Keywords : wind power generation, low voltage ride through LVRT, Profibus_DP protocol, Can open protocol
0 Preface
In recent years, with the rising prices of traditional energy sources and the resulting rising costs of power generation and the impact of environmental issues such as global warming, the development and utilization of renewable energy has risen to an unprecedented level. Wind power generation is one of the most mature, large-scale development and commercial development prospects of new energy development technology in the world today. The wind has the characteristics of random change, and the output power of the wind turbine is proportional to the cube of the wind speed, so the output power of the wind turbine usually changes greatly and rapidly with the wind speed. If a large amount of wind power is connected to the grid, it will affect the power quality of the grid. 3 grid stability and impact. Therefore, on the premise of controlling the proportion of wind power capacity in the system, it is very important to analyze the influence factors of wind power generation on the grid voltage and control it.
    Therefore, we need a device that can send out warning signals according to the characteristics of the wind power generation system, such as grid failure, grid frequency, excessive voltage deviation, excessive output power of the generator, reverse occurrence of active and reactive power flows, Remind the controller to take timely measures. This article introduces an AGP measurement and protection module designed for wind power generation systems. This module can measure traditional electrical parameters such as voltage, current, frequency, energy, etc., and alarms for system voltage, frequency, load and other faults. It also integrates 2 Under-voltage protection according to time improves the anti-interference ability of the control system against voltage flicker.
1 Circuit design principle
    AGP hardware circuit includes main control chip, power supply, voltage, current signal acquisition circuit, digital input module, relay output module, human-computer interaction unit, RS485 communication interface, Profibus_DP communication protocol interface, Can open communication interface (Figure 1).
Figure 1 Block diagram of hardware circuit
1.1 Main control chip
1.2 Power
    AGP adopts DC 24 V working power supply, uses Guangzhou Jinshengyang's wide voltage input DCDC module WRF2405P, working temperature range -40 ~ 85 ℃, isolation voltage 3000VDC, measured output ripple <1%, and the design of the power input part is added with discharge tube , PTC varistor, TVS tube, anti-reverse diode and other devices (Figure 2), with overvoltage, overcurrent and other protection.
Figure 2 Power circuit
1.3 Signal acquisition circuit (Figure 3)
    Signal acquisition includes voltage signal, current signal and frequency signal: voltage signal adopts voltage divider resistance input, current signal adopts transformer isolation input, after the AC signal is raised, the signal is amplified by the amplifier circuit, and finally the signal is sent to the CPU for Software differential operation.
Figure 3 Current signal circuit
1.4 Interface design
    AGP interfaces include human-computer interaction unit, RS485 communication interface, and switch input and output interfaces. While designing various interfaces, it is necessary to add components such as electromagnetic compatibility performance, voltage resistance, and contact protection to improve the reliability of the device.
2 Electric parameter calculation and software design
2.1 Calculation of fundamental wave, harmonic wave, phase angle difference, etc.
Definition of DFT
among them
The DFT is defined as Equation expanded formula
Write the system of equations in matrix form
Represented by vector Â
X = Wx
Expressed in plural:
    Represents can be seen, since a calculated X (k) value requires complex multiplications and N times (N-1) complex additions, thereby calculating the N X (k) value, total of N2 complex multiplications and N times (the matrix form N-1) Multiple addition. Each complex multiplication includes 4 real number multiplications and 2 real number additions, and each complex addition includes 2 real number additions, so calculating a DFT of N points requires a total of 4N2 real number multiplications and (2N2 + 2N · (N-1)) times Real number addition. When N is large, this is a very large amount of calculation.
    From the practical application, in order to meet the requirements of fast response of the wind power system, j can take 64 points, N only takes 2, only calculates the fundamental wave voltage, current and phase angle difference and other parameters, under the same conditions, the DFT calculation time is not optimized (Figure 4) Compared with the optimized DFT calculation time (Figure 5), after testing, it only takes 40us to calculate a single signal for one DFT calculation, which greatly improves the calculation speed
 Â
Figure 4 Unoptimized DFT operation time Figure 5 Optimized DFT operation time
2.2 Asymmetric fault calculation based on symmetric component method
Figure 6 Negative sequence voltage solution
   Â
Figure 6 shows the solution of negative sequence voltage. Similarly, the positive sequence component U + of voltage can be obtained.
Calculation of voltage unbalance:
U + ----- Positive sequence component of three-phase voltage;
U- ----- Negative sequence component of three-phase voltage;
As shown in the table below, after verification, the actual measured value error is less than 0.1%.
Table 1 Unbalance rate in various situations
Apply source signal | Theoretical unbalance rate (%) | Measured unbalance rate (%) | ||
Phase A | Phase B | Phase C | ||
220 | 0 | 0 | 100 | 99.9 |
220 | 220 | 0 | 50 | 49.9 |
220 | 220 | 110 | 20 | 20.0 |
2.3 Calculation of voltage distortion rate
Calculation of true RMS voltage:
Fundamental current calculation:
U 1 ----- Fundamental current
K ------ Fundamental current calibration factor
U DFT1 ------ The modulus of the 1st component of this DFT
Calculation of total harmonic distortion coefficient:
Calculation of voltage distortion rate:
2.4 Software process
    The software process of AGP mainly includes A / D signal acquisition program, TPM frequency measurement program, electrical parameter calculation program, protection processing program, various communication protocol processing programs, etc. Due to the large content, part of the program flow is now given.
Figure 7 Main program flowchart
    Each channel of the MCF51EM256 AD module has A and B channel inputs. After the acquisition of any channel is completed, the automatic switching time is adjusted by the built-in PDB module to achieve the voltage and current phase angle adjustment to achieve the power compensation function. This method is simple and feasible, interrupted At the same time, it is necessary to deal with AD exceptions, and now give the AD interrupt processing program flow.
Figure 8 Interrupt program flow chart
3 Technical regulations and applications of wind power generation systems
    With the continuous expansion of the installed capacity of wind power generation, the State Grid Corporation has put forward a series of requirements for wind turbines, "GB / T 19069-2003 Technical Requirements for Wind Turbine Generator Controllers" and "Implementation Rules for Technical Regulations for Wind Farm Access to the Grid -2009 "clarified the functions that the controller needs to have. Mainly include: grid frequency control, reactive power and grid voltage control, low voltage ride through (LVRT) control and power quality control.
3.1 Operating frequency of wind farms
Table 2 Wind farm operation at various frequencies
Grid frequency range | Claim |
Below 48Hz | It depends on the lowest frequency that the wind turbine in the wind farm is allowed to run. |
48Hzï¼49.5Hz | Every time the frequency is lower than 49.5Hz, it is required to be able to run for at least 10 minutes . |
49.5Hz-50.5Hz | continue to operate. |
50.5Hzï¼51Hz | Every time the frequency is higher than 50.5Hz, it is required to be able to run for at least 2 minutes ; and when the frequency is higher than 50.5Hz, the wind turbine in the stopped state is not allowed to be connected to the grid. |
Higher than 51Hz | Operate according to the power limit of the power grid dispatch department. |
3.2 Wind farm voltage range
    Wind power is required when the voltage deviation of the grid connection point of the wind farm is between -10% and + 10%, and the flicker value of the grid connection point meets the national standard power quality regulations on voltage fluctuation and flicker, harmonics of the public power grid, and three-phase voltage imbalance. The wind turbine in the field should be able to operate normally.
3.3 Wind farm low voltage ride through
    When the voltage of the grid connection point of the wind farm is in the area of ​​the voltage contour line and above in the figure, the wind turbine in the field must ensure uninterrupted grid operation; when the voltage of the grid connection point is below the voltage contour line in the figure, the wind turbine in the field is allowed to cut off from the grid Out 2 .
Figure 9 Regulations for wind farm low voltage ride through requirements
    For different European and American national grid companies, the prescribed low voltage drop amplitude and ride-through time are also different, 15% and 140 ms in the United Kingdom , 15% and 625 ms in Germany , 25% and 100 ms in Denmark , and Spain. 0% and 500 ms etc.
3.4 The current grid-connected system of doubly-fed wind turbines
Figure 10 AC excitation variable speed constant frequency wind power inverter circuit diagram
    Variable-speed constant-frequency power generation introduces advanced power electronics technology into generator control. The unit uses variable-speed operation to make the speed of the impeller of the wind turbine change with the change of wind speed, and maintain a substantially constant optimal tip speed ratio to obtain the maximum Wind energy utilization efficiency.
    During the operation of the variable-speed constant-frequency double-fed generator set, the stator winding is directly connected to the power grid, and the rotor winding is connected to the slip frequency power supply to achieve AC excitation. When the rotor frequency of the generator changes, the excitation current frequency is controlled to ensure that the stator output frequency is constant 4 .
3.5 Application Case
    AGP can be applied to various types of generator windings. Figure 10 below is a typical application case of the winding structure of a three-phase four-wire generator. The voltage signal is directly connected, and the current signal is connected to the module after being transformed by the transformer. Set relay 1 to overload, overvoltage, and overfrequency, relay 2 to undervoltage and underfrequency, relay 3 to reverse power, relay 4 to undervoltage protection A and B according to time, power the module to DC24V, and the device starts self-check. When the device fails the self-test, an alarm signal is issued and the generator set is prohibited from starting. When a fault occurs during the operation of the generator set, the controller executes the deceleration operation after receiving the relay 1 alarm. When the controller receives the relay 2 alarm, adjust the internal parameters of the control to make it normal. After receiving the alarm from relay 3, the generator stops and the grid-connecting switch is opened. When the grid voltage drops significantly, the module automatically calculates the depth and time of the drop, determines whether it can cross the low voltage, and gives the relay 4 diagnostic signal.
Figure 11 AGP300 typical application diagram
4 Conclusion
    AGP wind power measurement and protection module adopts advanced design scheme, can provide measurement and protection functions for different types of wind turbines (double-fed, permanent magnet direct drive), supports Modbus_RTU, Profibus_DP, Can open communication protocols, compatible with various PLC controls system. The product is stable and reliable, and is an ideal product for the nationalization of wind power equipment.
The article comes from: "Automation Expo" 20 12 years 9 issues.
references:
[1] GB / T 19069-2003 wind turbine generator controller technical conditions "
[2] "Implementation Rules of Technical Regulations for Wind Farm Access to Power Grid-2009"
[3] Wind power generation test technology Yao Xingjia waits Electronic Industry Press
[4] Flexible feed-in operation and control of doubly-fed wind turbine generator Ren Yongfeng An Zhongquan et al. Machinery Press
[5] Design, operation and maintenance of wind power generation system. Ye Hangye is waiting for Electronic Industry Press
[6] Ren Zhicheng, Zhou Zhong, Principles and Application Guide of Electric Power Measurement Digital Instruments, China Electric Power Press, 2007
About the Author:
Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system
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