Wang Yongjun, Yue Chaoyuan (Institute of Systems Engineering, Huazhong University of Science and Technology, Wuhan 430074, China) There have been some researches on the mechanism of hazards and impacts, and research on the costs and revenues of harmonics and harmonic electricity prices is still rare. . Discuss the influence of harmonics on electric power cost, theoretically analyze the influence of harmonics on electric energy meter measurement, and formulate harmonic electric price calculation model based on two pricing standards to provide theoretical basis for harmonic charging of electric power companies.
With the development of science and technology, the rapid growth of new types of electrical equipment (such as electronic devices, switching power supplies and speed control devices) has greatly improved social productivity on the one hand, and has enabled a large number of non-linear loads to be connected to the power grid. The power grid is seriously polluted and the quality of power is degraded. At present, harmonics, negative sequence, and flashover have become the three major public hazards in power systems, seriously threatening the safe and economic operation of the power grid and the safety of electricity for the majority of users, and the pollution caused by harmonics is particularly serious. In general, harmonics will reduce the quality of power, causing metering errors, large reactive power, and loss and temperature rise in power equipment and transmission lines, reducing the life of power supply equipment, threatening the safe operation of power grids, and the power supply costs of large power grids. In addition, with the rapid development of power electronics technology, the capacity of converter units in modern industrial production equipment is also increasing, and the number is increasing. These non-linear loads also generate a large amount of harmonic currents injected into the grid.
At present, harmonic problems are gradually being valued by people in China.
Harmonic control in power systems is an emerging and interdisciplinary issue that needs to be studied. The causes, hazards and impact mechanisms, the development and calculation of measurement and evaluation standards, and the methods of comprehensive management The exploration of other aspects should be deepened. Based on the influence of harmonics on power cost and the analysis of the influence of metering, the calculation of harmonic power price calculation model based on two pricing standards provides a theoretical basis for power companies to carry out harmonic charging.
1 Harm of power harmonics The current harmonic sources in China are mainly some rectification equipment, such as rectification equipment of chemical and smelting enterprises, various voltage regulators and electric locomotives. For example, electric locomotives not only generate harmonics but also asymmetric loads, and their location and working conditions will change over time. The electric arc furnace is also a source of harmonics, and its changes are very irregular, especially during the melting period, the harmonics are more serious. In addition, with the development of science and technology, new types of electrical equipment will be added rapidly, which may be a factor that can not be ignored in the future serious harmonic pollution.
At present, the harm caused by harmonics in China is basically recognized by everyone. Many also pointed out that the pollution caused by harmonics is very serious in China. For example, there are currently 2 large steel mills in Wuxi, and about 15 medium and small steel mills. The total capacity of electric arc furnaces is 338 (4 in 0t, 3 in 20t, 4 in 10t, 25 in 5t and below). It is mainly distributed on the 110, 35, and 10kV sides of each substation. The annual electricity consumption of electric arc furnaces has increased from 6 hours in 1990 to 120.5 billion kW-h at the end of 1993, with an average annual rate of 27.7% accounting for 18% of the annual electricity consumption in the region, which is a major electricity user in the region. As a non-linear load, the pollution caused by the harmonic current injected into the grid by the electric arc furnace is becoming increasingly serious. Due to harmonics, the distance protection false signal and malfunction, capacitor over current trip, transformer ferromagnetic sound and electrical equipment damage have occurred, which seriously threatens the safe and economic operation of the power grid.
In addition, the operation of the 500kV Luozong line of East China Power Grid has caused the distortion of the current waveform of the 220kV2774 line, which makes the high-frequency phase difference protection of the line unoperational. The reason is that the harmonic resonance is related to the power grid structure, and a strong harmonic source will certainly It will cause certain harm to the power grid, and a less powerful harmonic source will also constitute a resonance condition due to the improper coordination of the grid parameters, causing serious damage to the operation of the power grid.
Harmonics have different degrees of impact and harm on various electrical equipment. From an engineering point of view, the main aspects are as follows.
The main influence of a harmonic on the motor and transformer is to cause additional iron loss and copper loss, which makes the equipment temperature rise too high and reduce the insulation strength. Secondly, it generates vibration and emits noise. The long-term vibration will cause metal fatigue and mechanical damage. Harmonics can also cause harmonic overvoltages.
b. Harmonic amplification will increase the harmonic hazard. Harmonic amplification is a harmonic injected by a harmonic source. Its frequency is in the resonance region near the resonance point of the network, and the partial resonance generated by the inductor and the capacitor is excited. When the harmonic causes the system to resonate, the consequences are very serious, the harmonic voltage rises, the harmonic current is large, causing the relay protection to malfunction and causing equipment damage.
c Harmonic interference to relay protection and automatic devices. The interference of harmonics on relay protection and automatic devices can be divided into three categories: one is the interference generated by harmonics on the basis of positive sequence, the other is the interference generated by harmonics on the basis of negative sequence, and the third is the harmonics alone. The resulting interference. Harmonic sources such as electrified railways and electric arc furnaces are negative sequence power supplies. The harmonic interference superimposed on the negative sequence content has a serious impact on relay protection and automatic devices.
2 The impact of harmonics on the cost and revenue of power companies The above only explains the impact and harm of harmonics on various electrical equipment from the engineering point of view. In fact, the harm of harmonics to the power system is not only in engineering, but also in economics. Harmonic hazards have been studied as an engineering problem, but almost no research has been done on the economic aspects of this issue. Harmonics not only affects various electrical equipment, but also causes the power supply cost to the user to rise. Moreover, the electric energy meter generates errors due to the presence of harmonics, which causes the electric energy meter reading to be inconsistent with the actual load used, affecting the income of the power company. . Specifically, the impact of harmonics on the cost and revenue of power companies is mainly reflected in five aspects.
2.1 Harmonic causes various losses to increase the cost of electricity. Harmonic currents must generate additional losses in the transmission of transformers, motors and circuits. The harmonic resistance of the windings Rh is greater than the fundamental resistance Ri due to the skin effect of the current. For Rh=R\, the additional loss generated by the single-phase h-th harmonic current is the h-th harmonic current content in the APh winding.
It can be seen from the above equation that the additional loss of each harmonic is proportional to the square of the harmonic current content rate and the square root of the harmonic order.
The measured data of an electric appliance manufacturing company in Japan shows that when the Ih of the transformer reaches 0.1 in, the loss of the transformer is 10% larger than the rated loss. For the impact harmonic source of the electrified railway, the harmonic power loss caused by the harmonic It is even bigger. For example, in a certain area, the 5th harmonic of the electrified railway injected into the power grid causes resonance, so that the 3rd and 5th harmonic losses of the electrified railway injected into the power grid amount to 190kW, accounting for 1.9% of the average fundamental wave load of 10MW. The annual loss of electricity is 2.2. The capacity of some electrical equipment makes the power variable cost increase. The iron core in the electrical equipment will cause the core loss in the iron core coil under the action of the alternating magnetic flux, generating a large amount of heat. Core loss consists of two parts: eddy current loss and hysteresis loss.
Practice has proved that in normal operation of the motor and transformer, the eddy current loss is 2 to 3 times smaller than the hysteresis loss, and may be set to 2.5 times. Therefore, under the condition that the terminal voltage is kept constant, the iron loss generated by the nth harmonic is among them, and P0 is the iron loss under the fundamental wave condition, and the value can be referred to the national standard for the performance of the electrical steel.
1.7T magnetic induction, 50Hz, the iron loss per unit mass of material is shown in Table 1.
The measured data of the Xuedian traction station in the background harmonic analysis analysis report of the Harbin-Dalian railway electrification power supply system was adopted, and the actual iron loss calculation was carried out by taking the cold-rolled oriented silicon steel of DQ120-27 as an example. The actual calculation results are listed in Table 2. Table 1 Cold rolled oriented silicon steel loss table maximum iron loss - 27 cold rolled oriented silicon steel actual iron loss table harmonics harmonic current allowable value / A three maximum harmonic current / A Exceeding the standard is whether the unit mass iron core harmonic iron loss / (W ° kg | visible high harmonics obviously exceed the standard, the harmonics occupy the capacity of electrical equipment is very significant. With the increase of the number of harmonics The harmonics occupy a large amount of electrical equipment. The harmonics exceed the standard, causing the iron loss to be significantly large. At the same time, the heat generated by the electrical equipment is too large, so that the motor can not work at the rated power, that is, the iron loss occupies a large amount of electrical equipment capacity, so the harmonic Exceeding the wave will cause the cost of electricity to rise.
23 Harmonics reduce the life of electrical equipment and increase the fixed cost of electricity. Because the insulation structure used in electrical equipment is mostly composed of a variety of insulating materials, the insulation level of electrical equipment is the lowest heat-resistant material among the main insulating materials used. decided. Due to the existence of harmonics, the heat of various electrical equipment exceeds the design heating level, so it will greatly reduce the life of the insulating material and reduce the service life of the electrical equipment.
The working temperature has a great influence on the life of the insulating material. For example, the lifetime t(a) of the Class A insulation material has the following relationship with the use temperature 9 (C): 2718. This equation indicates that the life of the insulation material decreases exponentially with temperature. According to the test data a=.88, the insulation life of each 8C is shortened by half, and the temperature has a great influence on the insulation life.
The influence of 24 harmonics on relay protection and automatic devices causes power system accidents. In the devices in China's power system that are affected by harmonics and cause faults or performance degradation, relay protection and automatic devices are the most. For example, due to the influence of electrified railway harmonics, especially in traction substations, individual relay protection and automatic devices can be activated by a hundred or more times in a quarter, resulting in a large-scale blackout caused by false trips. The accidents are not uncommon. Regardless of whether it is in 220kV, 330kV or 500kV power grid, the harmonic protection affects the line protection or transformer protection, causing it to start or mis-trip, and its harmonics are derived from electrified railway or converter transformer. In some areas, the background harmonics will be amplified to cause some relay protection malfunctions under the condition that the capacitor group causes harmonic resonance. The impact of harmonics on relay protection and automatic devices is a serious threat to the safety and economic operation of the power grid. The losses caused by the accidents caused by it are sometimes invaluable.
2.4 Harmonic causes the metering error of the electric energy meter to affect the electric power company's income. The accurate measurement of the electric energy meter is the basis for fair receipt and payment of electricity. In addition to the above consideration of the impact of harmonics on the cost of electricity, the influence of harmonics on the metering of electric energy meters is also a factor that cannot be ignored when accounting for electricity prices. The influence of harmonics on measurement is mainly reflected in two aspects.
It is difficult to faithfully reflect the serious harmonic problems in the real power grid. For example, users such as electric locomotives and steel mills produce severe current waveform distortion, and the electric energy meter will generate large errors. The traditional electric energy measurement is based on the linear load. The voltage and current at both ends are assumed to be sinusoidal. Inductive energy meters are widely used at present, and are designed to work at power frequency conditions. In theory, only voltages and currents of the same frequency can constitute power. Power frequency voltage (current) and harmonic current (voltage) do not generate power. However, due to the nonlinearity of the ferromagnetic components of the inductive energy meter, there are some harmonics in the windings and discs of the watch when the power frequency voltage (or current) of the pure sinusoidal waveform is input.
Therefore, when one of the input voltage and current contains harmonics, although the true power of the subharmonic is zero in the circuit, in the electric energy meter, it is the same as the input pure sinusoidal power frequency due to distortion. The frequency harmonic components interact to form a false harmonic power, causing random or positive or negative errors in the energy meter.
On the other hand, the current usually passes through the transformer before entering the energy meter. When the higher harmonics enter the energy meter through the transformer, a large error will occur, which cannot accurately reflect the actual power consumption.
The frequency characteristics of the electric energy meter drop The harmonic voltage and current of the same frequency constitute the harmonic power and the corresponding harmonic electric energy, but due to the falling frequency characteristic of the inductive electric meter, the rotational speed of the disc under the harmonic power is higher than that of the same power frequency. The speed is much lower. The Northwest Electric Power Test Institute conducted harmonic influence tests on two single-phase inductive energy meters. The results show that the ratio of the rotational speed of the harmonic power of the same power meter to the fundamental power of the reaction is about 3 times. The harmonic is 0.65 harmonic and the 0.47 harmonic is 0.28. In the middle, due to its falling frequency characteristics, the ordinary inductive watt-hour meter can not accurately measure the high-order harmonic energy. In recent years, many research institutes in China have carried out research on the influence of harmonics on the metering of electric energy meters, and have done a lot of actual measurements. From the actual measurement results, the magnitude of the harmonic power determines the magnitude of the error generated by the harmonics on the energy meter. The greater the harmonic power, the greater the error generated on the energy meter, which is the main cause of the metering error of the meter. Under the condition that the voltage and current harmonic content are constant, the harmonic power direction also affects the error of the electric energy meter. When the harmonic power is the same or opposite to the fundamental power, the error of the electric energy meter is the largest. The positive and negative of the error depends on the direction of the harmonic power. When the harmonic power is injected into the user terminal by the power grid, the power meter error is a positive error; when the harmonic power is injected into the power grid by the user terminal, the power meter error is a negative error.
Taking the 3rd harmonic as an example to test the influence of harmonic power on the error of electric energy meter: 180 (analog load generates harmonic injection into the grid) (0 is the relative angle between harmonic component and fundamental wave) Table 3 3rd harmonic to electric energy meter The error influence project digital electromagnetic type can use the same method to obtain the influence of other higher harmonics on the energy meter error, and draw the error curve of (Pn) (n=2, 3, 4) under the above conditions.
The error curve can be used to estimate the error under the equivalent condition of the single harmonic. For the error caused by the load with more complex harmonic components on the electric energy meter, the total error curve can be obtained by the superposition principle according to the error curve drawn by the single harmonic. The experimental results prove that the estimated value is basically consistent with the actual measured value.
3 The cost of harmonic power companies is the basis for setting electricity prices. The price of all types of users should be able to truly reflect the cost of power supply and provide users with appropriate price signals.
The impact of this is implicit in the electricity bill income that the power company sells to all users. This is similar to the situation in which a few factories discharge wastewater into rivers and all residents along the river need to pay sewage charges. In this way, a reasonable price plan should reflect the interaction between harmonics on the income and cost of the power company. Therefore, the cost accounting standard should be the most basic standard for formulating the electricity price model, and it is also in line with relevant national policies.
As mentioned above, the increasing harmonic pollution in the power grid greatly affects the cost and revenue of the power company. Therefore, the future development of electricity price must consider the impact of harmonics on the cost of electricity. At present, we should formulate a new electricity price accounting method based on China's national conditions, carry out harmonic charging for users of large harmonic sources, strengthen research on the impact of harmonics on power companies' costs and revenues, and formulate harmonic electricity prices to force large harmonic sources. Reducing harmonic emissions by users and achieving economic harmonics control by economic means may be the most effective measure for harmonic control.
The impact of higher harmonics on the cost and revenue of power companies is mainly reflected in five aspects. It is difficult to make accurate estimates in the first four aspects. The punitive pricing standards for these effects are considered in harmonic electricity prices; The metering error of electric energy meter has been tested in China, and the error can be estimated by the measured data, which is reflected in the harmonic electricity price according to the cost accounting pricing standard.
According to the punitive pricing standard and the cost accounting pricing standard, the following formulas for calculating the harmonic electricity price are proposed: Pb unified sales price; Ki penalty coefficient, when the harmonic is stationary, Ki=0.2; when the harmonic is wave type, Ki= 0.3; When the harmonic content causes the estimation error of the system parameter resonance K2 harmonic on the metering of the electric energy meter, the value of K2 can be selected by measuring the total harmonic power to check the error curve.
The harmonic electricity price calculation model should be divided into the following two cases.
Many electrical equipments have certain harmonics, and their power consumption is different, and each harmonic voltage, harmonic current, and harmonic power are different, and it is necessary for all electrical equipment with harmonics. To charge, it is neither reasonable nor feasible to charge a device with a small harmonic content. Therefore, there must be a certain standard for harmonic charging, and harmonic charging is performed when a certain harmonic index exceeds the standard. To this end, the harmonic 2Un.In limit is used as the harmonic charge standard.
For setting the harmonic charging standard 2Un In limit according to the national standard GB/T1454993, there is an allowable value table for the harmonic current injected into the common connection point, see Table 4 according to Table 4 we can calculate the corresponding harmonic order The voltage content rate limit table is shown in Table 5. Since the influence of harmonics on the power supply cost is the result of the interaction of harmonic voltage and harmonic current, the voltage and current product Un / n of each harmonic is taken as the evaluation harmonic. The indicators of the influence of wave on power supply cost are shown in Table 6. Since the harmonic content of each harmonic of different harmonic power sources has different characteristics, we use 2UnVn as the standard for punitive charging for harmonic power supply. Table 7. For the sake of simplicity, a simple 2UnIn standard can be developed for different types of harmonic users. For example, for harmonic loads of AC arc furnaces, 2, 3, 4, 5, and 7 harmonics 2Un can be calculated. In value; for the electrified railway harmonic load, the 2Un.In value of 3, 5, 7, 9, 11, 13, and 15 harmonics can be calculated.
05 is to consider charging a load containing harmonics to use a certain measuring equipment, and invest a certain amount of work. Therefore, the additional cost of the harmonic charge is calculated, and it is calculated by 5% of the charge for the measurement error.
Table 4: Harmonic current input into the common connection point Allowable value Standard voltage reference is short 3203717191528132512141121 Table 5 Public power grid harmonic voltage limit Standard voltage reference Short 380V voltage level load is mainly for civilian use, even standard voltage / kV reference short circuit Capacity/MVA harmonic order and harmonic voltage current product/kVA standard voltage/kV reference short-circuit capacity harmonic order and harmonic voltage current product/kVA standard voltage/kV reference short-circuit capacity harmonic number and harmonic voltage current product/kVA The standard voltage reference short-circuit capacity/MVA industrial load is also mainly used internally by the user. Therefore, in the initial stage of harmonic charging, it may not be included in the harmonic charging standard limit table.
4 Conclusion It is necessary to consider the influence of harmonics on the cost and income of power companies from the economic point of view. The harmonic electricity price model and harmonic charging standard determined according to the two pricing standards can be used as the basis for charging the harmonic sources. Harmonic charging is a kind of power compensation and electricity surcharge for power companies. This is only economic compensation. Economic compensation is not the purpose of harmonic control. The ultimate goal of harmonic control is to reduce harmonic sources to power systems and their power users. Harmonic pollution.
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