**2.8. Frequency deviation**

The fundamental frequency varies from its rated (50 or 60 Hz) value. This frequency deviation is infrequent in stable and stiff interconnected power system networks. However, it can be noticed in weak power systems fed from local generators especially during sudden load application or rejection conditions.The operating frequency range should be kept within ±1% the rated frequency for 95% of week and -6%/+4% for 100% of week [15, 16]. The ratio of frequency deviation (RFD) can be defined as follows:

$$RFD = \frac{|f\_n - f\_r|}{f\_r} \times 100\tag{4}$$

quality arises, some people routinely predict that the issue is related to power system harmonic distortion. In the past, the terms of power quality and power system harmonics have

Introductory Chapter: Power System Harmonics—Analysis, Effects, and Mitigation Solutions…

http://dx.doi.org/10.5772/intechopen.76628

7

At present, as a consequence of the extensive use of power electronic-based components in all power system applications, most of today's loads are nonlinear. To generalize, three categories can be recognized as primary sources of harmonics in power systems [6]. They are given

• Magnetic core-based equipment as electric motors, power transformers, and generators.

On one hand, if the power system is characterized by series and shunt elements; thus, the nonlinearities exist in the system are mainly introduced by the shunt elements, such as loads. On the other hand, a series impedance of the power delivery system (impedance between the source and the load) is particularly linear, that is, short circuit or Thevenin impedance of a system. Even within a power transformer, the shunt branch (magnetizing impedance) of the standard T model is the source of harmonics, while the series leakage impedance is consid-

• The different forms of pulse modulation which are employed in active power and voltage

• Rectifiers and inverters of grid-connected solar photovoltaic cells and windmills.

been incorrectly interchanged.

• Arc and induction, and arc welders. • Power electronic-based equipment.

Today, the most prevailing harmonic sources are:

• High-frequency converters needed for induction heating.

• Converters (rectifiers and inverters).

control in transmission circuits.

• Magnetizing currents of the transformers.

• Excitation currents of the rotating machines.

• Flexible AC transmission systems (FACTS).

• Rectifiers and inverters of HVDC systems.

• Uninterruptible power supplies.

• Switch-mode power supplies.

• Thyristor controlled reactors.

*2.9.1. Harmonics sources*

ered as a linear element.

as follows:

where *f <sup>m</sup>* is the measured frequency which is time-varying quantity and *f r* is the rated system frequency.
