CN107657398A - The computational methods that a kind of power network quality of power supply influences - Google Patents
The computational methods that a kind of power network quality of power supply influences Download PDFInfo
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Abstract
The invention discloses a kind of power network quality of power supply influence computational methods, steps of the method are：Step 1, the power supply plan for determining Traction Station；Step 2, the externally fed scheme for determining several setting websites；Step 3, select the locomotive load of Traction Station horizontal；Step 4, the Harmonic Current Limits and negative-sequence current limit value for calculating according to the setting externally fed scheme of website, load level each setting website of Traction Station injection respectively；Step 5, after calculating harmonic current caused by train operation and negative-sequence current, calculate the negative-sequence current of Traction Station injection setting website and caused non-equilibrium among three phase voltages；The present invention is advantageous to the use and renewal of power quality controlling measure, be also beneficial to assess power network and develop circuit transport, also for relevant department it is determined that externally fed scheme and selection tractive transformer type when decision-making provide reference.
Description
Technical Field
The invention relates to the field of power grid electric energy, in particular to a method for calculating influence of power grid electric energy quality.
Background
In the prior art, a plurality of stations are distributed on a railway branch line along the line from a starting station to a terminal station, and the distances among the stations are different. Some railway lines mainly take passenger transport and consider a small amount of freight transport. Wherein, the passenger plane can adopt SS8 traction, and the long-term transition is carried out to the motor train unit; the cargo aircraft can adopt the HXD3 of AC-DC-AC transmission, and the traction constant number can be 4000t. The number of bus pairs can be 48 near term pairs and 76 far term pairs; the number of truck pairs can also be 17 near term pairs and 7 far term pairs. In order to provide power for the railway branch lines, a traction station is required to be newly built in a certain environment, and the position of the traction substation is set according to the specific railway branch line.
The railway branch line operation brings support and convenience to national economic construction and life of people, and simultaneously brings a severe test to the electric energy quality of a public power grid for supplying power to the railway branch line. The pollution of the electrified railway to the power quality of the power grid is mainly shown in the following steps: the power change causes the voltage change of the power supply bus of the public connection point of the power grid when the locomotive is started, stopped and shifted; harmonic current pollution of a locomotive rectification or variable frequency power supply injected into a power grid; and the negative sequence current injected into the power grid by the locomotive single-phase power supply system causes the unbalance of the three-phase voltage of the bus of the public connection point of the power grid. The degree of influence on the power quality of the power network depends on the one hand on the power quality control measures to be taken and on the other hand also on the external power supply scheme to be used and the internal power supply scheme of the traction station, in particular to a large extent on the type and connection of the traction transformer to be used.
Similar examples are numerous, and it is known from the above background that the calculation and evaluation of the impact on the power of the grid has a great impact on the development of the grid and the development of rail transportation.
Disclosure of Invention
The invention aims to provide a method for calculating the influence of the power quality of a power grid, which calculates the negative sequence current injected into a set station and the caused three-phase voltage unbalance degree of a traction station by selecting an external power supply scheme and a load level of the station and the type and the wiring mode of a traction transformer, is favorable for adopting and updating power quality control measures and is also favorable for evaluating the power grid and developing circuit transportation.
In order to achieve the purpose, the invention provides a method for calculating the influence of the power quality of a power grid, which comprises the following steps:
step 1, determining a power supply scheme of a traction station;
step 2, determining external power supply schemes of a plurality of set stations;
step 3, selecting the load level of the traction station;
step 4, respectively calculating the harmonic current limit value and the negative sequence current limit value of the traction station injected into each set station according to the external power supply scheme and the load level of the set station;
and 5, calculating harmonic current and negative sequence current generated when the train runs, and then calculating the negative sequence current injected into the set station by the traction station and the three-phase voltage unbalance degree caused by the negative sequence current.
Preferably, the step 1 comprises determining the voltage level of the traction station, the wiring mode of each side of the traction station, the power supply incoming line mode, the operation mode and the main capacity of the main transformer.
Preferably, the step 2 comprises:
step 2.1, planning a railway route covered by a power grid: determining the distance between each set station and the traction station;
and 2.2, determining the power grid data of each set station.
Preferably, in the step 3, the technical index parameters of the load level include: the length of a power supply arm, the average load current of the whole day, the maximum probability current of 95%, the maximum load current, the installation capacity of a parallel capacitor and the annual power consumption;
the loading levels include near loading levels and far loading levels.
Preferably, in the step 4, the harmonic current limit value is calculated based on a short-circuit current in a small mode of the primary side bus of the set station, a minimum short-circuit capacity of the primary side bus corresponding to the set station, a main transformer capacity, a minimum short-circuit capacity of the secondary side bus converted to the set station, and a traction transformer capacity of the corresponding traction station in the load level mode of each external power supply plan.
Preferably, in step 4, it is determined that each user connected to the common connection point causes the negative sequence voltage unbalance degree limit value of the point, and the positive sequence impedance and the negative sequence impedance of the common connection point are equal, then the negative sequence current can be converted by the following formula:
in the formula: I.C. A 2 Is a negative sequence current value; epsilon U2 Negative sequence voltage imbalance; s k Three-phase short-circuit capacity for a point of common connection; u shape L Is line voltage;
the calculation method of the harmonic current limit value comprises the following steps: and calculating the harmonic current limit value injected into each set station by the traction station according to the load level and the external power supply scheme of the set station.
Preferably, in the step 5, the harmonic current values of the respective orders are correspondingly calculated according to the harmonic characteristics of the traction mode, the load level and the power supply scheme.
Preferably, the traction mode is SS8 type electric locomotive traction: obtaining a load level through a traction load feeder line harmonic current content regression formula according to the harmonic characteristics of the SS8 type electric locomotive traction mode; the regression formula of the harmonic current content of the traction load feeder line is as follows:
K n ＝a+b×I f (％) (2)；
wherein, K n Is the n-th harmonic current content; a and b are regression coefficients; i is f Is the load current;
or the traction mode is a harmonious HXD3 type electric locomotive, and the load characteristics of the locomotive comprise high power factor, wide harmonic spectrum, low higher harmonic content and less than 2.5 percent of each harmonic current.
Preferably, in the step 5, the negative sequence current is calculated in a V/V traction transformer mode: the calculation method of the V/V traction transformer mode comprises the steps that when negative sequence current of the V/V traction transformer is analyzed, power supply ports on the secondary side are adopted for processing respectively, and then synthesis is carried out on the primary side;
in the V/V traction transformer mode, a traction substation is provided with a subarea pavilion, and a locomotive receives power under two power supply arms; the traction substation is provided with two power supply ports, and each port is powered by a single-phase traction transformer; two single-phase traction transformers form three phases on three pile heads with primary sides connected to a power supply; the method specifically comprises the following steps:
s1: for any port, selecting a first phase voltage of a primary side as a reference vector, and calculating a voltage of a secondary side power supply port;
s2: when power is supplied to the first phase and the third phase, the voltage of the secondary side first power supply port is calculated, and the negative sequence current of the secondary side first power supply port is calculated according to the power balance type of the two sides of the traction transformer;
s3: when power is supplied to the second phase and the third phase, the voltage of a second power supply port on the secondary side is obtained, and the negative sequence current of the second power supply port on the secondary side is calculated according to the power balance type on the two sides of the traction transformer;
s4: and superposing the negative sequence current generated by the first power supply port on the secondary side and the negative sequence current generated by the second power supply port on the secondary side to obtain the total negative sequence current on the primary side of the traction transformer.
Preferably, in step 5, the negative-sequence current is calculated in an impedance balancing traction transformer mode: respectively processing the power supply ports on the secondary side, and then synthesizing on the primary side;
in the impedance balance traction transformer mode, a first bushing is connected with a first phase, a second bushing is connected with a second phase, and a third bushing is connected with a third phase on a primary side; the three windings of the secondary side of the impedance balance transformer have the same number of turns and different impedances, the first phase and the third phase have the same impedance, and the second phase has the first phase impedanceDoubling; the number of turns of the epitaxial winding on both sides of the second phase winding being that of the inner delta windingTimes and the impedance is 0.4226 times the impedance of the first phase; the method specifically comprises the following steps:
t1: for any port, selecting a first phase voltage of a primary side as a reference vector, and calculating a voltage of a power supply port of a secondary side;
t2: when the power is supplied for the first phase, the voltage of a secondary side first power supply port is obtained, and the primary side second phase current is obtained; calculating the negative sequence current of a first power supply port on the secondary side according to the principle of magnetic potential equality and the principle of distribution of delta winding current in the secondary side according to impedance after calculating the power balance of the traction transformer;
t3: when the power is supplied to the third phase, the voltage of a second power supply port on the secondary side is obtained; calculating the negative sequence current of a second power supply port of the secondary side according to the principle of magnetic potential equality and the principle of distribution of delta winding current in the secondary side according to impedance after calculating the power balance of the traction transformer;
t4: and superposing the negative sequence current generated by the first power supply port on the secondary side and the negative sequence current generated by the second power supply port on the secondary side to obtain the total negative sequence current on the primary side of the traction transformer.
Compared with the prior art, the invention has the following beneficial effects: according to the invention, the negative sequence current injected into the set station and the caused three-phase voltage unbalance degree of the traction station are calculated by selecting the external power supply scheme and the load level of the station and the type and the wiring mode of the traction transformer, so that the method is beneficial to the adoption and the updating of electric energy quality control measures, is also beneficial to the evaluation of a power grid and the development of circuit transportation, and provides reference for the decision of relevant departments in determining the external power supply scheme and selecting the type of the traction transformer.
Drawings
FIG. 1 is a schematic diagram of a traction station power supply scheme of the present invention;
FIG. 2 is a schematic diagram of a second traction station power supply scheme of the present invention;
FIG. 3 is a schematic diagram of a primary wiring of the V/V traction transformer of the present invention;
fig. 4 is a schematic diagram of a primary wiring of the impedance balancing transformer of the present invention.
Detailed Description
The invention provides a method for calculating the influence of the power quality of a power grid, which is further described below by combining an attached drawing and a specific embodiment in order to make the invention more obvious and understandable.
The invention discloses a method for calculating the influence of power quality of a power grid, which is based on a section of electrified railway.
The invention discloses a method for calculating the influence of electric energy quality of a power grid on an electrified railway, which comprises the following steps:
step 1, determining a power supply scheme of a traction station;
step 2, determining external power supply schemes of a plurality of set stations;
and 3, selecting the locomotive load level of the traction station.
Step 4, respectively calculating a harmonic current limit value and a negative sequence current limit value of the traction station injected into each set station according to an external power supply scheme and a load level of the set station;
and 5, calculating harmonic current and negative sequence current generated when the train operates according to the selected load level and the external power supply scheme of the set station, and calculating the negative sequence current injected into the set station by the traction station and the three-phase voltage unbalance degree caused by the negative sequence current.
The invention takes the influence of the transformed railway traction station on the power quality of a power grid as an example, wherein the traction station, the first setting station, the second setting station and the third setting station are specific stations selected from actual railway lines. However, the present invention is not limited to the embodiment.
The method comprises the following specific steps:
according to the planning, the voltage grade of a 110kV traction station is 110/27.5kV, the 110kV side wiring mode of the traction station is a line transformer bank, 2-circuit power incoming lines, and the operation mode of 2 main transformers is a main supply mode and a hot standby mode. The main transformer capacity is 2 × 25MVA in the near term and 2 × 31.5MVA in the far term, the traction deformation mode is an impedance balance transformer, the impedance voltage is 8.4%, and the transformer has 2 times of overload capacity. A fixed capacitance compensation device is arranged on 27.5kV buses of a left arm and a right arm of a Ruan roadway power transformation station, the capacity is 1600kVar +1600kVar, the compensation degree is 0.12, filtering is carried out for 3 times, and a filtering site is reserved. The monthly average power factor of the high-voltage side of the traction substation is above 0.9 after compensation.
(II) setting power supply scheme of station
1. Power supply scheme one
The traction station receives power from the 220kV first setting station and the 220kV second setting station through 1-turn 110kV cable lines, respectively. The first set of station-to-pull station lines used 1 x 630mm2 cables, the line length being about 3km. The second set of stations to the pull station line uses 1 x 630mm2 cable, the line length is about 13km.
Wherein, the first station data of setting of 220kV is: main transformer capacity: 2 × 240MVA; voltage class: 220/110/35kV; impedance voltage: high-medium: 14.25 percent; high-low: 25.93 percent; medium-low: 9.25 percent. And, 220kV second set station data: the main transformer capacity is 2 multiplied by 240MVA; the voltage grade is 220/110/35kV; impedance voltage: high-medium: 11.91 percent; high-low: 21.96 percent; medium-low: 7.32 percent.
2. Power supply scheme two
The traction station receives power from the 220kV first setting station and the 220kV third setting station through 1-turn 110kV cable lines, respectively. The first set-up to the pull station line uses 1 x 630mm2 cables, the line length being about 3km. The third set-up station to the pull station line uses 1 x 630mm2 cables, the line length is about 10km.
The 220kV first setting station data is as follows: main transformer capacity: 2 × 240MVA; voltage class: 220/110/35kV; impedance voltage: high-medium: 14.25 percent; high-low: 25.93 percent; medium-low: 9.25 percent. And the data of the 220kV third setting station are as follows: main transformer capacity: 2 × 240MVA; voltage class: 220/110/35kV; impedance voltage: high-medium: 14 percent; high-low: 21 percent; medium-low: 8 percent.
And (III) the data of the planning electric load of the traction station is shown in the table 1.
TABLE 1 traction power supply technical index table
Wherein, an arm represents a first direction power supply arm of the railway and is a phase; the two arms represent the other direction power supply arm and are the other phase. The recent passenger plane adopts SS8 traction, and the long-term transition is carried out to the motor train unit; the cargo aircraft adopts the HXD3 of AC-DC-AC transmission, and the traction constant is 4000t. The number of bus pairs is 48 near term pairs and 76 far term pairs; the number of truck pairs is 17 near term pairs and 7 far term pairs.
(IV) electric energy quality index limit value
4.1 harmonic Current Limit
Considering that two main transformers of the traction station are in a main-standby operation mode, the traction station receives power from a 220kV first setting station and a 220kV second setting station respectively, and the traction station is considered according to a short-term load level and a long-term load level respectively, therefore, harmonic current limit values injected into a 110kV bus of the first setting station and a 110kV bus of the second setting station by the traction station are calculated respectively.
4.1.1, when power scheme one:
the short-circuit current of the 220kV bus of the first setting station in a small mode is 21.3kA, the minimum short-circuit capacity of the 220kV bus corresponding to the first setting station is 8485.3MVA, the main variable capacitance of the first setting station is 240MVA, and the minimum short-circuit capacity converted to the 110kV bus of the first setting station is 1405.28MVA. The short-circuit current of the 220kV bus of the second setting station under a small mode is 22.28kA, the minimum short-circuit capacity of the 220kV bus corresponding to the second setting station is 8875.72MVA, the main variable capacitance of the second setting station is 240MVA, and the minimum short-circuit capacity converted to the 110kV bus of the second setting station is 1642.26MVA. When the load level is near term, the protocol capacity is taken from the near term traction transformer capacity of 25MVA at the traction station. When the load level is in the forward phase, the protocol capacity is taken from the capacity of a forward traction transformer of the traction station, namely 31.5MVA. Therefore, harmonic current limits of 110kV buses injected into the first setting station and the second setting station by the traction station in the condition can be obtained respectively.
4.1.2, when power scheme two:
the short-circuit current of the 220kV bus of the first setting station in a small mode is 21.3kA, the minimum short-circuit capacity of the 220kV bus corresponding to the first setting station is 8485.3MVA, the main variable capacitance of the first setting station is 240MVA, and the minimum short-circuit capacity converted to the 110kV bus of the first setting station is 1405.28MVA. The short-circuit current of the 220kV bus of the third setting station is 23.32kA in a small mode, the minimum short-circuit capacity of the 220kV bus corresponding to the third setting station is 9290.03MVA, the main variable capacity of the third setting station is 240MVA, and the minimum short-circuit capacity converted to the 110kV bus of the third setting station is 1447.23MVA. When the load level is near term, the protocol capacity is taken from the near term traction transformer capacity of 25MVA at the traction station. When the load level is in the forward phase, the protocol capacity is taken from the capacity of a forward traction transformer of the traction station, namely 31.5MVA. Therefore, harmonic current limits of 110kV buses injected into the first setting station and the second setting station by the traction station in the condition can be obtained respectively.
4.2 negative sequence current limit
Each user connected to the common connection point causes a negative sequence voltage imbalance limit of typically 1.3% at that point;
assuming that the positive sequence impedance and the negative sequence impedance of the common connection point are equal, the negative sequence voltage and the negative sequence current are converted by the following equation.
In the formula: I.C. A 2 : negative sequence current value in units of A;
ε U2 : negative sequence voltage imbalance in%;
S k : the three-phase short circuit capacity of the public connection point is MVA;
U L : line voltage in kV;
4.2.1, when power scheme one:
the minimum short-circuit capacity of the first set station 110kV bus is 1405.28MVA, and the minimum short-circuit capacity of the second set station 110kV bus is 1642.26MVA. According to the formula (1), when the limit value of the unbalance degree of the negative sequence voltage of the bus is 1.3%, the negative sequence current limit value injected into the 110kV bus of the first setting station by the traction station is 95.89A, and the negative sequence current limit value injected into the 110kV bus of the second setting station is 112.06A.
4.2.2, when power scheme two:
the minimum short-circuit capacity of the first set station 110kV bus is 1405.28MVA, and the minimum short-circuit capacity of the third set station 110kV bus is 1447.23MVA. According to the formula (1), when the limit value of the negative-sequence voltage unbalance degree of the bus is 1.3%, the negative-sequence current limit value of the traction station injected into the 110kV bus of the first setting station is 95.89A, and the negative-sequence current limit value of the traction station injected into the 110kV bus of the third setting station is 98.75A.
4.3 harmonic Current calculation
4.3.1 harmonic characteristics of locomotive
The passenger train of the railway adopts SS8 traction of AC-DC transmission in the near term, and the long term is transited to a motor train unit, wherein the SS8 power is 3.6MW, and the motor train unit power is 4.8-5.5 MW (the long-range grouping power is 9.6-11 MW); the truck adopts a harmony number HXD3 with alternating current-direct current transmission, and the power is 9.6MW.
4.3.2 SS8 model electric locomotive
For SS8 traction, the regression formula of the harmonic current content of the traction load feeder line of the electrified railway is as follows:
K n ＝a+b×I f (％)(2)
wherein, K n The current content is the nth harmonic current; a and b are regression coefficients; I.C. A f Is the load current (A, ampere), I f &And l =300. The results of the regression coefficients for the model SS8 electric locomotive are shown in table 2;
TABLE 2 regression coefficient of SS8 type electric locomotive
| n harmonics | a | b |
| 3 | 23.9 | -0.042 |
| 5 | 13.9 | -0.025 |
| 7 | 10 | -0.017 |
Therefore, according to the harmonic characteristics of the SS8 electric locomotive, the current values of the harmonics of the passenger car at the recent load level can be obtained, as shown in table 3.
TABLE 3 harmonic current characteristics of passenger car (near term)
4.3.3 Harmonious HXD3 type electric locomotive
The HXD3 motor train unit adopted by the railway is a high-power streamline type AC-DC-AC motor train unit. The rectifier of the AC-DC link is not a phase control rectifier adopting a thyristor or an uncontrollable rectifier adopting a diode, but a rectifier based on the SPMW (pulse constant amplitude bandwidth modulation) technology of full-control devices such as a GTO thyristor and an IGBT, and a 25kV traction power supply is subjected to variable frequency transformation to be used as a power supply of an alternating current motor of the locomotive. The PWM rectifying circuit is characterized in that through proper control, the current on the AC power supply side can be sine wave and has the same phase with the voltage, and high power factor and low harmonic emission level are obtained.
The load characteristic of the AC-DC-AC transmission train is represented as high power factor, and the power factor is close to 1 when the train runs at full power; the harmonic frequency spectrum is wide, the higher harmonic content is low, the current content of each harmonic is below 2.5%, and the harmonic frequency spectrum of the alternating current transmission train is not published by the railway department, and the content of each harmonic is temporarily considered according to the numerical value given in the table 4. Thus, the harmonic characteristics of locomotives employed by railroads may be based on this data.
TABLE 4 harmonic current characteristics of passenger car (near term) and freight car (near term and long term)
Table 4 shows the harmonic current values of the passenger car (near term) and the freight car (near term and far term). The 27.5kV trailing arm harmonic current emissions for the near and far locomotives listed in table 9 are only applicable to single phase traction transformer connections, one traction supply arm. When a traction transformer is connected in a V/V mode, and an impedance balancing traction transformer or a Stent traction transformer exists, the harmonic current emission quantity of each power supply arm is converted according to the load distribution of the power supply arms under the condition that two traction arms exist.
(V) negative sequence current calculation
5.1 negative sequence Current calculation model
5.1.1V/V traction transformer
FIG. 3 is a schematic diagram of a primary wiring of a V/V traction substation; fig. 3 shows the case where phase C is grounded and power is supplied from phase AC and phase BC. The power supply mode of A-phase grounding or B-phase grounding can be connected.
Under the power supply mode of the V/V traction transformer, the traction substation is provided with a subarea pavilion, and the locomotive can receive power under two power supply arms. The traction substation is provided with two power supply ports, each port is equivalently powered by a single-phase traction transformer, but the two single-phase traction transformers are connected to three pile heads A, B and C of a primary side connected power supply. Therefore, when the negative sequence current of the V/V traction transformer is analyzed, the method of respectively processing the negative sequence current by adopting the power supply port on the secondary side and then synthesizing the negative sequence current on the primary side can be adopted. Wherein, I a 、I b Is the current of the secondary side; i all right angle A 、i B 、i C The currents of the corresponding pile heads on the primary side are respectively.
For each supply port, the primary side a-phase voltage is selected as the reference vector, i.e. the primary side a-phase voltage is selected as the reference vector
The secondary side power supply port voltage is:
in the formula: psi P Is the phase angle of the port voltage vector, K p Is the transformation ratio of the traction transformer.
5.1.1a, AC phase supply alone
When the AC phases are powered alone, there are:
in the formula (I), the compound is shown in the specification,is the voltage of the secondary side port,for transformation ratio of traction transformers, W 1 、W 2 Respectively, the number of turns of the primary winding and the number of turns of the secondary winding.
From equation (4), the secondary side port voltageHysteretic primary sideVoltage 30 electrical degrees. Balanced by the power on both sides of the traction transformer:
the following results were obtained:
in the formula (I), the compound is shown in the specification,voltages of three phases which are primary sides;is the port current of the secondary side of each phase;is the port current of the secondary side; in the formula (I), the compound is shown in the specification,is the power factor angle of the port locomotive load, and a is the coefficient.
According to the characteristic that zero sequence current does not exist when a single-phase traction transformer supplies power, when AC supplies power:
therefore:
5.1.1b, BC phase supply
When the BC phase is solely powered: psi P ＝-90°
From equation (8), the secondary side port voltageHysteretic primary sideThe voltage is 90 electrical degrees. I.e. the phase difference between the voltages at the two secondary side ports is 60 deg..
Also, by balancing the power across the traction transformer:
in the formula (I), the compound is shown in the specification,secondary side port currents for each phase; i is β Is the port current of the secondary side;is the power factor angle of the port locomotive load.
Simultaneously, the method comprises the following steps:
so that:
5.1.1c, primary side current
Superposing the negative sequence currents generated by the two power supply ports on the secondary side to obtain the total negative sequence current of the primary side of the traction transformer:
because of
Wherein the content of the first and second substances,currents on the primary side respectively;
if I β ＝ηI α (eta is the locomotive load ratio of the power supply arms at two sides):
I A ＝KI α (18)
I B ＝KηI α (19)
the primary side sequence current of the traction transformer is as follows:
positive sequence current
Negative sequence current
Positive sequence current effective value
Effective value of negative sequence current
Negative sequence current ratio
Under different load ratios eta of the power supply arms at two sides, the negative sequence current ratio epsilon and the power factor difference of the loads at two sidesThe relationship (c) is specifically as follows:
a. when the load ratio eta of the two side power supply arms =1, the negative sequence current ratio is minimum.
b. The larger the load ratio of the two power supply arms is, the larger the negative sequence current ratio is.
c. When η =1, andnamely when the load power factor angle of the power supply arm of the leading phase (AC phase) is smaller than the load power factor angle of the power supply arm of the lagging phase (BC phase) by 60 degrees, the negative sequence current ratio epsilon =0, namely the primary side forms a symmetrical three-phase system,there is no negative sequence current.
d. Under the general condition, when the load power factors of the two side power supply arms are basically equalAnd the load ratio η =1, the negative sequence current ratio ε =0.5, i.e., the negative sequence current amplitude is half the positive sequence current amplitude. And as the load ratio of the power supply arms on the two sides is increased, the negative sequence current ratio is also increased.
And when the V/V traction transformer supplies power, under different power factor difference angles of the power supply arms at two sides, the relationship between the negative sequence current ratio and the load ratio of the power supply arms at two sides is as follows:
a. when the load power factor difference angle of the two side power supply armsThe negative sequence current ratio is minimal.
b. When the load power factor difference angle of the two side power supply armsNamely, when the load power factor angle of the power supply arm of the leading phase (AC phase) is greater than the load power factor angle of the power supply arm of the lagging phase (BC phase) by 60 degrees, the negative sequence current ratio is constantly equal to 1 under any load ratio, namely the negative sequence current amplitude is equal to the positive sequence current amplitude.
c. When the temperature is higher than the set temperatureAnd the load ratio η =1, the negative-sequence current ratio ∈ =0.5.
5.1.2 impedance balance traction transformer
The primary connection of the impedance balancing traction transformer is schematically shown in fig. 4. Fig. 4 shows phase sequence power supply connected to a, B, and C in phase-change connection, that is, a primary side a sleeve is connected to a phase a, B sleeve is connected to B phase, and C sleeve is connected to C phase.
According to the phase change requirement, the phase sequence power supply mode of B, C and A or C, A and B can be connected.
The number of turns of three windings of delta in the secondary side of the impedance balancing transformer is the same (W) 2 ) But different impedances, where the a-phase and c-phase windings are equal in impedance (Z) L ) The free phase (b phase) being resistive for the side phases (a, c)And (4) doubling. The number of epitaxial winding turns on both sides of the free-phase winding being the number of internal delta winding turnsThe impedance is 0.4226 times the impedance of the side phases (a, c).
During the calculation of negative sequence current, the power supply ports on the secondary side are respectively processed, then the synthesis is carried out on the primary side, and the A-phase voltage on the primary side is still used as a reference vector, namelyThe following parameters will be used for the analysis: w is a group of 1 : the number of primary side winding turns; w 2 : the number of turns of the secondary side inner triangular winding; k: the ratio of the primary and secondary inner winding turns,Δ W: the number of turns of the secondary side free phase (B phase) epitaxial winding; k is W : the turn ratio of the secondary side free phase (B phase) epitaxial winding to the inner winding;Z L : secondary side inner winding impedance; k Z : the impedance ratio of the windings in the secondary side free phase (B phase) is (to make the zero-sequence current of the primary side 0),
when power is supplied by the A phase and the C phase, voltage vectors of two power supply ports are enabled due to the effect of the voltage of the middle phase extension windingAndthe phase difference angle of (a) is 90 deg..
5.1.2a, A phase supply
When the primary side is powered by the a-sleeve,is the secondary side port voltage;
the phase B current on the primary side is I 'due to the existence of the epitaxial winding on the free phase' B ＝I' Bi +I' Bo ；
Wherein: i' Bi : the current which flows through the inner winding in the secondary side and is reflected to the primary side; i' Bo : the current flowing through the outwardly extending winding in the secondary side reflects to the primary side.
According to the principle of magnetic potential equality, the following are provided: i' Bo ＝-KK W I α ；
According to the power balance formula of the traction transformer:
in the formula (I), the compound is shown in the specification,respectively are phase A and phase C current of the secondary side; u shape A 、U B 、U C The voltages of the primary side phase a, phase B, and phase C are provided.
After finishing, obtaining:
conjugation is taken from both sides to obtain:
in the formula (I), the compound is shown in the specification,primary side phase a and C currents, respectively.
According to the principle that the current of a delta winding in a secondary side is distributed according to impedance, the method comprises the following steps:
therefore:
5.1.2b, C phase supply
When the primary side is powered by the C sleeve, U β ＝K(-U C +K W U B ) (ii) a In the formula of U β Is the secondary side port voltage;
according to the principle of magnetic potential equality, the following are provided:
I" Bo ＝KK W I β (30)
similarly, based on the power balance, it can be obtained
According to the principle that the current of a delta winding in a secondary side is distributed according to impedance, the method comprises the following steps:
so that:
in the formula, I' Bo Reflecting the current flowing to the primary side for flowing through the external extended winding in the secondary side;is the secondary side port current;primary side currents of the respective phases;respectively, the secondary side port currents of the respective phases.
5.1.2c, primary side current
And superposing the negative sequence currents generated by the two power supply ports on the secondary side to obtain the total negative sequence current of each phase on the primary side of the traction transformer:
positive sequence current
Negative sequence current
Zero sequence current
Setting port current of secondary sideThe other port current of the secondary side is
Setting the load ratio of the two power supply arms as eta I, I can be obtained β ＝ηI α (ii) a Then:
due to the fact thatSubstituting into the formula to obtain:and
positive sequence current effective value
Effective value of negative sequence current
Negative sequence current ratio
When the power factor angle of the power supply arms changes between [ -90 degrees and 90 degrees ] under different load ratios eta of the power supply arms on the two sides, the change of the negative sequence current ratio is specifically as follows:
a. to be provided withFor the boundary, the curve is symmetrical left and right. Namely forWith the same negative sequence current ratio.
b. The two ports have the same load ratio and the same negative sequence current ratio.
c. For any load ratio eta, when the two ports have power factor angle differenceWhen the voltage is applied, the negative sequence current ratio is constant 1.0, namely the voltage is equivalent to a single-phase traction transformer.
d. When following withThe negative sequence current ratio is reduced whenThen, the minimum value of the negative sequence current ratio is obtained.
e. When the negative sequence current ratio increases with a decrease in the side load ratio η, the negative sequence current ratio ∈ curve with η =1 is the lowest, that is, the negative sequence current ratio is the smallest.
f. When η =1, andwhen the negative sequence current ratio epsilon =0 and the negative sequence current is 0, symmetrical three-phase currents are obtained at the primary side.
And under different power factor angles of the power supply arm, the change of the negative sequence current ratio along with the load ratio of the power supply arm is specifically as follows:
a. and the load ratio eta =1 of the ports on the two sides is taken as a demarcation point, and the curve is symmetrical. That is, the same load ratio on both sides will result in the same negative-sequence current ratio.
b. As the angular difference of the load power factors on the two sides increases, the negative sequence current ratio also increases. When the angular difference between the power factors of the two side loads is 0, namely the power factors of the two side loads are equal, the ratio of the negative sequence current caused by the angular difference is minimum.
c. Under any two-side load power factor angular difference, the minimum negative sequence current ratio is obtained as long as the load ratios of the two sides are equal, namely eta = 1.
e. When in useAnd η =1, the negative sequence current ratio is equal to 0, i.e., there is no negative sequence current, and the primary side three-phase current is symmetrical.
f. When in useUnder any load ratio, the negative sequence current ratio is equal to 1, namely the positive sequence current and the negative sequence current are equal, which is equivalent to the condition of single-phase traction power supply.
For zero sequence componentComprises the following steps:
to make the primary side I 0 =0, must haveThis is true.
Therefore:
for example, when the locomotive is supplied with power by the impedance balancing transformer, the primary side current is:
will be provided withSubstitution formula (48):
in the formula, the transformation ratio
Then when the locomotive is powered by the impedance balancing traction transformer, the transformer primary side negative sequence current is calculated as (49):
when in useAnd when the eta is more than or equal to 0 and less than or equal to 1, the calculation result of the negative sequence current injected into the 110KV bus of the first setting station and the second setting station and the three-phase voltage unbalance degree caused by the negative sequence current can be calculated.
The calculation result obtained by the method of the invention can reflect the degree of influence of which power supply scheme is adopted on harmonic waves and negative sequence current of the electric railway injection power grid, such as the harmonic current limit value caused by the change of the power supply mode and the negative sequence voltage unbalance rate caused by the bus.
While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention. Various modifications and alterations to this invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be determined from the following claims.
Claims (10)
1. A method for calculating the influence of the power quality of a power grid is characterized by comprising the following steps:
step 1, determining a power supply scheme of a traction station;
step 2, determining external power supply schemes of a plurality of set stations;
step 3, selecting the load level of the traction station;
step 4, respectively calculating the harmonic current limit value and the negative sequence current limit value of the traction station injected into each set station according to the external power supply scheme and the load level of the set station;
and 5, calculating harmonic current and negative sequence current generated when the train runs, and then calculating the negative sequence current injected into the set station by the traction station and the three-phase voltage unbalance degree caused by the negative sequence current.
2. Method for calculating the impact of the power quality of an electric network according to claim 1,
the step 1 comprises the steps of determining the voltage grade of the traction station, the wiring mode of each side of the traction station, the power supply wire inlet mode, the operation mode of a main transformer and the main capacity.
3. A method of calculating power quality impact of a power grid according to claim 1,
the step 2 comprises the following steps:
step 2.1, planning a railway route covered by a power grid: determining the distance between each set station and the traction station;
and 2.2, determining the power grid data of each set station.
4. A method of calculating power quality impact of a power grid according to claim 1,
in the step 3, the technical index parameters of the load level include: the method comprises the following steps of (1) length of a power supply arm, total daily average load current, 95% maximum probability current, maximum load current, installation capacity, parallel capacitor installation capacity and annual power consumption;
the loading levels include near loading levels and far loading levels.
5. A method of calculating power quality impact of a power grid according to claim 1,
in the step 4, the harmonic current limit value is calculated based on the load level mode according to the short-circuit current of the primary side bus of the set station of each external power supply scheme in the small mode, the minimum short-circuit capacity of the primary side bus corresponding to the set station, the main transformer capacity, the minimum short-circuit capacity of the secondary side bus converted to the set station, and the capacity of the traction transformer of the corresponding traction station.
6. A method of calculating power quality impact of a power grid according to claim 1,
in step 4, it is determined that each user connected to the common connection point causes the negative sequence voltage unbalance degree limit value of the point, and the positive sequence impedance and the negative sequence impedance of the common connection point are equal, so that the negative sequence current can be converted by the following formula:
in the formula: i is 2 Is a negative sequence current value; epsilon U2 Negative sequence voltage imbalance; s. the k Is disclosedThree-phase short circuit capacity of the common connection point; u shape L Is line voltage;
the calculation method of the harmonic current limit value comprises the following steps: and calculating the harmonic current limit value injected into each set station by the traction station according to the load level and the external power supply scheme of the set station.
7. A method of calculating power quality impact of a power grid according to claim 1,
in the step 5, each harmonic current value is correspondingly calculated according to the harmonic characteristics, the load level and the power supply scheme of the traction mode.
8. A method of calculating power quality impact of a power grid according to claim 7,
the traction mode is SS8 type electric locomotive traction: obtaining a load level through a traction load feeder line harmonic current content regression formula according to the harmonic characteristics of the SS8 type electric locomotive traction mode; the regression formula of the harmonic current content of the traction load feeder line is as follows:
K n ＝a+b×I f (％) (2)；
wherein, K n The current content is the nth harmonic current; a and b are regression coefficients; i is f Is the load current;
or the traction mode is a harmonious HXD3 type electric locomotive, and the load characteristics of the locomotive comprise high power factor, wide harmonic spectrum, low higher harmonic content and less than 2.5 percent of each harmonic current.
9. A method of calculating power quality impact of a power grid according to claim 1,
in the step 5, the negative sequence current is calculated in a mode of a V/V traction transformer:
the calculation method of the V/V traction transformer mode comprises the steps that when negative sequence current of the V/V traction transformer is analyzed, power supply ports on the secondary side are adopted for processing respectively, and then synthesis is carried out on the primary side;
in the V/V traction transformer mode, a traction substation is provided with a subarea pavilion, and a locomotive receives power under two power supply arms; the traction substation is provided with two power supply ports, and each port is powered by a single-phase traction transformer; two single-phase traction transformers form three phases on three pile heads with primary sides connected to a power supply; the method specifically comprises the following steps:
s1: for any port, selecting a first phase voltage of a primary side as a reference vector, and calculating a voltage of a secondary side power supply port;
s2: when power is supplied to the first phase and the third phase, the voltage of the secondary side first power supply port is calculated, and the negative sequence current of the secondary side first power supply port is calculated according to the power balance type of the two sides of the traction transformer;
s3: when power is supplied to the second phase and the third phase, the voltage of a second power supply port on the secondary side is obtained, and the negative sequence current of the second power supply port on the secondary side is calculated according to the power balance type on the two sides of the traction transformer;
s4: and superposing the negative sequence current generated by the first power supply port on the secondary side and the negative sequence current generated by the second power supply port on the secondary side to obtain the total negative sequence current on the primary side of the traction transformer.
10. A method of calculating power quality impact of a power grid according to claim 1,
in the step 5, the calculation method of the negative sequence current is in an impedance balance traction transformer mode: firstly, respectively processing power supply ports on the secondary side, and then synthesizing on the primary side;
in the impedance balance traction transformer mode, a first bushing is connected with a first phase, a second bushing is connected with a second phase, and a third bushing is connected with a third phase on a primary side; the three windings of the secondary side of the impedance balance transformer have the same number of turns and different impedances, the first phase and the third phase have the same impedance, and the second phase has the first phase impedanceDoubling; the number of turns of the epitaxial winding on both sides of the second phase winding being that of the inner delta windingTimes and the impedance is 0.4226 times of the impedance of the first phase; the method comprises the following steps:
t1: for any port, selecting a first phase voltage of a primary side as a reference vector, and calculating a voltage of a power supply port of a secondary side;
t2: when the power is supplied for the first phase, the voltage of a secondary side first power supply port is obtained, and the primary side second phase current is obtained; calculating the negative sequence current of a first power supply port on the secondary side according to the principle of magnetic potential equality and the principle of impedance distribution of delta winding current in the secondary side after the power balance calculation of the traction transformer;
t3: when the power is supplied to the third phase, the voltage of a second power supply port on the secondary side is obtained; calculating the negative sequence current of a second power supply port of the secondary side according to the principle of magnetic potential equality and the principle of impedance distribution of delta winding current in the secondary side after the power balance calculation of the traction transformer;
t4: and superposing the negative sequence current generated by the first power supply port on the secondary side and the negative sequence current generated by the second power supply port on the secondary side to obtain the total negative sequence current on the primary side of the traction transformer.
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