Harmonics

Harmonics in AC power systems are voltage or current waveforms that vary from the ideal sinusoidal shape due to the existence of frequencies greater than the fundamental frequency. Understanding harmonics, their origins, types, and effects on power systems is essential for ensuring electrical system reliability, effectiveness, and safety.

Sources

Harmonics in alternating current power systems are mostly caused by non-linear loads, which consume current in sudden pulses rather than smooth sinusoidal patterns. Variable speed drives (VSDs), compact fluorescent lights (CFLs), LED lights, computer power supplies, and inverters are some examples of modern electrical and electronic devices. Industrial procedures that involve rectifiers, arc furnaces, or welding equipment all contribute substantially to harmonic generation.

Types

Harmonics are classed according to their order, which is a multiple of the fundamental frequency. For example, if the fundamental frequency is 50 Hz, the second harmonic is 100 Hz, and the third harmonic is 150 Hz. Even-order harmonics (2^nd, 4^th, 6^th, etc.) and odd-order harmonics (3^rd, 5^th, 7^th, etc.) have distinct impacts on power systems. Odd-order harmonics, notably the third and its multiples, are especially problematic in three-phase systems because they tend to concentrate in the neutral conductor, which may lead to overheating.

Figure 22: Fundamental plus 3^rd harmonic

Effects on Power Systems

Harmonics might cause several detrimental effects on power systems, including:

Increased Heating: Harmonic currents can overheat electrical equipment like transformers, motors, and cables, resulting in lower efficiency, premature aging, and failure.

Equipment Malfunction: Harmonic distortion can cause sensitive equipment to malfunction or fail prematurely because it interferes with the operation of control systems and protective devices.

Poor Power Quality: Harmonic distortion can cause poor power quality, such as voltage swings, flickering lights, and decreased performance of power factor correction equipment.

Resonance: Harmonics can interact with the power system's inherent resonance frequencies, resulting in higher voltage and current levels that can damage equipment and disrupt operations.

Increased Losses: Harmonic currents cause greater power losses in the system, affecting overall transmission and distribution efficiency.

Harmonic Analysis

Harmonic analysis in AC power systems is a critical method for discovering, measuring, and comprehending harmonic distortion in electrical networks. The analysis uses a variety of techniques and tools to measure and assess the harmonics produced by nonlinear loads, as well as their influence on the power system. Effective harmonic analysis allows engineers to create solutions to reduce hazardous harmonics, maintaining power system stability, efficiency, and dependability.

Spectral Analysis: Spectral analysis is a fundamental approach for harmonic analysis that includes breaking down a signal into its constituent frequencies using Fourier transform methods like the Fast Fourier Transform (FFT). This approach produces a spectrum displaying the magnitude of each harmonic component in relation to the fundamental frequency. Spectral analysis is useful for determining the presence and quantity of harmonics over a large frequency range, allowing engineers to detect specific harmonic issues in the system.

Figure 23: FFT‑spectrum of a distorted sinusoidal signal with multiple harmonics

Power Quality Analyzers: Power quality analyzers are complex machines that test and record many aspects of power quality, including harmonic distortion levels. These devices can collect and analyze voltage and current waveforms in real time, revealing extensive data on harmonic content and other power quality issues such voltage sags, swells, and flicker. Power quality analyzers are extremely useful equipment for doing complete harmonic evaluations in both field and laboratory cases.

Harmonic Distortion Indices: Engineers utilize a number of harmonic distortion indices to measure it, including Total Harmonic Distortion (THD) and Total Demand Distortion (TDD). THD is a measure of a waveform's total harmonic distortion, given as a percentage of its fundamental frequency amplitude. The equation below can be used to calculate THD:

$$\text{THD} = \frac{\sqrt{V_2^2 + V_3^2 + V_4^2 + \ldots}}{V_1}$$

Here V_n denotes the RMS value of the n^th harmonic voltage, while V₁ is the RMS value of the fundamental component.

TDD, on the other hand, relates the harmonic current to the overall demand current, providing a more balanced perspective on harmonic distortion. These indices provide a standardized method for evaluating and comparing the degree of harmonic distortion across systems and conditions.

Network Impedance Scanning: Network impedance scanning is a technique for determining how the impedance of a power network changes with frequency. As network impedance effects how harmonics propagate and accumulate, understanding the impedance profile can aid in recognizing potential resonance conditions that may increase harmonic levels. This technique is especially beneficial during the planning stages of power system design and when adding additional non-linear loads to an existing system.

Computer Simulation and Modeling: Engineers can use advanced computer simulation and modeling tools to forecast harmonic distortion and visually assess mitigation solutions. Engineers may simulate harmonic production, propagation, and effect under a variety of operating situations by modeling the power system and its components, which include non-linear loads, transformers, and filters. Simulation and modeling are essential for developing harmonic mitigation methods and doing what-if analysis to predict the impact of system modifications on harmonic distortion.

Harmonic Mitigation Strategies

Harmonic distortion in AC power systems can have a variety of negative consequences, including shorter equipment life, lower system efficiency, and higher operational expenses. As a result, implementing proper harmonic mitigation solutions is critical for ensuring the integrity and performance of electrical networks. This section delves into the major methods for minimizing harmonics, with an emphasis on the usage of filters, adherence to harmonic standards, and the adoption of best practices for lowering distortion.

Filters

Filters are some of the most straightforward and effective instruments for harmonic reduction. They function by creating a low-impedance channel for harmonic frequencies, directing them away from the power supply and decreasing their influence. Harmonic mitigation is accomplished using a variety of filters:

Passive Filters: These are intended to address certain harmonic frequencies. Passive filters, which commonly consist of inductors, capacitors, and resistors, are tuned to the resonant frequency of the undesirable harmonic, providing a route of least resistance for that frequency. While effective and reasonably priced, they must be carefully built to minimize resonance with the power system at other frequencies.

Active Filters: Active harmonic filters employ power electronics to generate counter-harmonic currents in response to harmonics. These filters can adjust to changing load circumstances while also mitigating numerous harmonic orders, making them more effective but more costly than passive options.

Hybrid Filters: Hybrid filters, which include passive and active elements, provide an appropriate balance of performance and cost, efficiently reducing a wide range of harmonics while avoiding the resonance issues associated with exclusive passive filters.

Harmonic Standards

Harmonic standards play an important role in reducing harmonic distortion by establishing permissible harmonic values and offering guidance for harmonic management. The key standards include:

IEEE 519: This is one of the most often used standards for harmonic control in power systems. It defines permitted harmonic voltage and current levels for utilities and customers, with the goal of ensuring compatibility and reducing harmonics' impact on electrical systems and equipment.

IEC 61000: A set of international standards that address many elements of electromagnetic compatibility (EMC), including restrictions on harmonic current emissions for various types of equipment.

Adherence to these standards is crucial for system designers, equipment manufacturers, and end users to guarantee that electrical systems are resistant to harmonic-related challenges.

Best Practices for Reducing Harmonic Distortion

In addition to utilizing filters and following standards, numerous best practices may be used to reduce harmonic distortion:

Load Management: Distributing non-linear loads equally across all phases can aid in lowering overall harmonic levels in the system.

Power Factor Correction: Improving the system's power factor can lower the overall current flowing, minimizing the influence of harmonic currents.

Equipment Selection: Choosing equipment with low harmonic emissions and ensuring correct measurement and installation may greatly minimize harmonic production.

Regular Monitoring and Maintenance: Establishing a routine for maintaining electrical equipment and monitoring power quality can assist in identifying and resolving harmonic problems before they become a problem.