What is harmonics ?
Distortions in the electrical waveform for non-linear loads are called harmonics.In an ideal electrical system, the current and voltage waveforms are perfectly sinusoidal. However, when the waveform deviates due to additional frequencies that are integer multiples of the fundamental frequency (e.g., 50 Hz or 60 Hz), these are known as harmonics.
For example:
- The fundamental frequency is 50 Hz.
- The second harmonic is 100 Hz (2x the fundamental).
- The third harmonic is 150 Hz (3x the fundamental), and so on.
Overview
Harmonics are a common challenge in modern electrical power systems, caused by non-linear loads such as variable frequency drives (VFDs), UPS systems, and other electronic devices. These harmonics can distort waveforms, reduce power quality, and lead to operational inefficiencies. Harmonic filters, both active and passive, are critical in mitigating these issues. In this blog we will learn what is harmonics and how to reduce harmonics. Let’s explore their operation, necessity, and specific impacts of harmonics on power systems.
Harmonics can be broadly classified into:
- Odd Harmonics: These include the 3rd, 5th, 7th, etc. Odd harmonics are more common and have significant effects on power systems.
- Even Harmonics: These include the 2nd, 4th, 6th, etc., but are usually less prevalent.
Non-linear devices, such as switching power supplies, VFDs, and uninterruptible power supplies (UPS), introduce harmonics by altering the current waveform. The cumulative effect of harmonics leads to:
- Increased heat in transformers, motors, and cables.
- Reduced equipment lifespan.
- Potential resonance conditions, which can amplify harmonic levels.
Current Harmonics :
Current harmonics are distortions in the current waveform. Non-linear loads like Variable Frequency Drives (VFDs), rectifiers, and electronic devices often draw current in abrupt pulses, leading to harmonic distortion. Current harmonics can cause overheating of equipment, increased losses, and reduced efficiency in power systems.
Voltage Harmonics :
Voltage harmonics arise when current harmonics flow through the impedance of a power system. These distortions in the voltage waveform can affect sensitive equipment, causing malfunction or degradation. Voltage harmonics can also amplify issues like flickering lights and instability in power quality.
Harmonics in Variable Frequency Drives :
VFDs are commonly used in industrial applications to control motor speed and improve energy efficiency. However, they are also significant sources of harmonics. VFDs draw non-linear currents from the power supply due to their internal rectifiers and switching components. This leads to the generation of high-order harmonics, causing power quality issues if not mitigated.
Impacts of Voltage and Current Harmonics
- Voltage Harmonics:
- Increases in voltage harmonics lead to overheating of equipment, reduced lifespan of devices, and improper operation of sensitive equipment like medical instruments or precision tools.
- Can cause resonance issues in the system, leading to voltage amplification and equipment damage.
- Current Harmonics:
- High current harmonics result in increased losses in conductors, transformers, and other components.
- Leads to overheating, reduced efficiency, and malfunction of protective devices like circuit breakers.
- Contributes to higher energy costs and reduced system capacity.
Effects of Harmonics in Electrical Systems
Harmonics can have widespread effects, including:
- Thermal Stress: Increased heat generation in transformers, motors, and other equipment.
- Power Losses: Higher energy consumption due to additional losses in the system.
- Equipment Malfunction: Sensitive devices may malfunction due to distorted waveforms.
- Resonance: Harmonic resonance can amplify distortion, leading to severe damage.
Voltage Regulation Issues: Voltage drops and instability can occur, particularly in long-distance transmission systems.
The Most Dangerous Harmonics: 3rd, 5th, 7th, and 9th
- 3rd Harmonic (150 Hz in a 50 Hz system):
- The 3rd harmonic is a zero-sequence harmonic, meaning it adds in the neutral conductor in a three-phase system.
- This can lead to overheating of the neutral wire and transformers, causing insulation breakdown and fire hazards.
- 5th Harmonic (250 Hz):
- A negative-sequence harmonic that rotates in the opposite direction to the fundamental frequency.
- Causes torque pulsations in rotating machinery, leading to mechanical stress and inefficiency.
- 7th Harmonic (350 Hz):
- A positive-sequence harmonic that increases losses in electrical equipment.
- Can lead to overheating of transformers, motors, and capacitors.
- 9th Harmonic (450 Hz):
- Another zero-sequence harmonic that exacerbates issues in neutral conductors.
- Contributes to excessive transformer heating and reduced efficiency.
What is Total Harmonic Distortion (THD)?
Total Harmonic Distortion (THD) is a measure of the distortion of the voltage or current waveform caused by harmonics. It is expressed as a percentage and indicates the ratio of the root mean square (RMS) value of harmonic components to the RMS value of the fundamental frequency. A high THD indicates significant distortion, which can affect the performance and reliability of electrical systems.
How to Suppress Harmonics ?
Mitigating harmonics is essential for maintaining power quality and protecting equipment. Common methods include:
- Passive Filters: These use inductors, capacitors, and resistors to filter out specific harmonic frequencies.
- Active Filters: These are advanced filters that generate counter-harmonics to neutralize distortions.
- Line Reactors: Installed in series with non-linear loads, they reduce harmonic currents.
- Isolation Transformers: These prevent the propagation of harmonics from one system to another.
- Design Optimization: Using low-harmonic VFDs and properly sized cables can minimize harmonic generation.
Harmonic Filter Working Principle
- Passive Filters: Operate by creating a low-impedance path for harmonic frequencies, diverting them away from the main system.
- Active Filters: Use power electronics to inject a compensating current of equal magnitude but opposite phase to the harmonics, effectively canceling them out.
Harmonic filters are designed to attenuate or eliminate specific harmonic frequencies from the system.
Harmonic Filters: Active and Passive
Harmonic filters are employed to reduce harmonics and improve power quality. They can be classified into Active Harmonic Filters (AHFs) and Passive Harmonic Filters (PHFs).
Active Harmonic Filters (AHFs):
- AHFs use power electronics to inject counter-harmonic currents into the system to cancel out existing harmonics.
- Advantages:
- Dynamic and precise control of harmonics.
- Can address a wide range of harmonics.
- Adaptable to varying load conditions.
- How AHFs Work:
- They monitor the harmonic content in real-time using sensors.
- Generate a compensating current that is 180 degrees out of phase with the harmonic current.
- Inject this current back into the system to neutralize the harmonics.
Passive Harmonic Filters (PHFs):
- PHFs consist of passive components such as resistors, inductors, and capacitors to suppress specific harmonic frequencies.
- Advantages:
- Cost-effective and simple design.
- Effective for specific harmonic orders.
- How PHFs Work:
- PHFs are tuned to resonate at specific harmonic frequencies.
- They provide a low-impedance path for the targeted harmonics, diverting them from the system.
Which is Best for Industrial Use?
- Active Harmonic Filters (AHFs):
- Ideal for industries with variable and dynamic loads, such as manufacturing plants and data centers.
- Offer flexibility and effectiveness across a broad spectrum of harmonics.
- Passive Harmonic Filters (PHFs):
- Suitable for industries with steady and predictable loads.
- More cost-effective for addressing specific harmonic frequencies.
Conclusion
Harmonics are a significant concern in modern electrical systems, especially with the widespread use of non-linear devices like VFDs. Understanding the nature and effects of harmonics is crucial for designing efficient power systems and maintaining high power quality. Employing solutions like harmonic filters and optimizing system design can help mitigate these issues effectively.
Interview Questions on Harmonics
1. What are harmonics in electrical systems?
Answer:
Harmonics are voltage or current waveforms that operate at integer multiples of the fundamental frequency. They distort the ideal sinusoidal waveform of an electrical system, often caused by nonlinear loads like rectifiers, inverters, or variable frequency drives (VFDs).
2. How are harmonics generated in electrical systems?
Answer:
Harmonics are generated by nonlinear devices or loads that draw non-sinusoidal current, even when supplied with a sinusoidal voltage. Examples include electronic equipment, arc furnaces, and power converters. These devices cause periodic distortion in the current waveform, leading to harmonics.
3. What are the effects of harmonics on power systems?
Answer:
Harmonics can cause several issues, including:
- Overheating of equipment like transformers and motors.
- Increased losses in conductors and devices.
- Malfunction of protective devices.
- Voltage distortion, affecting sensitive equipment.
- Resonance in the power network.
4. What is Total Harmonic Distortion (THD)? Why is it important?
Answer:
THD is a measure of the distortion in a waveform due to harmonics. It is expressed as a percentage of the fundamental frequency’s amplitude.
THD is important because it quantifies the quality of power and its impact on system efficiency and reliability.
5. What are odd and even harmonics? Which are more common?
Answer:
- Odd Harmonics: These occur at odd multiples of the fundamental frequency (e.g., 3rd, 5th, 7th).
- Even Harmonics: These occur at even multiples of the fundamental frequency (e.g., 2nd, 4th).
Odd harmonics are more common because most nonlinear loads produce symmetrical current distortion, which cancels even harmonics.
6. What is the difference between current harmonics and voltage harmonics?
Answer:
- Current Harmonics: Arise from nonlinear loads that draw distorted current.
- Voltage Harmonics: Result from the interaction of current harmonics with system impedance, causing distortion in the voltage waveform.
7. What methods are used to mitigate harmonics in power systems?
Answer:
- Passive Filters: Use of inductors, capacitors, and resistors to filter specific harmonics.
- Active Filters: Electronic devices that dynamically inject compensating signals.
- Phase Shifting Transformers: Shift harmonics to cancel each other.
- Harmonic Mitigating Transformers: Reduce specific harmonics by design.
- Reducing Nonlinear Loads: Using more efficient or harmonic-free equipment.
8. What is a harmonic spectrum?
Answer:
A harmonic spectrum is a graphical representation of the amplitude of various harmonic components relative to the fundamental frequency. It shows the magnitude of each harmonic order and helps in analyzing the severity of distortion.
9. What are the IEEE standards related to harmonics?
Answer:
IEEE Standard 519 outlines limits for harmonic distortion in electrical systems:
- THD for voltage: Should not exceed 5% for systems below 69 kV.
- THD for current: Depends on the system’s short-circuit capacity and load conditions.
10. What is the role of power factor in relation to harmonics?
Answer:
Harmonics can lower the power factor because they increase the total current without increasing real power. This causes apparent power to rise, reducing system efficiency. Correcting power factor often involves harmonic mitigation techniques like filters.
