Positive and negative sequence separation control method during asymmetric fault

Abstract

The invention discloses a positive and negative sequence separation control method in an asymmetric fault. The method comprises the following steps: collecting a power grid line voltage and an AC side feedback current; 90-degree phase angle deviation is carried out on an input signal through an all-pass filter, and power grid voltage and feedback current are decomposed into a positive sequence component and a negative sequence component through a symmetric component method; according to the voltage drop depth, a required current given value is calculated according to national standard requirements; after errors obtained after comparison with the corresponding feedback values are adjusted by a PI adjuster, uP * d, uP * q, uN * d and uN * q are output; calculating with the grid voltage disturbance feed-forward compensation item of each group to obtain an alternating current side reference voltage, and performing coordinate transformation to obtain a component value in a coordinate system; sPWM modulation is carried out after midpoint balance control, and a driving signal is generated to control an inverter so as to complete positive and negative sequence separation control. The method has the beneficial effects that the filtering characteristic is good; the adaptability is high; the calculated amount is relatively small; the phase maintaining characteristic is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of power control, and particularly relates to a positive and negative sequence separation control method during asymmetrical faults. Background Art

[0002] Development Status and Challenges of New Energy Power Generation

[0003] In recent years, with the global emphasis on environmental protection and sustainable development, new energy power generation such as wind power and photovoltaic power has developed rapidly. New energy, with its clean and renewable characteristics, has become an important direction for future energy development. With the continuous increase in the penetration rate of new energy, power generation stations based on traditional synchronous generators will gradually be replaced by distributed micro-power sources and large-scale new energy power generation stations. New energy power generation units usually adopt grid-following and grid-forming control strategies to access the power system.

[0004] However, new energy power generation equipment has characteristics such as low inertia and weak damping, which pose great challenges to the safe and stable operation of the new power system. Traditional synchronous generators have a large moment of inertia and can store and release energy when the power grid is disturbed, playing a role in stabilizing the grid frequency and voltage. However, most new energy power generation equipment is connected to the grid through power electronic converters, and it does not have the inertial characteristics of traditional synchronous generators, resulting in a reduction in the overall inertia level of the power system. When a fault or disturbance occurs in the power grid, the change speed of the system frequency and voltage accelerates, and it is difficult for new energy power generation equipment to respond and adjust quickly, easily leading to system instability.

[0005] Harm of Grid Asymmetrical Faults

[0006] In the actual operation of the power system, power grid faults are inevitable, and asymmetrical faults are relatively common, such as single-phase grounding faults and two-phase short-circuit faults. These asymmetrical faults will cause the grid voltage to be unbalanced, generating positive sequence, negative sequence, and zero sequence components. The existence of the negative sequence component will have a serious impact on the equipment in the power system.

[0007] For generators, negative sequence current will generate additional losses and heat in the rotor, accelerate the aging of the rotor insulation material, reduce the service life of the generator. Being in this operating state for a long time may also trigger serious faults such as rotor winding short circuits, affecting the normal operation of the generator, and even causing the generator to shut down, resulting in large-scale power outages. Motors will also be affected by the negative sequence component, generating a reverse torque, interacting with the forward torque, resulting in unstable output torque, speed fluctuations, and reduced efficiency of the motor. At the same time, the additional losses and heat will increase the temperature of the motor, accelerate the damage of the motor insulation, increase the maintenance cost and failure rate of the motor. In addition, the grid voltage imbalance caused by asymmetric faults will also affect the power quality of electricity users. For some precision equipment with high requirements for voltage quality, such as computers, communication equipment, medical equipment, etc., it may cause abnormal equipment operation, data loss, and even equipment damage, bringing huge economic losses to users.

[0008] Limitations of traditional control strategies in dealing with asymmetric faults

[0009] When traditional control strategies are used to deal with asymmetric voltage faults, it is often difficult to achieve effective control of power equipment. Most traditional control methods are designed based on a symmetric three-phase system and can achieve good control of power equipment during normal grid operation. However, under asymmetric faults, if traditional single control methods are used, it may not be possible to simultaneously meet the precise control of active power, reactive power, and current.

[0010] For example, during asymmetric faults, the voltage imbalance in the grid will cause power fluctuations and current distortion. Traditional control strategies may not be able to accurately compensate for these imbalances and distortions, resulting in unstable equipment operation and even problems such as overcurrent and overvoltage, threatening equipment safety. Moreover, the response speed of traditional control strategies in dealing with faults is slow, unable to adapt to the changes in grid faults in a timely manner, and unable to effectively suppress the spread and impact of faults.

[0011] Requirements of grid connection guidelines for new energy power generation equipment

[0012] To ensure the stability and reliability of the power system, countries have formulated strict grid connection guidelines, requiring new energy grid-connected power generation equipment such as wind power and photovoltaic to have the ability to ride through voltage asymmetric faults. For example, it is stipulated that when the grid voltage drops, the power generation equipment does not disconnect from the grid and continues to operate within a certain voltage drop range and time interval, and can even provide reactive power support to the grid.

[0013] This is because the proportion of new energy power generation in the power system is increasing. If new energy power generation equipment cannot operate reliably during grid faults and a large number of them disconnect from the grid, it will further exacerbate the degree of grid faults and even trigger systemic power outages. Therefore, new energy power generation equipment needs to have the ability to ride through faults, maintain connection with the grid during grid faults, adjust its own output power through reasonable control strategies, provide necessary support for the grid, and help the grid resume stable operation. However, there are still certain deficiencies in the existing control methods in meeting the requirements of grid connection guidelines, and more effective control strategies need to be developed to improve the fault ride-through ability of new energy power generation equipment.

[0014] In summary, developing an efficient and accurate positive and negative sequence separation control method during asymmetrical faults has important practical significance. The present invention aims to solve the problems existing in the existing positive and negative sequence separation methods and improve the operation stability and reliability of the power system during asymmetrical faults. Summary of the Invention

[0015] To solve the above problems, especially the deficiencies of the existing technologies, the present invention provides a positive and negative sequence separation control method during asymmetrical faults that can solve the above problems.

[0016] To achieve the above object, the present invention adopts the following technical means:

[0017] A positive and negative sequence separation control method during asymmetrical faults, comprising the following steps:

[0018] Step 1: Collect the grid line voltages u AB , u BC , u CA , the AC side feedback currents i a , i b , i c ;

[0019] Step 2: Adopt an improved positive and negative sequence separation method, perform a 90-degree phase angle shift on the input signal through an all-pass filter, and then use the symmetrical component method to decompose the grid voltage and feedback current into positive sequence components u P d , u P q , i P d , i P q and negative sequence components u N d , u N q , i N d , i N q ;

[0020] Step 3: Calculate the required current reference value \(i\) according to the voltage drop depth generated during the unbalanced grid voltage fault, in accordance with the national standard requirements. P* d , \(i\) P* q , \(i\) N* d , \(i\) N* q ;

[0021] Step 4: The errors after being respectively compared with the corresponding feedback values are adjusted by a PI regulator and then output \(u\) P* d , \(u\) P* q , \(u\) N* d , \(u\) N* q ;

[0022] Step 5: Then, after performing arithmetic operations with the grid voltage disturbance feed-forward compensation terms \(u\) P d , \(u\) P q , \(u\) N d , \(u\) N q of each group, the AC-side reference voltage \(u\) P out_d , \(u\) P out_q , \(u\) N out_d , \(u\) N out_q is obtained. After coordinate transformation, the component values in the ABC coordinate system are obtained and synthesized to obtain \(u\) out_a , \(u\) out_b , \(u\) out_c ;

[0023] Step 6: Thus, after midpoint balance control, SPWM modulation is performed to generate drive signals to achieve the control of the inverter and complete the positive and negative sequence separation control.

[0024] A further solution of the present invention is that in the above Step 2, the all-pass filter expression is:

[0025]

[0026] In the formula, \(s\) is the complex frequency variable and \(\pi\) is the pi.

[0027] A further solution of the present invention is that in the above Step 2, the symmetrical component method formula is:

[0028]

[0029] where \(u\) αβ is the grid voltage in the stationary coordinate system; \(u\) + αβ , \(u\) - αβ are the corresponding positive - sequence and negative - sequence components; \(-j\) represents a 90 - degree phase shift (lag) of the original signal in the time domain. From the above formula, it can be seen that to extract the positive - sequence component of the grid voltage, a 90 - degree phase - angle shift of the input voltage signal is required to obtain two - phase orthogonal voltage signals. Generally speaking, this 90 - degree phase shift can be achieved by methods such as the T / 4 delay method, the differential method, etc. When the frequency changes, the dynamic response speed of the T / 4 delay method is slow, and the differential method is prone to generating noise when the system contains harmonics. Therefore, a method of using an all - pass filter for phase shift at 50 Hz is adopted.

[0030] Advantages of the present invention:

[0031] 1. The present invention has good filtering characteristics:

[0032] Accurately separate signals: The all - pass filter can accurately separate the positive - and negative - sequence components from complex three - phase signals according to the frequency characteristics of the signals. For three - phase unbalanced signals in the power system, etc., it can accurately extract the positive - and negative - sequence parts, providing accurate data for subsequent analysis and processing;

[0033] Suppress noise interference: During the process of separating the positive - and negative - sequences, the all - pass filter has a certain inhibitory effect on noise. It can effectively filter out noise components with frequencies different from those of the positive - and negative - sequence signals through specific frequency - selection characteristics, improving the quality and reliability of the signals; for example, in a power system with harmonic interference, the all - pass filter can suppress the influence of harmonics on measurement and analysis while separating the positive - and negative - sequences.

[0034] 2. The present invention has strong adaptability:

[0035] Adapt to different working conditions: The all - pass filter can adjust parameters according to different application scenarios and signal characteristics, and has strong adaptability. Whether it is a small unbalance degree during the normal operation of the power system or a large unbalance degree during a fault, etc., it can better achieve positive - and negative - sequence separation. For example, when a short - circuit fault occurs in the power system, by adjusting the parameters of the all - pass filter, the positive - and negative - sequence components after the fault can still be accurately separated, providing a basis for fault analysis and protection actions;

[0036] Be compatible with various signal forms: It can effectively separate the positive - and negative - sequences of different forms of three - phase signals, whether they are sine - wave signals or non - ideal signals with certain distortions; for some signal distortion situations caused by load characteristics, etc., the all - pass filter can still play a role and accurately extract the positive - and negative - sequence components.

[0037] 3. The calculation amount of the present invention is relatively small:

[0038] Simplify algorithm implementation: Compared with some other complex positive and negative sequence separation methods, the algorithm using all-pass filters for separation is relatively simple and has a small calculation amount; in actual engineering applications, especially in occasions with high requirements for real-time performance, such as real-time monitoring and control of power systems, a small calculation amount can reduce the burden on the processor and improve the response speed and processing efficiency of the system;

[0039] Easy to implement in engineering: The structure and algorithm of the all-pass filter are relatively simple and do not require a large number of matrix operations or complex transformation operations; this makes it easier to implement in hardware, reduces the hardware cost and design difficulty, and is convenient for wide application in various power equipment and monitoring systems.

[0040] 4. The present invention has the characteristic of phase preservation:

[0041] Accurately reflect the phase relationship: During the process of separating positive and negative sequence signals by the all-pass filter, the phase information of the signal can be well preserved; the phase relationship of positive and negative sequence components is very important for analyzing the operating state and fault conditions of the power system. The all-pass filter can ensure that the positive and negative sequence signals after separation are consistent with the positive and negative sequence components in the original signal in terms of phase, providing an accurate basis for subsequent phase analysis and fault location, etc.;

[0042] Conducive to subsequent analysis and processing: Accurate phase information is crucial for some analysis methods and protection algorithms based on phase comparison; for example, in the relay protection of power systems, by comparing the phase relationship of positive and negative sequence components, the type and location of faults can be judged. The phase preservation characteristic of the all-pass filter can provide a reliable basis for these analyses and algorithms, improving the accuracy and reliability of protection. Description of the Drawings

[0043] Figure 1 is the flowchart of the present invention. Detailed Embodiments

[0044] The technical solutions of the present invention will be clearly and completely described below with reference to the drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0045] Embodiment 1

[0046] As Figure 1 shown, a positive and negative sequence separation control method during an asymmetric fault includes the following steps:

[0047] Data acquisition step: Use high-precision voltage sensors and current sensors to collect the grid line voltage, u AB ,u BC ,u CA ,and the feedback current on the AC side, i a ,i b ,i c 。The accuracy of the sensors needs to meet the requirements of power system measurement to ensure the accuracy of the collected data. The collected data is transmitted to the subsequent processing module through the data transmission line.

[0048] Positive and negative sequence separation step: Input the collected voltage and current signals into the positive and negative sequence separation module. The all-pass filter in this module performs a 90-degree phase angle shift on the input signal according to the preset parameters. The parameters of the all-pass filter can be adjusted according to the operating frequency of the power system and the actual required filtering effect. For example, in a 50Hz power system, by reasonably designing the resistance, capacitance and other component parameters of the all-pass filter, the phase angle shift of the 50Hz signal and its harmonics is achieved. After completing the phase angle shift, use the symmetrical component method to decompose the grid voltage and feedback current into positive sequence components u P d ,u P q ,i P d ,i P q and negative sequence components u N d ,u N q ,i N d ,i N q 。The calculation process of the symmetrical component method is carried out according to its mathematical principle, and the three-phase unbalanced signal is decomposed into positive sequence, negative sequence and zero sequence components through matrix operation. Only the positive sequence and negative sequence components are extracted for subsequent processing in this process.

[0049] Current reference calculation step: According to the voltage dip depth generated during the grid voltage unbalance fault, determine the voltage dip depth by measuring the difference between the actual value and the rated value of the grid voltage. According to the national standard requirements, different voltage dip depths correspond to different current reference value calculation methods. For example, when the voltage dip depth is within a certain range, calculate the required positive sequence current reference value i P* d ,i P* q and negative sequence current reference value i N* d ,i N* q. The national standards stipulate detailed calculation criteria and constraint conditions to ensure the stable operation of the power system and the safety of equipment under fault conditions.

[0050] PI regulation steps: The calculated current reference values i P* d , i P* q , i N* d , i N* q are respectively compared with the corresponding feedback values i P d , i P q , i N d , i N q to obtain the error values. These error values are input into the PI regulator, and the PI regulator adjusts the error according to its proportional coefficient and integral coefficient. The proportional coefficient and integral coefficient of the PI regulator need to be set according to the dynamic characteristics and control requirements of the power system. For example, through experimental tests and simulation analyses, the proportional coefficient and integral coefficient that can make the system quickly stable and meet the control accuracy requirements under different load conditions and fault situations are determined. The adjusted output is u P* d , u P* q , u N* d , u N* q .

[0051] Voltage operation and transformation steps: The values u P* d , u P* q , u N* d , u N* q output by the PI regulation module are operated with the grid voltage disturbance feed-forward compensation terms u P d , u P q , u N d , u N q . The operation method is determined according to the specific control strategy, for example, it can be an addition operation. After the operation, the reference voltage u P out_d , u P out_q , u N out_d, u N out_q . Then, these reference voltages are subjected to coordinate transformation, transformed from the dq coordinate system to the ABC coordinate system, to obtain the corresponding component values. The process of coordinate transformation is carried out according to the mathematical formulas of Clarke transformation and Park transformation, and the coordinate conversion is achieved through matrix operations. Finally, these component values are synthesized to obtain u out_a , u out_ b , u out_c .

[0052] Control signal generation step: The synthesized voltage u out_a , u out_b , u out_c is subjected to midpoint balance control to ensure the balance of the three-phase voltages output by the inverter. Midpoint balance control can adopt various methods, such as adjusting by detecting the capacitor voltage. After completing the midpoint balance control, SPWM modulation is carried out. SPWM modulation generates a series of pulse width modulation signals according to the principle of comparing a sine wave with a triangular wave. These pulse width modulation signals are processed through power amplification and other processes to generate drive signals to achieve the control of the inverter, thereby completing the positive and negative sequence separation control.

[0053] Embodiment 2

[0054] A positive and negative sequence separation control system during an asymmetric fault, comprising: a data acquisition module, a positive and negative sequence separation module, a current reference calculation module, a PI regulation module, a voltage operation and transformation module, and a control signal generation module;

[0055] Data acquisition module:

[0056] For acquiring the grid line voltages u AB , u BC , u CA and the AC side feedback currents i a , i b , i c , and the acquired original data is sent to the positive and negative sequence separation module and the current reference calculation module through a data transmission line;

[0057] Positive and negative sequence separation module:

[0058] Receives the grid voltage and current signals from the data acquisition module, uses an all-pass filter to perform a 90-degree phase angle shift on the input signals, and then decomposes these signals into positive sequence components u P d , u P q , i P d , i P qand the negative-sequence component u N d , u N q , i N d , i N q ;

[0059] The positive-sequence and negative-sequence components obtained by decomposition are respectively sent to the current reference calculation module and the PI regulation module, providing key positive- and negative-sequence information for the subsequent module operations;

[0060] Current reference calculation module:

[0061] On the one hand, it receives the positive-sequence and negative-sequence components from the positive- and negative-sequence separation module. On the other hand, according to the voltage drop depth generated during the grid voltage asymmetry fault, it calculates the current reference value according to the national standard requirements;

[0062] The calculated positive-sequence current reference value i P* d , i P* q and the negative-sequence current reference value i N* d , i N* q are sent to the PI regulation module as the set values of the PI regulator;

[0063] PI regulation module:

[0064] Receives the current reference values i P* d , i P* q , i N* d , i N* q from the current reference calculation module and the corresponding feedback values i P d , i P q , i N d , i N q ;

[0065] Compares the current reference value with the feedback value to obtain an error value, and then performs PI regulation on the error value. The regulated output u P* d , u P* q , u N* d , u N* qSent to the voltage operation and transformation module;

[0066] Voltage operation and transformation module:

[0067] Receives the output value u from the PI regulation module P* d , u P* q , u N* d , u N* q And the grid voltage disturbance feed-forward compensation term u received from the positive and negative sequence separation module P d , u P q , u N d , u N q ;

[0068] Performs an operation on the value output by the PI regulation module and the grid voltage disturbance feed-forward compensation term to obtain the AC side reference voltage u P out_d , u P out_q , u N out_d , u N out_q ;

[0069] Then performs a coordinate transformation on the AC side reference voltage, transforming from the dq coordinate system to the ABC coordinate system to obtain the corresponding component values u out_a , u out_b , u out_c And sends these component values to the control signal generation module;

[0070] Control signal generation module:

[0071] Receives u from the voltage operation and transformation module out_a , u out_b , u out_c , performs midpoint balance control on these synthesized voltages to ensure the balance of the three-phase voltages output by the inverter;

[0072] After completing the midpoint balance control, performs SPWM modulation, generates a series of pulse width modulation signals according to the modulation result, and after processing such as power amplification, generates drive signals, and finally realizes the control of the inverter to complete the positive and negative sequence separation control.

[0073] Example 3

[0074] Application in the scenario of a small distributed photovoltaic power station

[0075] An asymmetrical fault positive and negative sequence separation control method, comprising the following steps:

[0076] Data acquisition: In a certain small-scale distributed photovoltaic power station, voltage sensors with an accuracy of 0.2 level and current sensors with an accuracy of 0.5 level are used. The voltage sensors are installed on the line connecting the AC output terminal of the photovoltaic inverter to the power grid for collecting the grid line voltages u AB ,u BC ,u CA 。The current sensors are installed on the output line of the inverter AC side for collecting the feedback currents i a ,i b ,i c 。The collected data is transmitted to the subsequent processing unit through the RS-485 communication bus.

[0077] Positive and negative sequence separation: A second-order active all-pass filter is adopted, and metal film resistors with an accuracy of 1% are selected for the resistors, and polypropylene capacitors are selected for the capacitors. By adjusting the resistor and capacitor values, an accurate 90-degree phase angle shift is achieved at a frequency of 50 Hz. For example, the resistors R 1 =R 2 =10 kΩ, and the capacitors C 1 =C 2 =0.159 μF. After the phase shift by the all-pass filter, a symmetrical component method calculation unit based on a digital signal processor (DSP) is used to decompose the grid voltage and the feedback current into positive and negative sequence components.

[0078] Current reference calculation: When a single-phase grounding fault occurs in the power grid, resulting in a voltage drop depth of 15%, according to the national standard regulations, for a photovoltaic power station of this capacity, the positive sequence current reference value i P* d needs to maintain 80% of the rated current, and i P* q is 0; the negative sequence current reference value i N* d is 10% of the rated current, and i N* q is 0.

[0079] PI regulation: A PI regulator with a proportionality coefficient K p =0.5 and an integral coefficient K i =0.01 is selected. The error after comparing the current reference value with the feedback value is input into the PI regulator, and after regulation, the corresponding voltage values u P* d ,u P* q ,u N* d ,u N* q 。

[0080] Voltage operation and transformation: The PI regulation output value and the grid voltage disturbance feedforward compensation term are added to obtain the AC side reference voltage u P out_d , u P out_q , u N out_d , u N out_q . Through the coordinate transformation algorithm integrated in the DSP, it is converted from the dq coordinate system to the ABC coordinate system, and u out_a , u out_b , u out_c .

[0081] Control signal generation: Midpoint balance control is performed on the synthesized voltage. By detecting the DC side capacitor voltage, the output voltage is adjusted using the voltage balance algorithm. Then SPWM modulation is performed, and the generated drive signal controls the IGBT module of the photovoltaic inverter to achieve positive and negative sequence separation control, enabling the photovoltaic power station to still operate stably under asymmetric faults and inject current meeting the requirements into the grid.

[0082] The examples given in the present invention are not intended to limit the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here, and the obvious changes or variations derived therefrom still fall within the protection scope of the present invention.

Claims

1. A positive and negative sequence separation control method for asymmetric faults, characterized in that: The following steps are involved: Step 1: Collect the grid line voltage u AB ,u BC ,u CA , AC side feedback current i a ,i b ,i c ; Step 2: Use the improved positive-negative sequence separation method to perform a 90-degree phase shift on the input signal through an all-pass filter, and then use the symmetrical component method to decompose the grid voltage and feedback current into the positive sequence component u P d ,u P q ,i P d ,i P q And the negative sequence component u N d ,u N q ,i N d ,i N q ; Step 3: According to the voltage drop depth caused by the asymmetric fault of the power grid voltage, calculate the required current given value i according to the national standard requirements P* d ,i P* q ,i N* d ,i N* q ; Step 4: The error after comparison with the corresponding feedback value is adjusted by the PI regulator and then output u P* d ,u P* q ,u N* d ,u N* q ; Step 5: Combine the grid voltage disturbance feedforward compensation term u of each group P d ,u P q ,u N d ,u N q After phase operation, the AC side reference voltage u is obtained P out_d ,u P out_q ,u N out_d ,u N out_q After coordinate transformation, we get the component values ​​in the ABC coordinate system, and synthesize them to get u out_a ,u out_b ,u out_c ; Step 6: After the midpoint balance control, SPWM modulation is performed to generate a drive signal to control the inverter to complete the positive and negative sequence separation control.

2. The positive-negative sequence separation control method for an asymmetric fault according to claim 1 is characterized in that: In step 2, the all-pass filter expression is: Where s is the complex frequency variable and pi is the circumference of a circle.

3. The positive and negative sequence separation control method for asymmetric fault according to claim 1 is characterized in that: In step 2, the formula of the symmetrical component method is: Where u αβ is the grid voltage in the stationary coordinate system; u + αβ ,u - αβ are the corresponding positive-sequence and negative-sequence components; -j means a 90-degree phase shift of the original signal in the time domain.

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