Popular Science of Harmonic Control Methods! Introduction to Harmonic Hazards

Harmonic control means that in an AC power system, a power supply device containing a large number of harmonics can be equivalent to a linear load and a series of harmonic current sources. It can be considered that a nonlinear load connected to an AC source draws fundamental current from the AC source and feeds back harmonic currents of various frequencies to the AC source. The greater the harmonic current value and the internal resistance of the power supply, the greater the distortion of the voltage waveform caused by the harmonics, and the greater the harm caused.

1. Introduction to Harmonic control

The quality of electric energy directly affects the quality of industrial products. There are three standards for evaluating electric energy quality. The first is the voltage aspect, which includes voltage fluctuations, voltage offsets, voltage flicker, etc.; secondly, frequency fluctuations; and finally, the voltage waveform quality, that is, the symmetry of the three-phase voltage waveform and the distortion rate of the sine wave. is the proportion of harmonics. my country has clear standards and norms for the three aspects of power quality.

With the development of science and technology, with the improvement of industrial production level and people's living standard, a large number of non-linear electrical equipment is put into operation in the power grid, which causes the harmonic component of the power grid to account for an increasing proportion. It not only increases the power supply loss of the power grid, but also interferes with the normal operation of the protection devices and automation devices of the power grid, causing malfunction and refusal of these devices, directly threatening the safe operation of the power grid.

2. Harmonic control causes of harmonics

Power grid harmonics come from three aspects:

2.1 The quality of the power generation source is not high enough to generate harmonics: it is difficult to achieve absolute symmetry in the production of the three-phase winding of the generator, and it is also difficult to achieve absolute uniformity in the iron core and other reasons, the power generation source will also generate some harmonics. But generally very little.

2.2 is the harmonics generated by the power transmission and distribution system: the power transformer mainly generates harmonics in the power transmission and distribution system. Due to the saturation of the transformer core, the nonlinearity of the magnetization curve, and the consideration of economy when designing the transformer, the working flux density is selected in On the near-saturation section of the magnetization curve, this makes the magnetization current a peak waveform, which contains odd harmonics. Its size is related to the structure of the magnetic circuit and the degree of saturation of the core. The higher the saturation degree of the iron core, the farther the transformer operating point deviates from linearity, and the greater the harmonic current, among which the third harmonic current can reach 0.5% of the rated current.

2.3 is the harmonic generated by electrical equipment: thyristor rectifier equipment. Since thyristor rectification has been widely used in many aspects such as electric locomotives, aluminum electrolytic cells, charging devices, switching power supplies, etc., it has caused a large number of harmonics to the power grid. We know that the thyristor rectifier adopts phase-shift control, and what is absorbed from the grid is a sine wave with a missing angle, so what is left for the grid is another part of the sine wave with a missing angle, which obviously contains a large number of harmonics. If the rectifier is a single-phase rectifier circuit, it will contain odd-order harmonic current when it is connected to an inductive load, and the content of the third-order harmonic can reach 30% of the fundamental wave; when it is connected to a capacitive load, it will contain odd-order harmonic voltage. Its harmonic content increases with the capacitance value. If the rectifier is a three-phase fully-controlled bridge 6-pulse rectifier, the primary side of the transformer and the power supply line contain 5th and above odd harmonic currents; if it is a 12-pulse rectifier, there are also 11 and above odd harmonic currents. Statistics show that: the harmonics generated by the rectifier account for nearly 40% of all harmonics, which is the largest source of harmonics.

3. Harmonic source of Harmonic control

Harmonics in the power system come from electrical equipment, that is to say, from generating equipment and electrical equipment. Since the magnetic field generated by the rotor of the generator cannot be a perfect sine wave, the voltage waveform emitted by the generator cannot be a sine wave without distortion. At present, there are two types of generators used in my country: hidden pole machines and salient pole machines. The hidden pole machine is mostly used for steam turbine generators, and the salient pole machine is mostly used for hydroelectric generators.

For the harmonic component, the hidden pole machine is better than the salient pole machine, but with the advancement of technology, the input of electronic excitation devices such as thyristors and IGBTs has increased the harmonic component of the generator. When the terminal voltage of the generator is higher than 10% of the rated voltage, the third harmonic of the voltage will increase significantly due to the magnetic saturation of the motor. Similarly, when the voltage on the power supply side of the transformer exceeds the rated voltage by more than 10%, the third harmonic of the secondary side voltage will also increase significantly. Since the grid voltage offset is below ±7%, the harmonic components generated by power generation and substation equipment are relatively small, much lower than the national assessment standards, so power generation and substation equipment are not the main factors that affect the quality of grid voltage waveforms contradiction.

For this reason, the main contradiction that affects the quality of the voltage waveform of the power grid is the nonlinear electrical equipment, that is to say, the nonlinear electrical equipment is the main harmonic source, and the nonlinear electrical equipment mainly includes the following four categories:

· Arc heating equipment: such as electric arc furnace, electric welding machine, etc.

· DC power equipment for AC rectification: such as electric locomotives, electrolysis, electroplating, etc.

· AC rectification and inverter electrical equipment: such as frequency conversion speed regulation, frequency conversion air conditioner, etc.

· Switching power supply equipment: such as intermediate frequency furnace, color TV, computer, electronic rectifier, etc.

These electrical equipment are non-linear electrical equipment, but the harmonics they generate are different. The specific examples are as follows:

The arc heating equipment is because the arc will start when the arc is above 70 volts, and there will be arc current, and the arc extinguishing voltage is slightly lower than the arc starting voltage, resulting in the nonlinearity of arc current and arc voltage.

In addition, the waveform of the arc current has certain asymmetry. It is precisely because the arc current is a non-sinusoidal wave that the arc heating equipment causes relatively large harmonic pollution to the power grid, and most of them are low-order harmonic pollution below the 18th order. In fact, electric welding machines have been widely used in the 1940s and 1950s. Due to the small amount of arc heating equipment at that time, the simultaneous application rate of the electric welding machine was even smaller, and the impact on the entire power grid was relatively small. However, it was found that the voltage and current of the local low-voltage power grid changed greatly during electric welding, and there was a large harmonic wave impact.

The reason for the harmonic generation of AC rectified DC power equipment is that the rectifier has a valve voltage, and when it is less than the valve voltage, the current is zero. In order to provide a stable DC power supply, this type of electrical equipment adds energy storage elements (filter capacitors and filter inductors) to the rectifier equipment, thereby increasing the valve voltage and stimulating the generation of harmonics. In order to control the voltage and current of DC power equipment, silicon controlled rectifiers are used in rectification equipment, which makes the harmonic pollution of this type of equipment more serious, and the order of harmonics is relatively low.

AC rectification and then inverter power equipment, the harmonics generated in the process of converting AC to DC are the same as the above-mentioned AC rectification and DC power equipment. When DC is converted into AC, the inverter waveform is reflected to the AC current. This type The harmonic components generated by the equipment include not only low-order harmonics, but also high-order harmonics.

Although the single capacity of this type of equipment is smaller than that of the above two types of equipment, its distribution is wide and its quantity is large. It is a technical means for promotion and use, so its harmonic pollution should attract enough attention.

Switching power supply equipment is widely used. Its working principle is to first rectify the AC into DC, and control the opening and closing of the primary current of the transformer through the switch tube, so as to induce the current on the secondary side of the transformer to supply the electrical equipment. In addition, the frequency of the switching power supply is relatively high, generally around 40kHz, which not only generates harmonics during rectification, but also reflects waves of around 40kHz to the power supply when the switching tube is turned on and off. This type of electrical equipment also has a small capacity, but it is the most widely used and largest non-linear electrical equipment. It also has a certain amount of third harmonics, which causes the center line current of the distribution transformer to remain high. Moreover, the third harmonic will pollute the 10kV power grid through the distribution transformer.

4. Analysis of the status quo of Harmonic control

Through the testing of the harmonic conditions of commonly used electrical appliances in the market, we learned that the harmonic pollution of industrial enterprises in China is very serious at present, especially in the early years, in order to save energy, the input of frequency conversion power supply and DC electrical appliances introduced, of which 5 The content of the 1st, 7th, and 11th harmonic currents account for 20%, 11%, and 6% of the fundamental wave respectively, which is not so good for low-power users, but for high-power users, the harm is even greater. It is too big. For intermediate frequency furnace users, it cannot be done with conventional reactive power compensation. Some users use conventional capacitor reactive power compensation and cannot invest in capacitors. Even if they are invested, the 5th harmonic current will be amplified. 1.8 to 3.8 times or more, greatly increasing the copper loss and iron loss of electrical appliances such as motors and transformers, shortening the service life of the equipment and paying more electricity bills.

5. Harmonic control of Harmonic control

5.1 General idea of harmonic control. Harmonic treatment should first consider prevention, control the source of harmonic generation, and minimize the harmonics generated in the system, so that it can be more conveniently treated or no further treatment is required. Therefore, when selecting equipment and building a system, reducing harmonics should be considered as an important condition. For two types of communication power supply equipment, AC and DC: Under the same or similar conditions, the UPS system should give priority to 12-pulse or Delta conversion equipment, and the DC system should give priority to better rectification circuits and perfect filtering measures The product.

Second, on the basis of prevention, remedial measures should be considered. Especially for the existing user low-voltage system, since the system structure has been basically fixed, the solution to the harmonic problem can only be controlled by adding reactors, filters and other remedial measures.

5.2 Preventive measures to control harmonics. Preventive solutions refer to measures to avoid harmonics and their consequences, as follows.

5.2.1 Phase cancellation in rectifiers (high pulse number rectifiers made of low pulse number rectifiers can cancel harmonics by choosing appropriate phase shifts) or harmonic control. A rectifier with a higher pulse number should be used, such as a 12-pulse rectifier instead of a 6-pulse rectifier.

5.2.2 Develop effective procedures and methods to control, reduce or eliminate harmonics of power system equipment.

5.2.3 Remedial measures to control harmonics. The remedial solution refers to the technology adopted to overcome the existing harmonic problems, including the use of LC passive filters, the use of active filters, and circuit detuning. See the following treatment methods for details.

6. Governance method of Harmonic control

6.1 Overview

For the harmonic voltage and harmonic current in the public power grid, there are relevant standards and regulations in the world and in my country. For example, the international IEEE Std519 requires that the maximum THD fed back by commercial and industrial users to the public power system should be less than 5%. In 1993, my country's State Bureau of Technical Supervision issued the national standard GB/T14549-93 "Power Quality Public Power Grid Harmonics" in 1993. According to the public power grids of different voltage levels, the maximum allowable harmonic current of each order is clearly stipulated. value.

In recent years, my country's communication power supply industry has gradually gained a certain understanding of harmonic currents. In the latest standards of the communication industry, the requirements for the harmonic content of the input current of UPS equipment have also been added. According to the size of UPS capacity and The importance of the place of use and other conditions divide the harmonic content index into 3 levels, namely 5%, 15% and 25% (in communication, because the UPS system with large capacity is often used, the power supply reliability required by the system is high, so Should be in accordance with the 5% index requirements).

In the field of communication, in order to prevent harmonics from interfering with the operation of equipment such as oil machines as much as possible, and to make the entire power supply system safer and more reliable, it is the best choice to control the current harmonic content of each point in the entire system within 5%. . Therefore, when building a new system, special requirements for harmonic indicators should be put forward for various equipment to ensure that the harmonics in the system are controlled during construction. For the existing system, because it is running, the difficulty and investment of transformation are relatively large. Therefore, it can be considered to appropriately reduce the requirements for harmonic control on the premise of ensuring the basic safety of the entire system.

6.2 Passive harmonics

The main structure of passive filtering is to use reactors and capacitors in series to form an LC series circuit and connect them in parallel to the system. The resonant frequency of the LC circuit is set at the harmonic frequency that needs to be filtered out, such as the 5th, 7th, At the 11th resonance point, the purpose of filtering out the 3rd harmonic is achieved. Its cost is low, but the filtering effect is not very good, if the resonant frequency is not set well, it will resonate with the system. This is the filtering method widely used in the market, mainly because of its low cost and easy acceptance by users. Although the effect of filtering is poor, as long as it meets the national limit standards for harmonics and the requirements for reactive power of the power sector. Due to its low cost, the market demand is great. Generally speaking, most low-voltage 0.4KV systems use passive filtering, and high-voltage 10KV systems almost always use this method to control harmonics. Since most of the small and medium-sized enterprises in our country are privately owned, the owners have insufficient understanding of the harm of harmonics, and are generally unwilling to spend a lot of money to control harmonics. It can't be effective, and the power supply department has very strict requirements for reactive power, and if it fails to meet it, it will be fined. Therefore, the owner had to ask for filtering. Therefore, its market prospect is considerable, and its economic benefits are also considerable.

The domestic high-level harmonic filtering device on the low-voltage side uses an optical fiber trigger system, which greatly reduces the misoperation caused by cable triggering due to harmonic interference.

6.3 Active Harmonics

The active harmonic filtering device is developed on the basis of passive filtering, and its filtering effect is good. Within its rated reactive power range, the filtering effect is 100%. It is mainly composed of power electronic components to generate a harmonic current with the same frequency and amplitude as the harmonic of the system, but the phase is opposite to the harmonic current in the system. However, due to the limitation of the withstand voltage and rated current of power electronic components, the cost is extremely high, and its production is much more complicated than passive filter devices, and the cost is much higher. Its main application scope is the power supply system of the computer control system, especially the power supply system of the office building and the computer control power supply system of the factory. For a single device, its profit is considerable, but users are generally unwilling to use active filtering. For the content of harmonics, it is not necessary to filter too cleanly, as long as it does not harm other electrical appliances.

6.4 Avoiding Resonance

Ordinary capacitors can amplify harmonics, and a certain reactor in series can not only protect the capacitors, but also effectively prevent system harmonics from being amplified. According to GB50053-94 "Code for Design of 10kV and Below Substations", "When the high-order harmonic content near the capacitor device exceeds the specified allowable value, a series reactor to suppress harmonics should be installed in the circuit." GB50227-95 The "Code for Design of Parallel Capacitor Devices" stipulates that "it is used to suppress harmonics. When the background harmonics at the place where the parallel capacitor When the harmonic is 3 times or above, it is advisable to take 12%".

To sum up, when constructing a communication power supply system, the use of active filters should be considered first on the basis of circuit detuning. It is best to consider it early in the construction process. For example, when building a UPS system, directly configure an active filter, etc., which can not only ensure the safety and reliability of the built system, but also increase the power supply reliability and energy-saving efficiency due to the convenient realization of terminal management.

7. Comparison of Harmonic control

7.1 Trends

The gradual increase of nonlinear loads in power grids is a common trend all over the world, such as variable frequency drives or thyristor rectifier DC drive equipment, computers, uninterruptible power supplies (UPS) for harmonic control important loads, energy-saving fluorescent lamp systems, etc. These nonlinear loads It will lead to grid pollution, power quality degradation, failure of power supply equipment, and even serious fire accidents. Some building fires in the world including our country have been proved to be related to power pollution.

7.2 Hazards

Problems and faults in the operation of power equipment, harmonic control are usually caused by fluctuations in electrical parameters of the power grid or instantaneous interference, such as: voltage fluctuations, surge shocks, harmonics, three-phase unbalance, low power factor, phase loss run etc.

For a long time, these situations that lead to equipment operation failure, increased maintenance workload and increased electricity consumption have attracted widespread attention from users and power supply departments.

Power pollution and power quality deterioration are mainly manifested in the following aspects: voltage fluctuations, surge shocks, harmonics, and three-phase unbalance.

7.3 Harm of Power Pollution

Power supply pollution will cause serious harm to electrical equipment, mainly including:

● Interfere with the normal operation of communication equipment, computer systems and other electronic equipment, resulting in data loss or crash.

● Affect the working performance of radio transmission system, radar system, nuclear magnetic resonance and other equipment, causing noise interference and image disorder.

● Cause electrical automatic device malfunction, or even serious accidents.

● Make electrical equipment overheat, increase vibration and noise, accelerate insulation aging, shorten service life, or even break down or burn out.

● Cause fluctuations in light brightness (flicker), affecting work efficiency.

● Causes increased power loss in the power supply system.

8. Introduction to the hazards of Harmonic control

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Harmonics are mainly caused by the distortion of the current waveform due to large-capacity rectification or converter equipment and other nonlinear loads. We perform Fourier series decomposition on these distorted alternating currents to obtain the fundamental wave component of 50 Hz and the harmonic component whose frequency is an integer multiple of the fundamental wave component.

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8.1 Common Effects of Harmonics on Systems

First of all, harmonics will increase the copper loss, iron loss and dielectric loss of the equipment and intensify the thermal stress, so that the rated output of the equipment needs to be reduced during operation. Secondly, harmonics can also increase the peak voltage. If the phase difference is ignored, the per unit value of the peak voltage increase is equal to the voltage peak factor. This increase in voltage will lead to an increase in insulation stress and may eventually cause cable insulation shock wear. Finally, harmonics can also cause damage to load equipment (here, load equipment damage is broadly defined as any equipment failure or malfunction caused by voltage distortion) and shorten the life of the equipment.

In addition, 3 times harmonics will cause the neutral line to carry current even in the case of load balance, and this current may be equal to or even greater than the phase current. This situation will lead to an increase in the zero-ground potential difference, and the selection of switches and cables on the neutral line needs to be adjusted appropriately. In addition, if the harmonics cause the resonance, the extremely large resonant current can cause more damage in the power system.

8.2 Harmonics affect the normal power supply of diesel generator sets

The internal resistance of the diesel generator set is much larger than that of the mains, and the voltage waveform distortion caused by harmonics is also much larger. Therefore, the influence of harmonics is not easy to be found when the mains supply power; but when the oil generator supplies power, the impact of harmonics on the power supply system will be much more obvious, for example, the voltage waveform output by the oil generator will be severely distorted. At this time, if the control part of the diesel engine judges the severely distorted output waveform, it may be considered as overvoltage, overfrequency, etc., which will cause the engine to stop; It relies on the discharge of the battery for power supply. If the mains power fails for too long, it will cause the UPS to shut down. Therefore, for equipment with a large input current harmonic content, it is required to increase the ratio of the oil engine to the equipment (that is, reduce the capacity of the oil engine to use), that is, increase the capacity of the oil engine to 2 to 5 times the equipment capacity, To reduce harmonic distortion and winding heating. But the cost of this method is very expensive, and it cannot guarantee the complete compatibility between UPS and diesel generator set. Due to the underload of diesel generator set, it will also lead to problems in the operation and maintenance of oil generator set.

In addition, harmonics increase the copper loss and iron loss of the generator. When the natural oscillation frequency of the generator is near the frequency of the pulsating magnetic field, the generator will undergo supersynchronous resonance.

8.3 Effect of Harmonics on Capacitor Banks

The impact of harmonics on capacitor banks is also serious, mainly in the following situations.

● Capacitors are overloaded by harmonic currents, because the reactance of capacitors decreases with increasing frequency, which makes capacitors known as absorption points for harmonics. At the same time, the large current generated by the harmonic voltage will cause the fuse of the capacitor to blow.

● Harmonic tends to increase dielectric loss, and the direct consequence is extra heat and shortened life of equipment.

● The combination of capacitor and power supply inductance constitutes a parallel resonant circuit, and its resonant frequency can be calculated. Harmonics are amplified at resonance and the resulting voltage can be significantly higher than the voltage rating and cause capacitor damage or fuse blown.

8.4 Effects of Harmonics on Transformers

Transformers in a harmonic environment will be damaged as follows.

● Load loss increases. Load loss includes copper loss and stray loss (coil eddy current loss). The stray loss is the most important factor to determine the extra heating loss of the transformer core caused by the nonlinear load.

● Hysteresis and eddy current losses increase. These losses will increase greatly with the increase of frequency, and the eddy current losses caused by harmonics are larger than the hysteresis losses caused by harmonics.

● Resonance may occur between the transformer inductance and the power factor correction capacitor.

● Insulation stress increases due to increased peak voltage.

These losses lead to heating of the transformer and a corresponding loss of life.

8.5 The influence of harmonics on protective devices, communication circuits and electronic equipment, etc.

Harmonics can also interfere with protective relays, measuring equipment, control circuits and communication circuits, and user electronic equipment, etc., and can also cause malfunctions or component failures of sensitive equipment. Harmonics affect protection and control devices, measurement equipment, communication circuits, and electronic loads in the following ways.

● Harmonics affect the breaking capacity of circuit breakers.

● Relays controlled by voltage and current peak or zero values are affected by harmonics. The time-delay characteristics of electromechanical relays change in the presence of harmonics. Zero-sequence current relays cannot distinguish between zero-sequence current and 3rd harmonic current, resulting in false tripping.

● Measuring instruments show different response characteristics to non-sinusoidal signals, resulting in inaccurate measurement.

● Harmonics interfere with telephone lines through inductive coupling.

● Due to the movement of the zero crossing point, harmonics affect the normal operation of electronic devices and control circuits.

● Harmonics interfere with user loads, which is especially worthy of attention for computer systems.

● Harmonics shorten the life of incandescent lamps and cause failure of fluorescent lamps.