CN116316708A - Adaptive control method, device and system for flexible direct-current power grid - Google Patents
Adaptive control method, device and system for flexible direct-current power grid Download PDFInfo
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- H—ELECTRICITY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention provides a self-adaptive control method, a device and a system for a flexible direct current power grid, wherein the method comprises the following steps: calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power; calculating the steady-state error of the active power of the multi-terminal flexible direct current network under the self-adaptive control to obtain the influence of the direct current line impedance on the steady-state error of the active power; based on the relation between the DC current error and the active power steady-state error and the influence of the DC line impedance on the active power steady-state error, the droop characteristic curves of the converters are translated to the intersection point of the volt-ampere characteristic curves and the DC current reference value, so that the actual DC current of each converter is consistent with the reference value all the time. The invention can obviously reduce steady-state error without changing the gradient of the sagging characteristic curve and simultaneously the dynamic performance of the network is not deteriorated.
Description
Technical Field
The invention relates to the field of direct current power grid stabilization and control, in particular to a flexible direct current power grid self-adaptive control method, device and system.
Background
Multi-terminal flexible direct current (VSC-HVDC) networks are a technology for large scale integration of renewable energy power delivery. Compared with the traditional alternating current transmission, the multi-terminal VSC-HVDC network has the advantages of high transmission efficiency, reactive power compensation of an alternating current system and the like. From the aspect of active power, the control method of the multi-terminal flexible direct current network which is remotely interconnected across the region is worthy of intensive study, and the existing control method of the flexible direct current network mainly comprises the following steps:
1. master/slave control method: the master/slave control method is widely applied to a point-to-point high-voltage flexible direct current system and can also be applied to a multi-terminal high-voltage flexible direct current network. The master/slave control algorithm is simple and can accurately control the power flow.
2. The self-adaptive control method comprises the following steps: adaptive control is a widely accepted method of voltage-based droop control, and the literature [ c.d. Barker, and r.s. whitehouse, 'Further Developments in Autonomous Converter Control in a Multi-Terminal HVDC System', in ALSTOM Grid, power Electronics Activities, stafford, UK ] describes the principle of adaptive control that allows each converter to meet the overall power balance requirement by independently adjusting its dc voltage. The dc voltage reference value of each converter can be automatically adjusted by measuring the sag characteristic curve generated by the dc current and voltage. The method is firstly applied to power distribution of a micro-grid, and then is further applied to a multi-terminal high-voltage flexible direct current power transmission network. The application of the adaptive control method to a multi-terminal high voltage flexible DC power transmission network is described in the literature [ Y.Zhang, L.Wang and W.Li, "Autonomous DC Line Power Flow Regulation Using Adaptive Droop Control in HVDC Grid," in IEEE Transactions on Power Delivery, vol. 36, no. 6, pp. 3550-3560, dec. 2021, doi: 10.1109/TPWRD.2020.3044978 ].
3. Correction method for increasing the slope of sagging characteristic curve: fig. 3 of the literature [ Gyusub Lee, dohoon Kwon, seung il moon DC Current and Voltage Droop Control Method of Hybrid HVDC Systems for an Offshore Wind Farm Connection to Enhance AC Voltage Stability [ J ]. IEEE Transactions on Energy Conversion,2020, pp (99) ] proposes a correction method for droop control, attempting to reduce steady state error by increasing the slope of the droop characteristic.
The prior art has the following defects:
(1) The master/slave control method causes the balance node to have overlarge pressure and to be seriously dependent on communication, so that the method cannot be well adapted to a more complex high-voltage direct-current power transmission network;
(2) The problem of influence of impedance on multi-terminal VSC-HVDC operation is not analyzed in detail in the traditional self-adaptive control;
(3) The correction method of increasing the slope of the droop characteristics reduces the damping of the power receiving converter and reduces the dynamic stability of the network while the steady state error is still significant.
Disclosure of Invention
The invention provides a self-adaptive control method, a device and a system for a flexible direct current power grid, which combine the advantages of independent communication of voltage droop control and master/slave control and accurate stress average and active power distribution of each converter, and realize the effects of remarkably reducing steady-state errors of the network and preventing the dynamic performance of the network from deteriorating under the condition of not changing the gradient of a droop characteristic curve.
A self-adaptive control method for a flexible direct current power grid comprises the following steps:
calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power;
calculating the steady-state error of the active power of the multi-terminal flexible direct current network under the self-adaptive control to obtain the influence of the direct current line impedance on the steady-state error of the active power;
based on the relation between the DC current error and the active power steady-state error and the influence of the DC line impedance on the active power steady-state error, the droop characteristic curves of the converters are translated to the intersection point of the volt-ampere characteristic curves and the DC current reference value, so that the actual DC current of each converter is consistent with the reference value all the time.
Further, the calculating the steady-state error of the active power of the flexible direct current network at two ends under the self-adaptive control to obtain the relationship between the direct current error and the steady-state error of the active power comprises the following steps:
the method comprises the steps of obtaining an intersection point which is an actual working point of a converter through volt-ampere characteristic curve expression and sagging characteristic curve expression of flexible direct current grid converters A and B at two ends under simultaneous self-adaptive control, calculating an active power steady-state error expression through the actual working point of the converter, and finally analyzing the active power steady-state error expression to obtain the relation between a direct current error and an active power steady-state error: the active power steady state error caused by the line impedance is represented by a dc current error and decreases when the slope k of the voltage droop control curve increases within the allowable range.
Further, in a two-terminal VSC-HVDC system, calculating the steady state error of the active power requires determining the actual operating point of the two converters: p (P) A (I ’ dc ,U dca ) And P B (I ’ dc ,U dcb ) Wherein I ’ dc For actual DC current, U dca And U dcb The actual working voltages of the current transformer A and the current transformer B are respectively, the actual working point of the current transformer is the intersection point of a volt-ampere characteristic curve and a sagging characteristic curve, and the sagging characteristic curve has the expression:
wherein k is the slope of the voltage sag control curve, I dc Indicating varying DC current, U s Indicating the DC voltage at the transmitting end relative to I dc Variation of U r Representing the direct current voltage at the receiving end relative to I dc Variation of U * dca And U * dcb Represents the intercept of the curve;
U dc0 representing the normal operating voltage of the whole HVDC network set by the dispatch center, the relation of the value and the voltage sag characteristic curve is expressed as follows:
DC reference value I dc0 Is that the converter works at the reference work P 0 Current at point:
the voltammetric characteristic curve expression is:
u in dca And U dcb Representing the actual DC voltages of converter A and converter B, R 0 The resistor is between the current transformer A and the current transformer B;
U dc0 the relation of the values of (2) to the voltammetric characteristic is expressed as:
combining the volt-ampere characteristic curve expression and the sagging characteristic curve expression, and obtaining the actual working points of the converter by the vertical type (3), the formula (4) and the formula (7) in parallel is as follows:
the active power steady state error calculation expression is:
from the formula it follows that: the active power steady state error caused by the line impedance can be almost represented by a dc current error, and decreases when the steady state increases within the allowable range.
Further, the calculating the steady-state error of the active power of the multi-terminal flexible direct current network under the adaptive control to obtain the influence of the direct current line impedance on the steady-state error of the active power includes:
the method comprises the steps of obtaining the actual working points of all converters by volt-ampere characteristic curve expression and sagging characteristic curve expression of all converters of a simultaneous multi-terminal system, calculating an active power steady-state error expression through the actual working points of the converters, and finally obtaining the influence of direct current line impedance on the active power steady-state error: in a long-distance interconnected multi-terminal VSC-HVDC system, the influence of the direct current line impedance cannot be ignored, and the steady-state error of the active power of the converter is mainly reflected on the direct current error.
Further, in the multi-terminal network, the steady-state error of the active power is calculated by determining the actual operating point of each converter, and for an N-terminal system, the voltage at the central connecting point is U t Ith variable flow
DC voltage U dci The relationship with the voltage at the center junction is expressed as:
wherein I is dci For the direct current of the ith converter flowing to the central connection point, R it Is the impedance between the ith current transformer flow and the center connection point;
the dc current relationship of each converter is expressed as:
combining (13) with (14), solving for the voltage at the center junction to be:
；
wherein Y is it Is admittance between the ith converter and the central connection point;
the volt-ampere characteristic of each converter is described as follows:
in U s1 ~U sN The direct current voltage of each converter is changed;
the sag characteristic curves of the converters are described as follows:
wherein U is * dc1 ~U * dcN Is the intercept, k, of the characteristic curve 1 ~k N Is the slope;
in this multi-terminal system, U dc0 The relationship with the voltage sag curve is expressed as:
U dci representing the actual DC voltage of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
the relationship between the DC reference values of the converters is expressed as:
the practical operating point of the ith converter is obtained by combining the simultaneous volt-ampere characteristic expression and the sagging characteristic expression and combining the expression (18) and the expression (19):
in which I ’ dci For the actual DC current of the ith converter in operation, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
u in dci Representing the actual DC voltage of the ith converter, I dci0 Representing the direct reference current, k, of the ith converter i Is the ithSlope of sagging characteristic curve of each converter, U dc0 Representing the normal operating voltage of the entire HVDC network set by the dispatch center;
active power steady state error ΔP of converter i The computational expression is:
p in the formula i Is the active power of the ith converter, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
wherein DeltaPi is the active power steady state error of the ith converter, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the dc reference current of the i-th converter.
Further, the step of translating the droop characteristic curves of the converters to intersections of the volt-ampere characteristic curves and the dc reference values based on the relationship between the dc error and the active power steady-state error and the influence of the dc line impedance on the active power steady-state error, so that the actual dc of each converter always coincides with the reference values, includes:
based on the relationship between the DC current error and the active power steady state error and the effect of the DC line impedance on the active power steady state error, the steady state error of the active power is reduced without changing the slope k of the voltage droop control curve:
DC reference value I of the ith converter dci0 Expressed as:
u in * dci For the intercept of the sagging characteristic curve of the ith converter, U dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
translating the sagging characteristic curve of each converter to enable the curve to pass through the intersection point of the volt-ampere characteristic curve and the corresponding direct current reference value, and obtaining the final position of the sagging characteristic curve, wherein the sagging curve of each converter is expressed as:
wherein Y is it Is the admittance, k between the ith converter and the central connection point i For the slope of the sagging characteristic curve of the ith converter, I dci0 Indicating the direct reference current of the ith converter, U dci Representing the actual DC voltage of the ith converter, I dci DC current representing variation of ith converter, U ’ si Is the current transformer I relative to I dci A new dc voltage that changes.
A flexible direct current grid adaptive control device, comprising:
the first steady-state error calculation module is used for calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power;
the second steady-state error calculation module is used for calculating steady-state errors of active power of the multi-terminal flexible direct current network under self-adaptive control to obtain the influence of direct current line impedance on the steady-state errors of the active power;
and the self-adaptive control module is used for translating the sagging characteristic curve of the current transformers to the intersection point of the volt-ampere characteristic curve and the direct current reference value based on the relation between the direct current error and the active power steady-state error and the influence of the direct current line impedance on the active power steady-state error, so that the actual direct current of each current transformer is consistent with the reference value all the time.
Further, the first steady-state error calculation module is specifically configured to
The method comprises the steps of obtaining an intersection point which is an actual working point of a converter through volt-ampere characteristic curve expression and sagging characteristic curve expression of flexible direct current grid converters A and B at two ends under simultaneous self-adaptive control, calculating an active power steady-state error expression through the actual working point of the converter, and finally analyzing the active power steady-state error expression to obtain the relation between a direct current error and an active power steady-state error: the active power steady state error caused by the line impedance is represented by a dc current error and decreases when the slope k of the voltage droop control curve increases within the allowable range.
Further, the second steady-state error calculation module is specifically configured to
The method comprises the steps of obtaining the actual working points of all converters by volt-ampere characteristic curve expression and sagging characteristic curve expression of all converters of a simultaneous multi-terminal system, calculating an active power steady-state error expression through the actual working points of the converters, and finally obtaining the influence of direct current line impedance on the active power steady-state error: in a long-distance interconnected multi-terminal VSC-HVDC system, the influence of the direct current line impedance cannot be ignored, and the steady-state error of the active power of the converter is mainly reflected on the direct current error.
The flexible direct current power grid self-adaptive control system comprises a memory, a processor and a computer program which is stored in the memory and can run on the processor, wherein the processor realizes the flexible direct current power grid self-adaptive control method when executing the computer program.
The invention combines the advantages of independent communication of voltage droop control and master/slave control, accurate stress average and active power distribution of each converter, and realizes the effects of remarkably reducing steady-state errors of the network and not deteriorating dynamic performance of the network under the condition of not changing the gradient of a droop characteristic curve.
Drawings
Fig. 1 is a control scheme of a converter C;
FIG. 2 is a main circuit of a two-terminal system;
fig. 3 (a) is a line diagram of the flow from the converter a to the converter B under ideal operation conditions, and fig. 3 (B) is a line diagram of the flow from the converter a to the converter B under actual operation conditions;
fig. 4 is a simplified circuit diagram of a multi-terminal HVDC network;
figure 5 is a graph of sag characteristics of converter C using the improved adaptive control method of the present invention.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The embodiment of the invention provides a self-adaptive control method for a flexible direct current power grid; the method comprises the following steps:
step (1): calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control, and analyzing the relation between the direct current errors and the steady-state errors of the active power: the active power steady state error caused by the line impedance is represented by a dc current error and decreases when the slope k of the voltage droop control curve increases within the allowable range.
The conventional adaptive control method is suitable for ignoring the influence of the impedance of the transmission line, but when the transmission line is long enough, the working point of each converter changes, and the impedance of the transmission line is not negligible.
Taking a two-terminal VSC-HVDC system as an example, the control scheme of the converter is shown in figure 1,the main circuit of the two-end system is shown in figure 2, and the calculation of the steady state error of the active power requires the determination of the actual operating point P of the two converters A (I ’ dc ,U dca ) And P B (I ’ dc ,U dcb )， I ’ dc For actual DC current, U dca And U dcb The actual working voltages of the current transformer A and the current transformer B are respectively. The actual working point of the converter is the intersection point of the volt-ampere characteristic curve and the sagging characteristic curve, wherein the sagging characteristic curve has the expression:
wherein k is the slope of the voltage sag control, I dc Indicating varying DC current, U s Indicating the DC voltage at the transmitting end relative to I dc Variation of U r Representing the direct current voltage at the receiving end relative to I dc Variation of U * dca And U * dcb Representing the intercept of the curve.
U dc0 Representing the normal operating voltage of the entire HVDC network set by the dispatch center, its value versus voltage sag curve is expressed as:
fig. 3 (B) is a line diagram showing the flow of current from converter a to converter B under actual operating conditions, and the dc reference value I dc0 Is that the converter operates at P in (b) of FIG. 3 0 Point (P) 0 Point is the reference operating point of the current transformer):
the volt-ampere characteristic expression is:
u in dca And U dcb Representing the actual DC voltages of converter A and converter B, R 0 Is the resistance between current transformer a and current transformer B.
U dc0 The relation of the values of (2) to the voltammetric characteristic is expressed as:
combining the volt-ampere characteristic expression and the sagging characteristic expression, connecting the vertical type (3), the formula (4) and the formula (7) in parallel to obtain the actual working point of the converter as follows:
the active power steady state error calculation expression is:
from the formula it follows that: the active power error caused by the line impedance can be almost represented by the steady state error of the direct current, and the steady state error decreases when the slope k increases within the allowable range.
Step (2): calculating steady-state errors of active power of the multi-terminal flexible direct current network under self-adaptive control, and revealing the influence of the impedance of the direct current line on the steady-state errors of the active power: in a long-distance interconnected multi-terminal VSC-HVDC system the influence of the line impedance cannot be neglected. The actual active power of the converter is increased and reduced, and the steady-state error of the active power of the converter is mainly reflected on the error of direct current.
Similar to step (1), in a multi-port network, the steady state error of the active power can also be calculated by determining the actual operating point of each converter. For an N-terminal system, the simplified circuit diagram is shown in FIG. 4, and the voltage at the center connection point is U t Direct current voltage U of ith converter dci The relationship with the voltage at the center junction is expressed as:
wherein I is dci For the direct current of the ith converter flowing to the central connection point, R it Is the impedance between the ith current transformer flow and the center connection point;
the dc current relationship of each converter is expressed as:
combining (13) with (14), solving for the voltage at the center junction to be:
wherein Y is it Is the admittance between the ith converter flow and the central connection point;
the volt-ampere characteristic of each converter is described as follows:
in U s1 ~U sN The direct current voltage of each converter is changed;
the sag characteristic curves of the converters are described as follows:
wherein U is * dc1 ~U * dcN Is the intercept, k, of the characteristic curve 1 ~k N Is the slope;
in this multi-terminal system, U dc0 The relationship with the voltage sag curve is expressed as:
U dci representing the actual DC voltage of the ith converter, I dci0 DC reference electric representing ith converterAnd (3) flow.
The relationship between the DC reference values of the converters is expressed as:
combining the simultaneous volt-ampere characteristic expression and the sagging characteristic expression with the formula (43) and the formula (44) to obtain the actual working points of the converters as follows:
in which I ’ dci For the actual DC current of the ith converter in operation, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
u in dci Representing the actual DC voltage of the ith converter, I dci0 Representing the direct reference current, k, of the ith converter i For the slope of the sagging characteristic curve of the ith converter, U dc0 Representing the normal operating voltage of the entire HVDC network set by the dispatch center;
active power steady state error ΔP of converter i The computational expression is:
p in the formula i Is the active power of the ith converter, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the dispatch center settingsNormal operating voltage, k, of the whole HVDC network i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
in DeltaP i Is the steady state error of the active power of the ith converter, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
table 1 shows the actual operating points of each converter in a four-terminal network:
TABLE 1
From table 1 it is concluded that: in a long-distance interconnected multi-terminal VSC-HVDC system the influence of the line impedance cannot be neglected. The actual active power of the converter is increased and reduced, and the steady-state error of the active power is mainly reflected on the error of the direct current.
Step (3): an adaptive control method for steady-state error correction is provided: and translating the sagging characteristic curve of the current transformers to the intersection point of the volt-ampere characteristic curve and the direct current reference value, so that the actual direct current of each current transformer is consistent with the reference value all the time.
Based on the analysis results of step (1) and step (2), the present step proposes an improved adaptive control method that reduces the steady state error of the active power without changing the slope k. DC reference value I of converter dci0 Expressed as:
and translating the sagging characteristic curve of each converter to enable the curve to pass through the intersection point of the volt-ampere characteristic curve and the corresponding direct current reference value, so as to obtain the final position of the sagging characteristic curve. The droop characteristic of the nth converter c after the improved adaptive control method is shown in fig. 5.
The sag curves of the converters can be expressed as:
wherein U is ’ si Is the current transformer I relative to I dci A new dc voltage that changes; this control method can effectively eliminate steady-state errors of the active power, because the actual direct current of each converter always coincides with the reference value.
The embodiment of the invention provides a flexible direct current power grid self-adaptive control method, which combines the advantages of independent communication of voltage droop control and master/slave control and accurate stress average and active power distribution of each converter, and achieves the effects of remarkably reducing steady-state errors of a network and preventing the dynamic performance of the network from deteriorating under the condition of not changing the gradient of a droop characteristic curve.
The embodiment of the invention also provides a self-adaptive control device of the flexible direct current power grid, which comprises the following components:
the first steady-state error calculation module is used for calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power;
the second steady-state error calculation module is used for calculating steady-state errors of active power of the multi-terminal flexible direct current network under self-adaptive control to obtain the influence of direct current line impedance on the steady-state errors of the active power;
and the self-adaptive control module is used for translating the sagging characteristic curve of the current transformers to the intersection point of the volt-ampere characteristic curve and the direct current reference value based on the relation between the direct current error and the active power steady-state error and the influence of the direct current line impedance on the active power steady-state error, so that the actual direct current of each current transformer is consistent with the reference value all the time.
Another embodiment of the present invention provides a flexible dc power grid adaptive control system, including: a computer readable storage medium and a processor;
the computer-readable storage medium is for storing executable instructions;
the processor is configured to read executable instructions stored in the computer readable storage medium, and execute the adaptive control method for the flexible direct current power grid according to the first aspect.
Another embodiment of the present invention provides a non-transitory computer readable storage medium having stored thereon a computer program which, when executed by a processor, implements the flexible direct current grid adaptive control method according to the first aspect.
It will be appreciated by those skilled in the art that embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only for illustrating the technical aspects of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those of ordinary skill in the art that: modifications and equivalents may be made to the specific embodiments of the invention without departing from the spirit and scope of the invention, which is intended to be covered by the claims.
Claims (10)
1. A flexible direct current power grid self-adaptive control method is characterized in that: the method comprises the following steps:
calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power;
calculating the steady-state error of the active power of the multi-terminal flexible direct current network under the self-adaptive control to obtain the influence of the direct current line impedance on the steady-state error of the active power;
based on the relation between the DC current error and the active power steady-state error and the influence of the DC line impedance on the active power steady-state error, the droop characteristic curves of the converters are translated to the intersection point of the volt-ampere characteristic curves and the DC current reference value, so that the actual DC current of each converter is consistent with the reference value all the time.
2. The flexible direct current power grid adaptive control method according to claim 1, wherein: calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power, wherein the calculating steady-state errors comprise the following steps:
the method comprises the steps of obtaining an intersection point which is an actual working point of a converter through volt-ampere characteristic curve expression and sagging characteristic curve expression of flexible direct current grid converters A and B at two ends under simultaneous self-adaptive control, calculating an active power steady-state error expression through the actual working point of the converter, and finally analyzing the active power steady-state error expression to obtain the relation between a direct current error and an active power steady-state error: the active power steady state error caused by the line impedance is represented by a dc current error and decreases when the slope k of the voltage droop control curve increases within the allowable range.
3. The flexible direct current power grid adaptive control method according to claim 2, wherein:
in a two-terminal VSC-HVDC system, calculating the steady state error of the active power requires determining the actual operating point of the two converters: p (P) A (I ’ dc ,U dca ) And P B (I ’ dc ,U dcb ) Wherein I ’ dc For actual DC current, U dca And U dcb The actual working voltages of the current transformer A and the current transformer B are respectively, the actual working point of the current transformer is the intersection point of a volt-ampere characteristic curve and a sagging characteristic curve, and the sagging characteristic curve has the expression:
wherein k is the slope of the voltage sag control curve, I dc Indicating varying DC current, U s Indicating the DC voltage at the transmitting end relative to I dc Variation of U r Representing the direct current voltage at the receiving end relative to I dc Variation of U * dca And U * dcb Represents the intercept of the curve;
U dc0 representing the normal operating voltage of the whole HVDC network set by the dispatch center, the relation of the value and the voltage sag characteristic curve is expressed as follows:
DC reference value I dc0 Is that the converter works at the reference work P 0 Current at point:
the voltammetric characteristic curve expression is:
u in dca And U dcb Representing the actual DC voltages of converter A and converter B, R 0 The resistor is between the current transformer A and the current transformer B;
U dc0 the relation of the values of (2) to the voltammetric characteristic is expressed as:
combining the volt-ampere characteristic curve expression and the sagging characteristic curve expression, and obtaining the actual working points of the converter by the vertical type (3), the formula (4) and the formula (7) in parallel is as follows:
the active power steady state error calculation expression is:
4. the flexible direct current power grid adaptive control method according to claim 1, wherein: calculating the steady-state error of the active power of the multi-terminal flexible direct current network under the self-adaptive control to obtain the influence of the direct current line impedance on the steady-state error of the active power, wherein the method comprises the following steps:
the method comprises the steps of obtaining the actual working points of all converters by volt-ampere characteristic curve expression and sagging characteristic curve expression of all converters of a simultaneous multi-terminal system, calculating an active power steady-state error expression through the actual working points of the converters, and finally obtaining the influence of direct current line impedance on the active power steady-state error: in a long-distance interconnected multi-terminal VSC-HVDC system, the influence of the direct current line impedance cannot be ignored, and the steady-state error of the active power of the converter is mainly reflected on the direct current error.
5. The flexible direct current power grid adaptive control method according to claim 4, wherein:
in a multi-terminal network, the steady-state error of active power is calculated by determining the actual operating point of each converter, and for an N-terminal system, the voltage at the central connection point is U t I-th converter DC voltage
U dci The relationship with the voltage at the center junction is expressed as:
wherein I is dci For the direct current of the ith converter flowing to the central connection point, R it Is the impedance between the ith current transformer flow and the center connection point;
the dc current relationship of each converter is expressed as:
combining (13) with (14), solving for the voltage at the center junction to be:
；
wherein Y is it Is admittance between the ith converter and the central connection point;
the volt-ampere characteristic of each converter is described as follows:
in U s1 ~U sN The direct current voltage of each converter is changed;
the sag characteristic curves of the converters are described as follows:
wherein U is * dc1 ~U * dcN Is the intercept, k, of the characteristic curve 1 ~k N Is the slope;
in this multi-terminal system, U dc0 The relationship with the voltage sag curve is expressed as:
U dci representing the actual DC voltage of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
the relationship between the DC reference values of the converters is expressed as:
the practical operating point of the ith converter is obtained by combining the simultaneous volt-ampere characteristic expression and the sagging characteristic expression and combining the expression (18) and the expression (19):
in which I ’ dci For the actual DC current of the ith converter in operation, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
u in dci Representing the actual DC voltage of the ith converter, I dci0 Representing the direct reference current, k, of the ith converter i For the slope of the sagging characteristic curve of the ith converter, U dc0 Representing the normal operating voltage of the entire HVDC network set by the dispatch center;
active power steady state error ΔP of converter i The computational expression is:
p in the formula i Is the active power of the ith converter, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
in DeltaP i Is the steady state error of the active power of the ith converter, U * dci Is the intercept of the sagging characteristic curve of the ith converter, Y it U is admittance between the ith converter and the central connection point dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the dc reference current of the i-th converter.
6. The flexible direct current power grid adaptive control method according to claim 4, wherein: the step of translating the droop characteristic curves of the current transformers to the intersection point of the volt-ampere characteristic curves and the direct current reference value based on the relation between the direct current error and the active power steady-state error and the influence of the direct current line impedance on the active power steady-state error, so that the actual direct current of each current transformer always accords with the reference value, and the method comprises the following steps:
based on the relationship between the DC current error and the active power steady state error and the effect of the DC line impedance on the active power steady state error, the steady state error of the active power is reduced without changing the slope k of the voltage droop control curve:
DC reference value I of the ith converter dci0 Expressed as:
u in * dci For the intercept of the sagging characteristic curve of the ith converter, U dc0 Representing the normal operating voltage, k, of the entire HVDC network set by the dispatch center i For the slope of the sagging characteristic curve of the ith converter, I dci0 Representing the direct current reference current of the ith converter;
translating the sagging characteristic curve of each converter to enable the curve to pass through the intersection point of the volt-ampere characteristic curve and the corresponding direct current reference value, and obtaining the final position of the sagging characteristic curve, wherein the sagging curve of each converter is expressed as:
wherein Y is it Is the admittance, k between the ith converter and the central connection point i For the slope of the sagging characteristic curve of the ith converter, I dci0 Indicating the direct reference current of the ith converter, U dci Representing the actual DC voltage of the ith converter, I dci DC current representing variation of ith converter, U ’ si Is the current transformer I relative to I dci A new dc voltage that changes.
7. The utility model provides a flexible direct current electric wire netting self-adaptation controlling means which characterized in that includes:
the first steady-state error calculation module is used for calculating steady-state errors of active power of the flexible direct current networks at two ends under self-adaptive control to obtain a relation between the direct current errors and the steady-state errors of the active power;
the second steady-state error calculation module is used for calculating steady-state errors of active power of the multi-terminal flexible direct current network under self-adaptive control to obtain the influence of direct current line impedance on the steady-state errors of the active power;
and the self-adaptive control module is used for translating the sagging characteristic curve of the current transformers to the intersection point of the volt-ampere characteristic curve and the direct current reference value based on the relation between the direct current error and the active power steady-state error and the influence of the direct current line impedance on the active power steady-state error, so that the actual direct current of each current transformer is consistent with the reference value all the time.
8. The flexible direct current power grid adaptive control device according to claim 7, wherein: the first steady-state error calculation module is specifically used for combining the volt-ampere characteristic curve expression and the sagging characteristic curve expression of the two-end flexible direct-current power grid converters A and B under the self-adaptive control, obtaining an intersection point to be an actual working point of the converters, calculating an active power steady-state error expression through the actual working point of the converters, and finally analyzing the active power steady-state error expression to obtain the relation between a direct-current error and an active power steady-state error: the active power steady state error caused by the line impedance is represented by a dc current error and decreases when the slope k of the voltage droop control curve increases within the allowable range.
9. The flexible direct current power grid adaptive control device according to claim 7, wherein: the second steady-state error calculation module is specifically used for solving the actual working points of the converters by combining the volt-ampere characteristic curve expression and the sagging characteristic curve expression of each converter of the multi-terminal system, calculating the steady-state error expression of the active power by the actual working points of the converters, and finally obtaining the influence of the impedance of the direct-current circuit on the steady-state error of the active power: in a long-distance interconnected multi-terminal VSC-HVDC system, the influence of the direct current line impedance cannot be ignored, and the steady-state error of the active power of the converter is mainly reflected on the direct current error.
10. A flexible direct current electric wire netting self-adaptation control system, its characterized in that: comprising a memory, a processor and a computer program stored on the memory and executable on the processor, the processor implementing the flexible direct current network adaptive control method according to any one of claims 1-6 when executing the computer program.
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