BATTERY CHARGING UNIT

PREAMBLE

The industry has a Battery charging Unit in which control components in the rectifier were reported to have failed due to overvoltage problems. Hence Wave Form & Harmonic Analysis was done at site to identify the probable phenomena of over-voltage for control component problems.

SITE ANALYSIS AND OBSERVATIONS

The Current THD Harmonics is 30% and Voltage THD Harmonics is 7% during the data logging. 

From the waveform recorded, the crest of the voltage waveform is increased by 24 volts due to these harmonics.

PHASEWISE VOLTAGE THD HARMONICS – EXIDE INDUSTRIES

PHASEWISE CURRENT THD HARMONICS – EXIDE INDUSTRIES

INFERENCE

Possible Causes for the Problem:

Harmonics
-The System Crest Voltage is continuously raised by approximately 8% over the fundamental crest of the voltage. This Crest of the Voltage causes stressing all the Electronic Components.  

-During nighttime, the voltage might reach fundamental Root Mean Square Value of 254 Volts.  This level of voltage will lead to a crest of 388 Volts for the distorted waveform.

Capacitor Electric Charges
-Capacitors on the LV side hold electric charge and will discharge to the Diode Rectifier during commutation in each cycle of the Waveform.  This additional Electric Charges flowing through the Rectifier Diode during commutation leads to failures in the control cards. 

-The over capacitance needs to be avoided in this transformer and an optimum P.F Setting needs to be maintained.

-The Peak Inverse Voltage (PIV) required for rectifier diodes needs to be increased to account for the capacitor electric charge.  It may necessitate change of rectifier diodes of higher PIV Values

Voltage Variation
The incoming supply is stable during the recording period and the fundamental RMS value is at 235 Volts.  The fundamental voltage – RMS value may raise to a maximum of 254 Volts for the same tapping during night time.  This may lead to further increase in the Peak Value of the actual Crest value of the waveform, due to Harmonics.

SYSTEM DESIGN AND RECOMMENDATION

Remedies for the Problem

Harmonics
The Crest of the voltage needs to be attenuated closer to the level of fundamental waveform crest. I.e., the raise of 24 volts needs to be reduced. This crest is caused by the current harmonics and predominantly due to 5th current harmonics.

From the existing level of 204 amps 5th Harmonic current, a suitable Auto Switched Harmonic Filter needs to bring down the harmonic current to less than 100 amps (5th harmonic current).
This reduction of 5th harmonic current will reduce the crest of the voltage from 24 volts to less than 10 volts.

This will ensure the maximum crest voltage will fall below the fundamental voltage crest within + 6%, thereby avoiding the rectifier problems due to harmonics.

Capacitor Electric Charges
The harmonic filter reactor acts as a damper, blocking the capacitor electric charge from flowing to the rectifier circuits.  This avoids a higher flow of harmonic current due to these capacitor charges in the Diodes and the Rectifier control circuits.  This leads to the reduction in the voltage across the rectifier diodes.
The PIV of the rectifier diodes needs to be matched for these blocking effects and the available rectifier diodes may be sufficient, since the blocking effect is carried out.

The Auto Switched Harmonic Filter System will be an ideal solution for these phenomena.

Voltage Variation
If it is anticipated or recorded that the night time voltage is 254 volts or above, it is necessary to install a Servo Stabiliser OR On Load Tap Changer for the Transformer.

Summary of Recommendation
Based on the explanations given above, the following modifications are to be carried out to avoid the problems associated with the diode rectifier:

1. Installation of Auto Switched Harmonic Filter System of 500 KVAR @ 400 Volts.
2. Provision of either On-Load Tap Changer in the Transformer or Servo Stabiliser.

PERFORMANCE

On installation of the recommended harmonic Filter System, the Voltage and Current distortion have reduced with significant reduction in the crest factor. Voltage crest which has increased above normal upto 24 V has been reduced to 3 V (normal V– crest is 330 V on 240 / 415 V supply) and thereby the problem of Overvoltage is controlled.