Voltage distortion

Figure 16.9 Voltage distortion caused by different types of thyristor drives (with the use of IGBTs, however, the distortion would be greatly reduced)...

For safe operation of power and control equipment and devices operating in such systems it is essential to limit the amplitude of the voltage distortions to a safe value by installing filter circuits based on the system s actual operating conditions. These limits are recommended by leading standards organizations are ... 

UK engineering recommendation - G5/3 (British Electricity Council Standard). For harmonic voltage distortions in the system as in Table 23.2. 

IEEE-519 Guide for harmonic control and reactive compensation of static power converters, for harmonic-voltage distortions of general and dedicated power system, as in Table 23.3. 

Table 23.2 Premissible individual and total harmonic voltage distortions - as in G5/3, UK engineering recommendations...

Table 23.3 Premissible voltage distortions as in IEEE-519 for general power systems and dedicated systems ...

FIGURE 4.9 Voltage distortion due to current distortion. The gradient graph indicates how distortion changes from source to load. 

IEEE 519 also provides guidelines for maximum voltage distortion at the PCC (see Table 4.12). Limiting the voltage distortion at the PCC is the concern of the utility. It can be expected that as long as a facility s harmonic current contribution is within the IEEE 519 limits the voltage distortion at the PCC will also be within the specified limits. 

Switchgear rated voltage (volts) Total harmonic voltage distortion (%) Individual harmonic voltage distortion (%) ... 

In a power network, Z includes inductive and capacitive components it is then frequency related and differs for each harmonic. The voltage distortion resulting from harmonic currents greatly depends on the impedance of the power network it is what other users connected to a common point will see. ... 

As described earlier, the THD (voltage distortion) is function of the network impedance at this early stage, available short circuit capacity at IPC (another way of specifying network impedance) is quite often the only available information. With the assumption that the network is purely inductive, this information can be used to calculate the impedance of large drive power transformers required to minimize the harmonic THD at the specified IPC network point. 

Figure 2. Typical voltage distortion at AFE PWM drive transformer secondary terminals— before and after addition of additional input impedance...

To minimize the effects of voltage distortion on weak AC systems introduced by VFD s applied on hoisting equipment, care should be taken during the specification phases of the VFD s and generation equipment procurement to ensure compatibflity and a trouble-free design. Also, the ratings of harmonic filtering equipment should be verified by a full harmonic study of the system. 

When source capacity is reduced, drive harmonics remain approximately the same, and can cause significant voltage distortion when connected to an emergency generator. Harmonic content can be the critical factor in calculating the required capacity of the standby or emergency generator. 

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