Sonel PQM analyzers case study #10: Disrupted industrial network

The times when linear loads, such as induction motors fed directly from the mains, incandescent lighting or thermal equipment, dominated in industry are long gone. Nowadays, most electrical appliances draw more or less non-linear current from the mains. This gives rise to a number of power quality issues, including the most annoying ones, i.e. damage to appliances.

Description of the identified problem

An industrial facility powered from its own transformer experiences frequent failures of the capacitor battery because it is a battery without chokes (from the period prior to the modernisation of the production lines). The vast majority of the electrical drives installed are powered by inverters. It is necessary to diagnose the cause of capacitor battery damage problems.

Measurement tools used:

• Sonel PQM-702 analyser (see successor Sonel PQM-710)
• Set of F-3A clamps (In=3000 A)
• Sonel Analysis software

 

Figure 1 Voltage and current waveforms on the transformer during production (currents 0.6...1.25 kA)

 

Figure 2 Active power and THD U waveforms during production

 

Preliminary conclusions:

1. One can notice the effect of the load on the drop in supply voltage of 8 V for 1kA currents.
2. A high load during production has a very prominent impact on the voltage distortion increase.
3. The total harmonic distortion factor (THD U) of 6...9% is very high (point 1). The EN 50160 standard allows a maximum of 8%.
4. The level of THD at no load confirms that the voltage distortion does not originate from the mains (point 2).

The analysis of the preliminary conclusions was followed by an analysis of the harmonic distributions during operation and during plant standstill (point 1 and 2 of Fig. 2, respectively).

Figure 3 shows the significant effect of three-phase inverters and variable single-phase loads on the harmonic distortion of the voltage during the plant duty cycle (point 1 from Figure 2).

Figure 4 shows a significantly lower proportion of higher odd harmonics during the plant standstill cycle (point 2 from Figure 2). In particular, harmonic 5 decreased from 4.4% to 1.4%. Harmonics of higher orders like 13, 15, 17 dropped almost to 0%.

 

Figure 3 Distribution of harmonics during production (for marker no. 1 from Fig. 2)

 

Figure 4 Distribution of harmonics during standstill (for marker no. 2 from Fig. 2)

 

Figure 5 Distribution of tg(φ) against active power on the transformer

 

Final conclusions:

1. A very high level of load-induced voltage variation indicates low short-circuit power.
2. The high level of THD U for the battery without chokes and the high content of higher harmonics in the voltage clearly account for the frequent causes of capacitor failures.
3. The current state of the capacitor battery indicates that tg(φ) is effectively maintained at around 0.35 during production. This is below the threshold of tg(φ)=0.4 required by Polish regulations.
4. Problems with overcompensation, asymmetry and maintenance of tg(φ)<0.4 at low load may indicate partial failure of the smallest capacitor battery stages.

Recommendations:

1. It is necessary to check whether the capacitor battery has been damaged.

 

Authors:
Krzysztof Lorek, Marcin Szkudniewski