**2.2. IEEE standard 519 harmonic distortion limits**

Large industrial loads are often connected to transmission networks due to large power requirements. Such loads are often non-linear and may include rolling mills driven by variable speed drives or could be an arc furnace. These non-linear loads are sources of harmonics. Harmonics that propagates from the industrial loads degrades the power quality at the electrical system. Harmonics can cause problems to different electrical equipment such as generators, motors, transformers, capacitors, and cables. In addition, it can lead to reduced capacity

36 Power System Harmonics - Analysis, Effects and Mitigation Solutions for Power Quality Improvement

The harmonic content in any network varies with time depending on the share of non-linear loads as well as system status. Examining temporal harmonic profile can help understanding system performance at different loading conditions. To examine the temporal profile, harmonics measurements were conducted at two industrial load grid stations of Oman Electricity Transmission Company (OETC) network in the Main Interconnected System (MIS) of Oman.

In this section, the indices conventionally used for measurement of voltage and current harmonic distortion and the harmonic distortion limits placed in IEEE standard 519, IEC stan-

The total harmonic distortion (THD) is used to define the effect of harmonics on the power system voltage. IEEE 519-2014 defines the THD as "the ratio of the root mean square of the harmonic content, considering harmonic components up to the 50th order and specifically excluding interharmonics, expressed as a percent of the fundamental". In other words, the THD is the contribution of all harmonics to the fundamental. The THD is calculated as

 is the rms value fundamental component of the voltage or current signal. To evaluate the current harmonic distortion, the total demand distortion (TDD) is commonly used. IEEE 519-2014 defines the TDD as "the ratio of the root mean square of the harmonic content, considering harmonic components up to the 50th order and specifically excluding

> \_\_\_\_\_\_\_ ∑*<sup>h</sup>*=2 *hmax I h* 2 \_\_\_\_\_\_ *I m*

= <sup>√</sup>

\_\_\_\_ *I*<sup>2</sup> − *I* 1 2 \_\_\_\_ *I m*

1

interharmonics, expressed as a percent of the maximum demand current"

*<sup>m</sup>* is the maximum demand load current and *<sup>I</sup>*

\_\_\_\_\_\_\_\_ ∑*h*=2 *hmax Mh* 2 \_\_\_\_\_\_\_ *M*1

(1)

(2)

is the rms value of the fundamental

**2. Harmonic limits according to the international standards and** 

dard 61000-3-6 and Oman's national regulations are presented.

**2.1. Voltage and current harmonic distortion indices**

and efficiency of power systems.

**Oman's national regulations**

described by the following formula:

where *M*<sup>1</sup>

where *I*

component.

*THD* = <sup>√</sup>

*TDD* = <sup>√</sup>

IEEE standard 519 gives harmonic distortion limits for both the current and the voltage signals in power systems [3]. **Tables 1** and **2** show relevant voltage and current distortion limits. The allowable voltage THD is based on the voltage level, while the current TDD limit is given based on the voltage level and the ratio of the short circuit current to the rated load current.

According to IEEE STD 519-2014, statistical analysis of 1-week short-time harmonic measurements is required to calculate the 95th and 99th percentile values for comparison with the recommended limits. While current harmonics are evaluated based on 95th and 99th percentiles, voltage harmonics are evaluated based on 95th percentile only.
