March 03, 2021

How to set the distribution system protection device of the inverter

1 The fact that the harmonics of the distribution system of the frequency converter cause the protection device to trip incorrectly

Case 1 When Chengdu International Shuangliu Airport was rebuilt at the airport, more than 100 sets of thyristor inverters of more than 75 kW were brought by foreign countries with the complete set of equipment. The distribution system was 110 kV incoming line, and the main transformer 10 kV power distribution and distribution system It is equipped with leakage protection. Because the input current of the thyristor inverter contains high harmonic components, the 5th harmonic of the input current reaches 20%, the 7th harmonic reaches 12%, and the total harmonic current distortion is about 30%. The integer multiple harmonic current of 3 is the zero sequence current, and the zero sequence currents are superimposed on each other in the neutral line.

The zero sequence makes the neutral current in the power supply system large. When there is a large harmonic current on the neutral line, the impedance of the neutral conductor can generate a large neutral voltage drop under harmonics. This neutral line voltage drop causes interference in the common mode, and some neutral lines. The current on it will also exceed the phase current. Because it is a cable power supply, the distributed capacitance is large, and the high-order harmonics are low-resistance, so the leakage current increases sharply, so that the leakage protection often trips incorrectly. The airport management staff and the Chengdu Power Supply Bureau have improved the leakage protection setting value, and the leakage protection does not trip, but the upper level differential protection jumped, and the accident with the system was broken. After consultation with the foreign company, the GTO inverter was replaced by the IGBT. Multi-frequency inverters, harmonics meet national standards, leakage protection, differential protection are implemented according to national standards.

Case 2 With the wide application of various types of variable frequency energy-saving equipment, the problem of harmonic pollution in the Shaoxing area of ​​Zhejiang is becoming more and more serious, which affects the quality of power supply. In 2005, the main transformer differential protection trip occurred in the 110 kV Dahe Substation. After the search and analysis, it was found that the 5th harmonic of the main transformer current of Dahe Substation reached 10%, and then the 10 kV outlet was tested and found to be 5 The 5th harmonic current of the 10 kV line exceeds 10%, and the 5th harmonic current of the Anzhong line with the largest harmonic current is 20%. The same is true because the harmonic current is too large, and the distribution line is a cable, the distributed capacitance is large, the harmonic current is low impedance, the leakage current exceeds the setting value, and a trip accident often occurs. With the agreement of the power supply bureau, the setting value is increased, and the leakage protection does not jump, but the upper level differential protection trips. Because the harmonic current in the system is ubiquitous, the electrical equipment failure caused by harmonic current is increasing day by day, and the potential hazard is large, which is easy to induce an accident. After investigation, it was found that the Sino-foreign joint venture Friendship Special Steel Co., Ltd. in the supply line has a new annual output of 200 kt stainless steel strip steel in the smelting process, especially during the melting period, random and large fluctuations in reactive power will cause The fluctuation of the power supply bus voltage, its main harmonic source equipment, including 3 20 t electric arc furnace, 2 25 t refining furnace. By evaluating its voltage fluctuations and flicker, the fluctuations are severe and constitute flicker interference. To this end, during the installation of the user's power distribution equipment, the user is required to install an SVC compensation device with a response time of about 10 ms, and the TCR part capacity is 10 000 kvar.

After the device is put into operation, the indicators meet the requirements of national standards.

Case 3 Hunan Huaihua Group's methylamine production line has two low-voltage power distribution rooms NF71 and NF73. Among them, NF71 has two transformers, each with a capacity of 1 600 kV·A and a voltage level of 6.3 kV/0.4 kV, using Yyn0. Connection method, the impedance voltage percentage is 4.3%, and the two transformers operate separately for a long time. There are also two transformers in the NF73, each with a capacity of 1 250 kV·A and a voltage rating of 6.3 kV/0.4 kV. The Yyn0 connection method is used, and the impedance voltage percentage is 4.4%, respectively. The two transformers operate separately for a long time. Since the main load under the four transformers is the inverter and part of the rectification equipment, this is the main harmonic source, so the parallel filter device is installed separately. Before installation, according to the test data, the 5th harmonic current is 235.6 A (95% probability value, the same below), the 7th harmonic current is 161.2 A, and the 5th and 7th harmonic currents have exceeded the national standard value. The specified range, the total harmonic current is 305.9 A, and the system power factor is around 0.92. During the whole test cycle, the total current of the system fluctuates between 665.1~1 023.7 A. In a certain period of time, the current changes in a short time is particularly large, mainly because the production process requires strict physical quantities such as speed and pressure. As a result, the harmonic current component corresponding to these periods is also very variable, which puts high demands on the fast response capability of the filtering device.

The Huaihua Group's methylamine production line transformation project selected Shanghai ZRAF series parallel active filters after considering all the above factors. After using the parallel active filter device, the various power quality indicators in the system have reached the assessment standard. The equipment of the entire production line is stable, and the existing transformers and motors are significantly improved in temperature and noise. The device is also operating normally. The various electrical faults caused by the influence of harmonic currents in the Huaiyi Group's methylamine production line have been significantly improved after the use of parallel active filter devices. The parallel active filter device is specifically designed to eliminate harmonic current, reduce electrical failure rate, and improve power usage efficiency. With the increasing use of frequency converters, the power quality problems caused by harmonics have received more and more attention from the society. Parallel active filter devices will have more and more applications in the management of power quality problems.

Case 4 Shanghai Pudong Financial Street Telecom Hub Project has more than 100 thyristor soft starters with more than 100 kW. The harmonic current also affects the protection circuit.

2 Inverter makes the leakage protection false trip principle

The main circuit and equivalent circuit of the inverter connected to the power distribution system are shown in Figure 1.

The frequency converter is a harmonic current source, as shown in Figure 2. Calculate the harmonic voltage of the victim point in the equivalent circuit

Case 1 The Chengdu Shuangliu International Airport Terminal is equipped with more than 100 thyristor inverters of more than 75 kW brought by foreign sets. The harmonic current is very large. Since the distribution line is cable-powered, there is distributed capacitance, such as As shown in Figure 3, because the impedance of the higher harmonic capacitance is small, abnormal current flows in, which often leads to overheating of the cable. The leakage protection device often trips according to the leakage current of 20 mA. The airport operator will set the leakage protection with the power supply bureau. The value is improved, although the leakage protection does not jump, but what happens if the real leakage causes harm to the person? Although the personal injury of the unit was not caused, the tripping of the differential protection of the busbar caused by the large harmonic current caused the system to be dismissed, resulting in a larger blackout.

3 The key to using the inverter is to control the harmonics.

3.1 Setting up an AC reactor

When a dedicated transformer is not installed on the input side of the inverter, an AC reactor (ACL) is connected to the input side to increase the rectification impedance, which suppresses the harmonic current. When the rectified impedance is increased, the rectification overlap angle U is increased to make the harmonic current smaller.

If the calculated voltage distortion rate does not meet the requirements of the national standard, the reactance value installed on the input side must be obtained according to the curve of Figure 4 to meet the requirements, but pay attention to increase the impedance, increase the voltage drop, so the voltage drop Must be controlled within 2%, it is best to find the voltage distortion rate and voltage drop are within the optimal value.

3.2 Setting AC Active and Passive Filters

The AC filters typically used in power circuits have tuned filters and quadratic high order filters, as shown in Figure 5. The tuned filter is suitable for the absorption of a single higher harmonic, while the high-order filter is suitable for the absorption of a plurality of higher harmonics. Generally, the two are combined and used as one device. The AC filter divides the higher harmonic components from the inverter from the impedance of the power system, so it is important to know the impedance of the power supply. In actual use, a method of setting a reactor on the power supply side to increase the impedance of the power supply is used. In addition, the fundamental power capacitance of the AC filter must be limited to the allowable value.

When designing an AC filter, it is necessary to know the amount of generation of higher harmonics (including non-theoretical harmonics) and to grasp the operating conditions of the system (variation of system impedance and filter transition state accompanying transformer inrush current), it is necessary to set in each Calculate in the environment.

3.3 Multiplexing of rectifiers

As mentioned above, for large-capacity thyristor inverters, it is possible to multiplex the rectifiers, that is, to divide the power-side rectifier into two, and to install Y, yd or D, yd winding transformers on the input side, and to suppress the flow to the power supply by multiplexing. The higher harmonics of the side. Because the rectifier needs to be separated, it is not used in general-purpose inverters.

When a plurality of inverters are provided and a dedicated transformer is provided at the input thereof, as shown in Fig. 6, the phase of the input current is shifted by the input transformer, which is also effective for suppressing harmonics.

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