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
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.
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:
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 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.
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.
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
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.
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.
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