Trystan Casey
Harmonic resonance is a phenomenon that can occur in electrical systems, leading to adverse effects such as blown fuses, capacitor failure, and transformer damage. It happens when the frequency of an external force or vibration matches the resonant frequency of the system, resulting in amplified current and voltage distortion. This can be caused by the simultaneous use of capacitive and inductive devices in distribution networks, creating either series or parallel resonance. To prevent harmonic resonance, it is crucial to understand the power system impedance, including the transformer and upstream impedance, to mitigate the risks associated with excessive vibrations or structural failure.
| Characteristics | Values |
|---|---|
| Definition | A phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency of the system. |
| Cause | The simultaneous use of capacitive and inductive devices in distribution networks. |
| Types | Series resonance and parallel resonance. |
| Series Resonance | Creates low impedance, drawing maximum current into the system. |
| Parallel Resonance | Creates high impedance, which can cause large harmonic voltage drops and voltage stress-related damage to capacitors. |
| Impact | Harmonic resonance can blow fuses and capacitors on a power system or damage transformers. |
| Self-correction | Most harmonic resonance problems are self-correcting, as the resonant condition will cause enough current/voltage in the system that could either blow the fuses, fail the capacitor, or cause other system damage that makes the system non-resonant. |
| Mitigation | Anti-harmonic reactors can be installed in series with capacitors to avoid resonance. |
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What You'll Learn
Series vs. parallel resonance
Harmonic resonance is a phenomenon observed in electrical systems, where certain frequencies create a resonance between two components, typically a capacitor and a transformer. This results in a significant increase in current, which can lead to equipment failure. To understand harmonic resonance, it is important to distinguish between series and parallel resonance in electrical circuits.
Series resonance occurs in electrical circuits containing a resistor, inductor, and capacitor connected in series. At a specific frequency, the inductive and capacitive effects cancel each other out, resulting in minimum impedance and maximum current flow through the circuit. This condition is characterised by the circuit behaving like a pure resistive circuit. The resonant frequency can be calculated using a formula that takes into account the values of inductance and capacitance.
On the other hand, parallel resonance occurs when the same components (resistor, inductor, and capacitor) are connected in parallel. In this case, the effects of the inductor and capacitor cancel each other out at a particular supply frequency, resulting in maximum impedance and minimum current flow. Similar to series resonance, the circuit behaves like a pure resistive circuit during parallel resonance. The resonant frequency for parallel resonance is determined by the same formula as series resonance, but the circuit is influenced by the currents flowing through each parallel branch.
The key difference between series and parallel resonance lies in the arrangement of the components (series vs. parallel) and the resulting impact on impedance and current flow. Series resonance leads to minimum impedance and maximum current, while parallel resonance results in maximum impedance and minimum current. These differences make each type of resonance suitable for different applications in electronic circuits.
While resonance can be desirable in some cases, such as musical instruments or radio receivers, it can also cause problems in electrical systems. For example, harmonic resonance can lead to capacitor failure, transformer heating, and fuse blowing. Therefore, it is important to understand and manage resonance in electrical systems to prevent equipment damage and ensure optimal performance.
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Capacitor failure
Harmonic resonance is a phenomenon that occurs when an object or system vibrates at a frequency that matches its resonant frequency, resulting in an amplified response. In electrical systems, this can lead to excessive currents and voltages, which can cause equipment failure.
Capacitors are a common component in electrical systems, and they can fail due to various reasons related to harmonic resonance. One of the primary causes of capacitor failure is the high impedance at the parallel resonant frequency, which results in high voltage and magnified current magnitudes. This can lead to the capacitor failing due to voltage stress damage to its insulating layers.
Additionally, the presence of power factor correction capacitors in a facility increases the potential for harmonic problems. Capacitors can cause the system to resonate near a harmonic frequency, resulting in high voltage and current distortion that can destroy the capacitor. This is particularly common when the natural frequency of the capacitor bank/power-system reactance combination is close to a particular harmonic, resulting in partial resonance with amplified voltage and current values. The elevated current can cause capacitor overheating, leading to dielectric degradation and eventual failure.
Furthermore, capacitors installed in power systems can alter the harmonic frequency response of the network or introduce transient disturbances. This can result in total or partial resonance at one of the harmonic frequencies, leading to capacitor failure. The installation of capacitors in a predominantly inductive impedance system can also lead to resonance and subsequent capacitor failure.
To mitigate these issues, it is important to consider the system's impedance, including the transformer and upstream impedance, to avoid creating resonance with the capacitor. Additionally, damping provided by resistances in the power system can help reduce the catastrophic effects of power system resonance and prevent capacitor failure.
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Transformer heating
Harmonic resonance is a phenomenon observed in various systems, including electrical systems. It occurs when an external force or vibration matches the resonant frequency of the system, resulting in the amplification of current. In the context of electrical systems, this can lead to issues such as fuse blowing, capacitor failure, transformer heating, and voltage distortion.
Winding losses refer to the heat generated in the transformer windings due to the increase in inductive reactance and total impedance caused by higher harmonic frequencies. This increase in impedance leads to higher winding losses and, consequently, increased heat production. Additionally, in a delta-wye transformer, "triplen" harmonics (odd multiples of the 3rd harmonics) can overload the neutral conductor, causing the current to circulate within the windings and resulting in transformer heating.
Core losses, on the other hand, are associated with the eddy currents induced in the transformer core by alternating magnetic fields. These eddy currents generate opposing magnetic fields, leading to increased resistance and heating within the transformer core. The higher the harmonic frequency, the greater the energy of the eddy currents and their heating effect.
To mitigate the effects of harmonic resonance and transformer heating, special K-rated transformers are used. These transformers are specifically tested and rated to handle harmonic currents and reduce their impact on the transformer. Additionally, techniques such as sequential energization of the three-phase transformer and the pre-insertion resistor method can be employed to manage harmonic resonance and its consequences, including transformer heating.
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Fuse blowing
Harmonic resonance is a phenomenon that occurs when an object or system vibrates at a frequency that matches its resonant frequency. In electrical systems, this can be caused by harmonic currents produced by non-linear electronic loads that are injected into the power system grid. This results in an amplified current, which can lead to damaging overvoltage or overcurrent conditions.
In a power system, the impedance of the system varies depending on the harmonic frequency. When the system inductive impedance and capacitive reactance become equal, a resonant condition can develop. This results in a very high voltage on the capacitor, which can lead to capacitor failure and subsequent fuse blowing.
Additionally, the magnification of current during parallel resonance can also cause capacitor failure due to overheating or voltage stress damage to the insulating layers inside the capacitor bank. This, in turn, can lead to fuse blowing as the system tries to protect itself from further damage.
To prevent fuse blowing due to harmonic resonance, it is important to check for series and parallel resonance and ensure that the system is properly designed to handle the harmonic frequencies. By analyzing the current and voltage on a power quality analyzer, the harmonic order causing resonance can usually be identified, allowing for appropriate mitigation strategies to be implemented.
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Voltage distortion
Voltage harmonics are distortions on the voltage waveform that occur at multiples of the fundamental frequency. These distortions are generally caused by non-linear loads drawing non-sinusoidal current from a sinusoidal voltage source. The primary cause of harmonic distortion is the presence of non-linear loads. These loads draw current in a non-sinusoidal manner, causing the current waveform to be distorted. Examples of non-linear loads include power electronic devices such as rectifiers, inverters, adjustable speed drives, and electronic equipment like computers, fluorescent lights, and LED lamps. The switching operations in these devices generate harmonics, which can then propagate through the power system.
Voltage harmonics can lead to several problems, including additional losses and heating in electrical machines, misoperation of protective devices, and malfunctions in sensitive electronic equipment. The presence of harmonic distortion can be measured using various harmonic indices, such as Total Harmonic Distortion (THD) and Individual Harmonic Distortion (IHD). THD measures the overall distortion of the voltage or current waveform, while IHD determines the magnitude of each harmonic.
In a normal alternating current power system, the current typically varies sinusoidally at a frequency of 50 or 60 Hz. When a linear load is connected, it draws a sinusoidal current at the same frequency as the voltage. However, when a non-linear load is connected, it draws a non-sinusoidal current, causing current waveform distortion. This distortion can be quite complex and can lead to power quality issues.
To avoid harmonic resonance and its negative consequences, engineers employ various methods such as harmonic filters, active filters, and series broadband drive filters. Additionally, adjustable speed drives may utilize reactors, pulse converters, phase-shifting transformers, and synchronous condensers to improve Power Factor (PF) and reduce harmonic resonance.
It is important to note that voltage harmonics are usually smaller in magnitude compared to current harmonics. In most cases, the voltage waveform can be approximated by the fundamental frequency of the voltage, and the impact of current harmonics on real power transferred to the load may be negligible. However, when there is significant impedance in the path from the power source to a nonlinear load, current distortions can also produce distortions in the voltage waveform, known as voltage harmonics.
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