Sonel PQM analyzers case study #4: Switching off the single-phase rectifier due to voltage dips

Overcurrent protections are designed to protect the receiving device and the power line from failure and damage. Their selection results from standard calculations. There may be times when specific receiver properties cause unexpected operation for no obvious reason.

Description of the identified problem

The DC rectifier supplied from the 230 V AC network is protected by a C10A circuit breaker. Both the operation and the connections are correct (Fig. 1). However, there are occasional circuit breakers shutdowns. Can it be explained? 

Measuring equipment used

• Sonel PQM-703 Power Quality Analyzer (see the product successor Sonel PQM-711)
• Set of C-6A current clamps
• Sonel Analysis software

 

Fig. 1. Picture of rectifier operation with captured overload and voltage dip

 

Fig. 2. Image of a current swell and a voltage dip in an enlarged view

 

Fig. 3. Picture of the same voltage dip as RMS_1/2

 

Initial conclusions

1. The device has a mechanical switch, it does not fall off due to dips.
2. Inrush currents have no limitation, phenomena similar to switching on occur at voltage recovery (Fig. 2 and 3).
3. A current swell of 15 A causes a voltage dip of 7.4 V.

 

Fig. 4. The image of the current increase to 8.33 A and a voltage dip of 1.8 V

 

Fig. 5. Oscillographic image of voltage distortion

 

Conclusions:

1. The device causes momentary increases in currents during switching on (Fig. 5).
2. Voltage dips are not closely related to these increases (Fig. 4).
3. The tested device is not able to trip the C10A protection by itself.
4. The influence of other devices supplied from the same section, reacting to voltage dips caused by switching on, cannot be excluded.

Recommendations:

1. Perform a detailed analysis of other devices in terms of generating high instantaneous currents.
2. Perform supplementary measurements directly on the C10A protection.

 

Author:
Krzysztof Lorek