The best step size selection guide in APFC panels

Best Step Size Selection Guide in APFC Panels:

The main part in the design of APFC panels is the size of the capacitor banks and the number of steps. Proper selection of step size and number of steps play a big role in the operation as the cost of APFC panel. This text focuses on the selection of the best size (maximum and minimum size) and the number of steps and requirements.

1. Maximum step size selection:

The maximum capacitor rating in an APFC panel depends on the following:

i. Maximum load variation that occurs at one time in the industry:

Large load variations require large capacitor measures that the target power factor is achieved soon, by the low number of switching operations.

ii. Current and voltage transient systems face the capabilities:

Current Transfers: Switching of large capacitors (usually above 100 kVar) introduces large amounts of intrusion current (current transistors) for the equatorial period. This leads to high thermal and electrical stress on the capacitor, short protection devices and, primarily, the switching device. This can lead to their nuisance tripping and premature failure.

Voltage transfers: Switching large amounts of capacitors can be transient to voltage, which can result in failure of sensitive electronic devices. Thus, the exact size of the capacitor step can be traded between points i and ii. The practically acceptable maximum possible rating in any APFC panel should be 100 kVAR. If this 100 kVAR rating capacitor is switched using a power contactor, the rinse interference current can reach up to 75 kA. Therefore, the capacitor should be replaced using a capacitor duty contact or a thyristor switching module.

However, a thyristor switching module or capacitor duty contactor for 100 kVAR rating is not readily available. So, the simplest thanks to replacing 100 KVAR bank is that two 50 KVAR TSM / contactors are connected in parallel, each with separate physical 50 KVAR capacitor banks. The control supply to both 50 KVAR TSM / contactors shall be normal (one relay output of the controller), in order that both devices be triggered at the same time. To understand this, the normal output of the APFC relay should be programmed as 100 kVAR. So, physically they are two different 50 KVAR banks, while electrically it behaves like a 100 KVAR bank.

2. Minimum step size selection:

The minimum capacitor rating depends on how the feature factor should be maintained. This minimum KVAR rating is based on the APFC controller's minimum current sensitivity (usually around 2.5%). However thanks to this minimum selected KVAR rating the PF can usually be within 3rd or 4th decimal places (depending on the panel size or KVAR requirement), while the power board only cares for the primary two digits of the PF (such as 0.99). Is.

In general, many industries want a clear target PF unity (equivalent to 1.00), so incentives are requested from some EBs. But in practice, the maximum target PF has gone from 0.96 to 0.99. These are healthy levels of the power factor as there will be a safety margin that avoids any dangerous harmonic amplification (due to high capacitance within the system) but also the leading power factor.

Therefore, as a rule of thumb, the minimum KVAR rating in the APFC panel is often chosen as 5-10% of the overall rating of the APFC panel. For smaller rated APFC panels (up to 100 KVAR), the smallest step is often 5 KVAR and for larger rated APFC panels (above 600 KVAR and up to 1000 KVAR) the minimum rating is often 25 KVAR.

AP. APFC Number of steps / branches in the panel:

Once the APFC panel's minimum and maximum KVAR ratings have been selected, the amount of branches is often determined on the basis of:

i. Technology of APFC controllers: APFC controllers will be self-ized optimized intelligent switching where the controller calculates a specific KVAR requirement and turns on / off the acceptable capacitor regardless of the capacitor already in the circuit.

Larger number of smaller ones are required for conventional controllers employing linear or circular switching

Measures for effective power factor correction (such as 1: 2: 2:…). While the latest controllers can have a combination of large and small steps, as the amount of steps in the capacitor bank is reduced, as is the cost of the associated switchgear.

ii. The size and cost of the APFC panel: The more steps the quantity will be the value of the APFC panel, thanks to the greater number of switchgears, the larger the size of the panel atl..

An APFC panel should have:

a. Maximum electrical steps (combination of physical steps) to ensure more accurate and versatile power factor correction

B. The minimum number of physical measures to measure the dimensions and price of the panel

Disclaimer:

For accurate compensation, the load profile of the industry must be studied. this provides a sign of the dimensions of peak loads, base loads, possible harmonics, power factor and thus, helps determine step sizes supported load variations. In some cases, the load requirements also can be established from equipment lists.

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