Can any capacitor bank with filters be used for power factor correction in networks affected by harmonics?

The specific characteristics of power factor correction in networks with high harmonic distortion levels, both in voltage and current, are now becoming more and more familiar to those who have to recommend the appropriate capacitor bank for each electrical installation. Generally speaking, most companies that manufacture automatic capacitor banks include devices designed to be used in networks with a certain level of harmonic distortion in their catalogues. CIRCUTOR, in particular, offers a complete range of automatic capacitor banks, with both contactor and thyristor operation, as well as fixed compensation units, equipped with rejection filters (also known as detuned filters) with 189 Hz tuning frequency (in 50 Hz networks), equivalent to an overvoltage factor of p = 7 %. This 189 Hz tuning frequency is CIRCUTOR's default choice; it offers a suitable, effective solution for the vast majority of installations requiring a capacitor bank fitted with detuned filters, ideal when faced with order 5 harmonics (250 Hz in 50 Hz networks) or higher, which are usually produced by the most common sources of harmonic currents, i.e. three-phase loads equipped with a 6-pulse bridge rectifier at their input: variable speed drives or frequency variators, AC/DC rectifiers, induction furnaces, ...

In the less likely event of order 3 harmonics (150 Hz in 50 Hz networks) prevailing, the installation of detuned filters tuned to 134 Hz is included as an alternative (overvoltage factor of 134 Hz p = 14 %).

• Does this standardised 189 Hz resonance frequency mean that the choice of capacitor banks should be made simply by choosing the necessary power from the standard models? The reply is simply: no.

• Is therefore wrong to choose this 189 Hz frequency as standard? Once again, the reply is simply: no.

 

Where does the problem lie then?

The answer to this question requires a brief look at the working principle of detuned filters. If we look at the impedance-frequency diagram of a reactor-condenser unit with p = 7 % (Fig. 1), we notice that it offers the lowest impedance at 189 Hz, and the impedance increases gradually on both of its sides, with the peculiarity that the impedance is capacitive for frequencies below 189 Hz, and inductive for higher frequencies.

"It is precisely this inductive behaviour in the presence of order 5 harmonic frequencies or higher that avoids resonance phenomena at any of these frequencies."

  But the value of this impedance at the different harmonic frequencies, as well as the short-circuit impedance value at the capacitor bank's network connection point (Xcc at PCC), are also fundamental for the detuned filter to operate correctly.

Fig. 1 - Frequency response of a detuned filter with p = 7 % (189 Hz)

In a network equipped with a detuned filter, a single-line diagram and an equivalent diagram as shown in Fig. 2, the standard behaviour is that the short-circuit impedance (Xcc) at the capacitor bank - network connection point (PCC) is significantly lower than the impedance at each step of the capacitor bank, so that each harmonic current step's absorption of the harmonic currents flowing through the network should be relatively low compared to the one flowing into the network, as this is the path with the lowest impedance. But the situation may change in the event of networks where the Xcc value is high, i.e. in networks where the short-circuit power (Scc) at the PCC is low. These types of networks are also known as soft networks.

Fig. 2 - Single-line diagram and equivalent diagram of an installation fitted with a detuned filter

Installations that may be vulnerable to these problems are those with low short-circuit power in the High Voltage distribution lines at the low voltage network connection point; or those fed by a power transformer whose K-factor value (harmonic overload factor), by default, is unsuitable for the harmonic contents of the loads it is supplying, or there are long cable sections between the transformer output and the capacitor bank - network PCC, leading to high impedance in this section. In these cases, the most common effect is an increase in harmonic currents absorbed by the capacitor bank steps. In some cases, such increase may be very serious, severely overloading the capacitors and reactors comprising each detuned filter, and, especially in the case of capacitors, increasing their deterioration, usually in the form of a decrease in capacity. This decrease in capacity even increases harmonic current absorption, because, as can be seen from the formula that determines resonance frequency (Fig. 1), a capacity decrease causes an increase in tuning frequency, meaning it is even closer to the harmonic frequencies in the network (remember that order 5 generally prevails), thereby reducing impedance at that frequency, which therefore increases consumption of currents of that order. In other words, the detuned filter starts to react in a similar way to a tuned or absorption filter, but, as it is not designed to be used as such, its capacity is exceeded, causing it to deteriorate. In addition to this effect, the fact that networks with low Scc values, in the event of high harmonic current circulation, usually display high harmonic distortion levels (THD(U)), is another factor leading to an increase in the harmonic current absorbed by capacitors. In short, any solution that prevents an installed capacitor bank from affecting the network, and, in turn, avoids the capacitor bank itself being affected by the presence of harmonics in the network, may not always solve the problem, with the consequent technical and commercial implications this will undoubtedly entail.

So what options do we have for these types of installations when considering power factor correction by means of a capacitor bank with detuned filters ? The first consideration is obviously to determine whether the installation to be corrected is one that is exposed, i.e. a soft network type. Unfortunately there is no infallible or simple method of doing so, but there are a number of determining factors that help us find out to a reasonably high degree of accuracy. The main ones are as follows:

• There is a noticeable decrease in voltage value between the no load and full load condition, and current harmonic distortion level (THD(I)) is above 15 % under full load status.
• Voltage harmonic distortion level (THD(U)), at the point where the capacitor bank is to be connected, is above 3% under the installation's no load status.
• Voltage harmonic distortion level (THD(U)), at the point where the capacitor bank is to be connected, is above 6% under the installation's normal load conditions.

If one or more of the above conditions are met, it is highly advisable to choose a capacitor bank fitted with detuned filters whose tuning differs from the standard 189 Hz (whenever, of course, the harmonics present in the network are of order 5 or above).

What tuning is recommended ? For such cases, CIRCUTOR proposes a 170 Hz tuning value, equivalent to p = 8.7 %, which gives high capacitor bank protection levels when installed in networks of this type. What do we achieve by this change in tuning? Going back to the frequency response diagram for a rejection filter (Fig. 1), when the resonance frequency decreases, filter impedance on order 5 harmonics or above increases, so high consumption of these harmonic currents is significantly reduced. Furthermore, this change in tuning is also coupled with the use of capacitors whose nominal voltage is higher than those used in standard p = 7% filters, and reactors whose inductance value (mH) also exceeds the standard. As a result, we end up with a capacitor bank that is considerably more robust than its p = 7 % power counterpart.

A real case is described below, where the implementation of two rejection filter capacitor banks, with thyristor operation, and reactor-capacitor units tuned to 170 Hz, has enabled perfect network compensation, along with considerable improvement in power supply quality (voltage quality). The installation is for a funicular railway in the city of Barcelona, whose simplified single-line diagram is shown in Fig. 3.

Fig. 3 - Simplified single-line diagram of the installation of a funicular railway in the city of Barcelona

Fig. 4 - Installation of the funicular. The capacitor bank is shown on the left of the photo

These types of installation show the same characteristics as those described above when it comes to identifying any possible problems should a capacitor bank with conventional detuned filters be installed, since they are usually located far from the high voltage substation that powers them, normally with a considerable distance between the MV/LV transformer and main load, in this case, the power transducer and operating motor, and, in particular, in the presence of a power transducer that causes a rather high current harmonic distortion level.

Fig. 5 shows the reactive and active inductive power evolution (1 s integration period) for one of the two transformers in the installation. The appropriate capacitor bank is a CIRCUTOR device with thyristor operation, 6 x 55 kvar / 500 V / 50 Hz / p = 8.7 %, disconnected.

Fig. 5 - Evolution for the Active Three-Phase Generated Power (red), Active Three-Phase Consumed Power (green), and Inductive Reactive Consumed Power (purple and blue)

Fig. 6 clearly indicates the effect of the current value supplied by the transformer on the mains voltage, another clear symptom of a soft network.

Fig. 6 - Evolution for Voltage between L1 and L2 phases (blue) and Current Intensity in L1 (green) at Point A

Fig.7 shows the evolution for voltage distortion levels THD(U), which are significantly higher at peak current consumption by the power transducer.

Fig. 7 - Voltage harmonic distortion evolution per phase at Point A

Fig. 8 - Voltage and current wave shapes at times of peak transducer consumption

Fig. 9 shows the reactive and active inductive power evolution (1 s integration period) for one of the two transformers in the installation. Capacitor bank now operational.

Fig. 9 - Evolution for the Active Three-Phase Generated Power (red), Active Three-Phase Consumed Power (green), and Inductive Reactive Consumed Power (purple and blue)

Fig. 10 shows how the decrease in current value supplied by the transformer significantly reduces network voltage variations, improving power quality.

Fig. 10 - Evolution for Voltage between L1 and L2 phases (blue) and Current Intensity in L1 (green) at Point A

Fig. 11 shows voltage distortion level THD(U) evolution when the power factor correction device is operating. Comparing these values with those in Fig. 7, a significant reduction in voltage harmonic distortion rates can be observed (around 40% for peak values). The connection of the capacitor bank has a double rate-reducing effect, by both absorbing a certain percentage of the harmonic current generated by the transducers on the part of the capacitors (in this case, no damage will be caused to the capacitors as they are specifically reinforced for such cases), and by reducing the current passing between the power transformer output and PCC, which significantly reduces the harmonic voltage drop in this cable, as well as reducing the transformer's own internal losses. In short, though high distortion levels are still present, network voltage quality improves to more acceptable values, leading to a noticeable improvement in the installation's power supply quality, thereby minimising the risk of the device malfunctioning.

Fig. 11 - Voltage harmonic distortion evolution per phase at Point A