harmonic reduction techniques:
1) Actions taken for faults due to power harmonics
(overheating of power capacitors, non-utility
generators, etc.)
2) Comply to the Japanese harmonic suppression
guidelines
EMI
Normally 40th to 50th degrees or
less (up to 3kHz or less)
Converter circuit
Rectifying circuit commutation
Depends on current capacity
Electrical channel
Line impedance
Power capacitor : Heat generation
Non-utility generator
: Heat generation
Install a reactor
508
fundamental wave and several harmonics is called a
distorted wave.
A distorted wave generally includes harmonics in a high-
frequency wave (KHz to MHz order). However, a distorted
wave handled as harmonics in a power distribution system
is usually of up to about 40th to 50th degrees (to several
kHz), and a distorted wave above that value generally
assumes an irregular form and must therefore be handled
as EMI.
Harmonics
(several Hz to MHz order)
Inverter circuit
Transistor switching
Depends on switching frequency and
voltage
Insulating material
Capacitance
Earth leakage circuit breaker
: Unnecessary operation
Thermal relay
: Unnecessary operation
Output side devices
(e.g. CT, meter) : Heat generation
Change detection sensitivity
Change switching frequency
The actions taken to achieve the two points on the left are
not always the same, so specific actions must be taken
after clarifying their purposes.
(1) Influence on and actions taken for the
power supply line
The power supply line to which the inverter is
connected with parallel loads such as a power
capacitor and a generator. And a harmonic current
generated in the inverter is divided into the power
supply line and parallel loads according to their
impedances. The influence of the harmonic current
on the electrical devices (parallel loads) and actions
taken against that current will be described below.
1) Power capacitor
For the maximum working voltages and maximum
working currents of power capacitors, JIS-C4902
SELECTION
Leakage Currents