What are the effects of harmonics on power transformers?
1. Harmonic currents increase the copper loss of transformers, causing local overheating, vibration, increased noise, and additional heating of windings.
2. The additional losses caused by harmonic voltage increase the hysteresis and eddy current losses of the transformer. When the system operating voltage is too high or the three phases are unbalanced, the harmonic components in the excitation current increase, the electrical stress on the insulation material increases, and the medium affecting partial discharge of the insulation increases. For delta-connected windings, zero-sequence harmonics form circulating currents in the windings, causing the winding temperature to rise.
3. The transformer excitation current contains harmonic current, which causes excessive harmonic current in the inrush current. Under the condition of resonance, this harmonic current will threaten the safe operation of the transformer.
What are the effects of harmonics on power surge arresters?
Large-capacity, high-voltage transformers in substations experience prolonged inrush current processes, which can last for several seconds or longer. This can sometimes lead to resonant overvoltages and cause damage to the related surge arresters due to excessively long discharge times. This problem presents significant challenges in selecting the parameters of surge arresters used for inductance or capacitance in the high-voltage protection filters.
What are the effects of harmonics on power transmission lines?
1. Harmonic pollution increases transmission line losses. Harmonic currents in transmission lines, combined with the skin effect, generate additional losses, thus increasing transmission line losses. This increase is particularly significant in power systems with directly grounded neutral points operating under three-phase asymmetrical conditions.
2. Harmonic pollution increases neutral current, causing neutral point drift. In low-voltage distribution networks, zero-sequence harmonic currents (3rd, 6th, 9th, etc.) not only greatly increase the neutral current, causing overload heating and increased losses, but also generate voltage drop, causing zero-potential drift and reducing the power quality of the supply.
What are the effects of harmonics on power capacitors?
When the proportion of nonlinear electrical loads in a power distribution system is large, and parallel capacitor banks are connected, on the one hand, due to the low harmonic impedance of the capacitor banks, the harmonic current injected into the capacitor banks is large, which overloads the capacitors and seriously affects their service life. On the other hand, when the harmonic capacitive reactance of the capacitor banks is equal to the equivalent harmonic inductive reactance of the system and resonance occurs, the harmonic current of the capacitors is severely amplified, causing the capacitors to overheat and be damaged. Therefore, exceeding the limits of voltage harmonics and current harmonics will cause the operating current of the capacitors to increase and become abnormal. For example, for commonly used self-healing parallel capacitors, the allowable overcurrent multiple is 1.3 times the frequency constant current. When the current of the capacitor exceeds this limit, it will cause a damage accident. At the same time, harmonics distort the power frequency sine wave, producing sawtooth peaks, which can easily induce partial discharge in the insulating medium. Prolonged partial discharge will also accelerate the aging of the insulating medium, reduce the self-healing performance, and easily lead to capacitor damage.
What are the effects of harmonics on power cables?
Harmonic pollution will increase the dielectric loss, transmission loss, leakage current, and temperature rise of cables, as well as the partial discharge of dry cables, increasing the likelihood of single-phase grounding faults.
Because the distributed capacitance of power cables amplifies harmonic currents, when the system load is low and the system voltage rises, the harmonic voltage also increases accordingly. The higher the rated voltage of the cable, the greater the risk of instability in the cable dielectric caused by harmonics, and the more prone it is to failure.
What are the effects of harmonics on other operating equipment in a power system?
1. Impact on Synchronous Generators: The negative sequence current and harmonic current injected into the synchronous generator by the user will generate additional losses, cause localized heating of the generator, and reduce insulation strength. At the same time, the additional harmonic components generated in the output voltage waveform will cause torsional vibration of the synchronous generator rotor under load, reducing its service life.
2. Impact on circuit breakers: Harmonics can cause the magnetic coils of some circuit breakers to malfunction, reducing the circuit breaker's breaking capacity and making it unable to interrupt fault currents with waveform distortion exceeding a certain limit. When medium-voltage circuit breakers interrupt inductive current, the re-ignition of harmonic surge voltage may occur, leading to burnout of circuit breaker contacts.
3. Impact on the arc suppression coil: When the harmonic components of the power grid are large, a single-phase ground fault will occur, and the inductance current of the arc suppression coil may not be effective. It will not be compensated at the grounding point, which will lead to the expansion of the system fault.
4. Impact on power line carrier communication: High harmonic content mainly interferes with power line carrier communication by generating noise during voice communication, causing data transmission distortion, reducing the authenticity and reliability of EMS and DAS real-time data, and causing data errors and other faults in centralized meter reading systems.
What are the effects of harmonics on relay protection and automatic devices?
1. Impact on the operating environment of relay protection and automatic devices
(1) It will be affected in local power grids with high harmonic content, such as electric arc furnace loads with serious harmonic exceedances and electrified railways.
(2) Substations that frequently experience severe inrush current in transformers and slow inrush current decay will be affected by harmonic interference generated by the inrush current.
(3) It will be affected in places where the system may be affected by large harmonic voltage due to the small short-circuit capacity.
(4) It will be affected in the vicinity of power distribution systems, power transmission systems and substation grids where harmonic resonance is prone to occur.
(5) It will be affected in the vicinity of a network where harmonics are amplified by capacitor banks or other reasons.
2. The starting quantity of relay protection and automatic devices is small.
Starting using negative-sequence current or voltage, zero-sequence current or voltage, or differential current or voltage is susceptible to harmonic interference. Among these, starting using negative-sequence quantities is the most sensitive to harmonics.
3. The relay or starting element itself is sensitive to harmonics.
(1) Transistor or integrated circuit protection devices have very small action quantities and very short action times, so their startup data are easily affected by harmonics and may have large errors.
(2) Control systems that use signal zero-crossing sampling and digital relays or microcomputer protection that use data zero-crossing points are both affected and interfered with by harmonics.
What are the effects of harmonics on relay protection settings?
During normal operation of relay protection, high harmonic components in the power supply may cause maloperation of overvoltage and overcurrent protection. When there is severe three-phase asymmetry, high positive-sequence harmonic content may interfere with protection devices whose starting element is the negative-sequence filter, leading to maloperation. For example, after the electrified railway in a certain area opened, a large amount of harmonic negative-sequence current injected into the system by the traction substation caused a severe deterioration in the power quality indicators of the power supply system, resulting in multiple instances of maloperation of generator negative-sequence current protection, main transformer overcurrent protection devices, line distance protection oscillation blocking devices, high-frequency protection transceivers, and bus differential protection fault filters.
In recent years, the widespread use of microprocessor-based protection devices has led to the potential for harmonic interference in signals to cause measurement errors and disrupt the normal operation of critical processing modules, thereby causing malfunctions or failures to operate the protection devices. For example, Shanghai Baosteel experienced an incident where harmonic currents generated by an electric arc furnace interfered with digital differential protection, causing the differential protection to trip.
What are the effects of harmonic currents on digital differential protection?
Digital differential protection devices are easy to set, have fast response time, and powerful functions, so they are now widely promoted and applied. However, under conditions of poor power quality, malfunctions may occur due to current waveform distortion.
What impact do harmonics have on electricity metering?
By testing and analyzing the frequency response of electronic energy meters for induction-type energy measurement accuracy, it was found that harmonics have a certain impact on the accuracy of energy measurement.
What are the impacts of load-side harmonic pollution on the power grid?
In recent years, the application of numerous nonlinear loads at the user end has become a significant factor in power quality pollution and even deterioration. From the clustered use of low-voltage, small-capacity household appliances to the various static converters in high-voltage, large-capacity industrial AC/DC conversion devices, all are sources of power quality pollution. Various static converters operate in a switching manner, causing distortion of the grid current and voltage waveforms. Large arc-generating equipment, such as electric arc furnaces and arc welding equipment, also become important sources of impulse and harmonics.
What are the impacts of harmonics on electricity users?
Pollution of the power supply by electrical equipment can affect the reliability of the equipment itself. Using power sources and equipment with poor power quality can, in turn, become a new source of pollution, harming the power system and other user equipment. Potential impacts include: effects on user motors, user compensation capacitors, user automatic control devices, impacts on residential life, and threats to electrical safety. Additionally, it can affect telecommunications and interfere with the precision manufacturing industries of broadcasting and television sets. Some of this interference manifests as differential-mode interference and common-mode interference. Differential-mode interference is the mutual interference of distributed capacitances in power frequency and long-distance transmission lines, while common-mode interference causes changes in circuit potential and is a major cause of malfunctions in microcomputer control units.
What are the effects of harmonics on the operation of users' motors?
Harmonic currents passing through an AC motor increase harmonic losses, leading to overheating, increased mechanical vibration, and increased noise. When the three-phase voltage is unbalanced, a negative-sequence current is generated in the stator windings, energizing a negative-sequence rotating magnetic field. This braking magnetic field reduces the motor's maximum torque overload capacity, increases copper losses, and can burn out the stator windings. The effects of negative-sequence harmonic components (5th, 7th, 11th, etc.) on the motor are similar to those of negative-sequence overvoltage. When the main low-frequency component causing voltage fluctuations coincides with the natural frequency of the motor's mechanical vibration, resonance is induced, which can damage the motor.
What are the effects of harmonics on user compensation capacitors?
Capacitors constitute the largest proportion of reactive power capacity in the power grid, with user-owned capacitors accounting for approximately two-thirds of all capacitors. The design of these capacitors often only considers reactive power compensation, neglecting the actual power quality pollution at the installation point. Therefore, low power quality at the operating point frequently leads to accidents such as failure to activate compensation devices, reduced capacitor lifespan, blown capacitor fuses, and even series resonance causing harmonic overvoltages and overcurrents, resulting in capacitor explosions. Furthermore, user-owned capacitor management is currently still based on average power factor assessment, with capacitors rarely switched on or off according to actual grid conditions, or even switched on but not off. This results in the grid voltage losing its proper regulatory capacity, making it difficult to control power quality indicators such as voltage deviation.
What are the effects of harmonics on user automatic control devices?
With the widespread use of digital control technology, many precision loads have placed higher demands on power quality indicators. Power quality contamination harms these devices primarily in three ways: firstly, it introduces distorted variables into the device's detection modules, interfering with normal analysis and calculations and leading to incorrect output results; secondly, it causes irreversible damage to the device's hardware, such as precision motors and switching power supplies; and thirdly, it interferes with the load's protection circuits, causing malfunctions.
