Switchgear for power factor correction systems

September 30th, 2014, Published in Articles: Vector

 

The importance of differentiating between various circuit types or capacitor assignments to inductive loads when implementing power factor correction (PFC).

A differentiation is made between individual PFC and group PFC. With individual PFC, the capacitor is assigned individually or switched to a load. With group PFC, the power factor of a load group is determined with varying power configurations. Multiple capacitors are automatically switched in or out by a VAr controller. The task of both application types is to improve the power factor and so achieve a reduction of the reactive power.

The level to which the power factor can be improved and the individual economic effects resulting from a cheaper electricity tariff should be clarified with the tariff consultants of the utility company.

This article examines the switchgear involved. The stresses and demands placed on the contactors and the capacitors differ for individual PFC and for group PFC. Both situations are dealt with here.

The differences in circuits and demands have been subject to particular attention as a result of new technologies employed in the manufacture of capacitors. In the last few years, the use of new materials and manufacturing processes has resulted in a reduction of the inductive internal resistances in the capacitors.

The peak inrush currents have increased dramatically as a result of the lower impedances, which have a lesser limiting effect on the current of the high frequency transients during switch on.

Individual PFC

With individual PFC, the capacitor draws its current from the mains supply. To also reduce the reactive power to the cables, the capacitor is installed as close as possible to the equipment with this circuit arrangement. With this application, the contactor is generally not in the proximity of the transformer, e.g. in the main distribution board, but rather in a sub-distribution board.

Fig. 1: Numerous individual impedances which add up exist on the current path from the medium-voltage transformer to the capacitor.

Fig. 1: Numerous individual impedances which add up exist on the current path from the medium-voltage transformer to the capacitor.

As shown in Fig. 1, numerous individual impedances which add up exist on the current path from the medium-voltage transformer to the capacitor, and generally have the effect of limiting the peak inrush currents to < 30 x IN of the capacitor. The peak inrush currents therefore flow for a few milliseconds during switch-on, which is generally within the range of the making capacity of the normal DIL M… contactor.

Normally, the capacitors are switched directly in parallel to the load with individual power factor correction. This means that the capacitor is switched with the same contactor as the motor. In accordance with Fig. 2, the motor protection must be set differently, depending on whether the capacitor is connected upstream or downstream of the overload relay, and whether the reactive current flows or does not flow through the thermal release.

From the point of view of demands placed on the contactor, normal DIL M… contactors can be used for individual PFC.

Group PFC

With group power factor correction, the physical arrangement of the contractors and capacitors is mostly in the proximity of the low-voltage transformer, e.g. in the low-voltage sub-distributor. At this point, it is important to observe that the operating voltage and the short-circuit rating are higher during a fault.

The higher voltage has been taken into consideration by the manufacturers of the capacitors in the higher rated operational voltage of the capacitor. The power ratings of the contactors for switching capacitors relate to these voltages. With group PFC, the charging current of the capacitors is not just supplied via the impedance-associated path by the mains supply, but also from the neighbouring low-impedance connected capacitors which are already charged. For this reason, the peak inrush currents are in the order of > 150 x IN.

The making capacity of a normal contactor is therefore exceeded. From a physical viewpoint, the high needle-shaped current peaks strip constituents from the contact material alloy which prevents welding of the contacts. After just a few hundred switching operations, only pure silver will remain and the contacts will weld. Special devices and measures are required here.

Special DILK… contactor for capacitors

Fig. 2: The motor protection must be according to whether the capacitor is connected upstream or downstream of the overload relay.

Fig. 2: The motor protection must be according to whether the capacitor is connected upstream or downstream of the overload relay.

These contactors for capacitors have been developed from the DILM… series contactors and fit perfectly and fully into the product range in terms of handling and accessories. In addition to a special anti-weld contact material, this contactor also contains series resistors. The capacitors are pre-charged via a special early-make auxiliary switch. The main contacts then close and conduct continuous current.

Choking of the PFC stages

The proportion of harmonics increases on the mains supply due to the ever-increasing proportion of “non-linear loads” (e.g. power electronics and switched mode power supply units). Under certain conditions, this can lead to thermal overload of the capacitors, as the resonance frequency and the oscillating circuit, which is formed by the line inductance and PFC capacity, can be in the frequency range of a harmonic. Depending on the size and number of stages, many resonance frequencies can be passed through with regulated reactive power factor correction equipment.

To avoid possible damage, PFC equipment is equipped with an upstream choke. Furthermore, the chokes ensure that the ripple control systems of the electricity supply company are not subject to interference and are therefore stipulated quite frequently in the technical connection guidelines. Power factor correction equipment with chokes correct reactive power; remove undesirable harmonics from the mains supply; avoid resonance phenomena with harmonics and are suited for electrical power networks with ripple control systems.

Systems equipped with chokes also have a further advantage: the inrush current peak which occurs when a capacitor stage is switched on is dampened considerably. Therefore, it is possible to use normal DILM… series contactors when switching choked PFC stages.

The same applies to non-choked systems when an additional inductivity of > 5 μH is added between the contactor and capacitor (corresponding to an air-core inductor with four windings and a diameter of 14 cm).

The contactors which are used must, however, be dimensioned correctly. The contactor must be capable of continuously conducting 1,5 times the capacitor on-load current conforming to EN 60931-1. It is possible to determine the maximum capacitor nominal output of a choked capacitor bank, which is to switch a contactor with the formula (kvar):

p_{c}=\sqrt{3U_{}N}x0,66xI_{AC-1}x 10^{-3}    [1]

Circuit requirements with the contactor control

Fig. 3: Numerous impedances exist on the current path from the MV transformer to the capacitor.

Fig. 3: Numerous impedances exist on the current path from the MV transformer to the capacitor.

When observing the switching operations of a capacitor bank, it is often noted that multiple contactors continuously remain on and only a few of them perform the control tasks. It is also advantageous for enhancement of the lifespan of a contactor to ensure that the number of switching operations is distributed evenly across all the contactors of PFC equipment. Various manufacturers of PFC regulators offer so-called ring regulators which exchange the switching sequence of the contactors cyclically.

Contact Marlene Coetzee, Eaton,  Tel 011 824-7400, marlenecoetzee@eaton.com

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