The Influence of Harmonics in Capacitors and Type of Filters for Mitigation

What is Harmonics?

Harmonics are sinusoidal currents whose frequency is an integral multiple of the fundamental frequency. In simpler terms, harmonics are a series of higher-frequency waveforms that are multiples of the original (fundamental) frequency in an electrical system. The fundamental frequency is typically 50 or 60 Hz, depending on the region.

Harmonic currents are created by non-linear loads, which are devices that do not have a linear relationship between their voltage and current. These non-linear loads modify the waveform of the electrical current, resulting in the generation of harmonic currents. Some examples of non-linear loads include Variable Frequency Drives (VFDs), Uninterruptible Power Supplies (UPS), DC Drives, Battery Chargers, Welding loads, and Electric Furnaces.

When harmonic currents flow through the system impedances (resistances, inductances, and capacitances), they create voltage harmonics. Voltage harmonics are sinusoidal voltages with frequencies that are integral multiples of the fundamental frequency. The presence of these voltage harmonics can alter the incoming sinusoidal voltage waveform, which can lead to various issues in the electrical system, such as overheating, equipment malfunction, or reduced efficiency.

The Effect of Harmonics in Capacitors

Capacitors are necessary for power factor correction. However, it is mandatory to determine the level of harmonics in the system so that the appropriate filters will be installed accordingly. 

Capacitors are in particular highly sensitive to the presence of Harmonics due to the fact that capacitive reactance, namely Xc is inversely proportional to the frequency of the harmonics present. As a result of this, the likely hood of amplification of Harmonic currents is very high when the natural resonance frequency of the capacitor and the network combined happens to be close to any of the harmonic frequencies present. If the harmonic power is substantial ie.. greater than 10% , this situation could result in severe over voltages and overloads which will lead to premature failure of capacitors and the equipment. 

Related Article: Power Factor Correction and Harmonic Filtering

In general, harmonics in an electrical network can have various negative effects on the performance, efficiency, and reliability of the system. Some of the main impacts of harmonics such as: 

1. Overheating of transformers, motors, capacitors, and other equipment - Harmonic currents can cause increased losses in the form of heat in transformers, motors, and other electrical components. This can lead to overheating, reduced efficiency, and even equipment failure. 
2. Reduced power factor - The presence of harmonics increases the reactive power in the electrical network, which can result in a lower power factor. A lower power factor can lead to increased energy costs and reduced system efficiency.
3. Interference with sensitive electronic equipment - Harmonic distortion can interfere with the operation of sensitive electronic equipment such as computers, PLCs, and control systems, causing malfunctions or erroneous readings.
4. Increased energy consumption- Harmonic currents cause additional losses in the electrical system, which can increase overall energy consumption. 
5. Resonance - In rare cases, harmonics can interact with system components to create resonance conditions, which can amplify harmonic distortion levels and lead to equipment damage or system instability. 
6. Nuisance tripping of circuit breakers - Harmonic currents can cause false tripping of circuit breakers, interrupting the power supply and causing downtime. 
7. Adverse effects on power generation and distribution equipment - Harmonic distortion can cause increased stress on generators, capacitors, and other components in the power distribution network, leading to premature wear and reduced lifespan. 
8. Distortion of voltage and current waveforms - The presence of harmonics distorts the voltage and current waveforms, which can negatively impact the performance of electrical equipment and increase the risk of equipment damage.

Figure 1. Summary of Issues Caused by Harmonics | Source: Schnieder Electric

To mitigate the effects of harmonics in an electrical network, various techniques can be employed, such as using harmonic filters, passive or active filters, or installing power factor correction equipment. Proper design and selection of electrical equipment, as well as regular maintenance, can also help reduce the impact of harmonics on the system.

Solutions to Harmonic Rich Environment (Schnieder Electric Solution Model)

Depending on the magnitude of harmonics in the network, different configurations shall be adopted. Schneider Electric (SE) proposed different type of filters suitable for different levels of harmonics in the system. 

According to SE, the harmonic filters are group according to the following:  

Passive Filters

• Detuned Filters
• Tuned Filters
• Series broad band filters

Active Filters

• Single Phase 
• Three phase, 3-wire
• Three phase- 4 wire

Hybrid Filters

• combination of passive and active filters

Detuned Filters

Detuned filters are the most preferred since they are cost effective solutions which work on the principle of avoiding resonance by achieving an inductive impedance at relevant harmonic frequencies. The tuning frequency is generally lower than 90% of the lowest harmonic frequency whose amplitude is significant and which operate in a stable manner under various network configurations and operating conditions. 

Detuned harmonic filter systems consist of Reactor (L) in serieswith a capacitor (C) as shown in figure 2 below. 

Figure 2. Detuned Filter

Such a filter has a unique self series resonance frequency at whichreactance of reactor equals reactance of capacitor, ie.. XL =XC.  

The resonance frequency Fr is given by the formula. 

Tuned Filters

Tuned passive filters are designed to target a specific harmonic frequency. These filters consist of a combination of capacitors and inductors connected in series or parallel, creating a resonant circuit. The filter is "tuned" to resonate at the frequency of the harmonic that needs to be mitigated. When the filter is tuned correctly, it presents a low impedance path for the targeted harmonic current, thereby diverting it away from the electrical system and reducing its impact on the network. Tuned filters are highly effective at reducing specific harmonics but have limited performance when dealing with a broad range of frequencies.

If the self-resonant frequency of LC filter is within 10% of the harmonic to be filtered, then the filter is called Tuned Filter. Theycare primarily used as harmonics absorption filters and are generally more bulky and costly. A harmonic study is required to design this filter. A computer simulation is required to verify the filter performance at all loading levels. 

Series Broadband Filters

If an installation requires to reduce the harmonic distortion without affecting the existing power factor, then specially designed broadband filters are recommended. The broadband filters will be connected in series with the non-linear load, hence the harmonic current generated by the nonlinear loads will be arrested at the point of generation.

Series broadband filters are connected in series with the load, and they work by injecting currents that are equal in magnitude but opposite in phase to the harmonic currents generated by the non-linear load. This cancellation technique effectively neutralizes the harmonic currents, mitigating their impact on the electrical system.

The main advantages of series broadband filters include: 

1. Wide frequency range - Series broadband filters can effectively mitigate a broad range of harmonic frequencies, making them suitable for systems with varying loads or multiple harmonic sources. 
2. Adaptive filtering - Series broadband filters can adapt to changes in load conditions and harmonic levels in real-time, providing continuous and efficient harmonic mitigation. 
3. Reactive power compensation - In addition to harmonic filtering, series broadband filters can also provide reactive power compensation, which helps improve the power factor of the system and reduce energy losses. 
4. Reduced size and weight - Series broadband filters are generally smaller and lighter compared to passive filters, making them suitable for space-constrained applications.

However, series broadband filters can be more expensive and complex to install and maintain compared to passive filters. Despite this, they offer a versatile and effective solution for mitigating harmonics in a wide range of electrical systems.

Active Filters

There are few instances where the passive filters cannot be used. For example, if a wide spectrum of harmonics has to be filtered, the passive based solution may not be effective and impose significant limitations. The Active harmonic filter can measure and filter the harmonics generated by non linear loads in real time mode. Active filter works on a principle of generating harmonic current out of phase with the harmonic current existing in the network. The Active filter comprises of active elements such as IGBT’s, DC Link capacitors, microprocessor based controller with DSP logic etc. 

Active filters employ active components such as power electronic devices, microprocessors, and control algorithms to actively mitigate harmonics. They are connected in series or parallel with the non-linear load and work by injecting compensating currents that are equal in magnitude but opposite in phase to the harmonic currents, effectively neutralizing them.

Advantages of active filters:

• Can effectively mitigate a broad range of harmonic frequencies.
• Adaptive to changes in load conditions and harmonic levels in real-time.
• Can provide reactive power compensation, improving the system's power factor.
• Reduced size and weight compared to passive filters.

Disadvantages of active filters:

• More expensive and complex to install and maintain compared to passive filters.
• Can generate heat and require proper cooling systems.

Harmonic Capacitors for Detuned Filter Application

Reactors have to be associated to capacitor banks for Power FactorCorrection in systems with significant non-linear loads generatingharmonics.

Capacitors and reactors are configured in a series resonant circuit, tuned so that the series resonant frequency is below the lowest harmonic frequency present in the system. This configuration is called "Detuned Capacitor Bank", and the reactors referred as "Detuned Reactors". 

Related Article: Reducing Harmonics in AC Drive Systems through Structural Modifications and Filtering Methods

The use of Detuned reactors prevents harmonic resonance problems, avoids the risk of overloading capacitors and leads to reduction in voltage harmonic distortion in the network. The tuning frequency can be expressed by the relative impedance of the reactor (in %), or by the tuning order, or directly in Hz. The most common values of relative impedance are 5.67%, 7% and14% (14% is used with high level of 3rd harmonic voltages). 

Figure 3. VarplusCan Harmonic Capacitor Model | Source: Schnieder Electric

Figure 4. Varplusbox Harmonic Capacitor Model | Source: Schneider Electric

The figure above shows 2 type of harmonic capacitor models that are specifically designed to carry wide spectrum of harmonic and fundamental currents without overloading. It is designed for higher voltage capacitor to allow increased voltage due to introduction of series reactor The kvar of the capacitor is suitably designed to deliver the rated kvar of the filter at the bus voltage.

Related Article: Power Factor Correction in AC System 

Detuned Reactors

As explained above, the detuned reactors (DR) are designed to mitigate harmonics, improve power factor and avoid electrical resonance in low voltage electrical networks.

Overall, harmonics significantly influence capacitors in electrical systems, potentially causing overvoltages, overloads, and premature failure. To mitigate the effects of harmonics, various types of filters can be employed, including passive filters (detuned, tuned, and series broadband filters), active filters (single-phase, three-phase), and hybrid filters. 

Each filter type has its own advantages and disadvantages, making it crucial to select the appropriate solution based on the specific needs of the electrical system. By implementing the right filter, it is possible to minimize the impact of harmonics on capacitors and other equipment, thereby enhancing the overall performance, efficiency, and reliability of the electrical network.

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• Title: Influence of Harmonics in Electrical Network
• Source: Schnieder Electric