CN109713693B - Three-phase unbalanced load switching method based on commutation switch networking - Google Patents
Three-phase unbalanced load switching method based on commutation switch networking Download PDFInfo
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Abstract
The low-voltage distribution station phase change switch system comprises a main control box, three-phase voltage and current measurement and unbalance degree calculation are carried out, a plurality of phase change boxes carry out uninterrupted switching of load phase sequences, the input end of each phase change box is connected with a A, B, C three-phase power grid, and the output end of each phase change box is connected with a single-phase load; when a plurality of phase change boxes operate in a networking mode, each phase change box collects the current value of each connected load and transmits the current value back to the main control switch box through 433MHz wireless communication, the main control switch box calculates the three-phase unbalance degree and decides which switch box needs to be switched, specifically which phase is switched to, and the optimal switch combination is selected; the method improves the accuracy of the calculation of the three-phase unbalance; the method aims to reduce the loss of the switching device, ensure the long-term safe and stable operation of the phase change box and improve the safety of the phase change switching system and the reliability of power supply.
Description
Technical Field
The invention belongs to the technical field of three-phase unbalance of a power system, and particularly relates to a three-phase unbalance load switching method of a commutation switch network.
Background
With the vigorous development of national economy, the demand of various industries and people's life in China for electric energy is more and more large, the demand of quantity is not only raised, but also the demand of the quality standard of the electric energy is gradually raised. The good electric energy quality can not only ensure the safe and stable operation of the whole power grid and power equipment, but also has very important significance for improving the industrial production and the product quality and the stability and the order of the daily life of people. The three-phase imbalance is one of the important indexes of the power quality. The three-phase imbalance is specifically the value: in the power system, the amplitudes of three-phase currents (or voltages) are inconsistent, and the amplitude difference exceeds a specified range.
At present, most of low-voltage power distribution networks in China are power supply networks with mixed power of three-phase production power and single-phase loads, and the problem of three-phase imbalance of the low-voltage power distribution networks is more and more prominent due to factors such as different load types, uneven load power distribution, non-timeliness of user power consumption, access of high-power single-phase loads, uncontrollable capacity increase of single-phase users and the like. Especially in rural areas and residential areas, etc., such situations are particularly acute. When a three-phase imbalance phenomenon occurs, the three-phase imbalance type power distribution system can be electrified for a period of time in a short period, but the distribution transformer can be burnt out when the power grid operates in an imbalance state for a long time, and great harm is generated. In order to solve the unbalanced three phase problem, the user load needs to be directly operated by taking the medicine from the source according to the symptoms, and based on the thought, the scheme of solving the unbalanced three phase problem by using the commutation switch device becomes a practical and feasible scheme.
When a plurality of commutation switch boxes operate in a networking mode, the traditional networking decision method only adjusts the three-phase unbalance degree to be less than 15%, and the method has obvious defects, and can cause the commutation switch boxes to frequently act, accelerate aging of devices and shorten the service life of the device. Another class of methods employs scheduling optimization algorithms, such as: although the problems can be solved theoretically by the analog transition algorithm, the particle swarm algorithm and the like, the three-phase imbalance problem of the power distribution network is an actual engineering problem, the final decision method is required to be simple and practical, and the method can be used in a microcontroller to practically solve the actual problem, so that the research of a practical three-phase imbalance load switching method for the commutation switch networking is very important.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a three-phase unbalanced load switching method based on commutation switch networking, so as to solve the problems of limitation and poor practicability of the existing method, and further improve the practicability and reliability of multi-commutation switch networking decision.
In order to achieve the purpose, the invention adopts the technical scheme that the three-phase unbalanced load switching method based on the commutation switch network is characterized by comprising the following steps of:
in the formula (2), σ represents the three-phase current unbalance, max { I A ,I B ,I C Denotes the maximum value of three-phase current of phase A, phase B and phase C, min { I } A ,I B ,I C Expressing the minimum value of three-phase currents of the A phase, the B phase and the C phase;
and 9, successfully changing the phase, and finishing the load switching in the current round.
The processing method of the time window N in the step 2 comprises the following steps: setting a time window N in the 1 st group of phase change boxes, the 2 nd group of phase change boxes, the 3 rd group of phase change boxes and the 4 th group of phase change boxes … …, wherein N is a natural number, and N sampling points calculate the current value at the outlet end of the phase change box once according to a root mean square weighted average algorithm to be used as a current measured value I s The sampling time window N can be set to be 2mins, 10mins and 20mins according to the load characteristics, and each switch in the corresponding transformer area only carries out 1 phase change operation, and the current measured value I s The method is implemented according to the following formula,
in the formula (1), I s Representing the measured value of the current at the outlet end of a phase change box, N representing N sampling points, I n Indicating the current value at each instant.
In the full-wave fourier algorithm of step 2, the sampling frequency is 1600HZ, and 32 points are sampled in one period for calculation.
The invention has the beneficial effects that:
the switching method of the invention calculates the current value to be uploaded through the set time window, rather than only taking a section current value at a certain moment as a basis, thus improving the accuracy of calculating the three-phase unbalance; secondly, in order to reduce the loss of the switching device and ensure the safe and stable long-term operation of the phase change box, the method considers the requirement that the unbalance degree of three phases is less than 15 percent in implementation and enables less phase change switch boxes to act as far as possible. The safety and the power supply reliability of the phase change switch system are improved.
The three-phase unbalanced load switching method based on the commutation switch networking provided by the invention adopts a full-wave Fourier algorithm to calculate the current value and adopts a method of setting a time window to process the current value. The three-phase unbalance of less than 15% is used as an optimization target, the load transfer direction and the load size to be transferred are determined, only one or 2 phase change boxes are selected for switching in each adjustment, the phase change boxes are prevented from frequently acting, and the three-phase unbalance can meet the requirements. The method is feasible, and the practicability and reliability of the phase change switch system are improved.
Drawings
Fig. 1 is a general schematic diagram of the commutation switch system of the present invention.
FIG. 2 is a flow chart of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the following drawings and specific examples, but the present invention is not limited to these embodiments.
The method for switching the three-phase unbalanced load based on the commutation switch networking is implemented according to the following steps as shown in fig. 2:
in the formula (2), σ represents the three-phase current unbalance, max { I A ,I B ,I C Denotes the maximum value of A, B, C phase three-phase current, min { I } A ,I B ,I C Represents A, B, C phase three-phase current minimum value.
and 9, successfully changing the phase, and finishing the load switching in the current round.
One end of phase line A, B and C is connected with 10KV distribution and transformation line 11 through transformer 10.
The time window N processing method in the step 2 comprises the following steps: setting a time window N in a commutation box I, a commutation box II, a commutation box III and a commutation box IV, wherein N is a natural number, and N sampling points calculate the current value of the outlet end of the commutation box once according to a root mean square weighted average algorithm to be used as a current measured value I s The sampling time window N can be set to be 2mins, 10mins and 20mins according to the load characteristics, and each switch in the corresponding transformer area only carries out 1 phase change operation, and the current measured value I s The method is implemented according to the following formula,
in the formula (1), I s Representing the measured value of the current at the outlet end of a phase change box, N representing N sampling points, I n Indicating the current value at each instant.
And (3) calculating the sampling frequency of the current value by the full-wave Fourier algorithm in the step 2 to be 1600Hz, and sampling 32 points in one period for calculation.
The full-wave Fourier algorithm of the step 2 comprises the following steps: the full-wave fourier algorithm is to add waveforms of one period and perform fourier decomposition to obtain the amplitude and phase of the fundamental wave and each harmonic, and to set an input signal as a periodic function:
in formulae (3) and (4), A k And
the amplitude and phase angle of the k harmonic wave, omega being the angular frequency of the fundamental component, a k And b k The amplitudes of the sine and cosine values of the respective order components, T representing time, x (T) representing the function value, k representing the order of the harmonics, and T representing the sampling period.
Sampling N =32 points in one period, sampling frequency 1600Hz, discretizing a current periodic signal to obtain expressions (5) and (6), wherein delta T is sampling interval, delta T = T/N, and k =1 is taken, so that the amplitude A of the fundamental wave can be calculated k The current value at any moment of the outlet end of the phase change box can be calculated,
in the formulae (5), (6) and (7), A k Representing the amplitude of the k harmonic, ω being the angular frequency of the fundamental component, a k And b k The amplitudes of the sine value and the cosine value of each sub-component are respectively, delta T represents a sampling interval, x (N) represents a discretized function value, k represents the number of harmonics, m represents a constant, and N represents the number of sampling points.
Examples
In order to verify the effectiveness of the method, the method is used for carrying out imbalance degree adjustment calculation according to the data of the outlet end of the transformer at a certain time of a data monitoring center of a certain distribution transformer area, the distribution transformer area comprises 10 phase change switch devices (1 main control box and 9 phase change boxes), and the phase sequence and the load current value of the 9 phase change boxes are shown in table 1.
Table 19 branch load current value and phase sequence
By adopting the method, the heavy-load phase is determined to be the C phase, the light-load phase is determined to be the A phase, the load transfer direction is from the C phase to the A phase, the average value of the three-phase current is 40.6A, the load capacity to be transferred is 17.2A, the difference item by item is obtained to obtain the number of the box to be transferred to be the number 8, and the load connection condition after phase change is shown in the table 2.
TABLE 2 phase sequence and degree of unbalance connected to the commutation boxes before and after commutation
Furthermore, the data of the outlet ends of the transformer at 10 moments in two days of a data monitoring center of a certain distribution transformer area are collected, and the effectiveness of the method is verified again. Table 3 shows the three-phase current and unbalance data before the adjustment by the method.
TABLE 3 three-phase current and unbalance data before adjustment
| Numbering | Time | Phase A current | Phase B current | C-phase current | Degree of unbalance of three |
| 1 | 06:48 | 10.68 | 18.9 | 29.1 | 63.3％ |
| 2 | 08:12 | 19.8 | 25.5 | 32.5 | 39.1％ |
| 3 | 11:05 | 21.8 | 42.3 | 57.8 | 62.3％ |
| 4 | 12:42 | 22.9 | 63.7 | 57.6 | 64.1％ |
| 5 | 17:38 | 43.6 | 69.5 | 65.7 | 37.3％ |
| 6 | 21:57 | 60.2 | 73.8 | 90.4 | 33.4％ |
| 7 | 05:18 | 41.8 | 81.1 | 87.8 | 52.4％ |
| 8 | 11:57 | 31.8 | 70.1 | 80.8 | 60.6％ |
| 9 | 15:09 | 32.7 | 64 | 61.6 | 48.9％ |
| 10 | 22:23 | 30.5 | 54.1 | 43.3 | 43.6％ |
The load amounts to be adjusted for the respective phases, the adjusted respective phases of currents, and the three-phase unbalance degrees by the above-described method are shown in table 4.
TABLE 4 adjusted three-phase current and unbalance data
The comparison of tables 1 to 4 shows that the unbalanced load switching method considering the time window can well solve the three-phase unbalanced problem of the power distribution network, and can acquire data as much as possible under the condition of large current fluctuation of the output end, thereby avoiding the one-sidedness of judgment. And only one or two loads are adjusted in each adjustment, so that the action frequency of the switch box is low, the service life of equipment is prolonged, the reduction of the unbalance degree of the three phases of the current is obvious in effect, and the number of the commutation boxes needing to act can be determined. The method is transplanted to a hardware platform of the automatic commutation load balancing device of the low-voltage distribution network to make a multi-commutation-box networking decision.
The foregoing is a preferred embodiment of the present invention, and it will be apparent to those skilled in the art that variations, modifications, substitutions and alterations can be made in the embodiment without departing from the principles and spirit of the invention as described herein.
Claims (3)
1. A three-phase unbalanced load switching method based on a commutation switch network is characterized by comprising the following steps:
step 1, a main control box is respectively connected with a phase line A, a phase line B and a phase line C of a low-voltage distribution network through a phase line TA1, a phase line TA2 and a phase line TA 3; the 1 st group of loads 1 are respectively connected with a phase line A phase, a phase line B phase and a phase line C phase through a1 st group of phase transformation boxes; the load 2 in the 2 nd group is respectively connected with a phase line A, a phase line B and a phase line C through a phase change box in the 2 nd group; the 3 rd group of loads 3 are respectively connected with a phase line A phase line, a phase line B phase line and a phase line C phase line through the 3 rd group of phase change boxes; the 4 th group of loads 4 are respectively connected with a phase line A phase, a phase line B phase and a phase line C phase through a 4 th group of phase transformation boxes; the nth group of loads n are respectively connected with the phase line A, the phase line B and the phase line C through the nth group of phase changing boxes; the main control box is in wireless communication with a1 st group of phase change boxes, a2 nd group of phase change boxes, a3 rd group of phase change boxes and a 4 th group of phase change boxes … … n group of phase change boxes in a mode of converting RS485 into 433MHz radio frequency; the main control box sends a current value calling command to each commutation box every T hours according to the load fluctuation condition;
step 2, the 1 st group of commutation boxes, the 2 nd group of commutation boxes, the 3 rd group of commutation boxes and the 4 th group of commutation boxes … … receive a current value calling command of a main control box, firstly, current values are collected according to a full-wave Fourier algorithm, and then, the current values are processed and uploaded according to a set time window N processing method;
the time window N processing method in the step 2 comprises the following steps: setting a time window N in the 1 st group of phase change boxes, the 2 nd group of phase change boxes, the 3 rd group of phase change boxes and the 4 th group of phase change boxes … …, wherein N is a natural number, and N sampling points calculate the current value at the outlet end of the primary phase change box according to a root mean square weighted average algorithm to be used as a current measured value I s The sampling time window N can be set to be 2mins, 10mins and 20mins according to the load characteristics, and each switch in the corresponding transformer area only carries out 1 phase change operation, and the current measured value I s The method is implemented according to the following formula,
in the formula (1), I s Representing the measured value of the current at the outlet end of a phase change box, N representing N sampling points, I n A current value representing each time;
the full-wave Fourier algorithm in the step 2 has the sampling frequency of 1600HZ, and 32 points are sampled in one period for calculation;
step 3, the main control box uploads the current value I = [ I ] according to the 1 st group of phase change boxes, the 2 nd group of phase change boxes, the 3 rd group of phase change boxes and the 4 th group of phase change boxes … … s1 ,I s2 ,I s3 ,…,I sn ]And the phase line connected with each current value, and calculating the total current I of each phase A ，I B ，I C And (4) calculating the unbalance degree sigma of the three-phase current according to the value, if the sigma is less than or equal to 15%, turning to step 9, if the sigma is more than 15%, turning to step 4, and expressing the target function of the unbalance degree of the three phases as follows:
in the formula (2), σ represents the three-phase current unbalance degree, max { I [ ] A ,I B ,I C Denotes the maximum value of three-phase currents of A phase, B phase and C phase, min { I } A ,I B ,I C Expressing the minimum value of three-phase currents of the A phase, the B phase and the C phase;
step 4, determining the load transfer direction, epsilon 1 ＝max{I A ,I B ,I C Is the heavy-duty phase, ε 2 ＝min{I A ,I B ,I C The transfer direction is the heavy load phase to the light load phase, if epsilon 1 ＝I A ，ε 2 ＝I B If the load transfer direction is from the phase A to the phase B;
step 5, determining the size of the load to be transferred, wherein the average value of the three-phase current is I av ＝(I A +I B +I C ) /3, toThe transfer amount is: Δ = I av -ε 2 ；
Step 6, determining the number of the commutation box to be acted, and screening out epsilon 1 The sum of any one load current value or any two load current values is equal to or close to delta, the difference is determined by using a method of making difference item by item, and the box number represented by the individual or the combination with the minimum delta difference value is selected as the commutation box number to be acted;
step 7, predicting the three-phase unbalance degree sigma after transfer 1 If σ is 1 Turning to step 8 when the concentration is less than or equal to 15 percent; if 15% < sigma 1 Recording the number of the current commutation box to be transferred when the number is less than or equal to sigma, taking the predicted three-phase current value after the transfer as the current three-phase current value, and turning to the step 4;
step 8, the main control box sends a commutation command to each commutation box needing load switching, and the commutation boxes carry out load switching;
and 9, successfully changing the phase, and finishing the load switching in the current round.
2. The commutation switch network-based three-phase unbalanced load switching method of claim 1, wherein the full-wave Fourier algorithm of the step 2 comprises the following steps: the full-wave fourier algorithm is to add waveforms of one period and perform fourier decomposition to obtain the amplitude and phase of the fundamental wave and each harmonic, and to set an input signal as a periodic function:
in formulae (3) and (4), A k And
the amplitude and phase angle of the k harmonic, omega being the angular frequency of the fundamental componentRate, a k And b k The amplitudes of the sine and cosine values of the respective order components, T representing time, x (T) representing the function value, k representing the order of the harmonics, and T representing the sampling period.
3. The method for switching the three-phase unbalanced load based on the commutation switch network according to claim 1, comprising the following steps:
step 1, a main control box is respectively connected with phase lines A, B and C of a low-voltage distribution network through TA1, TA2 and TA 3; the load I1 is respectively connected with the phase lines A, B and C through the phase changing box I; the load II 2 is respectively connected with the phase line A phase, the phase line B phase and the phase line C phase through the phase change box II; the load III 3 is respectively connected with the phase line A, the phase line B and the phase line C through the phase change box III; the load IV 4 is respectively connected with the phase line A, the phase line B and the phase line C through the phase changing box IV; the master control box wirelessly communicates with the phase change box I, the phase change box II, the phase change box III and the phase change box IV in a mode of converting RS485 into 433MHz radio frequency; the main control box sends a current value calling command to each commutation box every T hours according to the load fluctuation condition;
step 2, receiving a current value calling command of a main control box by a phase change box I, a phase change box II, a phase change box III and a phase change box IV, acquiring a current value according to a full-wave Fourier algorithm, and then processing and uploading the current value according to a set time window N processing method;
step 3, the main control box uploads current values I = [ I ] according to the phase change box I, the phase change box II, the phase change box III and the phase change box IV s1 ,I s2 ,I s3 ,…,I sn ]And the phase line connected with each current value, and calculating the total current I of each phase A ，I B ，I C And (4) calculating the unbalance degree sigma of the three-phase current according to the value, if the sigma is less than or equal to 15%, turning to step 9, if the sigma is more than 15%, turning to step 4, and expressing the target function of the unbalance degree of the three phases as follows:
in the formula (2), σ represents unevenness of three-phase currentScale, max { I } A ,I B ,I C Denotes the maximum value of A, B, C phase three-phase current, min { I } A ,I B ,I C Indicating A, B, C phase three-phase current minimum value;
step 4, determining the load transfer direction, epsilon 1 ＝max{I A ,I B ,I C Is the heavy-duty phase, ε 2 ＝min{I A ,I B ,I C The transfer direction is the heavy load phase to the light load phase, if epsilon 1 ＝I A ，ε 2 ＝I B If the load transfer direction is from the phase A to the phase B;
step 5, determining the size of the load to be transferred, wherein the average value of the three-phase current is I av ＝(I A +I B +I C ) And/3, the amount to be transferred is as follows: Δ = I av -ε 2 ；
Step 6, determining the commutation box number to be acted, and screening out epsilon 1 Any one of the load current values or the sum of any two of the load current values is equal to or close to Δ. Determining by using a method of making difference item by item, and selecting the box number represented by the individual or the combination with the minimum delta difference value as the commutation box number to be acted;
step 7, predicting the three-phase unbalance degree sigma after transfer 1 If σ is 1 Turning to step 8 when the concentration is less than or equal to 15 percent; if 15% < sigma 1 Recording the number of the current phase change box to be transferred when the sigma is less than or equal to sigma, taking the predicted three-phase current value after the transfer as the current three-phase current value, and turning to the step 4;
step 8, the main control box sends a commutation command to each commutation box needing load switching, and the commutation boxes carry out load switching;
and 9, successfully changing the phase, and finishing the load switching in the current round.
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