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TD, TQ and TQR. Transformers for current measurement

Current transformers for any type of installation

Solutions for low-voltage current measurement

The installation of current transformers allows the different measuring devices to provide reliable and traceable data on the evolution of consumption and production processes in electrical installations.

FD-td-tq-tqr-ico-disenoDesigned in collaboration with installers
In the continuous process of improvement of our products, and thanks to the accumulated experience of our installers, we have designed this new range of current transformers to be installed quickly, easily and robustly. Meeting the most demanding expectations of the current market

Designed in collaboration with installers

Solutions for every type of installation

TD transformers
Easier to install

Thanks to our partnership with installers, our TD current transformers have a new and improved design to cover any need that may arise during their installation. The different models take into account aspects involving both their easy installation and their power optimisation when being connected to any electronic measurement device.

TD transformers
TQ and TQR transformers
Installation without interruption

The split-core TQ and TQR transformers have been designed to be connected to installations already in operation. A simple, two-step process makes for easy installation that saves on indirect costs, avoiding to disconnect the supply before start-up.

TQ and TQR transformers

TD. Narrow section transformers

Easier to install

TD. Narrow section transformers

FD-td-tq-tqr-ico-sujecionAttachment using ties

New tie fastening system built in at the transformer itself for an easy, fast and secure installation.

Attachment using ties


The inside of the transformers can be encapsulated for installation in very humid or saline environments.

FD-td-tq-tqr-ico-bajas-pLow losses

Ideal for installation with any type of device, especially for low-energy electronic equipment.


Best measuring accuracy guaranteed when connected to any type of receiver.


Multiple formats for connecting the transformer.

  • DIN rail: Two-way fastening with an accessory for connecting to the DIN rail, whether connecting vertically or horizontally.
  • Panel: The transformers have individual parts for installation at the bottom of a panel.
  • Busbar/Cable: Enclosure with different window options for installing directly on a busbar or cable, using insulated-tip screws or ties, for secure fastening.

Accessories for TD current transformers

Accessory for installing TD transformers to DIN rail. We can bidirectionally fix the device to a DIN rail with just this accessory, as it provides the possibility of fixing it either horizontally or vertically.

Accessories for TD current transformers


It has optional accessories for sealing the terminals and the transformer label.



FD-td-tq-tqr-ico-testTest report online

Download the test reports for Circutor's TD transformers free of charge from:


Test report online

Test report online

Accessories for TD current transformers 

Accessories for sealing TD series current transformers. The TD-Cover kit consists of a transparent cover that is placed at the top of the transformer, disabling access to the secondary connection terminals, and it can be sealed to avoid any manipulation. It also includes two caps, common to any TD series model, to prevent access to the secondary terminals that remain unused once the measurement devices are connected.

Accessories for TD current transformers

TQ. Split-core current transformers

Installation without interruption

TQ. Split-core current transformers

FD-td-tq-tqr-ico-aperturaPush-button opening

Simple installation with instant opening using the push button, avoiding the use of removable parts.

Push-button opening

Push-button opening


Installation to DIN rail or directly on conductors. Feature non-metallic parts to ensure fastening in busbars with plates.


FD-td-tq-tqr-ico-ligerosLightweight and compact

New design that reduces its weight and size for easier installation in any electrical panel.

Lightweight and compact


Guarantee the best measuring accuracy when connected to any type of receiver.

FD-td-tq-tqr-ico-bajas-pLow losses

Ideal for installation with any type of device, especially for low-energy electronic equipment.


Prevents tampering with the electrical connections by sealing the terminal block of the current transformer.

TQR. Split-core current transformers

Installation without interruption

TQR. Split-core current transformers

FD-td-tq-tqr-ico-aperturaPush-button opening

Simple installation with instant opening using the push button, avoiding the use of removable parts.

Push-button opening

FD-td-tq-tqr-ico-sujecionAttachment using ties

New tie fastening system for an easy, fast and secure installation.

Attachment using ties



Designed with a circular cross-section to fully adapt to the wiring cross-section, improving the measurement accuracy.


Low losses

FD-td-tq-tqr-ico-bajas-pLow losses

Ideal for installation with any type of device, especially for low-energy electronic equipment.


Guarantee the best measuring accuracy when connected to any type of receiver.

FD-td-tq-tqr-ico-ipHigh IP rating

Transformers with high IP65 protection, thanks to a sealing joint that keeps particles out of the connection terminals.


More information: TD, TQ and TQR. Transformers for current measurement


MYeBOX®. Energy Audits and Class A Power Quality

Everyone is well aware of the importance of measuring electrical parameters correctly to help us make the right decisions regarding energy efficiency and its consequent short-term cost-effectiveness, but we often find that not only is it necessary to perform energy audits to quantify the energy consumed by our different installations, but power quality or transient events must also be detected and recorded at our installations.

Such power quality faults, although often referred to as hidden costs, lead to production downtime, loss of material, unproductive staff hours, etc., and in some cases may be much costlier for companies than poor energy management.

MYeBOX® is a new system enabling energy audits to be performed which comply with the ISO 50001 certification, quality analysis according to the EN 50160 Standard and now also class A certification under the IEC 61000-4-30 Standard.

Remote management

The MYeBOX system stands out from its competitors with its new connectivity features, allowing devices to be fully managed in a simple, intuitive way from any location via a mobile application or the MYeBOX Cloud platform. These tools allow the user to remotely access the device and verify connection, device configuration, parameterise desired logging intervals, enable and configure power quality or transient event detection, alarms and even start or stop data logging. The possibility to remotely view the parameters measured by the device on a mobile terminal allows the user to detect faulty installation and/or device configuration and correct any problems immediately. This leads to important savings in time and travel costs, other devices only detecting such faults after downloading the data and obliging the user to make several trips to the installation to retake measurements.


One of MYeBOX's most outstanding features is that the device's wiring may be modified by firmware. What advantages does this have? Once the device has been installed, if the user detects that the parameters measured by the device are incorrect due to faulty wiring, data logging may be stopped, the device's wiring can be remotely modified and data logging resumed, thereby saving a trip without the need to retake measurements.

Single solution for simultaneous measurements

MYeBOX®By allowing remote configuration, the internal clocks of the devices can be synchronised via the mobile terminal or web platform, guaranteeing that all devices simultaneously logging at an installation have the same timestamp for all their logs. This is essential when determining the consequences or effects of a disturbance on the rest of the installation. If the devices being measured are not synchronised, it is impossible to draw cause/effect conclusions.

Remote management

One of the most recurrent needs of an energy audit is the need to carry out different measurements at different points in the same installation. This need usually requires long, costly journeys to the installations where the devices are measuring in order to stop data logging, move them to the new measuring point and restart logging. MYeBOX enables data logging to be stopped remotely and any company maintenance personnel (qualified and following safety guidelines) may then be asked to change the device's location. Once the device is in the new location, its correct wiring and configuration can be remotely checked, and data logging started again.

Multiple analysis

With a conventional analyser, the user is required to set a recording interval that applies to all variables. Although this may seem unimportant, it does penalise the user in that the recording interval for an energy audit to comply with EN50160 must be every 10 minutes. What happens if the user also needs to record some variables such as voltage and current every second? It simply cannot be done simultaneously. Such variables need to be recorded again and a one-second interval must be selected. MYeBOX is a precise, all-in-one device in that it allows the user to perform various types of installation analysis. How does it do so? It is the only analyser on the market that allows "per se" configuration of different recording intervals for different variables or variable sets. Logging of variables such as voltage and current per second may be configured and other variables can be recorded at 10-minute intervals.


MYeBOX enables the configuration of certain alarms related to the value of some electrical magnitudes measured by the device. These alarms may be e-mailed to different users of that particular analyser, thereby actively controlling the installation.


MYeBOX may therefore be tailored to meet any requirements that help installers and maintenance managers make the right decisions at the right time, saving both indirect and direct costs in the most flexible, efficient way.


More information: MYeBOX®. Portable power analyzer


Harmonics: origin, effects and solutions

Harmonic currents are one of the most important factors affecting network quality in installations, especially when it comes to wave shape. These currents cause distortions that distort the wave shape's ideal sinusoidal reference. This article will address harmonics from their origin to their consequences, as well as the tools available to electrical energy consumers to minimise their effects.

The presence of this type of parasitic currents in electrical installations (both domestic and industrial) has increased in recent years due to the growing implementation of so-called non-linear loads requiring the use of electronic converters to transform AC into DC and vice versa in order to work correctly. After the aforementioned transformation, the loads end up consuming current with a distorted wave shape.

Devices as common as computers require AC-DC conversion
Devices as common as computers require AC-DC conversion

Thanks to the mathematician Jean-Baptiste Fourier, this wave shape can be broken down into a sum of currents with multiple frequencies of the fundamental frequency (50 - 60 Hz).

We are therefore dealing with disturbances originating in the installations themselves, unlike other network quality factors such as amplitude, frequency or symmetry, which are usually caused by the power supplier.

In addition to the above-mentioned effects on the current's wave shape, current harmonics also have a distortion effect on the voltage wave, due to voltage drops that occur when these currents flow through the impedance of lines and transformers.

These distortions can be measured by network analysers and are mainly calculated as a percentage of distortion or harmonic distortion rate (THD). At an international level, there are standards that establish harmonic distortion limit values, which must be minimised to prevent them from affecting installations close to those being used by users (see IEC-61000-2-2; 2-4; 3-2; 3-4; 3-12; IEEE-519-2014).

After this conceptual introduction, let's now look at the most common non-linear loads:

  • Static transducers (rectifier devices, variable speed drives, soft starters, battery chargers...)
  • Single-phase electronic equipment such as computers, printers, programmable logic controllers, etc. Internally, they operate with direct current and have a filter capacitor and a rectifier at the input.
  • Lighting installations with discharge lamps.
  • Arc furnaces and welding equipment.
  • Transformers and reactors with iron core, and non-linear magnetisation.

most common non-linear loads

One of the most serious effects of current distortion caused by the above-mentioned loads is an increase in the network's effective current, leading to an unnecessary increase in consumption and problems related to cable and transformer sizing.

Its main consequences are:

Conductor overload

An increase in the effective current may cause currents flowing through the conductors to exceed their maximum acceptable limit, requiring an increased cross-section if the effect caused by the harmonic currents has not been accounted for. This problem may be particularly important for neutral conductors, since triplen harmonics (odd order, multiple of 3: 3, 9, 15), primarily caused by single-phase loads, cause harmonic currents to return through the neutral, all adding up together as one. It is essential to control overcurrent levels in the neutral, as excessive heating can cause serious degradation, even to the point of the neutral cutting off unless properly controlled. Neutral cut-off would lead to permanent overvoltage in the network, destroying equipment that is not suitably protected for such cases.

Derating of transformers

The presence of harmonics in the network increases hysteresis loss values and losses due to eddy currents in the transformers, increasing their operating temperature, which, in turn, reduces their useful life. This causes transformers affected by harmonic currents to suffer derating, that is, reduction in the power at which they can operate without causing overheating.

Triggering of protections

The effective current flowing through conductors may be seriously affected by an increase in current caused by harmonics in the installation, and may even exceed circuit-breaker temperature limits, thereby causing them to trigger. Although it is more unlikely, the presence of harmonics in circuit-breakers, due to their magnetic protection, may cause them to trigger, if the current's waveform crest factor exceeds the limit. High crest factors are often found in single-phase loads such as computers or discharge lighting. Harmonic currents have an indirect effect on RCCB triggering, since they flow through circuit breakers and do not directly cause them to trigger. On the other hand, it does mean that network behaviour upstream of the circuit-breaker has high impedance against harmonic currents, causing them to flow through parasitic capacitances or capacitance devices connected to earth (EMC filters), increasing the level of leakage found in the earth leakage device, thereby causing unwanted tripping.

Resonance and overload of capacitor banks

Capacitors are components that may display parallel resonance with the inductive behaviour of the transformer and cabling of the installation's power supply. This resonance greatly increases the unit's impedance to a given frequency that varies depending on the power of the capacitor bank or the power supply's impedance characteristics. Due to these characteristics of capacitor components, together with the presence of harmonics in the network, the installation may be affected by two detrimental phenomena:

  • Firstly, an increase in voltage distortion rate in any section of the installation affected by the resonance, which may affect other loads.
  • On the other hand, the capacitors themselves and other components in the capacitor bank, such as operating elements, may suffer damage as a result of their lower impedance against harmonic currents and high voltage distortion rate, leading to increased capacitor current consumption and possible capacitor burn-out.

Having dealt with the consequences of current harmonics, let's now look at the main problems regarding voltage harmonics:

Voltage distortion

Voltage distortion is a consequence of harmonic currents flowing through the impedances comprising the installation's different distribution and power supply components. Voltage distortion is particularly important, since high levels can cause devices in installations to malfunction, which is why there are compatibility level standards for this type of disturbances. The EN 50160 standard establishes conditions to be met by both the consumer and distributor at the coupling point (PCC), while the 61000-2-4 standard stipulates maximum distortion limits for correct operation of different types of loads. Different types of environment are also specified in this standard. By way of example, the voltage distortion limit for class 1, which includes sensitive loads such as automated systems, computers, etc., is 5%. This means that for higher distortion rates, such loads may be affected and operate incorrectly.

Effect on induction motors

Induction motors will suffer higher losses from an increase in parasitic currents. Furthermore, depending on the rotation sequences induced by the magnetic fields caused by voltage harmonics, the motor may suffer accelerations (positive sequence), braking (negative sequence), or both simultaneously, causing vibrations and eccentricities leading to mechanical wear of components. Derating of motors compared to voltage distortion rate is described in EN 60034-12 and NEMA MG1. In short, the factors identified lead to a loss of torque in the motor and reduced performance.

Zero-crossing disturbances

Many electronic devices have controllers that trigger load operation when voltage crosses zero. This is used to minimise switching current peaks in many inductive loads, thereby reducing the negative effects on electromagnetic compatibility levels. In the event of voltage distortion, such devices may operate incorrectly, causing them to break down, enter a loop, reset, etc.

Zero-crossing disturbances

Once the origins and effects of harmonics have been analysed, it should be pointed out that the purpose for eliminating them from electrical installations is not merely economic, but also helps to guarantee high-quality electric power supply. Unlike what happens with the power factor, nowadays there are no penalties for problems related to the presence of harmonics in electrical energy consumer networks.

In terms of cost saving, although we have already mentioned that harmonics increase the effective current and thus lead to increased energy consumption, it is not advisable to use harmonic filtering solutions in order to reduce additional losses, since the equipment needed to do so would hardly save any costs in consumption.

So why should we reduce the presence of harmonics in our installations?

The answer to this question lies in the advantages of having high-quality electrical energy flowing through our power supply:

Avoiding needless tripping of protections and ensuring devices run smoothly will help to maintain service continuity, so important in any industrial activity.

Keeping distortion rates to a minimum will lead to substantial saving in equipment maintenance costs, ensure that devices always run smoothly under optimum conditions, and avoid premature breakdown affecting both service continuity and high repair or replacement costs.

Besides these considerations, safety in electrical installations must be considered a top priority, especially considering the presence of personnel working with machinery and the need to avoid serious incidents such as fires. With this in mind, correctly dimensioning cabling and devices to suit operating conditions is crucial when it comes to reducing insulation faults and overheating of components.

CIRCUTOR's proposal: Active AFQm filters

Circutor's latest innovation in harmonic filtering comes with the release of the new AFQm active filters. The new AFQ series has been revamped and now offers extra versatility thanks to a more compact, lighter, efficient modular design and the proven top-quality track record of its predecessor, the AFQevo.

The working principle of the AFQm filter is the injection of a counter-phase set of currents into the harmonic currents flowing through the network. The device measures the distortion rate that occurs and then counterbalances it to obtain the best possible setting for an ideal sine wave, as illustrated in the figure below:

Working principle of an active filter
Working principle of an active filter

In this way, high-precision filtering is achieved, helping to maintain a top-quality power supply, which, in turn, leads to more efficiency and better overall functioning of the installation's components, as explained earlier in this article.

Due to the high level of harmonics in today's electrical installations, AFQm active filters can be used in a wide range of applications, especially in industries where an optimum wave shape is essential.high-precision filtering

Characteristics of the active AFQm filter


The AFQm eliminates harmonics and guarantees your installation's power supply quality.

Active multifunction filter, with primary selection of the following items:

  • Harmonic current filtering
  • Phase balancing
  • Reactive power compensation



Quick installation and easy step start-up.

The device just needs to be connected to the filter's network and measurement transformers, configured using its touch screen and then started up. The device itself will check that start-up is safely carried out thanks to an internal self-diagnosis system.


Its colour display allows the device to be configured and its installation status shown in real time.

  • Easy device configuration with clear, guided steps. The harmonics to be filtered can be selected individually for optimum device operation.
  • Troubleshooting connection problems: Faced with a common problem such as incorrect measurement transformer connection, you only need to enter the configuration menu to correct it in a few clicks.
  • Real-time display: Filter status, main electrical parameter readouts, phasor diagrams, wave shapes and harmonic spectrum are also displayed on the touch screen in real time. Information is displayed to the user in a very visual way by graphs and diagrams, so the behaviour of the installation and device may be instantly checked . The device also displays information for the 5 seconds prior to alarm activation to totally control installation status.

Troubleshooting connection problemsReal-time display


Find the combination that best suits your filtering needs


The compact AFQm range consists of 3 wall-mount models: 30A, 60A and 100A. Compared to the previous model, the new active filters are now more compact, lighter, quieter, and, since several arrays are possible, a lot more versatile too. For installations with higher filtering capacity requirements, 100A models may be installed in a cabinet-type array, obtaining cabinets reaching 400A. In such configurations, there will only be one master module, responsible for managing the whole filtering system. In this way, far fewer measurement transformers and much less electrical wiring are required, since only 3 measurement transformers need to be connected to the master and the CAN bus wiring to the slave modules. In the event of an even higher filtering demand, the master-slave function can be expanded to 100 parallel connected units.



Manage the device wherever you are via PC or mobile devices

The AFQm includes Ethernet TCP/IP and Modbus TCP communications for on-line web site monitoring , and format may be directly downloaded (without the need for software). Furthermore,the device may be fully configured with all on-screen configuration features, including filter status that may be monitored in real time and remotely.

As an example, the device may be started up and monitored remotely, on-site personnel only being required to physically install the filter. As a result, the cost of sending technical personnel to the installation is spared, allocating such resources only when strictly necessary.


All readings are stored in the device's memory so no data is lost

The filter stores readings with a one-minute interval and has a 7-year data recording capacity thanks to its 2 Gb internal memory. These data records may be accessed via communications for in-depth analysis of the installation's behaviour.


Thanks to the device's inbuilt, low-maintenance systems, the product's service needs are kept to a minimum

The AFQm has a variety of systems to guarantee filter safety during operation:

  • Protection system to prevent start-up if there is a problem
  • Anti-resonance system: the device is designed to bypass specific frequencies where resonance is detected
  • Smart thermal management system: fan speed and power regulation under high temperature conditions
  • Safe mode activation in case of fault detection
  • The device performs self-diagnostic tasks for the code and hardware executing it

Safe mode activation

Consult all information regarding Circutor's new AFQm active filters:

Discover Line: The first integral Energy Management System (EMSi) on the market

Line: Energy Efficiency is that easy as that !

Discover Line: Integral Energy Management System


We are pleased to introduce our new commitment towards innovation. A solution developed with the aim of simplifying the configuration and implementation of an energy management systems.

We will launch the Line solution on next July 1st. the first and unique EMSi in the market (integral Energy Management System)


Integral energy management
Integral installation control
Integral maintenance

Line: integral Energy Management System
We are pleased to welcome you to the presentation of the new Line solution


Do not miss the presentation in a unique online event in which we will discover the power of this new integral energy manaments tool (EMSi).

The online presentation will take place on July 1st. Book your place and join the event to discover the new integral Energy Management System.

Join us for the presentation of the new Line solution


Register and reserve your place.


(Limited places up to 1000 attendees)

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

The typical power factor correction solution for networks with harmonic distortion is usually based on standard devices, but the use of specific equipment is sometimes required in certain cases.

Capacitor banks with detuned filters

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?

Electrical network types

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)
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

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.

Special solutions to be implemented

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.

Case study

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. 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
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.

Situation prior to the installation of the capacitor bank

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

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

Present situation, after capacitor bank installation

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

Final conclusions

Having considered the foregoing, the best conclusion from those considered would be CIRCUTOR's standard, frequent recommendation to analyse, wherever possible, any installation whose power factor correction requires the use of a capacitor bank, to discard any doubts or fears we might have about possible effects of harmonic distortion in the network; an analysis that gives us the information required to make the right, safe choice of the most appropriate device to meet each particular case. With this in mind, please remember that CIRCUTOR has a full market range of network analysers, using state-of-the-art technology, which, together with effective data management software, enable any study to be carried out on the topics described in this article.

CIRCUTOR, your most reliable ally when requirements are related to power factor correction.

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