In this study case a grid harmonics evaluation was done to prevent equipment damage and processes shutdown. A power plant company suffered from constant shutdown and damages to their equipment like computer and motors. To prevent these consequence harmonic distortion were measured and analysed at different location, modelled power plant’s grid and performed calculation with EA-PSM. After comparing measurement and calculation result with no error, based on calculation results we produced minimal protection parameters, which would prevent the problems.

Modelled grid

Evaluated grid was powered through two 10/0.4kV voltage transformers in parallel. Main plant’s grid was 0.4kV voltage mostly populated with motor, motor with frequency drives and capacitor banks. Full grid is depicted in Figure 1 and simplified grid is depicted in Figure 2.

Frequency drives in the grid was the main cause of the distortions. In EA-PSM frequency drives are modelled as inverter, which is harmonic current source. Typical harmonic current values for frequency drives are given in Table 1. In reality harmonic current values are larger, because of voltage asymmetry. To get real harmonic current values we measured harmonic distortion from the frequency drives, given in Table 2.

Fig. 1. Full modelled grid
Fig. 1. Full modelled grid
Table 1. Typical 6-pulse inverter harmonic currents
HARMONIC 5 7 11 13 17 19 23 25
I/IH, % 17.5 11.1 4.5 2.9 1.5 1 0.9 0.8
Table 2. Measured frequency drives harmonic currents
HARMONIC 5 7 11 13 17 19 23 25
I/IH, % 21.1 13.2 5.5 2.9 1.5 1 0.9 0.8
Fig. 2. Simplified modelled grid.
Fig. 2. Simplified modelled grid.

One of the frequency drives was connected through a filter, which filters a portion of harmonics currents. In this case we cannot apply typical harmonic currents, measured currents of this frequency drive are given in Table 3, and are used in the program.

Table 3. Measured frequency drive with filter harmonic currents.
H. 3 5 7 11 13 15 17 19 21 23 25
I/IH, % 5.5 12.57 9.06 4.5 1.9 0.33 1.9 1.8 0.25 1.28 1.06

Other motors were not connected through frequency drives and did not generate harmonic currents. Simple motors only decrease grid harmonic distortion, because the motor itself consumed portion of the distortions.

Result comparison

Measured and calculated results were compared to validate that EA-PSM calculation are correct. Current distortion comparison results are depicted in Table 4 and voltage distortion results in Table 5.

Table 4. THDi results comparison
LOCATION THDI, % THDI ERROR, %
Measured Calculated
TRANSFORMER 1 0,4KV SIDE 25.73 25.745 0.06
Transformer 2 0,4kV side 7.19 7.27 1.11
FC with filter 17.35 17.435 0.49
Table 5. THDu results comparison
LOCATION THDU, % THDU ERROR, %
Measured Calculated
TRANSFORMER 1 0,4KV SIDE 4.09 4.087 0.07
Transformer 2 0,4kV side 2.20 2.214 0.6
FC with filter 2.65 2.638 0.45

Comparison shows that measured values and calculated values match and maximum error for current distortion is only 1.11%, for voltage distortion – 0.6%.

Protection selection

In this case active harmonic filter is selected to filter harmonic distortions. Based on EA-PSM calculation results filter’s nominal current is selected, which is the main filter parameter. Nominal current is selected by calculating maximum harmonic distortion in the grid. Usually it is the grid mode where most frequency drives are connected and working and least simple motors are connected. Grid’s maximum harmonic distortions are given in Table 6.

Table 6. Maximum current harmonic distortions in the grid.
HARMONIC CURRENT, A I/IH, %
1 845.742 100.000%
5 147.872 17.484%
7 93.905 11.103%
11 38.039 4.498%
13 24.531 2.901%
17 12.609 1.491%
19 8.224 0.972%
23 7.511 0.888%
25 6.736 0.797%
THD 181.828 21.499%

The nominal filter current should be not less than 230 A.

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