**2. Harmonic limits according to the international standards and Oman's national regulations**

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

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

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 described by the following formula:

$$\text{THD} = \frac{\sqrt{\sum\_{k=2}^{n} M\_k^2}}{M\_1} \tag{1}$$

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

voltage harmonics are evaluated based on 95th percentile only.

**PCC voltage Individual harmonic magnitude (%) THD (%)**

V ≤ 1 kV 5 8 1 < V ≤ 69 kV 3 5 69 < V ≤ 161 kV 1.5 2.5 V > 161 kV 1 1.5

**2.3. IEC standard 61000-3-6 harmonic distortion limits**

**Table 1.** Voltage THD limits according to IEEE 519-2014 [3].

**Table 2.** Current distortion limits according to IEEE 519-2014 [3].

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,

Harmonics Temporal Profile in High-Voltage Networks: Case Study

http://dx.doi.org/10.5772/intechopen.72568

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IEC Standard 61000-3-6 specifies the allowable harmonic distortion limits as shown in **Table 3** [4].

**Voltage Isc/I(load) TDD (%) <11 11 ≤ h < 17 17 ≤ h < 35 35 ≤ h ≤ 50** <69 kV <20 5 4 2 1.5 0.6

69–161 kV <20 2.5 2 1 0.75 0.3

>161 kV <25 1.5 1 0.5 0.38 0.1

20–50 8 7 3.5 2.5 1 50–100 12 10 4.5 4 1.5 100–1000 15 12 5.5 5 2 >1000 20 15 7 6 2.5

20–50 4 3.5 1.75 1.25 0.5 50–100 6 5 2.25 2 0.75 100–1000 7.5 6 2.75 2.5 1 >1000 10 7.5 3.5 3 1.25

25–50 2.5 2 1 0.75 0.3 ≥50 3.75 3 1.5 1.15 0.45

where *M*<sup>1</sup> 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 interharmonics, expressed as a percent of the maximum demand current"

$$TDD = \frac{\sqrt{\sum\_{i=2}^{I\_n} I\_i^2}}{I\_n} = \frac{\sqrt{I^2 - I\_i^2}}{I\_n} \tag{2}$$

where *I <sup>m</sup>* is the maximum demand load current and *<sup>I</sup>* 1 is the rms value of the fundamental component.
