Frequently Asked Questions
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1 How to suppress higher harmonic in electric power system
The higher harmonic has a relatively large impact on the power system and users. With the increase of nonlinear users, the higher harmonic will become more and more serious. Therefore, the higher harmonic in power system must be suppressed. There are many methods for suppressing the higher harmonics of power system, the commonly used are the following:
- (1) Increase of the phase number or pulse number of converter unit can reduce the harmonic current generated by converter unit, thereby reduce the harmonic current injected into the power grid.
- (2) Change the access point of nonlinear loads connected to the grid. Because the high-voltage power grid has great short-circuit capacity, and is able to bear great harmonic, connect equipment generating high-capacity harmonic to the bus of a power grid of one-level higher, or increase the electrical distance from nonlinear load to the place where the load is sensitive to harmonic .
- (3)Install inductance, capacitance or other types of filter at the harmonic source or on appropriate bus to absorb harmonic current and prevent harmonic current from being injected into the public grid. For example, install filter inside electric railway locomotive or on the bus of traction substation. The way, capacity, and location for installation of filter should be determined according to the situation of harmonic source, the situation of the measured harmonic, operation way of power system and the requirement degree of other load nearby the harmonic source on harmonic. Measurement should be conducted after the installation of filter to prevent the occurrence of resonance of certain several times of harmonic and amplification of the current of these several times of harmonic after being put into operation. At the place where reactive compensation capacitor is installed, it should be measured whether there is a situation of harmonic being amplified after the capacitor is put into operation. At the place where both reactive compensation capacitor and filter need to be installed, the same set of capacitors can be adopted for playing the roles in the two aspects.
- (4) For load with great reactive power impact, sometimes it is necessary to install static reactive power compensation device and filter at the same time in order to effectively suppress harmonic.
2 What are the hazards of the harmonics of frequency converter
With its superior speed adjustment performance, energy-saving effect, frequency converter is widely applied in industry. But because the higher harmonic current generated by frequency converter in input circuit has certain interference on the power supply system and the like equipment, it makes harmonic interference problem become very serious. Then in what aspects does the harm of frequency converter harmonic mainly reflected?
- (1) Increase of the loss of transmission lines, transformers and power capacitors makes the temperature of equipment become too hot and reduces the equipment utilization rate and economic benefit.
- (2) Impact the work reliability of relay protection and automatic device: especially for electromagnetic relay, power harmonics often cause misoperation or maloperation of relay protection and automatic device and make its action lose selectivity, reliability reduce. It is easy to cause the accident of the system, and the safe operation of power system is seriously threatened.
- (3) Generate interference to the work of communication system: When the large-amplitude odd low-frequency harmonic current through power line is passing through the magnetic field coupling, interference voltage will be generated in the communication lines of the nearby power lines. It will interfere the work of communication system, impact the intelligibility of call in communication line and will even threaten the safety of communication equipment and personnel in extreme condition.
- (4) Influence on electric equipment: Power harmonic will cause graphic distortion of TV, computer, fluctuation of screen brightness, and make components inside the device become overheat, cause the occurrence of error in computer and data processing system, even damage the machine in serious condition.
- (5) Power harmonics will also have adverse effects on indication of measuring and metering instruments and rectifying device, etc.
- From the above five points, we can see that the harmonics of frequency converter has become a big public nuisance influencing the power quality of the current electric power system, and we believe that everyone have had a certain understanding on the harm of the harmonics of frequency converter. In the future usage, you should try to avoid the harmonics of frequency converter, and need to find ways for elimination of harmonics of frequency converter.
3 Importance of arc protection product
There are numerous low and medium voltage switch cabinets in factory usage and power distribution of power plants and large users. They are the core equipment in the whole power supply system and are very important to the safe operation of the whole factory / power distribution system. However, in the current conventional protection scheme, low and medium voltage bus is not configured with special protection and is usually realized by the backup protection setting of incoming line switch; And the protections of incoming and outgoing line switches need to cooperate with each other. Therefore, the low and medium voltage bus failure of factory usage/ power distribution system will be cleared with delay. The differential of quick-break protection delay is usually at least 300ms, even 500ms; While the cooperation differentials of over-current protection are even up to 1 to 2 seconds.
In view of the important position of low and medium voltage bus in power distribution system, the clearing of any failure with delay will seriously endanger the safety of personnel and equipment , and cause huge losses, so there is sufficient reason for paying attention to the protection on the operation of low and medium voltage bus.
At the same time, fault arc light has the characteristics of rapid development, unpredictability, big destructive power, very easy spreading, the temperature of arc center exceeds 10,000 ℃, the light intensity of arc light can be more than 2,000 times of normal lighting intensity. Therefore, early discovery, early treatment of the fault arc light have become the key to suppress arc light development. Equipment and personal safety can only be protected to the maximum extent if we try to win every millisecond to clear the failure through trip of circuit breaker of power supply.
Due to inevitability of arc generation, harmfulness of arc light fault occurrence, a kind of device is urgently needed for the protection of arc light fault. The fast protection system of low and medium voltage bus arc short circuit of Tysen-kld’s technology combines arc light detection and high-speed over-current detection, and uses high speed, breaking output to make the protection action be fast, safe, reliable. After the occurrence of arc light, its action time can be up to 1ms. Its fast action ability can protect safety of personnel and equipment to the maximum extent.
4 Which equipment is included in harmonic source
- (1) Power Electronic Equipment
The power electronic equipment mainly include rectifier, frequency converter, switch power supply, static converter, SCR system and other SCR control systems, etc. Since the industrial and civil power equipment often adopt these power electronic equipment and circuits, such as rectifier and frequency converter circuit, their load characters are generally divided into inductive load and capacitive load, the single-phase rectifier circuit of inductive load is the current-type harmonic source with odd harmonic. Because capacitor voltage will feedback to power supply through rectifier tube, the single-phase rectifier circuit of capacitive load belongs to voltage type harmonics source. Its harmonic content relates to the size of capacitance value. The greater the capacitor value, the greater the harmonic content. Because what the harmonics source of the frequency conversion circuit adopts is phase control, its harmonic component contains not only integer harmonic, but also non-integer interharmonic.
- (2) Saturable Equipment
The Saturable equipment mainly include transformer, motor and generator etc. The saturable equipment are non-linear ones, whose core materials have non-linear magnetization curve and hysteresis loop. Under the sine-wave voltage, the exciting current is of symmetric function, and satisfies. It only contains odd items in the application of Fourier series for decomposition. For three-phase symmetrical transformer, the odd-time (3 times, 6 times, 9 times...) harmonics of the third harmonic are all of zero sequence. The transformer can be considered to be current source type harmonic source only generating odd harmonics. The number of times of transformers is also affected by the wiring mode at the primary, secondary side. The size of harmonic relates to the structure type of magnetic circuit, saturation degree of core. The higher the saturation degree of core, the greater the harmonic current. Compared with power electronic equipment and arc equipment, the amplitude of harmonics on saturable equipment can often be ignored under unsaturated condition.
- (3) Electric arc furnace equipment and gas electric light source equipment
(a) The nonlinear effect of electric arc furnace will generate a large number of harmonics in metal smelting process.
(b) Gas electric light source comprises fluorescent lamp, halide lamp, neon lamp. According to the volt- ampere properties of this kind of gas discharge light source, its nonlinear characteristic is very serious, and it also contains negative volt-ampere properties. Only when gas lamp is connected with inductance ballast in series during working and its comprehensive volt-ampere properties are no longer negative, can it work properly. Because the nonlinearity of ballast is quite serious, among which, the third harmonic content is above 20%, it features symmetric function, and only contains odd harmonic, therefore, gas electric light source equipment belongs to the current source type harmonic source.
5 What are the hazards of harmonic to power, electrical equipment
- It makes the mechanical efficiency of the motor drop, resulting in a waste of electricity. Harmonic mainly causes additional heating of motor, and easily generates overvoltage or overcurrent. The generation of harmonic will also lead to insulation layer damage caused by overheat at uneven winding, resulting in abnormal operation of the frequency converter. Its maloperation is easily caused by harmonic distortion, thus generating additional harmonic loss to the line of power supply and malfunction of protective electrical equipment. 1) current harmonic of transformer will increase copper loss.
- Frequency Converter When distortion of the input voltage of frequency converter occurs, it will lead to the increase of the line voltage drop.
- There is stray capacitance in motor windings, its comprehensive result is that it makes the temperature of transformer rise.
In addition, insulation is destructed till burned. Harmonics may bring about a consequence that the capacitor installed in the switchgear and power system is much bigger than the impedance in the system, resulting in overheat of the capacitor. But resonance may occur at the time of high harmonic frequency. Harmonic may also cause resonance between transformer windings and wire-to-wire capacitor. Resonance causes the entrance of abnormal current into the capacitor, thus affecting the working performance and service life of frequency converter. Harmonic voltage will increase iron loss, influence measurement accuracy of metering instrument. And the skin effect of conductor to the high-frequency harmonic current causes the increase of the equivalent impedance of the line, increase of the motor torque pulse and noise. This increases the burden of frequency converter rectifier diode and electrolytic capacitor, and the reactance value increases several times, while capacitance value greatly reduces, resulting in additional temperature rise of motor.
- Power capacitor is under power frequency state, thus noise pollution is produced. The section of output cable should be increased accordingly.
- The high-frequency harmonic current in power supply line makes the line impedance increase with the increase of frequency. Because frequency converter is a power electronic device, the peak of input current will increase.
6 What are the measurement methods for the harmonic of public grid
- Harmonic voltage (or current) should be measured in the possible minimum operating mode during normal power supply of the power grid and should be conducted in the time period when the amount of harmonic generated in the work cycle of harmonic source is great (for example: electric arc steel-smelting furnace should be measured during melting period).
When a capacitor bank is fitted near the measuring point, it should be measured in various operation modes of the capacitor bank.
- The times number of the measured harmonic is generally from the second to nineteenth. The measurement range of the times number of harmonic can be appropriately changed according to the characteristics of harmonic sources or test analysis results.
- For harmonic source of fast change load (for example: steelmaking arc furnace, rolling mill with power supplied by thyristor converter equipment, electric locomotive etc.), the interval between measurements is no greater than 2min, the times number of measurements should meet the requirements of mathematical statistics, generally no less than 30 times.
For harmonic source of slow change load (for example: chemical rectifier, DC transmission converter station etc.), the measurement interval and duration are not specified.
- The data of harmonic measurement should take the maximum phase value in 95% probability value of measured value of each phase during the measurement period as the basis for judging whether the harmonic exceeds the allowable value.
But for harmonic source of slow change load, you can choose five close measured values, and take their arithmetic mean value.
Note: For practicality and convenience, 95% probability value of the measured values can be selected according to the following methods: Arrange the measured values based in descending order, abandon 5% of large values in the front, take the maximum value among the remained measured values.
7 Technical principle of configuration of reactive power compensation for the state grid
Chapter 1 General
- Article 1 In order to ensure the voltage quality and stable operation of the power grid, improve the economic efficiency of power grid operation, this technical principle is specially formulated according to the Electric Power Law of the People's Republic of China and other relevant laws and regulations of the state, Guideline for Security and Stability of Power System, information source: Technical Guidelines for Power System Voltage and Reactive Power, Management Regulations for Power System Voltage Quality and Reactive Power of the State Power Grid Corporation and other related technical standards and regulations.
- Article 2 State Grid Corporation power grid enterprises of each level, power generation companies of grid-connected operation, power users shall abide by this technical principle.
Chapter 2 Basic Principle for Configuration of Reactive Power Compensation
- Article 3 The reactive power compensation devices configured for power system should be able to ensure reactive power balance of sub (voltage)layer and sub(power supply ) zone under the operation modes of the system active load peak and load valley. The key point of sub (voltage) layer reactive balance is the reactive balance of the layer of 220kV and the above voltage level. The key point of sub (power supply) zone local balance is the reactive balance of power distribution system of 110kV and the below. Reactive power compensation configuration should implement the combination of disperse local compensation and substation centralized compensation, combination of power grid compensation and user compensation, combination of high voltage compensation and low voltage compensation according to the situation of power grid, to meet the needs of loss reduction and voltage regulation.
- Article 4 Power grid of each level should avoid long-distance transmission of reactive power through transmission line. There should be no large amount of reactive power exchange between 500 (330) kV voltage level and system of the next level. The charging power of UHV transmission line of 500 (330) kV voltage level should be basically compensated by the adoption of high and low voltage shunt reactor according to the principle of local compensation.
- Article 5 The receiver system should have enough reactive power reserve capacity. When there are voltage stability problems in the receiver system, consider the configuration of dynamic reactive power compensation device at the hub substation of the receiver system through the comparison of technology and economy.
- Article 6 Substation of each voltage level should be reasonably configured with reactive power compensation device of appropriate size and type by combining power grid planning and power supply construction. The installed reactive power compensation equipment should cause no obvious amplification of the system harmonic and should avoid large amount of reactive power running through the transformer. For 35kV ~ 220kV substation, when the main transformer is of maximum load , its power factor at the high voltage side should not be less than 0.95, and its power factor should not be more than 0.95 during valley load.
- Article 7 For urban power grid adopting a large number of 10kV ~ 220kV cable lines, during the construction of new substation of 110kV and above voltage level, inductive reactive power compensation device of appropriate capacity should be dispersedly configured at relevant substation based on cable incoming and outgoing situation.
- Article 8 For Substation of 35kV and above voltage level, the high voltage side of main transformer should have acquisition and measurement function of operation parameters such as bidirectional active power and reactive power (or power factor) etc.
- Article 9 In order to ensure that the system has enough accident reserve reactive power capacity and voltage regulation ability, the generating units connected to power grid should have the ability of operation with power factor being 0.85 (lagging phase) to 0.97 (leading phase) during full load, the new unit should satisfy the ability of operation with leading phase being 0.95. In order to balance the charging power of transmission line of 500 (330)kV voltage level, installation of shunt reactor of certain capacity can be considered at the power plant side.
- Article 10 Power users should adopt appropriate reactive power compensation way and capacity according to its load nature. Under any circumstance, user should not send reactive power back to power grid and should promise not to absorb reactive power from the power grid during peak load peak the power grid.
- Article 11 Parallel capacitor bank and parallel reactor bank should adopt automatic switching on off manners.
Chapter 3 Reactive Power Compensation of 500 (330) kV Voltage Level
- Article 12 Capacitive Reactive Power Compensation of Substation of 500 (330) kV Voltage Level
The main function of capacitive reactive power compensation of substation of 500 (330) kV voltage level is to compensate the reactive power loss of main transformer and reactive power shortage of power grid during transmission of great capacity through transmission line. The capacitive reactive power compensation capacity should be configured according to 10%~20% of the main transformer capacity or determined after calculation.
- Article 13 Inductive Reactive Power Compensation Configuration of Substation of 500 (330) kV Voltage Level
The main function of high voltage shunt reactor of 500 (330) kV voltage level (including neutral reactor) is to limit the frequency over-voltage and reduce secondary arc current, recover voltage and balance the charging power of EHV transmission lines. The capacity of High voltage shunt reactor should be determined according to the above requirements. The function of the shunt reactor bank at the low voltage side of main transformer is mainly to compensate the remaining charging power of EHV transmission lines. Its capacity should be determined according to the power grid structure and operation needs.
- Article 14 When the short lines for 500 (330) kV voltage level in local area are in great amount, high voltage shunt reactor should be installed at appropriate place according to the structure of power grid for reactive power compensation. High voltage shunt reactor mainly compensating the reactive power should be equipped with circuit breaker.
- Article 15 When substation of 500 (330) kV voltage level is equipped with two or more sets of transformer, the reactive power compensation capacity configured for each transformer should be basically the same.
Chapter 4 Reactive Power Compensation of 220kV Substation
- Article 16 The capacitive reactive power compensation of 220kV substation mainly compensates reactive loss of main transformer, and appropriately compensates the reactive power loss of partial lines. The compensation capacity should be configured according to 10% ~ 25% of the main transformer capacity, and satisfy that its power factor at high voltage side is not less than 0.95 when the 220kV main transformer is of maximum load.
- Article 17 When the access bus of reactive power compensation device 灶220kV substation has direct-fed Load, the capacitive reactive power compensation capacity can be configured according to the upper limit; When the access bus of reactive power compensation device has no direct-fed load or the outgoing line at each side of transformer are mainly cable, the capacitive reactive power compensation capacity can be configured according to the lower limit.
- Article 18 For 220kV substation with cable as main incoming and outgoing lines, the corresponding inductive reactive power compensation device can be configured according to the cable length. The inductive reactive power compensation device capacity of each transformer should not be greater than 20% of the capacity of the main transformer, or determined after comparison of technology and economy.
- Article 19 The grouping capacity selection of the reactive power compensation device of 220kV substation should be determined according to calculation. The bus voltage change caused by switching on off of the maximum single group reactive power compensation device should not exceed 2.5% of the rated voltage. Under normal condition, the single group capacity of reactive power compensation should be not greater than 20Mvar when being connected to the 66kV voltage level, not greater than 12Mvar when being connected to 35kV voltage level, not greater than 8Mvar when being connected to the 10kV voltage level.
- Article 20 When 220kV substation is equipped with two or more transformers, the reactive power compensation capacity configured for each transformer should be basically the same.
Chapter 5 Reactive Power Compensation of 35kV ~ 110kV Substation
- Article 21 The capacitive reactive power compensation device of 35kV~110kV substation mainly compensates the reactive power loss of transformer, and appropriately gives consideration to the reactive power compensation at load side. The capacity of capacitive reactive power compensation device should be configured according to 10%～30% of the capacity of the main transformer, and satisfy that when 35kV～110kV main transformer is of the maximum load, its power factor at high voltage side is not less than 0.95.
- Article 22 When the capacity of single main transformer of 110kV substation is of 40MVA and above, each main transformer should be equipped with capacitive reactive power compensation device of not less than two sets.
- Article 23 The single group capacity of reactive power compensation device of 110kV substation should be not greater than 6Mvar. The single group capacity of the reactive power compensation device of 35kV substation should not be more than 3Mvar. The selection of single group capacity should also consider the need of reactive power compensation during small load of the substation load.
- Article 24 During the construction of new 110kV substation, inductive reactive power compensation device of appropriate capacity should be configured according to cable incoming and outgoing situation.
Chapter 6 Reactive Power Compensation of Distribution Network of 10kV and Other Voltage Level
- Article 25 Reactive power compensation of distribution network mainly relies on centralized compensates at the low voltage side of distribution transformer, and takes high voltage compensation as subsidiary.-The reactive power compensation device capacity of distribution transformer can be considered based on the maximum load rate of transformer being 75%, load natural power factor being 0.85. When being compensated to the maximum load of the transformer, its power factor at high voltage side is not less than 0.95, or is configured according to 20%~40% of the transformer capacity.
- Article 26 The capacitor bank of distribution transformer should be fitted with automatic switching on off control device being grouped according to the reactive power (or reactive current) and with voltage as constraint conditions.
Chapter 7 Reactive Power Compensation of Power User
- Article 271 Power users should reasonably configure the reactive power compensation device according to its load characteristics, and meet the following requirements:
For power users of high voltage power supply of 100kVA and above, the power factor at high voltage side of the transformer should not be less than 0.95 during peak user load; For other power users, the power factor should not be less than 0.90.
Chapter 8 Supplementary Provisions
- Article 281 State Grid Corporation of China is responsible for the interpretation of this technical principle.
- Article 29 this technical principle is executed from the date of issuance.
8 What are the benefits of improving power factor
Power supply bureau requires the user to improve the power factor to improve their cost-effectiveness. then what are the benefits for end users to improve power factor?
- ① Through the improvement of the power factor, reduce the total current in the line and the electrical components in power supply system, such as the capacity of transformer, electrical equipment, wire etc..Therefore, not only the investment cost is reduced, but the power loss itself is reduced.
- ② Being guaranteed by good power factor value, the voltage loss in power supply system is reduced. This can make the load voltage more stable, and improve the quality of electric energy.
- ③ It can increase the margin of the system, and has dug out the potential of power generation and supply equipment. If the power factor of the system is low, then under the condition with the existing equipment capacity remaining unchanged, the installation of the capacitor can improve power factor, increase the load capacity.
For example, when the power factor of 1000KVA transformer is improved from 0.8 to 0.98:
Before compensation: 1000 * 0.8=800KW
After compensation: 1000 * 0.98=980KW
For the same set 1000KVA transformer, after the change of power factor, it can bear another 180KW load.
- It reduces user’s electricity expense; preferential electric charge through the reduction of the loss of the above each element and the improvement of power factor.
In addition, some power electronic devices such as rectifier, frequency converter, switch power supply etc.; Saturable equipment such as transformers, motors, generators etc.; Arc equipment and electric light source equipment such as electric arc furnace, fluorescent lamp etc. All these equipment are the main harmonic sources, plenty of harmonic will be generated during their operation. Harmonics harm all the electrical equipment connected to the power grid such as generator, transformer, motor, capacitor etc. to different extents. They mainly reflect as producing harmonic additional loss, making the equipment overload, overheating and acceleration of equipment insulation aging due to harmonic overvoltage.
The capacitor in parallel connected to the line for reactive compensation has amplification effect on harmonics, making the distortion of voltage and current of the system more serious. In addition, the superposition of harmonic current on the fundamental current of the capacitor will make the effective value of current of the capacitor increase, and cause increase of temperature, thus reduce the service life of the capacitor.
The harmonic current makes the copper loss of transformer increase, which causes local overheat, vibration, increase of noise, additional heating of winding etc.
The harmonic pollution will also increase the loss of transmission line such as cable etc. And the harmonic pollution has influence on the communication quality. When the current harmonic component is high, it may cause malfunction of over-voltage protection, over-current protection of relay protection.
Therefore, if the harmonic content of the system measured is too high, besides that the capacitor end needs to be in series connected with suitable tuning (detuned) reactor, it is necessary to conduct special discussion on the installation of harmonic improvement device aiming at load characteristic.
9 What are the methods for reactive compensation of low and middle voltage distribution network
The reactive power compensation ways commonly used in distribution network are: installation of reactive compensation device in partial transformer and distribution power stations of the system, in each user; dispersed installation of shunt capacitor units in high and low voltage distribution lines; installation of shunt capacitor between low voltage side of distribution transformer and power distribution panel in workshop as well as installation of shunt capacitor nearby single motor, for centralized or decentralized local compensation.
- (1) Local Compensation
For large motor or high-power electric equipment, local compensation device should be installed. Local compensation is the most economic, simplest and most effective compensation way. In local compensation way, the capacitor is directly connected on the electric equipment, only a fuse is connected in series between them for protection. When the electric equipment operates, the capacitor will operate along with it. They will be cut off together at the time of cut-off, then the most convenient reactive power automatic compensation is realized. At the time cut-off, the coil of electric equipment is the discharge coil of the capacitor.
- (2) Dispersed Compensation
When each user terminal is far away from main transformer, dispersed compensation device should be installed at the power supply terminal. Combining with the low-voltage compensation at user end, it can reduce the line loss significantly, and can also give consideration to enhancing the terminal voltage.
- (3) Centralized Compensation
The reactive power compensation in substation mainly compensates the requirement of main transformer on reactive power capacity. The reactive power compensation capacity is determined on account of the reactive power compensation level of reactive power flow and distribution lines and user in supply voltage area. For 35KV substation, it is generally determined based on 10%-15% of the main transformer capacity; For 110KV substation, it can be determined based on 15%-20%.
10 Analysis of the causes of frequent fuse burning of reactive power compensation cabinet after usage of frequency converter
Very strong harmonic current emission will be produced when frequency converter is working. When the harmonic current emitted by frequency converter is injected into the system, large current will be produced on capacitor. This is caused by the following two reasons:
- First, the frequency of harmonic current is high, compensation capacitor has smaller impedance to high-frequency current, therefore, larger current will be produced.
- Second, harmonic current will resonate in the system, resulting in the amplification of harmonic current. This is usually the main reason of the problem.
In power system, frequency converter and reactive compensation capacitor form a LC parallel circuit, as shown in figure 1. This circuit has a inherent resonant frequency of F0. When F0 is equal to the frequency of the harmonic, the harmonic current will be amplified. Thereby, large current is generated in the loop formed by LC. Generally it will reach 10~20 times of harmonic current, which greatly damages the power system and the capacitor.
The blowout of the fuse on the compensation capacitor branch just shows that the fuse plays a role of protection. Otherwise, it will lead to more serious consequences, such as explosion of capacitor, overheating of transformer etc.
When the above problems occur, there are two solutions:
- The first solution: connect reactor L1 in series to capacitor, and make the resonance point of the series circuit formed by L1 and C be lower than the frequency of minimum-order harmonic current. In this solution, it is less than the fifth harmonic. The harmonic frequency of the circuit formed by reactor (L+L1) and compensation capacitor C is bound to be less than the frequency of harmonic source, therefore, no resonance will occur.
- The second solution is to reduce the harmonic current of harmonic source. The specific method is to install harmonic protective device at the power input end of the frequency converter, or install active filter at the system line incoming side for unified centralized management.
Usually, the second solution is generally adopted. According to statistics, the reactive power compensation cabinet works normally since adoption of this measure, and fuse blowout has never occurred basically.
For the requirement on the limit of the harmonic current of the electricity saving cabinet, we can adopt GB17625 standard or IEEE519 standard. According to the survey, more European enterprises adopt IEEE519 standard. Its main content is to specify the total harmonic current distortion THID<8%.
As long as preventive measures are taken in the design of energy-saving scheme, the risk caused by harmonic current can be completely avoided.
11 Generation and harm of the third harmonic
The harmonic in power grid mainly refers to the harmonic with frequency of integer components of power frequency(fundamental frequency) and inter-harmonic with frequency of non integer components of power frequency. They are all important reasons resulting in power quality pollution of power grid. The third harmonic in power grid is one of the main components of harmonic influence. In addition to load arc furnace load of the electrified railway being the main harmonic source, power transformer is also one important harmonic source of the third harmonic in power system based on the analysis results of a large number of on-site tests. The asymmetry of exciting current, core saturation of power transformer, and the load magnetic circuit of three-phase circuit brings about the consequence that the third harmonic components still exist in the line voltage and line current of transformer delta Z windings. Especially, when being under load valley, with the increase of power grid voltage, the saturation degree of transformer core will increase, the generated third harmonic content will also increase with it. With the operation of a large number of capacitors in power grid, through measurement of onsite harmonic, it was discovered that the zero sequence component of the third harmonic can not only be looped by transformer delta winding, but it spreads to the whole network, and brings influence and threat to the normal operation of capacitors and power grid. For example, after the reactors with series reactance ratio of 5% ~ 6% is blindly adopted for access of capacitor to power grid, the amplification of the third harmonic and occurrence of resonance caused by it have been proved by a large number of field accidents.
The generation of the third harmonic, also includes high-power thyristor rectifying devices and a large number of developed and applied power electronic devices. The increase of the capacity of steelmaking arc furnace and rolling mill, the development and application of electrified railway traffic, large increase of the application of power electronic equipment and electric appliances such as UPS power supply, electronic control equipment, energy-saving lamps and computer, microwave oven in household appliances etc. lead to the increase of harmonics injected into power grid by various nonlinear load, resulting in increasing influence of power quality pollution of power grid. Harmonic pollution has become very serious in the area where these devices are intensively used, such as factory workshop, apartment buildings, residential quarters, office buildings, hotels and commercial buildings etc.. The influence of harmonic pollution makes power quality decrease significantly, therefore, the suppression and governance of harmonic pollution of power quality have become urgent.
12 Calculation of reactive power compensation capacity of low-voltage capacitor
Usually, estimation methods for the capacity of centralized compensation capacitor of substation are as follows:
1. The natural power factor of machinery factory is 0.75. If the compensation is 0.96, we can learn from the following table:
K is 0.59. When the transformer's installed capacity is Se, under normal operation, it is considered based on 80% of rated capacity, then
For civil building, the natural power factor is about 0.78. When it is compensated to 0.95, K is 0.474. Based on the range of 80% of the rated capacity during the operation of the transformer, then
From this, compensation capacity can be preliminarily estimated. For machinery factory, the capacitor capacity is generally 35% of transformer capacity, while for civil building, it is generally 30% of transformer capacity. For example, the transformer capacity of civil building is 1000kVA, the compensation capacity of low-voltage capacitor can be taken as 300kvar. When the power factor is compensated to a certain value, and then the capacity of the capacitor is increased, the improvement efficiency of power factor will not be obvious. This will make investment increase too much, and has little significance to the increase of power factor. Therefore, the power supply sector's requirement on power factor at low-voltage side being not less than 0.90 has its reasonable aspect. Overcompensation is more unnecessary. It not only increases investment, but also raises the voltage of the power grid system and cause damage to electric equipment because reactive power will be returned to power grid.
13 What are the factors influencing the quality of electric energy
Power quality directly relates to the power supply safety and power supply quality of the power system. From a technical perspective,the factors affecting power quality mainly include three aspects:
- (1) Factors of natural phenomena, e.g. The influence of lightning, storm, rain and snow on power quality causes accident to power grid, resulting in the reduction of power supply reliability.
- (2) Factors of automatic protection and normal operation of the power equipment and devices, e.g. The influence of startup and shutdown of large power equipment, tripping and reclosing of automatic switch etc. on power quality temporarily reduces the rated voltage, generates fluctuation and flicker.
- (3) Factors of large amount of operation of nonlinear load, impact load etc. of power users, e.g. The influence of operation of steelmaking arc furnace, electric locomotive etc. on the quality of power energy makes utility grid generate large amount of harmonic interference, generate voltage disturbance, generate voltage fluctuation and flicker.
14 Basic knowledge of power quality
What is the power quality (Power Quality, abbreviated as PQ)? In a general sense, power quality is power supply of high quality, i.e. the quality of the AC power supplied to user end through public grid.
The public grid under ideal state should supply power to user at constant frequency, sine waveform and standard voltage. At the same time, in three-phase AC system, the amplitude of voltage and current of each phase should be equal, with symmetrical phase and phase difference of 120 degrees. But due to nonlinearity or asymmetry of equipment such as generator, transformers and lines etc. in the system, the load nature is variable. Together with the reasons such as imperfect control measure and operation, external interference and various faults, this kind of ideal state does not exist. Therefore, various problems have occurred in power grid operation, power equipment and power supply, then the concept of power quality is created.
Around the meaning of power quality, understanding from different perspectives usually includes:
- (1) Voltage Quality: A concept of reflecting whether the power supplied by power supply enterprise to user is up to standard with the deviation between actual voltage and ideal voltage. This definition can include most problems of power quality, but not include the problems of power quality caused by frequency, also not include the influence and pollution of electrical equipment to power quality.
- (2) Current Quality: Reflects the current change closely related to voltage quality. In addition to requirement of constant rated frequency, sine waveform on AC power supply, power user also requires same phase of current waveform and power voltage waveform to ensure high power factor operation. This definition is helpful to improve power quality and reduce line loss of power grid, but can not generalize most power quality problems caused by voltage.
- (3) Power Supply Quality: Its technical meaning refers to the voltage quality and power supply reliability, non-technical meaning refers to its quality of service, including the responding speed of power supply enterprise to consumer complaints, and transparency, rationality of electricity price composition.
- (4) Electricity Usage Quality: Including current quality and userality: 的权, responsibilities and obligations reflecting the interaction and influence of both sides, power user and supplier, including whether power users pay the exact amount of electricity charge on time etc.
- (5) Public connection point: The joint where userction point: he exact amount of electricity charge on time etc.
- (6) Fundamental (component): The components obtained in Fourier series decomposition of periodic alternating quantity, with the same frequency as power frequency. For the generalized periodic alternating quantity, its fundamental frequency is not necessarily equal to the power frequency, so the generalized fundamental component is: The component with the ordinal number of 1 in Fourier series of periodic quantity. When Fourier series adopts the type of exponential function, in exponential function ejncy, so the generalized fundamental compo.
- (7) Harmonic (component): The components obtained in Fourier series decomposition of periodic alternating quantity, with frequency being more than one integer multiples of the fundamental frequency, i.e. the components with frequency being two or above integer multiples of fundamental frequency. When this periodic quantity is expressed with Fourier series, the harmonic is the component with ordinal number being greater than one in the series, or each item with exponential function ejheach item wi.
- (8) Harmonic Order: Integer ratio of harmonic frequency to fundamental frequency. In some literature, the term with the same meaning is "harmonic ordinal number" (harmonic number). It should be pointed out that, from the contrast of English and Chinese, order should be "ordinal number", while "number" should be "times number". The symbol used in the previous literature is n.
- (9) Harmonic Content : the content obtained from subtracting fundamental components from periodic alternating quantity, i.e. being equal to the square root of the obtained difference after subtracting the square of root-mean-square value of the contained fundamental component from the square of root-mean-square value of alternating quantity. It should be pointed out that in the definition of IEEE, "content" is not an actual value, but "percentage of fundamental amplitude".
- (10) Harmonic Ratio (HR): The ratio of the root-mean-square value of order h harmonic component contained in periodic alternating quantity to the root-mean-square value of the fundamental component (in percentage). The voltage content percentage of the order h harmonic is in HRUh;The current content percentage of the order h harmonic is in HRIh.Their expression are:
For the same meaning, the term adopted by IEEE is "harmonic Percentage".
- (11) (11) Total Harmonic Distortion (THD): The ratio of root-mean-square value of harmonic content in periodic alternating quantity to the root-mean-square value of its fundamental component (in percentage). The total harmonic distortion of voltage is in THDu, the total harmonic distortion of voltage is in THDi. In the IEEE standard, it is called "Harmonic Distortion Percentage ". In addition to the distortion caused by harmonic, the distortion caused by interharmonics etc. should also be considered. To be rather exactly, the following expressions can be used for definition
- (12) Harmonic Source: The electrical equipment injecting harmonic current into public grid or generating harmonic voltage in public power grid. A broader definition is: equipment, apparatus, devices or circuits generating harmonics during their working. In some literatures, it is called "harmonic generator".
- (13) Short-time Harmonic: harmonic contained in current with impact duration of not more than 2s and interval between two impacts of not less than 30s, and the harmonic voltage caused by it. IEC 77A working team adopted the following definition: harmonic content contained in pulsating quantity lasting 1s or shorter. The definition of IEC is more broader. For example, it can be "fast changing harmonic" or "very short harmonic burst", not only limited to only being caused by the impact current. In harmonic standards, its allowable value is generally not specified. But in actual harmonic work, whether research on harmonic influence or suppression measure, all should be treated respectively.
- (14) Quasi-steady-state Harmonic: The harmonic changing slowly can be regarded as harmonic of steady state in short period.
- (15) Odd Harmonics: The harmonic with odd harmonic order (h=2k + 1, K is any positive integer).
- (16) Even harmonic: The harmonic with even harmonic order (h=2k, K is any positive integer).
- (17) Interharmonic: The periodic alternating quantity with frequency of non-integer multiples or integer divisors of the fundamental frequency. In the past literature, it is called "non harmonic", also called "pseudo harmonic". For the influence of IEEE harmonic on equipment, the working team literally translate “pseudo-harmonics” as " fractional harmonic", which can be interpreted as a "harmonic with frequency of non-integer multiples of fundamental frequency".
- (18) Characteristic Harmonic: Under ideal external and internal working conditions, the harmonic of some certain order determined by the working characteristics of the harmonic source itself is called the characteristic harmonic of the harmonic source. The most important example is that the characteristic harmonic at AC side of the converter with pulse number of P is the order harmonics, and the characteristic harmonic at its DC side is p order harmonic, K is any positive integer. In some literature, it is called "theoretical harmonic".
- (19) Non-Characteristic Harmonic: Each harmonic being different from the characteristic harmonic order of its harmonic source is called the non-characteristic harmonics of this harmonic source. The actual working conditions of harmonic source will always deviate from ideal condition to different extents, so some big or small non-characteristic harmonic content will occur.