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Tech Tip 05

Low Voltage Capacitors Can Amplify Switching Transients


SYMPTOMS

  • Applying low voltage capacitors much smaller than high voltage caps
  • Nuisance drive shutdowns
  • Damage to sensitive equipment

CAUSES

  • Magnification of high voltage capacitor switching transients

SOLUTIONS

  • Determine likelihood of overvoltage problems
            (contact Technical Services for assistance)
  • Apply capacitors as a tuned filter

BACKGROUND

A rate study performed for a sewage treatment facility revealed that installing power factor correction capacitors would be beneficial. Calculations showed installing 100 kVAR of capacitance would improve the customer's power factor to near unity, thereby reducing their bill.

Analysis of potential steady-state harmonic concerns revealed there was little danger of exciting a resonance characteristic of the customer's adjustable speed drive systems (5th, 7th, 11th, 13th, etc.). However, a study of capacitor switching transients revealed that energizing a 12 kV capacitor with the proposed 100 kVAR low voltage capacitance could cause transient overvoltages at the customer's 480 volt bus 50% higher than without low voltage capacitance. This could easily damage some electronics.

The transient magnification phenomenon occurs when the series resonance formed by the low voltage capacitor in series with the distribution transformer is on the same order of that formed by the 12 kV capacitor in series with the system impedance. Nearly equal resonant frequencies, combined with a low voltage capacitor much smaller than the high voltage capacitor, in theory can cause transient overvoltages as high as 6 per unit.

 

Figure 1:  Equivalent Capacitor Switching Circuit

To avoid matching the high voltage resonance, some new installations may allow for installing a different amount of low voltage capacitance. However, in this case, installing 70 kVAR would bring the power factor up to 0.98 lagging but could still cause severe amplification of 12 kV capacitor switching transients.

SOLUTION

One effective method of reducing the transient at the low voltage bus is to install the low voltage capacitors as a harmonic filter. The filter is usually tuned to the lowest characteristic harmonic (typically the 5th) to mitigate potential harmonic concerns in addition to preventing capacitor switching transient magnification.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.