Transient Inrush Reactors - TI Reactors by Northeast Power Systems, Inc.

NEPSI’s Transient Inrush Reactors are designed to increase the life expectancy of capacitor switches by limiting both the magnitude and frequency of the transient inrush currents associated with back-to-back capacitor bank

switching. These reactors are most commonly applied in multi-stage medium voltage (2.4kV through 34.5kV) capacitor banks or in fixed medium voltage capacitor banks that are connected on the same switchgear bus as other fixed capacitor banks, as shown in Figure 1.

Application Considerations
Medium voltage multi-stage capacitor banks with voltages of 2.4kV through 34.5kV are typically switched by single phase or three phase capacitor switches that are tested in accordance with ANSI Standard C37.77. These switches are usually rated 200, 400 and 600 amps, with the 200 amp single phase group operated switch being the most common. ANSI C37.77 requires that the switch be tested for switching at a high frequency transient making current of 12,000 amps (peak) at 6000 Hertz. This equates to an IT Product of 7.2x10⁷ amps/second. In many back-to-back capacitor bank switching applications, either the 12,000 amps or the 6,000 Hertz rating or both are exceeded. The TI Reactor when placed in line with the capacitor switch as shown in Figure 1 will decrease the IT Product, inrush frequency, and inrush current magnitude through the energizing switch.

Figure 1 - High Frequency Inrush Currents That May Damage Capacitor Switches Can Be Reduced With NEPSI's TI-ReactorsÔ

As a conservative approach, NEPSI recommends the TI Reactor be sized to limit the IT product of the capacitor inrush current to 3.6x10⁷ amps/second. This is 50% of the maximum tested value. Table one shows the inductance requirements for this IT Product level at various voltage levels.

T he transient inrush reactor application table below can be used to aid the capacitor design engineer in sizing and specifying the proper NEPSI TI-ReactorÔ for single step and multi-step capacitor banks. The values in the table are based on Table 1 and the use of 200 amp single-phase capacitor switches. For contactors and circuit breakers, or other switch ratings, consult the manufacturer for ratings and recommended inductance. The values listed in Table 1 and 2 are conservitive in nature, and lesser values are possible with detailed calculations or EMTP simulations. For single-step and two-step automatic capacitor banks, the equations listed in Table 4 can be used to calculate the magnitude and frequncy associated with capacitor switching. Contact NEPSI's Engineering Services for more complex systems requiring EMTP simulations.

TI Reactorä Ordering Guide
The TI ReactorÔ has both an inductance rating and a current rating. The proper inductance, if not known can be sized from the transient inrush reactor application table in the following section or from the equations listed in Table 4. With proper inductance known, the ampacity is chosen based on the capacitor stage current. An additional 35% of current should be added to the stage current to account for capacitor kvar, voltage variations, and harmonic currents. Fill in the part number below for quotations and ordering.

Transient Inrush Reactor Application Table
T he chart below can be used to determine the amount of equivalent inductance required between capacitor banks to keep the IT Product of the transient inrush current to 3.6x10⁷ amps/second. This is 50% of the ANSI tested capacitor switch capability since they are tested for 12000 amps of inrush at a frequency of 6000 hertz as specified in ANSI C37.66 (American National Standard, Requirements for Oil-Filled Capacitor Switches for Alternating-Current Systems).

Figure 2 - Reactor Inductance Versus Diameter and
Number of Turns

T he design of the NEPSI TIÔ Reactor will closely follow the reactor specification plots shown in the Figure 2 above. For example, a 101 microhenry reactor with a 6 diameter would require approximately 28 turns. Note, that as diameter and the number of turns increase, so does the reactance. If a specific diameter or length is required, please specify with RFQ.

Figure 3 - Plot to right shows inductance requirements change with voltage for back-to-back switching applications

Table 4 – Formulas for Calculating Transient Magnitude and Frequency Associated with Capacitor Bank Switching
Switching Conditions	Equation
Energizing an isolated capacitor bank or capacitor bank stage

Energizing a capacitor bank or capacitor bank stage with another capacitor bank or stage on the same bus

f_s = System Frequency L_eq = Total equivalent inductance per phase between capacitor banks or stages, in microhenrys I₁, I₂ = Currents of bank or stages being switched and of bank or stage already energized, respectively. I_{max peak}= A peak value calculated without damping. In practical circuits it will be about 90 percent of this value. V_LL = Rated maximum voltge in kilovolts I_sc = Symmetrical rms short-circuit current, in amperes.

Northeast Power Systems, Inc.
66 Carey Road
Queensbury, New York 12804
Phone: 518-792-4776
Fax: 518-792-5767

E-mail: sales@nepsi.com
Website : www.nepsi.com