CN109193682B - UPQC seamless switching power coordination control strategy based on PAC - Google Patents

UPQC seamless switching power coordination control strategy based on PAC Download PDF

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CN109193682B
CN109193682B CN201811354442.0A CN201811354442A CN109193682B CN 109193682 B CN109193682 B CN 109193682B CN 201811354442 A CN201811354442 A CN 201811354442A CN 109193682 B CN109193682 B CN 109193682B
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power
voltage
compensator
series
angle
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CN109193682A (en
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曲正伟
王宝娜
王云静
孙秀飞
侯烁
张鸿羽
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Beijing Xinxing Hezhong Technology Co.,Ltd.
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Yanshan University
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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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1807—Arrangements for adjusting, eliminating or compensating reactive power in networks using series compensators
    • 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation

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Abstract

The invention discloses a UPQC seamless switching power coordination control strategy based on PAC, which comprises the following contents: establishing a UPQC system of the integrated direct current energy storage unit; under the working condition of stable operation of the voltage of the power grid, the most sensitive power angle is introduced to control the series compensator to output reactive power, so that the necessary premise is provided for quick and smooth switching among different operation states while the design capacity of the device is fully utilized; determining a selection method of the most sensitive angle; the flow conditions of active power and reactive power in the system under different operating conditions are detailed; establishing a simple model of the direct current energy storage unit, and adopting conventional double-loop control; the complementary PWM control method is adopted to control the bidirectional flow of energy and coordinate and control the flow of energy in the system; the control system realizes flexible seamless switching among different operating conditions of voltage stable operation, voltage sag and voltage sag. The invention improves the utilization rate of UPQC, obviously reduces the total design capacity of the device, improves the dynamic response speed of the device and reduces the cost input of the device.

Description

UPQC seamless switching power coordination control strategy based on PAC
Technical Field
The invention belongs to the field of power electronics and the field of power systems, and particularly relates to a UPQC seamless switching power coordination control strategy based on PAC.
Background
In the rapid development of social economy and the increasingly improved living standard of human beings, the application permeability of sensitive equipment in a power system is greatly increased, more and more sensitive users are provided, and meanwhile, the grid-connected proportion of distributed energy is gradually increased, which provides higher new requirements for the quality of electric energy. The quality of electric energy is becoming a focus of much attention in the field of electric power. A Unified Power Quality Conditioner (UPQC) has received wide attention from researchers at home and abroad as an effective device of a comprehensive Power Quality controller having both a control function of a voltage Power Quality problem related to a Power supply side and a current Power Quality problem management function related to a load side. The UPQC is equivalent to setting a fault isolation between a power supply and a load, and can prevent the mutual influence of the power quality problems of a power grid side and a current side. In the conventional control strategy of the UPQC, no matter how the input voltage of a power grid is, the series compensator does not process reactive power, the compensation of the load reactive power is completely undertaken by the parallel compensator, and under the normal stable working condition, the UPQC is in an idle state, so that certain capacity and resource waste are caused.
Therefore, the invention provides a UPQC seamless switching power coordination control strategy based on PAC. Under the normal and stable working condition of the system, the function of the series compensator is multiplexed to reduce the reactive load of the parallel compensator, the reactive power output of the series compensator and the parallel compensator is reasonably distributed, so that the actual capacity of UPQC is fully utilized on the premise of meeting the same compensation effect, the utilization rate of equipment is increased, the reliability is improved, the sensitive power angle is selected to realize voltage and current compensation under the state, when the sudden problems of temporary drop or temporary rise and the like occur to the power voltage, the control system realizes seamless switching under different operating conditions, the impact is avoided, and the compensation sensitivity and the dynamic response speed of the device are improved. Meanwhile, the active circulation between the UPQC device and the feeder line can be effectively stabilized by coordinately controlling the energy flow of the series compensator, the parallel compensator and the direct-current energy storage unit, and the UPQC of the integrated distributed power generation system has the capability of compensating the voltage interruption of a power grid.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a UPQC seamless switching power coordination control strategy based on PAC, which can realize seamless and flexible switching between different operating conditions by controlling a series compensator to bear a part of load reactive power requirements under a voltage stable operating condition and selecting a sensitive power angle control system. Meanwhile, power flow among the series-parallel converter and the direct current energy storage unit is coordinated and controlled, active circulation is stabilized on the premise of realizing UPQC conventional compensation, impact is avoided, compensation sensitivity and dynamic response speed of the device are improved, UPQC design capacity is reduced, existing capacity of the device is fully utilized, and UPQC cost is reduced, so that the UPQC is beneficial to popularization and application in engineering.
The invention provides a UPQC seamless switching power coordination control strategy based on PAC, which comprises the following steps:
s1, determining whether the reactive power required by the load exceeds the capacity limit of the series compensator, and calculating the maximum power angle delta which can be output by the series compensatormaxThe most sensitive power angle is selected to control under the normal operation condition of voltage, and seamless flexible switching between different operation states is realized;
maximum power angle delta that the series compensator can outputmaxIs determined by formula (1), wherein UsrmaxRepresenting the maximum compensation voltage, U, that the series compensator can outputsRepresenting the actual magnitude of the grid voltage,
Figure BDA0001865719450000021
s2, performing power flow analysis on power among the series-parallel compensator and the direct-current energy storage unit, and determining a power coordination control strategy model, wherein the power coordination control strategy model comprises a series compensation unit, a parallel compensation unit and a direct-current energy storage unit, the power coordination control strategy model adopts a series-parallel part to apply a most sensitive power angle control method, the direct-current energy storage unit adopts a conventional voltage outer ring and a current inner ring to perform double-ring control to maintain a direct-current bus constant, and energy among the series-parallel compensator is coordinately controlled according to a voltage fluctuation condition and active circulating current in the model to effectively stabilize the active circulating current;
s3, selecting the most sensitive power angle as a UPQC power control angle under a stable operation working condition to realize flexible seamless switching of control of the UPQC whole device between different operation working conditions of the power grid voltage;
and S4, repeating the steps S2-S3 under different voltage operation conditions, correcting the power coordination control strategy model to obtain an optimal power coordination control strategy model, and controlling by adopting the most sensitive power angle, so that the dynamic response speed is improved, the UPQC capacity is reasonably distributed, and the overall capacity of the device is reduced on the premise of realizing dynamic adjustment.
Preferably, the following operating conditions of the grid voltage are determined according to different ranges of the voltage sag coefficient k:
when k is 1, the voltage of the power grid normally and stably operates, the series compensator is controlled to bear a part of load reactive power requirement, and meanwhile, certain active power is absorbed from the power grid;
when k is less than 1 and US≥ULWhen cos delta is generated, the voltage of the power grid falls shallowly, the series compensator outputs reactive power and absorbs active power at the same time, the parallel compensator sends out reactive power, the series compensator and the parallel compensator share the reactive power of a load, and the direct-current energy storage unit provides energy required by voltage compensation;
when k is less than 1 and US＜ULWhen cos delta is generated, the voltage of the power grid drops deeply, the series compensator outputs active power and reactive power at the same time, the parallel compensator absorbs the active power output by the series compensator, and the direct-current energy storage unit is used for maintaining the voltage at the direct-current side stable and unchanged;
when k is larger than 1, the voltage of the power grid is temporarily reduced, and the working condition is consistent with that when k is 1, the voltage of the power grid normally and stably operates;
wherein k represents a ratio of an actually detected actual voltage amplitude of the power network to a nominal voltage amplitude of the power network, with the purpose of determining an actual state of a sag or sag of the power network voltage, USAnd the actual voltage amplitude of the power grid is obtained.
Preferably, the dc energy storage unit includes a controlled voltage source and a constant internal resistance, and the controlled voltage source and the constant internal resistance are connected in series.
The method aims to establish a conventional energy storage power supply model with a controlled source and a constant internal resistance connected in series, and adopts dual-loop control of a conventional voltage outer loop and a current inner loop to maintain the voltage stability of a direct current bus, relieve the problem of power accumulation on the direct current side and effectively stabilize active circulation.
Preferably, the selection principle of the most sensitive power angle in step S3 is as follows:
when the capacity of the series compensator meets the load reactive power demand, i.e. Qse_max≥QLSelecting one half of a power angle when only the series compensator independently compensates all reactive power of the load;
when the capacity of the series compensator is not sufficient to meet the reactive power required by the load, i.e. Qse_max＜QLSelecting one half of the power angle when the series compensator has the maximum compensation power;
the most sensitive power angle control is adopted under the normal and stable operation condition, so that seamless switching among different control states can be realized when sudden faults such as sag or rise of the power supply voltage occur.
Preferably, the specific calculation step of step S3 is:
s31, maintaining the load voltage
Figure BDA0001865719450000031
On the basis of constant amplitude, the output voltage of the series compensator
Figure BDA0001865719450000032
And at an angle to the supply voltage of
Figure BDA0001865719450000033
The power angle is determined by equation (2):
Figure BDA0001865719450000034
s32, defining a seamless switching power coordination control strategy analysis and design matrix as formula (3):
Figure BDA0001865719450000035
s33, defining the voltage sag factor k as shown in equation (4):
Figure BDA0001865719450000036
s34, controlling the output voltage of the series compensator
Figure BDA0001865719450000037
And the current of the power supply
Figure BDA0001865719450000038
There is an angle difference between
Figure BDA0001865719450000039
Therefore, the amplitude and angle of the voltage output by the series compensator are respectively shown in the formula (5) and the formula (6):
Figure BDA0001865719450000041
Figure BDA0001865719450000042
s35, further calculating the active power and the reactive power injected by the series compensator as shown in the formula (7) and the formula (8):
Figure BDA0001865719450000043
Figure BDA0001865719450000044
s36 amplitude of current injected by parallel compensator
Figure BDA0001865719450000045
And angle
Figure BDA0001865719450000046
As shown in formulas (9) and (10), respectively:
Figure BDA0001865719450000047
Figure BDA0001865719450000048
s37, further calculating the active power and the reactive power injected by the parallel compensator as shown in the formula (11) and the formula (12):
Figure BDA0001865719450000049
Figure BDA00018657194500000410
wherein P isseRepresenting the active power, Q, of the output of the series compensatorseRepresenting reactive power, P, of the series compensator outputshRepresenting active power, Q, of the output of the parallel compensatorshRepresenting the reactive power output by the shunt compensator,
Figure BDA00018657194500000411
representing the current output by the shunt compensator,
Figure BDA00018657194500000412
representing the current injection angle of the shunt compensator.
Preferably, S4 specifically includes the following steps:
s41, the series compensator is controlled to bear a part of load reactive power requirement under the voltage stable operation condition, and the reactive power burden of the parallel compensator is reduced;
s42, selecting the most sensitive power angle control system to realize seamless flexible switching among different operation conditions;
and S43, power flow among the series-parallel compensator and the direct current energy storage unit is coordinated and controlled, and active circulation current is stabilized on the premise of realizing UPQC conventional compensation.
Preferably, in step S4, when k is 1, the grid voltage is normally and stably operated, and in the stable operation condition, the series compensator bears a part of the load reactive power demand, and simultaneously, along with absorbing a certain active power, the most sensitive power angle control is adopted, so that the system is easy to realize smooth and seamless switching between different operation states,
at this time, the output voltage of the series compensator is:
Figure BDA0001865719450000051
the active power and the reactive power transmitted by the series compensator are respectively as follows:
Figure BDA0001865719450000052
Figure BDA0001865719450000053
the output current of the parallel compensator is as follows:
Figure BDA0001865719450000054
the active power and the reactive power generated by the parallel compensator are respectively as follows:
Figure BDA0001865719450000055
Figure BDA0001865719450000056
equations (14) and (17) show that the active power absorbed by the series compensator is equal to the active power delivered by the parallel converter, i.e. | P, ignoring UPQC device lossessh|＝|PseForcing this part of the energy circulation to exchange with the dc energy storage unit to avoid the influence of the active circulation on the system by step S2,
when k is less than 1, the sag degrees are different, the series compensator correspondingly controls to send out or absorb certain active power, and the part of active power is used for maintaining the active current unchanged; in order to realize seamless switching of the control state, the power angle of steady operation is kept unchanged at the moment of voltage drop of the power grid, only the output voltage and the amplitude of the series compensator are changed, and when the energy to be compensated exceeds the actual capacity of the series compensator, the size of the power angle delta to be corrected is determined by the formula (1) to realize dynamic adjustment;
when k is larger than 1, the voltage of the power grid rises temporarily, and the control power angle delta is determined according to the capacity of the UPQC series compensator to realize dynamic adjustment.
The total reactive power output by the series-parallel compensator in step S5 is:
Figure BDA0001865719450000057
a power factor of 0.85 to 0.9
Figure BDA0001865719450000061
Wherein Q represents the total reactive power emitted by the UPQC, QseRepresenting reactive power, Q, of the output of the series compensatorshRepresents the martial power output by the parallel compensator,
Figure BDA0001865719450000062
representing the power factor angle and delta the power angle.
Its value is exactly equal to the load reactive power demand. The requirement of high power factor is usually satisfied, the power factor is generally required to be 0.85-0.9
Figure BDA0001865719450000063
Herein the following
Figure BDA0001865719450000064
For example, assuming that the compensation capacity required by the load does not exceed the maximum capacity of the series compensator, δ is 17.632 °. At the moment, the reactive power output by the series compensator and the parallel compensator is 0.2623UI and 0.2377UI respectively, and the active circulating current P is at the momenthIs 0.0407 UI. When the load reactive demand is compensated only by the parallel compensator, the capacity of the parallel compensator is 0.5 UI. Compared with the traditional control strategy, the control strategy has the advantages that the capacity of the parallel converter is reduced by about 52%, the reactive power output by the series compensator is larger than that of the series compensator when the series compensator equally divides the reactive power for control, and the burden of the parallel compensator is better lightened.
Compared with the prior art, the invention has the following beneficial effects:
(1) the condition that the series compensator is in standby for a long time is effectively improved, the series compensator is controlled to output reactive power under the working condition of normal and stable voltage operation so as to compensate the reactive power demand of partial load, the reactive load of the parallel compensator is reduced, and the reliability and the utilization rate of the device are improved;
(2) under the working condition of stable voltage operation, the most sensitive power angle control is adopted, and the series compensator is always in a state of being easy to control and convert to sudden faults such as voltage sag or voltage rise and the like. And the quick and smooth switching of the UPQC between different voltage operation conditions can be realized. The switching loss and the impact generation in the starting process of the series converter are effectively reduced, the dynamic response speed of the device is improved, the overall design capacity of the device is reduced, and the cost investment is reduced;
(3) energy planning of the direct current energy storage unit is added, energy flow between the direct current energy storage unit and the series-parallel compensator is coordinated and controlled, active circulation between the equipment and the feeder line is effectively stabilized, and meanwhile, the compensation capability of voltage interruption of the UPQC equipment is realized, so that the problem of electric energy quality during grid connection of distributed energy sources is solved, and the grid-connected operation capability of the distributed energy sources is enhanced;
(4) the electric energy quality of distributed power generation is improved, and the safe operation of the load is guaranteed. The UPQC has the capability of compensating the power grid voltage interruption, continuously supplies power to the load, can feed redundant electric energy into the power grid, and can continuously supply power to the load in an island mode.
Drawings
FIG. 1 shows that the voltage sag depth satisfies US＜ULSystem energy flow diagram at cos δ;
FIG. 2 shows that the voltage sag depth satisfies US≥ULSystem energy flow diagram at cos δ;
FIG. 3 is a system energy flow diagram during normal steady state operation of the voltage;
FIG. 4 is a flow chart of system energy during voltage ramp;
FIG. 5 is a dual-loop control block diagram of the DC energy storage unit;
FIG. 6 is a UPQC operation vector diagram under different operating conditions;
FIG. 7 is a vector diagram illustrating compensation of the series compensator in the event of a voltage sag;
fig. 8 is a vector diagram of compensation of the parallel compensator in the event of a voltage sag.
Detailed Description
Exemplary embodiments, features and aspects of the present invention will be described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers can indicate functionally identical or similar elements. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
The invention further describes a UPQC seamless switching power coordination control strategy based on PAC and UPQC device capacity calculation analysis in detail with reference to the attached drawings as follows:
the invention comprises the following steps:
(1) in order to realize seamless switching of different operation conditions, the most sensitive power angle is selected to control under the normal and stable operation condition of voltage;
in normal steady operation, assuming the load remains constant, the reactive power required by the load remains constant as the series compensator can compensate for the load voltage amplitude being maintained at a desired level. At this time, the power angle is also a constant value. In order to realize the flexible state switching of the series compensator, the closer the value of the power angle is to the most sensitive angle, the more beneficial to realizing the seamless switching control. The most sensitive angle is selected according to the principle that when the capacity of the series compensator is larger than the reactive power requirement of the load, one half of the power angle when only the series compensator singly compensates the whole reactive power of the load is selected, and if the capacity of the series compensator is not enough to meet the reactive power required by the load, one half of the maximum power angle which can be compensated by the series compensator is selected as the power angle when the voltage is stably and normally operated. Therefore, when sudden faults such as temporary drop or temporary rise occur to the power supply voltage, the control state conversion can be rapidly and stably realized. Maximum power angle delta capable of being output by series compensatormaxDetermined by formula (1). Wherein U issrmaxRepresenting the maximum compensation voltage that the series compensator can output. U shapesRepresenting the actual magnitude of the grid voltage.
Figure BDA0001865719450000071
(2) Power flow analysis of power among the series-parallel compensator and the direct-current energy storage unit;
under different operating conditions, energy is coordinately distributed between the series-parallel compensation unit and the direct-current energy storage unit;
fig. 1-4 show the detailed flow of the active and reactive power of the UPQC for different sag levels, normal operation and voltage sag conditions of the grid voltage. The load voltage amplitude UL is constant with a constant load. When the voltage sag value meets US < ULcos delta, the series compensator outputs active power and reactive power at the same time, the parallel compensator absorbs the active power, the active power between the series and parallel connection is transmitted and buffered through the direct-current energy storage unit, the reactive power provided by the series and parallel connection is supplied to a load, and the direct-current energy storage unit mainly acts on maintaining the voltage at the direct-current side stable and unchanged at the moment; when the voltage sag value meets the condition that US is larger than or equal to ULcos delta, the series compensator outputs reactive power and absorbs active power, the parallel compensator sends out reactive power, the active power between series connection and parallel connection is still transmitted through the direct current unit, the reactive power provided by the series compensator and the parallel compensator is supplied to a load, the direct current energy storage unit provides energy required by voltage compensation, and meanwhile, the active circulation current is stabilized; when the system voltage stably operates, except that the direct current energy storage unit is charged by the power grid, the other energy flows and the voltage drop US is the same when the voltage drop US is larger than or equal to ULcos delta; when the voltage of the power grid is temporarily increased, the energy flow analysis is similar to the voltage stabilization condition of the system; when voltage interruption occurs, the distributed power supply can supply power to the load through the parallel compensator, and island operation is achieved.
A power coordination control strategy;
the invention adopts a conventional energy storage power supply model with a controlled voltage source and a constant internal resistance connected in series, the control adopts double-loop control of a conventional voltage outer loop and a current inner loop to maintain the constant direct current bus, and meanwhile, the energy between the series-parallel compensators is coordinately controlled according to the voltage fluctuation condition and the active circulation in the device, thereby effectively stabilizing the active circulation. The dual loop control block diagram is shown in fig. 5. Wherein
Figure BDA0001865719450000081
And
Figure BDA0001865719450000082
for the dc side voltage and current reference values,
Figure BDA0001865719450000083
and
Figure BDA0001865719450000084
the actual voltage and current on the direct current side. DC1 and DC2 respectively represent two switching tubes of DC/DC, and a complementary PWM control method is adopted, so that bidirectional switching of charging and discharging states can be realized without a logic unit.
(3) Designing a seamless switching power coordination control strategy scheme;
on one hand, reactive power of a load in the traditional control is provided by the shunt compensator, so the series compensator is usually in an idle state and is not fully utilized. On the other hand, active circulating current is formed between the UPQC device and the feeder line, which increases the capacity burden and loss of the series-parallel compensator, and meanwhile, the loss is large at the moment when the compensator is turned off and turned on, and even shock happens. Aiming at the two aspects, the invention researches a UPQC seamless switching power coordination control strategy based on PAC. The strategy of the invention coordinates and controls the power flow among the series part, the parallel part and the direct current energy storage unit. The DC/DC conversion and the control of the DC energy storage unit are added, so that the energy accumulation at the DC side can be buffered, the energy between the series-parallel compensators can be transferred, and the energy required by the voltage compensation of the series-parallel compensators can be provided, the voltage at the DC side can be stabilized in a small fluctuation range, and meanwhile, the UPS function can be realized to supply power to the load when the voltage is interrupted for a short time. More importantly, the control system realizes smooth transition among different operation states. The analysis process assumes that the system operates under ideal conditions, i.e., assumes that the transformer is an ideal transformer, disregards the internal and system power losses of the UPQC, and has a power factor of 1 on the power supply side.
The analysis is performed by a UPQC seamless switching power coordination control strategy based on PAC according to different conditions of normal operation, voltage sag and voltage rise of the power distribution network in combination with FIG. 6. Fig. 6 is a working vector diagram of the UPQC under different operating conditions of the PAC-based UPQC seamless switching power coordination control strategy. At the maintenance of the load voltage
Figure BDA0001865719450000085
On the basis of constant amplitude, the output voltage of the series compensator
Figure BDA0001865719450000091
And at an angle to the supply voltage of
Figure BDA0001865719450000092
At the moment, active power and reactive power flow through the series compensator at the same time, and the reactive demand of the load is shared by the series compensator and the parallel compensator, so that the utilization rate of the series compensator is improved, the reactive burden of the parallel compensator is reduced, and a necessary premise is provided for realizing seamless switching among different running states. The determination of the power angle δ in this control is the primary solution, and is determined by equation (2).
Figure BDA0001865719450000093
The UPQC seamless switching power coordination control strategy analysis and design process based on PAC is as follows:
the following definitions are made in conjunction with fig. 6:
Figure BDA0001865719450000094
meanwhile, the voltage sag coefficient k is defined as shown in the following formula:
Figure BDA0001865719450000095
for the series compensator to output reactive power, its output voltage is controlled
Figure BDA0001865719450000096
And the current of the power supply
Figure BDA0001865719450000097
There is an angle difference between
Figure BDA0001865719450000098
Therefore, the amplitude and angle of the voltage output by the series converter are respectively shown as follows:
Figure BDA0001865719450000099
Figure BDA00018657194500000910
therefore, the active power and the reactive power injected by the series compensator can be further obtained as shown in the following formula:
Figure BDA00018657194500000911
Figure BDA00018657194500000912
to maintain the DC bus voltage at a constant level, the parallel compensator should inject current at the same time
Figure BDA00018657194500000913
This part of the active power is fed back to the power supply. The magnitude and angle of the current output by the shunt compensator are thus given by:
Figure BDA00018657194500000914
Figure BDA00018657194500000915
similarly, the active power and the reactive power injected by the parallel compensator can be further obtained as follows:
Figure BDA0001865719450000101
Figure BDA0001865719450000102
as can be seen from FIG. 6, the desired load current is made in the PAC control
Figure BDA0001865719450000103
And a power supply voltage
Figure BDA0001865719450000104
The angle between them is reduced from the power factor angle phi to alpha. The reactive power output of the shunt compensator is reduced.
(4) Under different voltage operating conditions, the control analysis and design of the series-parallel compensator are carried out;
firstly, controlling a series-parallel compensator when voltage stably operates;
through the analysis of the step three, when k is 1, the power grid voltage normally and stably operates. From the upper energy-saving flow analysis, under the stable operation working condition, the series compensator bears a part of load reactive power demand and simultaneously absorbs certain active power.
At this time, the output voltage of the series compensator is:
Figure BDA0001865719450000105
the active power and the reactive power transmitted by the series compensator are respectively as follows:
Figure BDA0001865719450000106
Figure BDA0001865719450000107
the output current of the parallel compensator is as follows:
Figure BDA0001865719450000108
the active power and the reactive power generated by the parallel compensator are respectively as follows:
Figure BDA0001865719450000109
Figure BDA00018657194500001010
equations (14) and (17) show that the active power absorbed by the series compensator is equal to the active power delivered by the parallel converter, i.e. | P, ignoring UPQC device lossessh|＝|PseL. This will create an active circulating current on the UPQC apparatus and the distribution line and this energy will not be exchanged with the dc energy storage unit, but will only flow independently between the series-parallel compensators. The direct current energy storage unit is added to force the energy to be exchanged with the direct current energy storage unit, so that the problem of direct current voltage rise caused by energy flowing on the direct current side can be effectively relieved, and the influence of active circulation can be avoided.
Controlling the series-parallel compensator when the voltage fluctuates;
through the analysis of the step three, when k is less than 1, the voltage of the power grid is subjected to temporary drop, the energy flow analysis shows that the properties of different active powers of the voltage temporary drop depth are changed, the series compensator generates active power when the temporary drop depth exceeds ULcos delta, the series compensator absorbs certain active power when the temporary drop depth is shallow, and the direct-current energy storage unit provides the active power to maintain the active current unchanged. In order to realize seamless switching and smooth transition of the control state, the power angle of the steady-state operation is kept unchanged at the moment of voltage drop, only the amplitude and the phase angle of the output voltage of the series compensator are changed, and a parameter diagram of the series compensator in voltage sag is respectively shown by combining fig. 7 and fig. 8. As can be seen from fig. 7 and 8, the reactive power distribution between the series-parallel compensators and the output energy are kept constant. Therefore, the system can realize smooth switching from the stable operation state to the voltage sag state without generating impact.
When the voltage sag happens, the voltage sag coefficient k is detected firstly, so that the compensation reference voltage of the series compensator is determined, and when the required compensation energy exceeds the actual capacity of the series compensator, the delta can be controlled to reduce the reactive power output of the series compensator, and simultaneously, the reactive power output of the parallel compensator is increased, so that dynamic adjustment is realized, and the requirements of voltage sag compensation and load reactive power are met. When the detected k value is 1, the control and power distribution are switched to the stable operation state. And similarly, when k is detected to be more than 1, the voltage of the power grid rises temporarily, the energy analysis is simpler when the voltage rises temporarily, and the control implementation steps are similar to those of voltage stable operation.
(5) Designing UPQC capacity;
the total reactive power output by the series-parallel compensator is as follows:
Figure BDA0001865719450000111
its value is exactly equal to the load reactive power demand. In consideration of actual working conditions, the capacity distribution of the series-parallel compensator can be properly adjusted according to needs, so that a proper capacity value can be flexibly designed according to compensation requirements. At the moment, reactive power output by the power grid meets the active requirement of a load, and meanwhile, the direct-current energy storage unit is charged through the parallel compensator, so that the energy requirements of the direct-current energy storage unit for balancing direct-current partial energy, compensating voltage during voltage sag and supplying power for a short time are met.
The requirement of high power factor is usually satisfied, the power factor is generally required to be 0.85-0.9
Figure BDA0001865719450000112
Herein the following
Figure BDA0001865719450000113
For example, assume that the compensation capacity required by the load does not exceed the series compensationThe maximum capacity of the device is 17.632 °. The reactive power output by the series-parallel compensator at the moment is 0.2623UI and 0.2377UI respectively, and the active circulating current at the moment is 0.042 UI. When the load reactive demand is compensated only by the parallel compensator, the capacity of the parallel compensator is 0.5 UI. Compared with the traditional control strategy, the control strategy has the advantages that the capacity of the parallel converter is reduced by about 52%, the reactive power output by the series compensator is larger than that of the series compensator when the series compensator equally divides the reactive power for control, and the burden of the parallel compensator is better lightened. Table 1 shows the comparison of the effect parameters of the control strategy used in the conventional parallel compensator for independently bearing the load reactive power, the series-parallel compensator for equally dividing the load reactive power, and the control strategy used in this document. It can be seen that the control used herein has significant effects in terms of active circulation reduction, share of reactive power by the series compensator, and overall capacity reduction of the UPQC. Meanwhile, the existing capacity of the series compensator is not increased, so that the cost of the device is greatly reduced.
Figure BDA0001865719450000121
TABLE 1 capacity of UPQC under different controls
(6) And a reasonable experiment platform is set to verify the feasibility and the effectiveness of the seamless control strategy through the correctness of a simulation verification theory.
The setting experiment of the invention comprises UPQC function verification under three conditions of voltage sag, voltage rise and stable operation. Compared with a control strategy that a series compensator does not process reactive power, the control method disclosed by the invention can improve the utilization rate of the device and reduce the design capacity of the device. Under the voltage stable operation condition, the most sensitive power angle control is selected, and the system control can be quickly and smoothly switched to the voltage dip operation condition or the voltage lift operation condition. The control strategy of the invention is verified to fully utilize the existing capacity of the device and improve the dynamic response speed of the system on the premise of realizing the traditional function.
Finally, it should be noted that: the above-mentioned embodiments are only used for illustrating the technical solution of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims (5)

1. A UPQC seamless switching power coordination control strategy based on PAC is characterized in that: which comprises the following steps:
s1, determining whether the reactive power required by the load exceeds the capacity limit of the series compensator, and calculating the maximum power angle delta which can be output by the series compensatormaxThe most sensitive power angle is selected to control under the normal operation condition of voltage, and seamless flexible switching between different operation states is realized;
maximum power angle delta that the series compensator can outputmaxIs determined by formula (1), wherein UsrmaxRepresenting the maximum compensation voltage, U, that the series compensator can outputsRepresenting the actual magnitude of the grid voltage,
Figure FDA0003007372890000011
s2, performing power flow analysis on power among the series-parallel compensator and the direct-current energy storage unit, and determining a power coordination control strategy model, wherein the power coordination control strategy model comprises a series compensation unit, a parallel compensation unit and a direct-current energy storage unit, the power coordination control strategy model adopts a series-parallel part to apply a most sensitive power angle control method, the direct-current energy storage unit adopts a conventional voltage outer ring and a current inner ring to perform double-ring control to maintain a direct-current bus constant, and energy among the series-parallel compensator is coordinately controlled according to a voltage fluctuation condition and active circulating current in the model to effectively stabilize the active circulating current;
s3, selecting the most sensitive power angle as a UPQC power control angle under a stable operation working condition to realize flexible seamless switching of control of the UPQC whole device between different operation working conditions of the power grid voltage;
s4, under different voltage operation conditions, repeating the steps S2-S3, correcting the power coordination control strategy model to obtain an optimal power coordination control strategy model, controlling by adopting the most sensitive power angle, improving the dynamic response speed on the premise of realizing dynamic adjustment, reasonably distributing the UPQC capacity and reducing the whole capacity of the device;
the most sensitive power angle in step S3 is selected as follows:
when the capacity of the series compensator meets the load reactive power demand, i.e. Qse_max≥QLSelecting one half of a power angle when only the series compensator independently compensates all reactive power of the load;
when the capacity of the series compensator is not sufficient to meet the reactive power required by the load, i.e. Qse_max＜QLSelecting one half of the power angle when the series compensator has the maximum compensation power;
the method is characterized in that the most sensitive power angle control is adopted under the normal and stable operation condition, so that seamless switching among different control states can be realized when sudden faults such as sag or rise of the power supply voltage occur;
the specific calculation step for selecting the most sensitive power angle in step S3 is:
s31, maintaining the load voltage
Figure FDA0003007372890000012
On the basis of constant amplitude, the output voltage of the series compensator
Figure FDA0003007372890000013
And at an angle to the supply voltage of
Figure FDA0003007372890000014
The power angle is determined by equation (2):
Figure FDA0003007372890000021
s32, defining a seamless switching power coordination control strategy analysis and design matrix as formula (3):
Figure FDA0003007372890000022
s33, defining the voltage sag factor k as shown in equation (4):
Figure FDA0003007372890000023
s34, controlling the output voltage of the series compensator
Figure FDA0003007372890000024
And the current of the power supply
Figure FDA0003007372890000025
There is an angle difference between
Figure FDA0003007372890000026
Therefore, the amplitude and angle of the voltage output by the series compensator are respectively shown in the formula (5) and the formula (6):
Figure FDA0003007372890000027
Figure FDA0003007372890000028
s35, further calculating the active power and the reactive power injected by the series compensator as shown in the formula (7) and the formula (8):
Figure FDA0003007372890000029
Figure FDA00030073728900000210
s36 amplitude of current injected by parallel compensator
Figure FDA00030073728900000211
And angle
Figure FDA00030073728900000212
As shown in formulas (9) and (10), respectively:
Figure FDA00030073728900000213
Figure FDA00030073728900000214
s37, further calculating the active power and the reactive power injected by the parallel compensator as shown in the formula (11) and the formula (12):
Figure FDA00030073728900000215
Figure FDA00030073728900000216
wherein P isseRepresenting the active power, Q, of the output of the series compensatorseRepresenting reactive power, P, of the series compensator outputshRepresenting active power, Q, of the output of the parallel compensatorshRepresenting the reactive power output by the shunt compensator,
Figure FDA00030073728900000217
representing the current output by the shunt compensator,
Figure FDA00030073728900000218
indicating the current injection angle, U, of the parallel compensatorsRepresenting the actual magnitude of the grid voltage.
2. The PAC-based UPQC seamless switching power coordination control strategy of claim 1, wherein:
determining the following operating conditions of the power grid voltage according to different ranges of the voltage sag coefficient k:
when k is 1, the voltage of the power grid normally and stably operates, the series compensator is controlled to bear a part of load reactive power requirement, and meanwhile, certain active power is absorbed from the power grid;
when k is less than 1 and US≥ULWhen cos delta is generated, the voltage of the power grid falls shallowly, the series compensator outputs reactive power and absorbs active power at the same time, the parallel compensator sends out reactive power, the series compensator and the parallel compensator share the reactive power of a load, and the direct-current energy storage unit provides energy required by voltage compensation;
when k is less than 1 and US＜ULWhen cos delta is generated, the voltage of the power grid drops deeply, the series compensator outputs active power and reactive power at the same time, the parallel compensator absorbs the active power output by the series compensator, and the direct-current energy storage unit is used for maintaining the voltage at the direct-current side stable and unchanged;
when k is larger than 1, the voltage of the power grid is temporarily reduced, and the working condition is consistent with that when k is 1, the voltage of the power grid normally and stably operates;
wherein k represents a ratio of an actually detected actual voltage amplitude of the power network to a nominal voltage amplitude of the power network, with the purpose of determining an actual state of a sag or sag of the power network voltage, USAnd the actual voltage amplitude of the power grid is obtained.
3. The PAC-based UPQC seamless switching power coordination control strategy of claim 1, wherein: the direct current energy storage unit comprises a controlled voltage source and a fixed value internal resistance, and the controlled voltage source and the fixed value internal resistance are connected in series.
4. The PAC-based UPQC seamless switching power coordination control strategy of claim 1, wherein: s4 specifically includes the following steps:
s41, the series compensator is controlled to bear a part of load reactive power requirement under the voltage stable operation condition, and the reactive power burden of the parallel compensator is reduced;
s42, selecting the most sensitive power angle control system to realize seamless flexible switching among different operation conditions;
and S43, power flow among the series-parallel compensator and the direct current energy storage unit is coordinated and controlled, and active circulation current is stabilized on the premise of realizing UPQC conventional compensation.
5. The PAC-based UPQC seamless switching power coordination control strategy of claim 4, wherein: in step S4, when k is 1, the grid voltage operates normally and stably, under the stable operation condition, the series compensator bears a part of the reactive power demand of the load, and simultaneously adopts the most sensitive power angle control along with absorbing a certain active power, so that the system is easy to realize smooth and seamless switching between different operation states,
at this time, the output voltage of the series compensator is:
Figure FDA0003007372890000031
the active power and the reactive power transmitted by the series compensator are respectively as follows:
Figure FDA0003007372890000041
Figure FDA0003007372890000042
the output current of the parallel compensator is as follows:
Figure FDA0003007372890000043
the active power and the reactive power generated by the parallel compensator are respectively as follows:
Figure FDA0003007372890000044
Figure FDA0003007372890000045
equations (14) and (17) show that the active power absorbed by the series compensator is equal to the active power delivered by the parallel converter, i.e. | P, ignoring UPQC device lossessh|＝|PseForcing this part of the energy circulation to exchange with the dc energy storage unit to avoid the influence of the active circulation on the system by step S2,
when k is less than 1, the sag degrees are different, the series compensator correspondingly controls to send out or absorb certain active power, and the part of active power is used for maintaining the active current unchanged; in order to realize seamless switching of the control state, the power angle of steady operation is kept unchanged at the moment of voltage drop of the power grid, only the output voltage and the amplitude of the series compensator are changed, and when the energy to be compensated exceeds the actual capacity of the series compensator, the size of the power angle delta to be corrected is determined by the formula (1) to realize dynamic adjustment;
when k is larger than 1, the voltage of the power grid rises temporarily, and the control power angle delta is determined according to the capacity of the UPQC series compensator to realize dynamic adjustment.
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