Learn How Pre-Insertion Inductors Solve an Array of Power Quality Problems Associated with the Switching of Substation Capacitor Banks

One of the most notable features of the new Mark VI Circuit-Switcher is its ability to be applied with pre-insertion inductors, similar to the Mark V.

Pre-insertion inductors are ideal for limiting inrush current and overvoltage at the capacitor-bank bus, for limiting phase-to-phase switching-surge overvoltages at remote transformers, and for controlling overvoltages on long, open-ended lines and on the user’s utilization-voltage bus. Let’s look at a few examples:

Energizing Back-to-Back Capacitor Banks

Graph 1.
Figure 1. Peak inrush current without transient control.

Graph 2.
Figure 2. With 40-mH, 5.5-ohm pre-insertion inductors

In shunt capacitor banks, the inrush currents usually do not cause serious problems because the magnitudes and frequencies are moderate. There can, however, be problems in the case of back-to-back switching because the magnitude and frequency of the inrush current are on the order of tens of kiloamperes and tens of kilohertz, respectively. Here’s an example of inrush current transients during back-to-back capacitor bank energization without pre-insertion inductors.

In back-to-back capacitor bank switching, the peak inrush current transient can get as high as 28.4 kA — a level that can be significantly higher than fault currents, due to the high-magnitude, high-frequency nature of the inrush current.

With the application of pre-insertion inductors, the inrush current in this example is controlled to a maximum peak of 1.1 kA.

If not mitigated, transient overvoltages may cause degradation of line and equipment insulation, nuisance operation of surge arrestors, and interference in substation control wiring. Transient overvoltages also cause increases in step potential — a possible shock hazard. Transients can cause severe closing duty on other switching devices, and the nuisance tripping of adjustable-speed drives.

Bus Overvoltage in Shunt Capacitor Bank

Graph 3.
Figure 3. Bus Overvoltage without transient control.

Graph 4.
Figure 4. With 40-mH, 5.5-ohm pre-insertion inductors

When a shunt capacitor bank is energized without any form of transient control, the user’s local bus voltage abruptly changes to that of the capacitor bank . . . which is usually at zero voltage. For many industrial customers, the sharp collapse in voltage transmits a fast transient into the system through the lines and cables connected to the capacitor-bank bus and can cause nuisance tripping of adjustable-speed drives.

The use of pre-insertion inductors reduces the collapse in bus voltage and enhances the damping characteristics of the system, reducing the magnitude of capacitor-switching transients.

How does the pre-insertion inductor work?

First, an open-air arc makes up the initial circuit through the pre-insertion inductor. The inductor core picks up the inrush current before it reaches the main contact. Then, the pre-insertion inductor is bypassed, and disconnected as the moving arcing rod rotates. As the main contact is made, the pre-insertion inductor is completely removed from the circuit. Let’s take a look at it in action:

The sliding contact arrangement places the pre-insertion inductor in the circuit for only a few cycles. That’s how long it takes to provide high-quality transient control.