CN107317345B - Method for controlling frequency of isolated power grid by participation of electrolysis load - Google Patents
Method for controlling frequency of isolated power grid by participation of electrolysis load Download PDFInfo
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- 238000005868 electrolysis reaction Methods 0.000 title claims abstract description 7
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- 230000002159 abnormal effect Effects 0.000 claims description 4
- AZDRQVAHHNSJOQ-UHFFFAOYSA-N alumane Chemical compound [AlH3] AZDRQVAHHNSJOQ-UHFFFAOYSA-N 0.000 claims description 4
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H02J3/382—
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
- H02J3/48—Controlling the sharing of the in-phase component
-
- 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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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- 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
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Abstract
The invention belongs to the technical field of power system operation and control, and particularly relates to a method for enabling electrolysis loads to participate in isolated power grid frequency control, wherein system frequency adjustment is realized through an electrolytic aluminum load control system, and the electrolytic aluminum load control system comprises a network control unit NCU, a dead zone unit, a proportional amplification unit, a WASHOUT unit and an amplitude limiting unit; the proportion amplification unit is connected with the WASHOUT unit in parallel, one end of the proportion amplification unit is sequentially connected with the dead zone unit and the network control unit NCU, and the other end of the proportion amplification unit is connected with the amplitude limiting unit; the network control unit NCU is connected with the isolated power grid system, and the amplitude limiting unit is connected with the WAMS control master station; the method specifically comprises the following steps: 1. establishing an isolated power grid system model; 2. designing control logics of the electrolytic aluminum load controller under different power disturbance modes; 3. and designing parameters of the electrolytic aluminum load controller under different power disturbance modes. The method utilizes the heat energy storage characteristic of the electrolytic aluminum load to realize effective power regulation of the electrolytic aluminum load and maintain the frequency stability of the system.
Description
Technical Field
The invention belongs to the technical field of operation and control of power systems, and particularly relates to a method for controlling the frequency of an isolated power grid by electrolytic loads.
Background
The mode that renewable energy is consumed on the spot by forming an isolated power grid by using high energy consumption load, thermal power and renewable energy has attracted wide attention, however, due to the randomness and volatility of the renewable energy and the defects of small inertia, low reserve capacity and the like of the isolated power grid, along with the gradual increase of the proportion of the renewable energy in the isolated power grid, the frequency stability of the isolated power grid containing high-permeability wind power faces a great challenge.
For the problem of safe and stable operation of an isolated power grid containing high-permeability renewable energy, one method is to equip a local power grid with an energy storage device, however, for the local power grid containing centralized renewable energy, the capacity of the local power grid is often as high as million kilowatts, and the capacity of the conventional energy storage device is difficult to match.
The other method is to develop local load control capability to enable the load to participate in system frequency adjustment, and corresponding research is available for direct load control based on commercial loads such as HVAC, electric pumps and the like at home and abroad. However, the commercial load and the residential load are small in single capacity and are relatively dispersed, and it is difficult to perform large-scale aggregation to stabilize power fluctuation of the centralized renewable energy source.
The research of the participation of the electrolytic aluminum load in the system frequency control draws great attention, however, the existing control method only aims at the instant power disturbance of an isolated power grid, and the problem of continuous power disturbance of the isolated power grid is difficult to solve.
Disclosure of Invention
The invention aims to provide a control structure based on participation of electrolytic aluminum in frequency control, and designs control logics of a controller and a calculation method of control parameters under different power disturbance scenes.
In order to achieve the purpose, the invention adopts the technical scheme that: a method for controlling electrolytic load to participate in isolated power grid frequency realizes system frequency regulation through an electrolytic aluminum load control system, wherein the electrolytic aluminum load control system comprises a network control unit NCU, a dead zone unit, a proportional amplification unit, a WASHOUT unit and an amplitude limiting unit; the proportion amplification unit is connected with the WASHOUT unit in parallel, one end of the proportion amplification unit is sequentially connected with the dead zone unit and the network control unit NCU, and the other end of the proportion amplification unit is connected with the amplitude limiting unit; the network control unit NCU is connected with the isolated power grid system, and the amplitude limiting unit is connected with the WAMS control master station; the method specifically comprises the following steps:
and 3, designing parameters of the electrolytic aluminum load controller in different power disturbance modes.
In the method for participating in isolated grid frequency control of electrolysis loads, the implementation of step 1 comprises the following steps:
step 1.1, obtaining an electrolytic aluminum load power-voltage mathematical model by establishing an electrolytic aluminum load topological structure; the method comprises the following specific steps:
1) the electrolytic aluminum load topological structure is equivalent to that a back electromotive force E is connected with an equivalent resistor R in series, and is connected with an alternating current bus line through a rectifier bridge, a saturable reactor and an on-load tap changer;
2) active power P of electrolytic aluminium loadASLAnd a DC side voltage VBThe relationship is as follows:
3) electrolytic aluminum load DC side bus voltage VBBus voltage V on the AC side of loadAHIs as in formula (2);
(2) in the formula VSRThe voltage drop of the saturable reactor is shown, and k is the transformation ratio of the on-load tap changing transformer;
4) changing the voltage of the high voltage side of the load alternating current bus by changing the voltage of the generator terminal; obtaining a voltage change Δ V of the generator terminal according to a voltage sensitivity methodGVoltage delta V of high voltage side of AC bus with loadAHThe relationship of (a) is as in formula (3);
ΔVAH＝Ksens·ΔVG(3)
(3) in the formula, KsensThe voltage sensitivity coefficient of the bus i to the bus j is obtained;
5) the generator terminal voltage variation delta V can be obtained by the formulas (2) and (3)GActive power variation delta P with loadASLThe corresponding relationship of (A) is:
ΔPASL＝KASLΔVG(4)
(4) in the formula, KASLThe load comprehensive proportionality coefficient;
step 1.2, acquiring electrical parameters of an isolated power grid system, and establishing a mathematical model of the isolated power grid system, wherein the mathematical model comprises a thermal power unit excitation system, a thermal power unit speed regulator, a thermal power unit, a transmission line, a transformer, a double-fed wind driven generator and an electrolytic aluminum load;
obtaining parameters of an inertia link by using least square curve fitting, wherein the parameters are as shown in a formula (5);
ΔPASL(t)＝-0.054+0.054·e-2.782t(5)
written in the form of a transfer function, as in equation (6);
(6) in the formula, KASLIs the load amplification factor of the electrolytic aluminum; t isASLThe time constant of active power of electrolytic aluminum load.
In the method for participating in isolated grid frequency control of electrolysis loads, the step 2 is realized by:
step 2.1, establishing an electrolytic aluminum load closed-loop control system;
the electrolytic aluminum load closed-loop control system comprises a conventional WAMS system, an additional WAMS control master station, a downlink channel and a network control unit NCU; the conventional WAMS system comprises a PMU substation, an upstream channel and a WAMS main station; the WAMS main station communicates with the PMU substation through a TCP/IP protocol to realize system data acquisition and state monitoring; the WAMS control main station communicates with the PMU substation by adopting a UDP protocol, and directly acquires the state quantity of the PMU substation; the WAMS controls the main station to calculate control parameters in real time and sends the control parameters to a network control unit NCU;
the network control unit NCU is arranged on the side of the generator excitation cabinet to realize the control of the active power of the electrolytic aluminum load; the output port of the NCU is accessed to the reference voltage superposition point of the generator excitation system, and the acquired frequency signal is used as a feedback signal to be input; when power disturbance occurs in the system, the frequency deviation acts on a reference voltage superposition point of an excitation system of the thermal power generating unit through a Network Control Unit (NCU) to change the excitation voltage, so that the voltage of the alternating current side of the electrolytic aluminum load is changed, and the active power of the electrolytic aluminum load is adjusted to stabilize the unbalanced power of the system;
step 2.2, designing the control logic of the electrolytic aluminum load controller;
I) monitoring system power disturbance according to PMU substation information, and judging the system state;
II) when the system state is instantaneous power disturbance or continuous power fluctuation, the WAMS controls the master station to issue control parameters;
III) collecting local frequency signals, and judging the communication state of the WAMS control master station;
IV) if the communication state of the WAMS control master station is normal, receiving the WAMS master station parameters, and updating the NCU parameters of the network control unit to control the excitation voltage of the generator;
v) if the communication state of the main station is abnormal, the excitation voltage of the generator is controlled by locally storing control parameters by the network control unit NCU.
In the method for participating in isolated grid frequency control of electrolysis loads, the step 3 is realized by:
step 3.1, designing parameters of a proportional amplification unit of the electrolytic aluminum load controller;
the proportion amplifying unit provides steady-state power support for the system, and the power disturbance quantity delta P generated by the system comprises the load adjustment quantity delta P of the electrolytic aluminumASLregPrimary regulating quantity delta P of thermal power generating unitGreg(ii) a The primary regulating quantity and the system frequency variable quantity of the thermal power generating unit satisfy the following formula:
(7) in the formula: f. ofNTo a rated frequency, fregIs a steady-state value of the system frequency after primary frequency modulation, PGNRated power of the generator set, RGThe difference adjustment coefficient of the thermal power generating unit is obtained;
the relationship between the electrolytic aluminum load adjustment amount and the system frequency variation satisfies the following formula (8):
(8) in the formula: Δ fASLLbFor the dead band range of the load controller, PASLNRated power for electrolytic aluminum load, RASLThe load equivalent adjustment coefficient of the electrolytic aluminum is obtained;
the power regulation amount borne by the electrolytic aluminum load is as follows:
ΔPASLreg＝ΔP-ΔPGreg(9)
(9) in the formula, the system power disturbance quantity delta P is calculated in two situations;
a. under the condition of instantaneous power disturbance, the system power disturbance quantity is delta PSThen, there is formula (10):
ΔP＝ΔPS(10)
b. counting the maximum fluctuation quantity delta p of the wind power for a period of time under continuous power fluctuationwindmax(ii) a The fluctuation amount is used as a system power disturbance amount delta P, and an amplification factor K is calculatedPThen, formula (11):
ΔP＝ΔPwindmax(11)
the load equivalent adjustment coefficient of the electrolytic aluminum is obtained by the following equations (9), (10) and (11):
for an electrolytic aluminum load controller acting on a generator excitation system, equation (13) is satisfied:
when the electrolytic aluminum load active power changesThe chemical quantity is delta PASLregIn time, the variation of the terminal voltage of the thermal power generating unit is as follows:
the proportional amplification factor K is obtained from the formula (13) and the formula (14)P:
Step 3.2, designing parameters of a WASHOUT unit of the electrolytic aluminum load controller;
calculating WASHOUT unit parameters through an SFR model;
WASHOUT unit parameter TDFor high-pass filtering, generally selecting 6-7 s;
Δ f(s) and Δ PLThe relationship of(s) is:
(16) in the formula:
(17) in the formulas (18), M is the equivalent inertia coefficient of the system, D is the load damping coefficient, and TD、TR、FH、KMThe time constant of the generator is R is the droop coefficient of the speed regulator of the generator;
ignoring the pole-zero far from the imaginary axis, equation (16) can be converted to:
(19) in the formula, the parameter z2,ζ,ωn,ωdAll of which can pass through SFR modelCalculating parameters to obtain;
the lagrange transform of the output signal of the SFR model under the unit step function is obtained from equation (19):
in formula (20):
performing reverse Laplace transformation on the formula (20) to obtain
(21) In the formula:
let t bezAt this time, the system frequency offset is maximum, and the slope of the frequency change amount at this time is zero, which can be expressed by equation (22):
the maximum offset occurrence time t of the system frequency can be obtained by the formula (22)zBy adjusting WASHOUT unit KDParameter, maximum frequency offset of system Δ fdevmaxIs maintained at Δ fmaxAccordingly, formula (23) can be obtained:
the WASHOUT unit coefficient K can be obtained from the expressions (22) - (23)D。
The invention has the beneficial effects that:
1. the load controller fully considers the complex working conditions of the actual site, and the industrial application of the load controller can be realized.
2. The method can effectively identify the instantaneous power disturbance and the continuous power disturbance of an isolated power grid system, automatically use different control modes for different disturbances, realize effective power regulation of electrolytic aluminum load and maintain the stable frequency of the system.
3. On-site tests have been carried out on the industrial site, and the test results prove the effectiveness of the control method of the invention.
Drawings
FIG. 1 is a mathematical model of the load of electrolytic aluminum created according to one embodiment of the present invention in conjunction with the principles of the electrolytic aluminum process;
FIG. 2 is a dynamic power fitting curve of the load of the electrolytic aluminum, which is established by the measured data according to one embodiment of the invention;
FIG. 3 is a diagram of the topology of an electrolytic aluminum load controller in accordance with one embodiment of the present invention;
FIG. 4 is a control logic diagram of an electrolytic aluminum load controller in accordance with an embodiment of the present invention;
FIG. 5(a) is a wind power graph under a closed loop control test considering a rapid drop in wind power according to an embodiment of the present invention;
FIG. 5(b) is a system frequency variation graph under a closed-loop control test considering a rapid drop in wind power according to an embodiment of the present invention;
FIG. 6(a) is a graph of voltage variation of generator terminal under a closed-loop control test considering rapid decrease of wind power according to an embodiment of the present invention;
FIG. 6(b) is a graph of the voltage variation of the load bus under a closed-loop control test considering the rapid decrease of the wind power according to an embodiment of the present invention;
FIG. 7(a) is a graph of active power change of electrolytic aluminum in a closed loop control test considering rapid wind power droop in accordance with an embodiment of the present invention;
FIG. 7(b) is a graph of the active power change of the generator under a closed-loop control test considering the rapid wind power drop according to an embodiment of the present invention;
FIG. 8 is a graph of wind power change under a closed loop control test with consideration of wind farm removal in accordance with an embodiment of the present invention;
FIG. 9(a) is a graph of the frequency change of a loaded controller system under a closed loop control test with consideration of wind farm shedding for one embodiment of the present invention;
FIG. 9(b) is a graph of the frequency change of a system without a load controller under a closed loop control test with consideration of wind farm shedding in accordance with an embodiment of the present invention;
FIG. 10(a) is a graph of active power change for an electrolytic aluminum load with a load controller under a closed loop control test with consideration of wind farm removal in accordance with an embodiment of the present invention;
FIG. 10(b) is a graph of the change in active power without the load controller electrolytic aluminum load under a closed loop control test with consideration of wind farm removal, in accordance with an embodiment of the present invention.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
The embodiment is realized by the following technical scheme that the method for controlling the frequency of the isolated power grid by the electrolytic load is characterized in that the system frequency is adjusted by an electrolytic aluminum load control system, wherein the electrolytic aluminum load control system comprises a network control unit NCU, a dead zone unit, a proportional amplification unit, a WASHOUT unit and an amplitude limiting unit; the proportion amplification unit is connected with the WASHOUT unit in parallel, one end of the proportion amplification unit is sequentially connected with the dead zone unit and the network control unit NCU, and the other end of the proportion amplification unit is connected with the amplitude limiting unit; the network control unit NCU is connected with the isolated power grid system, and the amplitude limiting unit is connected with the WAMS control master station; the method specifically comprises the following steps:
and 3, designing parameters of the electrolytic aluminum load controller in different power disturbance modes.
Further, the implementation of step 1 includes:
step 1.1, obtaining an electrolytic aluminum load power-voltage mathematical model by establishing an electrolytic aluminum load topological structure; the method comprises the following specific steps:
1) the electrolytic aluminum load topological structure is equivalent to that a back electromotive force E is connected with an equivalent resistor R in series, and is connected with an alternating current bus line through a rectifier bridge, a saturable reactor and an on-load tap changer;
2) active power P of electrolytic aluminium loadASLAnd a DC side voltage VBThe relationship is as follows:
3) electrolytic aluminum load DC side bus voltage VBBus voltage V on the AC side of loadAHIs as in formula (2);
(2) in the formula VSRThe voltage drop of the saturable reactor is shown, and k is the transformation ratio of the on-load tap changing transformer;
4) changing the voltage of the high voltage side of the load alternating current bus by changing the voltage of the generator terminal; obtaining a voltage change Δ V of the generator terminal according to a voltage sensitivity methodGVoltage delta V of high voltage side of AC bus with loadAHThe relationship of (a) is as in formula (3);
ΔVAH＝Ksens·ΔVG(3)
(3) in the formula, KsensThe voltage sensitivity coefficient of the bus i to the bus j is obtained;
5) the generator terminal voltage variation delta V can be obtained by the formulas (2) and (3)GActive power variation delta P with loadASLThe corresponding relationship of (A) is:
ΔPASL＝KASLΔVG(4)
(4) in the formula, KASLThe load comprehensive proportionality coefficient;
step 1.2, acquiring electrical parameters of an isolated power grid system, and establishing a mathematical model of the isolated power grid system, wherein the mathematical model comprises a thermal power unit excitation system, a thermal power unit speed regulator, a thermal power unit, a transmission line, a transformer, a double-fed wind driven generator and an electrolytic aluminum load;
obtaining parameters of an inertia link by using least square curve fitting, wherein the parameters are as shown in a formula (5);
ΔPASL(t)＝-0.054+0.054·e-2.782t(5)
written in the form of a transfer function, as in equation (6);
(6) in the formula, KASLIs the load amplification factor of the electrolytic aluminum; t isASLThe time constant of active power of electrolytic aluminum load.
Further, the implementation of step 2 includes:
step 2.1, establishing an electrolytic aluminum load closed-loop control system;
the electrolytic aluminum load closed-loop control system comprises a conventional WAMS system, an additional WAMS control master station, a downlink channel and a network control unit NCU; the conventional WAMS system comprises a PMU substation, an upstream channel and a WAMS main station; the WAMS main station communicates with the PMU substation through a TCP/IP protocol to realize system data acquisition and state monitoring; the WAMS control main station communicates with the PMU substation by adopting a UDP protocol, and directly acquires the state quantity of the PMU substation; the WAMS controls the main station to calculate control parameters in real time and sends the control parameters to a network control unit NCU;
the network control unit NCU is arranged on the side of the generator excitation cabinet to realize the control of the active power of the electrolytic aluminum load; the output port of the NCU is accessed to the reference voltage superposition point of the generator excitation system, and the acquired frequency signal is used as a feedback signal to be input; when power disturbance occurs in the system, the frequency deviation acts on a reference voltage superposition point of an excitation system of the thermal power generating unit through a Network Control Unit (NCU) to change the excitation voltage, so that the voltage of the alternating current side of the electrolytic aluminum load is changed, and the active power of the electrolytic aluminum load is adjusted to stabilize the unbalanced power of the system;
step 2.2, designing the control logic of the electrolytic aluminum load controller;
I) monitoring system power disturbance according to PMU substation information, and judging the system state;
II) when the system state is instantaneous power disturbance or continuous power fluctuation, the WAMS controls the master station to issue control parameters;
III) collecting local frequency signals, and judging the communication state of the WAMS control master station;
IV) if the communication state of the WAMS control master station is normal, receiving the WAMS master station parameters, and updating the NCU parameters of the network control unit to control the excitation voltage of the generator;
v) if the communication state of the main station is abnormal, the excitation voltage of the generator is controlled by locally storing control parameters by the network control unit NCU.
Further, the implementation of step 3 includes:
step 3.1, designing parameters of a proportional amplification unit of the electrolytic aluminum load controller;
the proportion amplifying unit provides steady-state power support for the system, and the power disturbance quantity delta P generated by the system comprises the load adjustment quantity delta P of the electrolytic aluminumASLregPrimary regulating quantity delta P of thermal power generating unitGreg(ii) a The primary regulating quantity and the system frequency variable quantity of the thermal power generating unit satisfy the following formula:
(7) in the formula: f. ofNTo a rated frequency, fregIs a steady-state value of the system frequency after primary frequency modulation, PGNRated power of the generator set, RGThe difference adjustment coefficient of the thermal power generating unit is obtained;
the relationship between the electrolytic aluminum load adjustment amount and the system frequency variation satisfies the following formula (8):
(8) in the formula: Δ fASLLbFor the dead band range of the load controller, PASLNRated power for electrolytic aluminum load, RASLThe load equivalent adjustment coefficient of the electrolytic aluminum is obtained;
the power regulation amount borne by the electrolytic aluminum load is as follows:
ΔPASLreg＝ΔP-ΔPGreg(9)
(9) in the formula, the system power disturbance quantity delta P is calculated in two situations;
a. under the condition of instantaneous power disturbance, the system power disturbance quantity is delta PSThen, there is formula (10):
ΔP＝ΔPS(10)
b. under continuous power fluctuation, counting the maximum fluctuation quantity delta pwindmax of the wind power for a period of time; the fluctuation amount is used as a system power disturbance amount delta P, and an amplification factor K is calculatedPThen, formula (11):
ΔP＝ΔPwindmax(11)
the load equivalent adjustment coefficient of the electrolytic aluminum is obtained by the following equations (9), (10) and (11):
for an electrolytic aluminum load controller acting on a generator excitation system, equation (13) is satisfied:
when the active power variation of the electrolytic aluminum load is delta PASLregIn time, the variation of the terminal voltage of the thermal power generating unit is as follows:
the proportional amplification factor K is obtained from the formula (13) and the formula (14)P:
Step 3.2, designing parameters of a WASHOUT unit of the electrolytic aluminum load controller;
calculating WASHOUT unit parameters through an SFR model;
WASHOUT unit parameter TDFor high-pass filtering, generally selecting 6-7 s;
Δ f(s) and Δ PLThe relationship of(s) is:
(16) in the formula:
(17) in the formulas (18), M is the equivalent inertia coefficient of the system, D is the load damping coefficient, and TD、TR、FH、KMThe time constant of the generator is R is the droop coefficient of the speed regulator of the generator;
ignoring the pole-zero far from the imaginary axis, equation (16) can be converted to:
(19) in the formula, the parameter z2,ζ,ωn,ωdAll can be obtained by the parameter calculation of the SFR model;
the lagrange transform of the output signal of the SFR model under the unit step function is obtained from equation (19):
in formula (20):
performing reverse Laplace transformation on the formula (20) to obtain
(21) In the formula:
let t bezAt this time, the system frequency offset is maximum, and the slope of the frequency change amount at this time is zero, which can be expressed by equation (22):
the maximum offset occurrence time t of the system frequency can be obtained by the formula (22)zBy adjusting WASHOUT unit KDParameter, maximum frequency offset of system Δ fdevmaxIs maintained at Δ fmaxAccordingly, formula (23) can be obtained:
the WASHOUT unit coefficient K can be obtained from the expressions (22) - (23)D。
In specific implementation, the method for controlling the frequency of the isolated power grid by the electrolytic load comprises the following steps:
s1, modeling an isolated power grid system: obtaining an electrolytic aluminum load topological structure, and establishing an electrolytic aluminum load power-voltage mathematical model; obtaining electrical parameters of an isolated power grid, establishing a mathematical model of an isolated power grid system, wherein the mathematical model comprises a thermal power unit excitation system, a thermal power unit speed regulator, a thermal power unit, a transmission line, a transformer, a double-fed wind driven generator and an electrolytic aluminum load, and establishing a simulation model of the isolated power grid system in a real-time digital simulation system (RTDS).
S2, designing a topological structure of the electrolytic aluminum load controller. And designing a topological structure of the controller according to the transient process power support and the steady process power support by combining the wind power fluctuation characteristic of the isolated power grid. The topological structure of the controller is divided into a proportion amplifying part, and power support is mainly provided in a steady-state process; the wasbout portion, mainly provides power support during transients.
And S3, designing action logics of the electrolytic aluminum load controller under different disturbance conditions. The electrolytic aluminum load controller can automatically identify the power disturbance of the system, and the power disturbance is divided into instant power disturbance and continuous power disturbance. Under different power disturbance modes, the control parameters of the electrolytic aluminum load controller are different. Meanwhile, the electrolytic aluminum load controller has the functions of local control and online control. When the communication is normal, the electrolytic aluminum load controller adopts an online control function; when the communication is abnormal, the electrolytic aluminum load controller adopts a local control mode.
And S4, designing parameters of the electrolytic aluminum load controller under different power disturbance modes. When the system has instantaneous power disturbance, the proportion amplifying part of the electrolytic aluminum load controller and the WASHOUT part act together, and the adjusting power born by the thermal power generating unit and the electrolytic aluminum load controller is calculated according to the unbalanced power generated by the wide area measurement system. And then calculating a parameter KP of the proportional amplification part. And calculating the parameters of the WASHOUT part according to the maximum offset of the system frequency greater than a certain limit value. When the system has continuous power fluctuation, the parameter of the proportional amplification part is calculated according to historical wind power data, and the WASHOUT part does not act in the mode.
And S5, respectively carrying out comparison tests on the designed electrolytic aluminum load controller on an RTDS simulation platform and an industrial field, and checking the control effect of the load controller under the instantaneous disturbance of the wind power and the continuous fluctuation of the wind power.
The following is further explained in conjunction with the accompanying drawings.
Modeling of electrolytic aluminum load
The electrolytic aluminum load is a heat storage type load, the power regulation speed is high, and dynamic power support can be provided for an isolated power grid system. The electrolytic aluminum load can be equivalent to that the back electromotive force E is connected with the equivalent resistor R in series, and is connected with an alternating current bus line through a rectifier bridge, a saturable reactor and an on-load tap changing transformer, and an equivalent circuit diagram is shown in figure 1.
Active power P of electrolytic aluminium loadASLAnd a DC side voltage VBThe relationship is as follows:
electrolytic aluminum load DC side bus voltage VBBus voltage on the AC side of loadVAHIs as in formula (2):
in the formula VSRAnd k is the voltage drop of the saturable reactor, and k is the transformation ratio of the on-load tap changing transformer.
Because each bus of the isolated power grid is small in electrical distance, the voltage of the high-voltage side of the load alternating-current bus can be changed by changing the voltage of the generator terminal. According to the voltage sensitivity method, the voltage change quantity delta V of the generator terminal can be obtainedGVoltage delta V of high voltage side of AC bus with loadAHThe relationship is as in formula (3).
ΔVAH＝Ksens·ΔVG(3)
Wherein, KsensIs the voltage sensitivity coefficient of bus i to bus j.
The generator terminal voltage variation Δ V can be obtained by the equations (2) and (3)GActive power variation delta P with loadASLThe corresponding relationship of (1).
ΔPASL＝KASLΔVG(4)
Wherein, KASLThe load comprehensive proportionality coefficient is obtained.
In FIG. 2, the thick line is the change curve of the active power of the electrolytic aluminum load when the voltage of the generator terminal measured on site has a-0.15 p.u. step. As can be seen from the figure, the dynamic response of the electrolytic aluminum load has an obvious inertia link. A first-order inertia link is adopted to describe the dynamic response of the electrolytic aluminum load, and each parameter of the inertia link is obtained by using least square curve fitting, as shown in formula (5), and a thin line in figure 2 is a fitting curve.
ΔPASL(t)＝-0.054+0.054·e-2.782t(5)
Written in the form of a transfer function, as shown in equation (6).
In the formula, KASLFor load amplification of electrolytic aluminiumA coefficient; t isASLThe time constant of active power of electrolytic aluminum load.
Topological structure design of electrolytic aluminum load controller
The long-term frequent participation of the electrolytic aluminum load in the system frequency adjustment has certain influence on the production efficiency of the electrolytic aluminum, and the control range of the electrolytic aluminum load needs to be limited. In addition, as can be seen from fig. 2, the electrolytic aluminum load dynamic response is faster than the primary frequency modulation of the thermal power generating unit, and more power support should be provided during the transient of the frequency modulation. The logic block diagram of the electrolytic aluminum load controller is shown in figure 3 according to the dynamic characteristics of the electrolytic aluminum load. The load controller comprises a proportional amplification unit, a WASHOUT unit, a dead zone unit and a limiting unit.
The dead zone unit realizes the locking of the electrolytic aluminum load controller, when the system is in a normal operation condition, the frequency deviation of the system is small, and the dead zone unit locking load controller is adjusted only by primary frequency modulation of the thermal power generating unit.
The WASHOUT unit presents a high-pass filtering characteristic and provides transient power support only when the system has an emergency condition, which causes the frequency to change violently. The wasbout cell effect diminishes as the frequency levels back off. Controlling parameter K by WASHOUT unitDThe maximum variation of the system frequency can be reduced, and the system frequency is prevented from being reduced too low.
The function of the proportional amplification unit is similar to that of a primary frequency modulation coefficient R of the thermal power generating unit, and the proportional amplification unit and the primary frequency modulation coefficient R of the thermal power generating unit jointly provide steady-state power support for the system. By controlling the proportional amplification unit, the primary frequency modulation spare capacity of the system can be improved.
Control logic of electrolytic aluminum load controller
The following is an electrolytic aluminum load closed loop control system applied to the site. The closed-loop control system is additionally provided with a WAMS control main station, a downlink channel and a network control unit NCU on the basis of a conventional WAMS system (comprising a PMU substation, an uplink channel and a WAMS main station).
The conventional WAMS main station and the PMU realize the functions of system data acquisition and state monitoring through TCP/IP protocol communication. The WAMS control main station communicates with the PMU by adopting a UDP protocol, and directly obtains the state quantity of the PMU substation. The WAMS control main station comprises a high-level application program, calculates control parameters in real time according to the system state and sends the control parameters to the network control unit NCU.
And the network control unit NCU is arranged on the side of the generator excitation cabinet to realize the control of the active power of the electrolytic aluminum load. The network control unit NCU has the functions of frequency signal acquisition and real-time communication with the WAMS control master station, has independent logical operation capacity and can convert digital quantity into physical quantity for output. And the output port of the NCU is accessed to the reference voltage superposition point of the generator excitation system, and the acquired frequency signal is used as a feedback signal to be input. When power disturbance occurs in the system, the frequency deviation acts on a reference voltage superposition point of an excitation system of the thermal power generating unit through the network control unit NCU, so that the excitation voltage is changed, the voltage of the alternating current side of the electrolytic aluminum load is changed, and the active power of the electrolytic aluminum load is adjusted to stabilize the unbalanced power of the system.
In order to improve the reliability of the closed-loop control system, a control mode which takes local control as a main mode and takes on-line control as an auxiliary mode is adopted, and a local device network control unit NCU comprises a main control logic and can still independently complete local closed-loop control under the condition of communication interruption with a WAMS control main station. And the WAMS control master station modifies the control parameters of the NCU of the network control unit on line according to the running state of the system, and optimizes the effect of the closed-loop control system. The control logic of the real-time closed-loop control system based on the electrolytic aluminum load is shown in FIG. 4.
Fourth, design of parameters of electrolytic aluminum load controller
4.1. Parameter design of proportional amplification unit of electrolytic aluminum load controller
The scale up unit provides steady state power support for the system. The power disturbance quantity delta P generated by the system is adjusted by the load of the electrolytic aluminumASLregPrimary regulation delta P of thermal power generating unitGregAnd (4) sharing the common charge. The primary regulating quantity and the system frequency variable quantity of the thermal power generating unit satisfy the following formula:
wherein: f. ofNTo a rated frequency, fregIs a steady-state value of the system frequency after primary frequency modulation, PGNRated power of the generator set, RGAnd the difference adjustment coefficient is the thermal power generating unit.
The relationship between the electrolytic aluminum load adjustment amount and the system frequency variation satisfies the following formula (8):
wherein: Δ fASLLbFor the dead band range of the load controller, PASLNRated power for electrolytic aluminum load, RASLThe load equivalent adjustment coefficient of the electrolytic aluminum is shown.
The power regulation amount borne by the electrolytic aluminum load is shown as the formula (9):
ΔPASLreg＝ΔP-ΔPGreg(9)
the system power disturbance amount Δ P is calculated in two cases.
i) Under the condition of instantaneous power disturbance, the real-time monitoring system can be used for monitoring the power disturbance generated by the system to directly obtain the system power disturbance quantity delta P, and the system power disturbance quantity is recorded as delta P under the conditionSThen, there is formula (10):
ΔP＝ΔPS(10)
ii) under the continuous fluctuation of the wind power, the fluctuation quantity of the wind power is difficult to measure and calculate in real time, and the maximum fluctuation quantity delta Pwindmax of the wind power is counted for a period of time on the basis of historical change data of the wind power. The fluctuation amount is used as a system power disturbance amount delta P, and an amplification factor K is calculatedPThen, formula (11):
ΔP＝ΔPwindmax(11)
the load equivalent adjustment coefficient of the electrolytic aluminum is obtained by the following equations (9), (10) and (11):
for an electrolytic aluminum load controller acting on a generator excitation system, equation (13) is satisfied:
when the active power variation of the electrolytic aluminum load is delta PASLregIn time, the variation of the terminal voltage of the thermal power generating unit is as follows:
the proportional amplification factor K is obtained from the formula (13) and the formula (14)P:
4.2. Design of WASHOUT unit of electrolytic aluminum load control system
The wasbouut unit provides transient power support only when the system frequency changes rapidly, reducing the maximum frequency offset. The WASHOUT unit parameters may be calculated by an SFR model.
Assume a coherent response of all generators to system load changes and equate them to one machine. Further, for such isolated power systems, automatic generation control is not invested, and therefore the role of the secondary frequency modulation will not be considered in the SFR model. The prime movers of all generators in the isolated grid under study are reheat turbines. Due to the fact that the response time of the thermal power unit speed regulator is short, the SFR model ignores the dynamic response process of the thermal power unit speed regulator. WASHOUT unit parameter TDSelecting 6-7s for high-pass filtering according to engineering experience;
Δ f(s) and Δ PLThe relationship of(s) is shown by the following formula:
wherein:
wherein M is the equivalent inertia coefficient of the system, D is the load damping coefficient, and TD、TR、FH、KMAnd R is the droop coefficient of the speed regulator of the generator.
Since the system has a pole-zero far from the imaginary axis, neglecting the pole-zero far from the imaginary axis, equation (16) can be expressed as follows:
in the formula, the parameter z2,ζ,ωn,ωdAll can be obtained by the parameter calculation of the SFR model.
The lagrange transform of the output signal of the SFR model under the unit step function is obtained from equation (19):
wherein:
performing reverse Laplace transformation on the formula (20) to obtain
Wherein:
let t bezAt this time, the system frequency offset is maximum, and the slope of the frequency change amount at this time is zero, which can be expressed by equation (22):
the maximum offset occurrence time t of the system frequency can be obtained by the formula (22)zBy adjusting WASHOUT unit KDParameter, maximum frequency offset of system Δ fdevmaxIs maintained at Δ fmaxAccordingly, formula (23) can be obtained:
the WASHOUT unit coefficient K can be obtained from the expressions (22) - (23)D。
Five examples and simulations
The following research is conducted based on a certain actual isolated grid in China.
Example 1: considering a closed-loop control test of the rapid reduction of the wind power;
the wind farm output at the initial moment was 61 MW. The power of the wind power plant is manually intervened, the active output of the wind power plant is reduced by 30MW within 30 seconds, and the active output curve of the wind power plant is shown in fig. 5 (a). When the closed loop control system is engaged, the system frequency curve is shown in fig. 5 (b). The generator terminal voltage change curve is shown by a solid line in FIG. 6(a), and the electrolytic aluminum load bus voltage change curve is shown by a solid line in FIG. 6 (b). The active power of the electrolytic aluminum load is shown by a solid line in FIG. 7 (a). The active power of the thermal power generating unit is shown by a solid line in fig. 7 (b). Because the thermal power generating unit and the electrolytic aluminum load jointly stabilize the disturbance of wind power, the system frequency is finally stabilized at 49.81Hz, and the system keeps stable operation.
As a comparative test, when the closed-loop control system is not put into operation, the response curves of the system frequency, the generator terminal voltage, the load bus voltage, the load active power and the thermal power unit active power are shown by dotted lines in fig. 5(b), 6(a) (b), 7(a) (b). Comparing the frequency response curves represented by the solid line and the dotted line in fig. 5(b), it can be seen that the proposed control method can effectively control the electrolytic aluminum load to participate in the frequency control of the isolated power grid, share the primary frequency modulation pressure of the thermal power generating unit, and significantly improve the frequency response characteristics of the isolated power grid.
Example 2: considering a closed-loop control test for cutting off a wind power plant;
in order to verify the effect of participating in system frequency control when large disturbance of a control system occurs, a field test for cutting off a wind power plant is carried out. Considering the danger of the field test, the comparative test that the electrolytic aluminum load does not participate in the frequency control adopts RTDS-based hardware-in-loop simulation. And cutting off the active power output of the wind power plant before cutting off, and cutting off the wind power plant when t is 5s, as shown in FIG. 8. The sudden change of the wind power causes the system frequency to drop rapidly, and under the action of the closed-loop control system, the electrolytic aluminum load can actively and rapidly adjust the drop of the automatic response system frequency, so that active support is provided for the system, the active power of the system drops by 30MW within 3s, and sufficient active support can be provided in the transient process, as shown by the solid curve of fig. 10 (a). And then, the primary frequency modulation of the thermal power generating unit also plays a role gradually, part of power adjustment amount is born, and finally, under the common response of the thermal power generating unit and the electrolytic aluminum load, the system frequency is stabilized at about 49.9 Hz. And simultaneously, simulation tests under the same working condition are carried out on the RTDS-based hardware-in-loop simulation platform. The system frequency response curves of the hardware-in-loop platform simulation at the time of the control method input and output are respectively shown by dotted lines in fig. 9(a) and (b). As can be seen from fig. 9(a), under the condition that the boundary conditions and the fault settings are the same, the frequency response curve of the system obtained by simulation can more accurately simulate the frequency response curve actually measured on site. Without a closed loop control system, the electrolytic aluminum load power would not respond to the frequency change of the system, as shown by the dashed curve in FIG. 9 (b). Although the frequency of the system does not collapse under the action of the primary frequency modulation of the thermal power generating unit, compared with the situation that the control method is put into use, the frequency of the system is greatly deviated, and the frequency is reduced to 49.5Hz at the lowest, as shown in FIG. 9 (b). For large power disturbances, a too low frequency drop may cause the low frequency load shedding device to act.
In summary, the control method of the embodiment can provide reference for stability research and control strategy formulation and verification of a system similar to the isolated power grid.
It should be understood that parts of the specification not set forth in detail are well within the prior art.
Although specific embodiments of the present invention have been described above with reference to the accompanying drawings, it will be appreciated by those skilled in the art that these are merely illustrative and that various changes or modifications may be made to these embodiments without departing from the principles and spirit of the invention. The scope of the invention is only limited by the appended claims.
Claims (2)
1. A method for enabling electrolysis loads to participate in frequency control of an isolated power grid is characterized in that system frequency adjustment is achieved through an electrolytic aluminum load control system, and the electrolytic aluminum load control system comprises a network control unit NCU, a dead zone unit, a proportional amplification unit, a WASHOUT unit and an amplitude limiting unit; the proportion amplification unit is connected with the WASHOUT unit in parallel, one end of the proportion amplification unit is sequentially connected with the dead zone unit and the network control unit NCU, and the other end of the proportion amplification unit is connected with the amplitude limiting unit; the network control unit NCU is connected with the isolated power grid system, and the amplitude limiting unit is connected with the WAMS control master station; the method specifically comprises the following steps:
step 1, establishing an isolated power grid system model;
step 2, designing control logics of the electrolytic aluminum load controller under different power disturbance modes;
step 3, designing parameters of the electrolytic aluminum load controller in different power disturbance modes;
the implementation of step 1 comprises:
step 1.1, obtaining an electrolytic aluminum load power-voltage mathematical model by establishing an electrolytic aluminum load topological structure; the method comprises the following specific steps:
1) the electrolytic aluminum load topological structure is equivalent to that a back electromotive force E is connected with an equivalent resistor R in series, and is connected with an alternating current bus line through a rectifier bridge, a saturable reactor and an on-load tap changer;
2) active power P of electrolytic aluminium loadASLAnd a DC side voltage VBThe relationship is as follows:
3) voltage V on DC side of electrolytic aluminium loadBBus voltage on the AC side of loadVAHIs as in formula (2);
(2) in the formula VSRThe voltage drop of the saturable reactor is shown, and k is the transformation ratio of the on-load tap changing transformer;
4) changing the voltage of the high voltage side of the load alternating current bus by changing the voltage of the generator terminal; obtaining a voltage change Δ V of the generator terminal according to a voltage sensitivity methodGVoltage delta V of high voltage side of AC bus with loadAHThe relationship of (a) is as in formula (3);
ΔVAH＝Ksens·ΔVG(3)
(3) in the formula, KsensThe voltage sensitivity coefficient of the bus i to the bus j is obtained;
5) the generator terminal voltage variation delta V can be obtained by the formulas (2) and (3)GActive power variation delta P with loadASLThe corresponding relationship of (A) is:
ΔPASL＝KASLΔVG(4)
(4) in the formula, KASLThe load comprehensive proportionality coefficient;
step 1.2, acquiring electrical parameters of an isolated power grid system, and establishing a mathematical model of the isolated power grid system, wherein the mathematical model comprises a thermal power unit excitation system, a thermal power unit speed regulator, a thermal power unit, a transmission line, a transformer, a double-fed wind driven generator and an electrolytic aluminum load;
obtaining parameters of an inertia link by using least square curve fitting, wherein the parameters are as shown in a formula (5);
ΔPASL(t)＝-0.054+0.054·e-2.782t(5)
written in the form of a transfer function, as in equation (6);
(6) in the formula, KASLThe load comprehensive proportionality coefficient; t isASLIs the time constant of active power of electrolytic aluminum load;
the implementation of step 2 comprises:
step 2.1, establishing an electrolytic aluminum load closed-loop control system;
the electrolytic aluminum load closed-loop control system comprises a conventional WAMS system, an additional WAMS control master station, a downlink channel and a network control unit NCU; the conventional WAMS system comprises a PMU substation, an upstream channel and a WAMS main station; the WAMS main station communicates with the PMU substation through a TCP/IP protocol to realize system data acquisition and state monitoring; the WAMS control main station communicates with the PMU substation by adopting a UDP protocol, and directly acquires the state quantity of the PMU substation; the WAMS controls the main station to calculate control parameters in real time and sends the control parameters to a network control unit NCU;
the network control unit NCU is arranged on the side of the generator excitation cabinet to realize the control of the active power of the electrolytic aluminum load; the output port of the NCU is accessed to the reference voltage superposition point of the generator excitation system, and the acquired frequency signal is used as a feedback signal to be input; when power disturbance occurs in the system, the frequency deviation acts on a reference voltage superposition point of an excitation system of the thermal power generating unit through a Network Control Unit (NCU) to change the excitation voltage, so that the voltage of the alternating current side of the electrolytic aluminum load is changed, and the active power of the electrolytic aluminum load is adjusted to stabilize the unbalanced power of the system;
step 2.2, designing the control logic of the electrolytic aluminum load controller;
I) monitoring system power disturbance according to PMU substation information, and judging the system state;
II) when the system state is instantaneous power disturbance or continuous power fluctuation, the WAMS controls the master station to issue control parameters;
III) collecting local frequency signals, and judging the communication state of the WAMS control master station;
IV) if the communication state of the WAMS control master station is normal, receiving the WAMS master station parameters, and updating the NCU parameters of the network control unit to control the excitation voltage of the generator;
v) if the communication state of the main station is abnormal, the excitation voltage of the generator is controlled by locally storing control parameters by the network control unit NCU.
2. The method for participating in isolated grid frequency control of electrolysis-type loads according to claim 1, wherein the step 3 is implemented by:
step 3.1, designing parameters of a proportional amplification unit of the electrolytic aluminum load controller;
the proportion amplifying unit provides steady-state power support for the system, and the power disturbance quantity delta P generated by the system comprises the load adjustment quantity delta P of the electrolytic aluminumASLregPrimary regulating quantity delta P of thermal power generating unitGreg(ii) a The primary regulating quantity and the system frequency variable quantity of the thermal power generating unit satisfy the following formula:
(7) in the formula: f. ofNTo a rated frequency, fregIs a steady-state value of the system frequency after primary frequency modulation, PGNRated power of the generator set, RGThe difference adjustment coefficient of the thermal power generating unit is obtained;
the relationship between the electrolytic aluminum load adjustment amount and the system frequency variation satisfies the following formula (8):
(8) in the formula: Δ fASLLbFor the dead band range of the load controller, PASLNRated power for electrolytic aluminum load, RASLThe load equivalent adjustment coefficient of the electrolytic aluminum is obtained;
the power regulation amount borne by the electrolytic aluminum load is as follows:
ΔPASLreg＝ΔP-ΔPGreg(9)
(9) in the formula, the system power disturbance quantity delta P is calculated in two situations;
a. under the condition of instantaneous power disturbance, the system power disturbance quantity is delta PSThen, there is formula (10):
ΔP＝ΔPS(10)；
b. counting the maximum fluctuation quantity delta p of the wind power for a period of time under continuous power fluctuationwindmax(ii) a The fluctuation amount is used as a system power disturbance amount delta P, and an amplification factor K is calculatedPThen, formula (11):
ΔP＝ΔPwindmax(11)；
the load equivalent adjustment coefficient of the electrolytic aluminum is obtained by the following equations (9), (10) and (11):
for an electrolytic aluminum load controller acting on a generator excitation system, equation (13) is satisfied:
when the active power variation of the electrolytic aluminum load is delta PASLregIn time, the variation of the terminal voltage of the thermal power generating unit is as follows:
the proportional amplification factor K is obtained from the formula (13) and the formula (14)P:
Step 3.2, designing parameters of a WASHOUT unit of the electrolytic aluminum load controller;
calculating WASHOUT unit parameters through an SFR model;
WASHOUT unit parameter TDFor high-pass filtering, generally selecting 6-7 s;
Δ f(s) and Δ PLThe relationship of(s) is:
(16) in the formula:
(17) in the formulas (18), M is the equivalent inertia coefficient of the system, D is the load damping coefficient, and TD、TR、FH、KMThe time constant of the generator is R is the droop coefficient of the speed regulator of the generator;
ignoring the pole-zero far from the imaginary axis, equation (16) can be converted to:
(19) in the formula, the parameter z2,ζ,ωn,ωdAll can be obtained by the parameter calculation of the SFR model;
the lagrange transform of the output signal of the SFR model under the unit step function is obtained from equation (19):
in formula (20):
performing reverse Laplace transformation on the formula (20) to obtain
(21) In the formula:
let t bezAt this time, the system frequency offset is maximum, and the slope of the frequency change amount at this time is zero, which can be expressed by equation (22):
the maximum offset occurrence time t of the system frequency can be obtained by the formula (22)zBy adjusting WASHOUT unit KDParameter, maximum frequency offset of system Δ fdevmaxIs maintained at Δ fmaxAccordingly, formula (23) can be obtained:
the WASHOUT unit coefficient K can be obtained from the expressions (22) - (23)D。
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