Harmonic sources

The most important harmonic sources are basically converters and power supplies for numerous electrical equipment. This equipment is a source for harmonics, and at the same time, its operation principles may be very sensitive to harmonics,

especially to voltage harmonics. Still, some devices can be designed to decrease their characteristic harmonics.

13.3.1.

Single-phase power supplies

A major harmonic concern in commercial buildings is that power supplies for single-phase electronic equipment will produce too much distortion for the wiring. Direct current power for modern electronic and microprocessor-based office equipment is commonly derived from single-phase full-wave diode bridge rectifiers. Modern technology for single-phase power supplies is based on switch-mode. A distinctive characteristic of switch-mode power supplies is the very high third- harmonic content in the current. Other characteristic harmonics are the 5th and 7th harmonics. Switch-mode power supplies are beginning to find applications in fluorescent lighting systems. Typical current harmonics and the waveform for a switch-mode power supply are shown in Fig. 13.3.1.-1.

Fig. 13.3.1.-1 Typical current harmonics and the waveform for a switch-mode power supply 0 0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9 10 11 12 13 Harmonic

Magnitude p.u. of fundamental

13.3.2.

Three-phase power converters

Three-phase electronic power converters differ from single-phase converters mainly because they do not generate the third harmonic or the third harmonic is quite small. There are many designs and types of converters for AC or DC drives with different power ratings. Harmonics may vary significantly between designs and operation conditions. Still, some examples are given below.

Six-pulse and twelve-pulse converters

Harmonic components of the AC current waveform with q-pulse rectifier are:

and the magnitudes of the harmonic currents are:

where

The most significant harmonics for six-pulse converters are the 5th, 7th, 11th and 13th. For twelve-pulse converters, the 11th, 13th, 23rd and 25th harmonics are the most significant.

PWM-type ASD

Typical current harmonics and the waveform for a Pulse Width Modulation-type Adjustable Speed Drive with rated speed are shown in Fig. 13.3.2.-1 .

Fig. 13.3.2.-1 Current harmonics and the waveform for a PWM-type ASD

CSI-type ASD

Typical current harmonics and the waveform for a Current Source Inverter-type Adjustable Speed Drive are shown in Fig. 13.3.2.-2.

h the harmonic order

k any positive integer

q the pulse number of the rectifier circuit

I_h the amplitude of the harmonic current of order h

I1 the amplitude of the fundamental current

h

=

kq 1±

I

I

h

---

=

Magnitude p.u. of fundamental

H

Fig. 13.3.2.-2 Current harmonics and the waveform for a CSI-type ASD

Cycloconverter harmonics

The expressions of cycloconverter current harmonics are complex. They vary as a function of the frequency ratio of the cycloconverter:

where

This means that harmonics may vary significantly and interharmonics (non-integer multiple of fundamental frequency) may also appear. Characteristic harmonics for a six-pulse cycloconverter are harmonics from fundamental to 2nd, 5th to 7th, and 11th to 13th.

13.3.3.

Other harmonic sources

There are many other harmonic sources in addition to converters and power supplies. These sources are mainly arching devices like arc furnaces and welding equipment.

Arc furnaces

The harmonics produced by electric arc furnaces used for the production of steel are unpredictable. The steel scrap to be molten is a very non-linear load and thus the melting arc changes constantly. The arc current may be non-periodic and may include both harmonics and interharmonics. Still, in most applications, the low- order harmonics starting with the second and ending with the seventh predominate the non-integer harmonics. Fig. 13.3.3.-1 presents typical harmonics for an arc furnace during the initial melting period and the refining period. These harmonics have quite a low percentage magnitude compared to the fundamental component. Arc furnaces form a large load with fundamental currents of several kA, which makes arc furnaces a significant harmonic source for the power system.

fh the harmonic frequency imposed on the AC system

fi the input frequency of the cycloconverter

k, n integers

q the pulse number of the converter

fo the output frequency of the cycloconverter

0 0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9 10 11 12 13 Harmonic

Magnitude p.u. of fundamental

HarmCSI.CNV

Fig. 13.3.3.-1 Typical harmonics for arc furnaces. The first picture is for the melting phase and the second for the refining phase.

Other arching devices similar to arc furnaces are arc welding equipment.

Saturable devices

Equipment in this class includes transformers and other electromagnetic devices with a steel core, including motors. Harmonics are generated due to the non-linear magnetising characteristics of the steel. Harmonics are due to exciting current, which is very rich in harmonics like the 3rd, 5th, 7th and 9th. Transformers are not as much a concern as electronic power converters because exciting current is small compared to the rated full load current. However, their effect will be noticeable particularly on utility distribution systems that have hundreds of transformers. A significant increase in triplen harmonic currents is often noticed during the early morning hours when the load is low and thus the percentage of harmonics compared to the fundamental is high.

Motors and synchronous generators also exhibit some distortion, although it is generally of little consequence.