**3. Harmonics temporal profile at grid station A**

## **3.1. Harmonics measurements at grid station A**

**Figure 1** shows MIS system and the locations of grid stations under study. The grid station A consists of four 132/33 kV parallel transformers (TX1 to TX4). Each transformer is 75 MVA capacity. As the grid station is located adjacent to a 600 MW power generation facility, the maximum short circuit level on 132 kV side is 27.54 kA. The grid station is supplying electricity to a cement factory, an industrial area, industrial area housing, a university campus, and a hospital.

Harmonics Temporal Profile in High-Voltage Networks: Case Study http://dx.doi.org/10.5772/intechopen.72568 39

**Figure 1.** Main interconnected system [7].

**Odd harmonics non-multiple of** 

<sup>h</sup> <sup>−</sup> 0.2

1.2 \_\_17

**Table 4.** Voltage THD limits according to national Omani codes.

**Table 3.** IEC 61000-3-6 voltage harmonic limits [4].

**Odd harmonics multiple of** 

h MV HV-EHV H MV HV-EHV h MV HV-EHV 5 2 3 4 2 2 1.8 1.4 4 2 9 1.2 1 4 1 0.8 3 1.5 15 0.3 0.3 6 0.5 0.4 2.5 1.5 21 0.2 0.2 8 0.5 0.4

<sup>h</sup> <sup>21</sup> <sup>&</sup>lt; <sup>h</sup> <sup>≤</sup> <sup>45</sup> 0.2 0.2 <sup>10</sup> <sup>≤</sup> <sup>h</sup> <sup>≤</sup> <sup>50</sup> 0.25 \_\_10

**Even harmonics (%)**

<sup>h</sup> <sup>+</sup> 0.22

0.19 \_\_10 <sup>h</sup> <sup>+</sup> 0.16

**three (%)**

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

**2.4. Harmonic distortion limits according to Oman's national regulations**

**PCC voltage Individual harmonic magnitude (%) VTHD (%)** Low voltage (415 V) — 2.5 Distribution level (11, 33, 66 kV) 1.5 2 Transmission level (132, 220 kV) 1.5 2

for both transmission and distribution networks (**Table 4**).

**3. Harmonics temporal profile at grid station A**

**3.1. Harmonics measurements at grid station A**

a hospital.

Allowable harmonic distortion levels in Oman are dictated by the grid code [5] and the distribution code [6] for high-voltage and medium-voltage networks, respectively. The grid code specifies that the maximum THD should not exceed 2% with no individual harmonic greater than 1.5% for transmission networks (220 and 132 kV). The distribution code dictates that the maximum THD in distribution networks (66, 33 and 11 kV) systems should not exceed 2.0% with no individual harmonic greater than 1.5%. For low voltage line (415 V), the total harmonic distortion limit is 2.5%. Individual harmonic distortion level should be below 1.5%

**Figure 1** shows MIS system and the locations of grid stations under study. The grid station A consists of four 132/33 kV parallel transformers (TX1 to TX4). Each transformer is 75 MVA capacity. As the grid station is located adjacent to a 600 MW power generation facility, the maximum short circuit level on 132 kV side is 27.54 kA. The grid station is supplying electricity to a cement factory, an industrial area, industrial area housing, a university campus, and

**three (%)**

17 ≤ h ≤ 49 1.9 \_\_17

Measurements were conducted using Hioki 3196 Power Quality Analyzer Meter [8]. The current clamps were installed on transformer one (TX1). Ten-minute average values were recoded over a period of 1 week starting from 17th of January 2012. This study was part of power quality study in MIS [9–12]. A summary of measurements is presented in **Figure 2**.


**Figure 2.** Summary of THD measurements at grid station a. (a) Voltage THD (b) current THD.

For the voltage signals, **Figure 1** shows that the average THD at the high-voltage side is between 0.84 and 0.68%, the maximum THD is between 1.07 and 0.92%, and the minimum THD level is between 0.40 and 0.52%. For the current signals, the THD at the high-voltage side (132 kV) ranges between 0.84 and 2.52%.

Histograms of voltage THD and current TDD at grid station A are presented in **Figure 3**. Since the measurements were conducted at 132 kV voltage level, the corresponding voltage THD limit is 2.5%. Using the voltage THD histograms presented in **Figure 3**, the 95th percentiles are calculated for different phases. A comparison between the 95th percentile of voltage THD and IEEE Std 519 limit is presented in **Table 5**.

current is 369 A. The short circuit level at grid station A is obtained from OETC capability

When system loading increases, more voltage drop occurs. Therefore, the temporal voltage profile reflects the daily load profile. The voltage profile measured at grid station A is pre-

The voltage THD exhibits a daily profile similar to that of the load as demonstrated in **Figure 5**. Considering individual harmonics, the figure shows that the dominant harmonic components are the 5th and the 7th followed by the 3rd and 11th. There is a small trace of the

Unlike voltage THD, current THD does not exhibit a clear daily profile. **Figure 6** demonstrates that the dominant harmonic components are the 5th and the 3rd. There is a small trace of the 7th harmonics. It is worth mentioning that a cement factory is connected to this feeder.

The temporal variations of the voltage and current THD are associated with the variations of the harmonic-producing loads. Normally, the individual harmonic distortion is linked to specific harmonic-producing loads. Station A is a grid station that is connected to a cement factory and an industrial area where many non-linear loads are fed from this station. Such loads are variable frequency drive (VFD) and switch-mode power supplies (SMPS), in which both are considered main sources of the 5th harmonics (H5). This explains the dominance of

*<sup>L</sup>* = 39 is 4%. A comparison between the 95th and 99th

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*SC*/*I*

percentiles of current TDD and IEEE Std 519 limits is presented in **Table 6**.

**3.2. Harmonics temporal voltage profile at grid station A**

**Table 5.** Measured voltage THD 95th percentile versus IEEE Std 519-2014.

**Phase VTHD P95 VTHD IEEE limits**

A 1.03 2.5 B 0.85 2.5 C 1.03 2.5

**3.3. Current harmonics temporal profile at grid station A**

the 5th and the 3rd harmonic currents contamination.

A 1.25 1.42 4 B 1.24 1.30 4 C 1.36 1.52 4

**Table 6.** Measured current TDD 95th and 99th percentiles vs. IEEE Std 519-2014.

**Phase TDD P95 TDD P99 TDD IEE limits**

statement. The TDD limit based on *I*

sented in **Figure 4**.

13th and 9th harmonics.

**3.4. Reasons for temporal variations**

To calculate the current TDD, Eq. (2) is used. The TDD is a function of individual harmonics and maximum demand load current. Using measurements, the maximum demand load

**Figure 3.** Histograms of THD measurements at grid station A. (a) Voltage THD (b) current TDD.


**Table 5.** Measured voltage THD 95th percentile versus IEEE Std 519-2014.

For the voltage signals, **Figure 1** shows that the average THD at the high-voltage side is between 0.84 and 0.68%, the maximum THD is between 1.07 and 0.92%, and the minimum THD level is between 0.40 and 0.52%. For the current signals, the THD at the high-voltage side

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

Histograms of voltage THD and current TDD at grid station A are presented in **Figure 3**. Since the measurements were conducted at 132 kV voltage level, the corresponding voltage THD limit is 2.5%. Using the voltage THD histograms presented in **Figure 3**, the 95th percentiles are calculated for different phases. A comparison between the 95th percentile of voltage THD

To calculate the current TDD, Eq. (2) is used. The TDD is a function of individual harmonics and maximum demand load current. Using measurements, the maximum demand load

**Figure 3.** Histograms of THD measurements at grid station A. (a) Voltage THD (b) current TDD.

(132 kV) ranges between 0.84 and 2.52%.

and IEEE Std 519 limit is presented in **Table 5**.

current is 369 A. The short circuit level at grid station A is obtained from OETC capability statement. The TDD limit based on *I SC*/*I <sup>L</sup>* = 39 is 4%. A comparison between the 95th and 99th percentiles of current TDD and IEEE Std 519 limits is presented in **Table 6**.

#### **3.2. Harmonics temporal voltage profile at grid station A**

When system loading increases, more voltage drop occurs. Therefore, the temporal voltage profile reflects the daily load profile. The voltage profile measured at grid station A is presented in **Figure 4**.

The voltage THD exhibits a daily profile similar to that of the load as demonstrated in **Figure 5**. Considering individual harmonics, the figure shows that the dominant harmonic components are the 5th and the 7th followed by the 3rd and 11th. There is a small trace of the 13th and 9th harmonics.

#### **3.3. Current harmonics temporal profile at grid station A**

Unlike voltage THD, current THD does not exhibit a clear daily profile. **Figure 6** demonstrates that the dominant harmonic components are the 5th and the 3rd. There is a small trace of the 7th harmonics. It is worth mentioning that a cement factory is connected to this feeder.

#### **3.4. Reasons for temporal variations**

The temporal variations of the voltage and current THD are associated with the variations of the harmonic-producing loads. Normally, the individual harmonic distortion is linked to specific harmonic-producing loads. Station A is a grid station that is connected to a cement factory and an industrial area where many non-linear loads are fed from this station. Such loads are variable frequency drive (VFD) and switch-mode power supplies (SMPS), in which both are considered main sources of the 5th harmonics (H5). This explains the dominance of the 5th and the 3rd harmonic currents contamination.


**Table 6.** Measured current TDD 95th and 99th percentiles vs. IEEE Std 519-2014.

**4. Harmonics temporal profile at grid station B**

Grid station B consists of two 220/132 kV parallel transformers (TX1 and TX2). Each transformer is 500 MVA capacity. The grid station B is located close to two power generation facilities. The maximum short circuit level on 132 kV side at the time of measurements is 17.96 kA. The grid station is supplying electricity to large industrial customers via 132 kV feeders as well as smaller industrial customers via two 132/33 kV transformers. Ten-minute average values were recoded using Hioki 3196 Power Quality Analyzer Meter [8]. The current clamps of the analyzer were connected to a 132 kV feeder supplying a steel smelter. Measurements were recoded over a period of 1 week starting from 28 January 2012. A summary of measurements is presented in

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For the voltage signals, **Figure 7** shows that the average THD at the high-voltage side is between 0.97 and 0.90%, the maximum THD is between 0.97 and 0.90%, and the minimum THD level is between 0.28 and 0.32%. For the current signals, the THD at the high-voltage side (132 kV) ranges between 33.09 and 2.28%. Despite the high-current distortion, the voltage

It is worth noting that the dominant harmonic components are the 5th, the 7th, the 11th, the 13th and the 17th orders. In addition, there is a clear content of the 2nd, the 3rd, and other

Histograms of voltage THD and current TDD at station B are presented in **Figure 10**. Using the voltage THD histograms, the 95th percentiles are calculated for different phases. A comparison between the 95th percentile of voltage THD and IEEE Std 519 limit is presented in

The current waveform at grid station B is highly distorted as shown in **Figures 7**–**9**.

**4.1. Harmonic measurements at grid station B**

**Figure 6.** Current harmonics temporal profile at grid station A.

THD is small due to the high short circuit capacity.

**Figure 7**.

harmonic orders.

**Table 7**.

**Figure 4.** Voltage temporal profile at grid station A.

**Figure 5.** Voltage harmonics temporal profile at grid station A. (a) Voltage THD and (b) individual harmonics.

**Figure 6.** Current harmonics temporal profile at grid station A.

**Figure 4.** Voltage temporal profile at grid station A.

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

**Figure 5.** Voltage harmonics temporal profile at grid station A. (a) Voltage THD and (b) individual harmonics.
