CN109347096B - Optimization management method for power quality of active power distribution network - Google Patents
Optimization management method for power quality of active power distribution network Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention discloses an optimal management method for power quality of a dynamic power distribution network, which relates to the field of optimal management of power grids, and comprises the following steps: step 1, establishing an optimization management model of the electric energy quality of an active power distribution network in a prediction time domain, wherein the optimization management model takes minimization of network power loss in the prediction time domain as an optimization target and takes conditions related to the electric energy quality and safe operation of a system as constraint conditions; step 2, solving the optimized management model by adopting a cuckoo algorithm to obtain an optimal management model result; and 3, on the basis of the optimal management model result, reducing the influence caused by the prediction uncertainty problem by adopting model prediction control with a feedback characteristic in a matching way. The optimization management method for the power quality of the active power distribution network can effectively reduce power loss caused by harmonic waves, the obtained scheduling plan also considers the power quality factor, and the influence of uncertain factors is reduced by adopting model prediction control.
Description
Technical Field
The invention relates to the field of power grid optimization management, in particular to an optimization management method for the power quality of an active power distribution network.
Background
Distributed Generation (DG) and Energy Storage (ES) are gaining increasing attention in many countries as important components of smart grids. The navigat research institute predicts that DG installed capacity in 2023 will exceed 165GW compared to DG installed capacity in 87.3GW in 2014. Furthermore, by 2025, the ES market in china will reach $ 87 billion and 31GWh, which is three times that of the ES market in 2015. DG and ES may provide benefits to the power system, for example, DG may reduce power losses and thereby reduce the corresponding equipment investment. The ES may integrate a renewable type DG such as a wind turbine into the power system. However, since DG and ES are connected to the distribution network through inverters, they will also have some negative effects including harmonic pollution.
Harmonic regulation has a significant impact on the operation and planning of power distribution networks for the following reasons: 1) in general, harmonic injection is positively correlated with the fundamental power of the harmonic resource. A large fundamental output implies a large harmonic injection of harmonic resources. Thus, when the harmonic constraints are close to their limits, the output of both the inverter-based DG and ES with poor harmonic spectra will be limited; 2) if the capacitor is not configured correctly, the small harmonic injection will be amplified due to the resonance between the capacitor and the inductive elements in the power distribution network. This means that even small harmonic injection of ES and DG can cause severe Power Quality (PQ) problems. Therefore, it is necessary to consider the harmonic pollution problem caused by DG in the operation and planning of the distribution network.
Voltage imbalance level is another important PQ indicator for power distribution networks. Voltage imbalances may cause negative effects, such as more power loss. Therefore, in order to provide a good power supply to the end user, it is important to ensure that the voltage levels are within a reasonable range, while limiting the voltage imbalance levels of the distribution network.
Currently, the existing research mainly focuses on optimal addressing capacity of the DG considering harmonic constraints, optimal operation of the distribution network considering voltage imbalance levels, determination of maximum permeability level of the DG based on an inverter in the distribution network, estimation of DG configuration capacity in the distribution network, and reactive power control of the distribution network considering wind energy uncertainty. The existing literature relates to an optimized operation method of a power distribution network considering voltage unbalance level and harmonic pollution, and related literature also considers less uncertain problems of processing renewable energy sources.
In view of the importance of the power quality problem, it is necessary to research an active power distribution network optimization management method considering the power quality problem.
Disclosure of Invention
In view of the above drawbacks of the prior art, the technical problem to be solved by the present invention is how to perform optimal management on the power quality of an active power distribution network when considering both the harmonic pollution and the voltage imbalance level in the active power distribution network and the uncertainty problem caused by the renewable energy being connected to the active power distribution network.
In order to achieve the above object, the present invention provides an optimized management method for power quality of an active power distribution network, including the following steps:
and 3, on the basis of the optimal management model result, reducing the influence caused by the prediction uncertainty problem by adopting model prediction control with a feedback characteristic in a matching way, wherein the prediction uncertainty problem is the uncertainty caused when renewable energy is accessed into the active power distribution network.
Further, the objective function of the optimization management model is as follows:
wherein the content of the first and second substances,is the net active loss associated with the fundamental frequency at time t,is the active power loss associated with the harmonic frequency h at time t; Ω is the set of relevant harmonic frequencies; t is a prediction time domain related to model prediction control, where the scheduling interval may be 1h or 0.5h, that is, the optimal management model may execute once with the minimum active power loss in the prediction time domain T as an objective function of 1h or 0.5h, but only issue a scheduling plan at the next time, and repeat the process until a boundary point of the next time comes.
Further, the overall voltage harmonic distortion constraint can be used to reflect the overall harmonic level of node i, as follows:
wherein THD is an abbreviation for Total voltage harmonic distortion, referring to the overall voltage harmonic distortion constraint; vi p,t,Fundamental frequency and harmonic frequency h voltage value, THD of p phase at time t node ii,maxIs the upper limit of THD;
the individual harmonic distortion constraint is an index that evaluates the harmonic level of each relevant frequency, and the individual harmonic distortion constraint for each node i satisfies the following constraint:
wherein IHD is an abbreviation of Indvidual Harmonic Distortion Constraints and refers to Individual Harmonic Distortion Constraints;is the upper limit of the individual voltage harmonic distortion of the harmonic frequency h;
the voltage imbalance factor constraint may be used to reflect the voltage imbalance level, and to ensure a relatively good voltage imbalance level, the following constraints should be satisfied:
wherein VUF is an abbreviation of Voltage Unbalance Factor Constraints and refers to Voltage imbalance Factor Constraints; vi 1,t,Vi 2,tPositive sequence voltage values and negative sequence voltage values of a node i at the moment t respectively; UM% is the upper limit in the form of percentage VUF; since the definition of the voltage imbalance factor is for a sine wave, only the voltage at the fundamental frequency is used to calculate VUF.
Further, the voltage effective value constraint is:
wherein the content of the first and second substances,is the effective value of the voltage of the p phase of the node i at the moment t;the upper limit and the lower limit of the voltage effective value are respectively;
the current effective value constraint is as follows:
wherein the content of the first and second substances,the effective value of the current of the p-th phase of the ij branch at the moment t is shown;the upper limit and the lower limit of the current effective value are respectively;is the current value of the p-th phase fundamental frequency and harmonic frequency h of the branch ij at the time t.
Further, the constraints of the optimization management model also include battery constraints, which generally include energy value constraints and power value constraints;
the energy value constraint is:
wherein, the first and the second end of the pipe are connected with each other,is the energy stored in the battery at the end of time t; ei,max,Ei,minAre respectivelyUpper and lower limits of (P)i ES,tIs the active power of the accumulator at time t, epsilonin,εou tThe charging and discharging efficiency of the storage battery respectively; equation (7) describes the relationship between two continuous-time energy values and the limit that the energy value of the storage battery satisfies the upper and lower limits, respectively;
in order to ensure that the active power, reactive power and apparent power of the battery are within limits, the power value constraints should satisfy the following constraints:
wherein, the first and the second end of the pipe are connected with each other,is the rated active power of the storage battery,is the battery reactive power at time t,are respectivelyThe upper and lower limit values of (a),is the limit of the apparent power of the battery.
Further, the constraints of the optimization management model further include controllable distributed power supply constraints:
wherein, Pi DDG,tIs the active power of the controllable distributed power supply at time t,are respectively Pi DDG,tThe upper and lower limit values of (c),the upper and lower limit values of the gradient rate of the distributed power supply, wherein the controllable distributed power supply can be a micro gas turbine or a fuel cell;is the power factor angle of the distributed power supply at time t,is thatUpper and lower limit values of (d);
the constraints of the optimal management model further include capacitor constraints:
wherein the content of the first and second substances,is the capacitor capacity available at node i at time t,respectively, the limit of the capacity; for operation of the distribution network, since the number of available capacitors is discrete, the distribution network is provided with a plurality of capacitorsIs a discrete decision variable;is the number of times the capacitor state is changed,is the maximum number of transitions, the purpose of which is to ensure that the capacitor has a good life cycle.
Further, the constraints of the optimization management model also include balance constraints; the balance constraint is a power flow balance formula of the fundamental frequency and the harmonic frequency at each moment;
for power flow at the fundamental frequency, the node current injection equation can be written as follows:
V1 abc,t=[Vi a,t Vi b,t Vi c,t]T (12)
wherein the content of the first and second substances,is a sub-matrix of the fundamental frequency node admittance matrix at time t,Vi abc,ta node injection current vector and a voltage vector of the fundamental frequency are respectively at a time t node i; the injected node current typically comes from loads, renewable power sources (wind turbines or photovoltaics), controllable distributed power sources (gas turbines or fuel cells), and batteries;node injection current values V of A, B and C phases of the fundamental frequency at the time t node ii a,t,Vi b,t,Vi c ,tRespectively at time tnode iVoltage values of fundamental wave frequencies A, B and C phases;
similarly, the power flow equation for the harmonic frequency h can be expressed as:
wherein, the first and the second end of the pipe are connected with each other,the node injection current values of A, B and C phases of the harmonic frequency h of the node i at the time t are respectively,voltage values of A, B and C phases of i harmonic frequency h of a node t at the moment are respectively;is the node admittance matrix of the harmonic frequency h at time t; harmonic components of distributed power sources and batteries are modeled as current sources with one or more branch circuits.
Further, the cuckoo algorithm is an algorithm obtained by improving a standard cuckoo algorithm;
the standard cuckoo algorithm continuously searches the optimal nest process by simulating cuckoos, and performs an idealized assumption:
firstly, only one egg can be produced by one cuckoo at a time, and only one egg can be produced in one bird nest;
secondly, the best nest in each generation is reserved to the next generation;
③ host birds with a certain probability PaFinding an intruder, the host bird will discard the entire birdBird nest and immediately searching a new nest;
the new nest is generated by means of Levy flight:
in the formula:the position of the ith bird nest in the t iteration is represented, a step scale factor alpha is larger than 0 and is directly related to the severity of the problem, wherein alpha is generally selected to be L/100, and L is a characteristic range of the severity of the problem;characterizing point-to-point multiplication, Levy (λ) is a value generated by a Levy random process, obeying a Levy distribution:
L(λ):u=t-λ,(1<λ<3) (16)
the Levy flight consists of high-frequency short-distance flight and low-frequency long-distance flight;
when the step size scale factor alpha takes a larger value, the global search capability of the standard cuckoo algorithm can be enhanced, and a global optimal solution area can be quickly determined; when the step size scale factor alpha is a small value, the local searching capability of the standard cuckoo algorithm can be enhanced, and the convergence speed of the algorithm is improved;
the cuckoo algorithm sets the step size scale factor alpha as a variable, takes a larger value at the initial stage of iteration, accelerates the positioning of a globally optimal region, takes a smaller value at the later stage of iteration, and accelerates the convergence of the algorithm;
the step size scaling factor α is set to:
wherein alpha ismaxAnd alphaminIs the maximum and minimum of the step-size scale factor, NmaxThe number of the maximum iterations of the cuckoo algorithm is N, and the number of the current iterations of the cuckoo algorithm is N.
Further, the step 2 further comprises the following steps:
step 2.1, reading in system data, and deciding variables and upper and lower limits; collecting an energy value of a storage battery at the last optimization moment and an actual output value of a distributed power supply, and forming closed-loop feedback of model predictive control by using the energy value and the actual output value as known variables during optimization at the current moment; initializing parameters of the cuckoo algorithm;
step 2.2, initializing the iteration number k to be 0, randomly generating n nests in the feasible region, and setting the position as x (x)1,x2,...,xn)TEach bird nest contains m state variables;
2.3, calculating the fitness of each nest based on the power flow analysis of the fundamental frequency and the harmonic frequency; penalizing a fitness function if the power flow is not aggregated;
step 2.4, sorting the individual advantages and disadvantages, selecting the optimal nest position, and reserving the nest position to the next generation;
step 2.5, updating other nest positions by utilizing the Levy flight, combining with a parent generation, searching for the best and reserving to the next generation;
step 2.6, judging whether the bird is found by the host bird, if so, selecting a abandoned bird nest, and updating the position by utilizing the Levy flight;
step 2.7, selecting the current global optimal bird nest and judging whether the algorithm ending condition is met; if not, the step 2.3 is returned to.
Further, in the step 3, when each optimization time t comes, the last optimization time t-1, that is, the energy value of the storage battery and the actual output value of the distributed power source at the beginning of the optimization time t are collected, and the collected values are substituted into relevant constraints as known variables during optimization at the current optimization time t; the optimization program takes the minimum network loss in the predicted time domain T as an objective function, adopts the cuckoo algorithm for optimization, only issues a scheduling plan of the optimization time T, and temporarily repeats the optimization process when the next optimization time T +1 comes; the real storage battery energy value of the system and the actual output value of the distributed power supply are acquired during each optimization, so that effective closed-loop feedback is formed, and the uncertainty influence brought by the renewable energy is reduced.
The optimization management method for the power quality of the active power distribution network can effectively reduce power loss caused by harmonic waves, the obtained scheduling plan also considers the power quality factor, and the influence of uncertain factors is reduced by adopting model prediction control.
The conception, the specific structure and the technical effects of the present invention will be further described with reference to the accompanying drawings to fully understand the objects, the features and the effects of the present invention.
Drawings
FIG. 1 is a schematic illustration of a Levy flight trajectory;
FIG. 2 is a schematic diagram of a calculation flow based on the improved cuckoo algorithm;
fig. 3 is a time domain diagram of model predictive control.
Detailed Description
The technical contents of the preferred embodiments of the present invention will be more clearly and easily understood by referring to the drawings attached to the specification. The present invention may be embodied in many different forms of embodiments and the scope of the invention is not limited to the embodiments set forth herein.
In order to ensure the power quality level of the power distribution network, the scheme establishes an optimal power distribution network operation model, takes the network power loss in a minimized prediction time domain as an optimization target, and takes conditions related to the power quality and the safe operation of a system as constraints, and the method is concretely as follows.
An optimization management method for the power quality of an active power distribution network comprises the following steps:
and 3, on the basis of the optimal management model result, reducing the influence caused by the prediction uncertainty problem by adopting model prediction control with a feedback characteristic in a matching way, wherein the prediction uncertainty problem is the uncertainty caused when the renewable energy is accessed into the active power distribution network.
The objective function of the optimization management model is as follows:
wherein the content of the first and second substances,is the net active loss associated with the fundamental frequency at time t,is the active power loss associated with the harmonic frequency h at time t; Ω is the set of relevant harmonic frequencies; t is a prediction time domain related to model prediction control, where the scheduling interval may be 1h or 0.5h, that is, the optimal management model may execute once with the minimum active power loss in the prediction time domain T as an objective function of 1h or 0.5h, but only issue a scheduling plan at the next time, and repeat the process until a boundary point of the next time comes.
The overall voltage harmonic distortion constraint can be used to reflect the overall harmonic level of node i, as follows:
wherein, THD is an abbreviation of Total voltage harmonic distortion, referring to the integral voltage harmonic distortion constraint; vi p,t,Fundamental frequency and harmonic frequency h voltage value, THD of p phase at time t node ii,maxIs the upper limit of THD;
the individual harmonic distortion constraint is an index that evaluates the harmonic level of each relevant frequency, and the individual harmonic distortion constraint for each node i satisfies the following constraint:
wherein IHD is an abbreviation of Indvidual Harmonic Distortion Constraints and refers to Individual Harmonic Distortion Constraints;is the upper limit of the individual voltage harmonic distortion of the harmonic frequency h;
the voltage imbalance factor constraint may be used to reflect the voltage imbalance level, and to ensure a relatively good voltage imbalance level, the following constraints should be satisfied:
wherein VUF is an abbreviation of Voltage Unbalance Factor Constraints and refers to Voltage imbalance Factor Constraints; vi 1,t,Vi 2,tPositive sequence voltage values and negative sequence voltage values of a node i at the moment t respectively; UM% is the upper limit in the form of percentage VUF; since the definition of the voltage imbalance factor is for a sine wave, only the voltage at the fundamental frequency is used to calculate VUF.
The voltage effective value constraint is as follows:
wherein the content of the first and second substances,is the effective value of the voltage of the p phase of the i node at the moment t;respectively are the upper limit and the lower limit of the effective value of the voltage;
the current effective value is constrained as:
wherein the content of the first and second substances,the effective value of the current of the p-th phase of the ij branch at the moment t is shown;the upper limit and the lower limit of the current effective value are respectively;is the current value of the p-th phase fundamental frequency and harmonic frequency h of the branch ij at the time t.
The constraint conditions of the optimization management model also comprise storage battery constraints, and the storage battery constraints generally comprise energy value constraints and power value constraints;
the energy value constraint is:
wherein the content of the first and second substances,is the energy stored in the battery at the end of time t; ei,max,Ei,minAre respectivelyUpper and lower limits of (P)i ES,tIs the active power of the accumulator at time t, epsilonin,εout is the charge-discharge efficiency of the storage battery respectively; equation (7) describes the relationship between two continuous-time energy values and the limit that the energy value of the storage battery satisfies the upper and lower limits, respectively;
to ensure that the active power, reactive power and apparent power of the battery are within limits, the power value constraints should satisfy the following constraints:
wherein the content of the first and second substances,is the rated active power of the storage battery,is the battery reactive power at time t,are respectivelyThe upper and lower limit values of (a),is the limit of the apparent power of the battery.
The constraints of the optimization management model also include controllable distributed power supply constraints:
wherein, Pi DDG,tIs the active power of the controllable distributed power supply at time t,are respectively Pi DDG,tThe upper and lower limit values of (c),is the upper and lower limit values of the climbing rate of the distributed power supply, wherein the controllable distributed power supply can be a micro gas turbine or a fuel cell;is the power factor angle of the distributed power supply at time t,is thatUpper and lower limit values of (d);
the constraints for optimizing the management model also include capacitor constraints:
wherein the content of the first and second substances,is the capacitor capacity available at node i at time t,respectively, the limit of the capacity; for operation of the distribution network, since the number of available capacitors is discrete, the distribution network is provided with a plurality of capacitorsIs a discrete decision variable;is the number of times the capacitor state is changed,is the maximum number of transitions, the purpose of which is to ensure that the capacitor has a good life cycle.
The constraint conditions of the optimization management model also comprise balance constraints; the balance constraint is a power flow balance formula of fundamental frequency and harmonic frequency at each moment;
for power flow at the fundamental frequency, the node current injection equation can be written as follows:
V1 abc,t=[Vi a,t Vi b,t Vi c,t]T (12)
wherein the content of the first and second substances,is a sub-matrix of the frequency node admittance matrix at time t,Vi abc,ta node injection current vector and a voltage vector of the i fundamental wave frequency of a node at the moment t respectively; the injected node current typically comes from the load, renewable power (wind turbine or photovoltaic), controllable distributed power (gas turbine or fuel cell), and battery;node injection current values V of A, B and C phases of i fundamental wave frequency of node at time ti a,t,Vi b,t,Vi c,tThe voltage values of i fundamental wave frequencies A, B and C of a node t at the moment are respectively;
similarly, the power flow equation for harmonic frequency h can be expressed as:
wherein the content of the first and second substances,the node injection current values of A, B and C phases of the harmonic frequency h of the node i at the time t are respectively,voltage values of A, B and C phases of i harmonic frequency h of a node t at the moment are respectively;is the node admittance matrix of the harmonic frequency h at time t; harmonic components of distributed power sources and batteries are modeled as current sources with one or more branch circuits.
The cuckoo algorithm in the step 2 is an algorithm obtained by improving a standard cuckoo algorithm;
the Cuckoo algorithm is also called Cuckoo Search algorithm (CS), is a newly proposed bionic intelligent algorithm, is proposed in 2009 by Xin-She Yang professor and Deb Suash, and finds an optimal solution of a problem by using a special parasitic brooding mode of Cuckoo and combining with a Levy flight process followed by bird flight.
Cuckoo search algorithms have been proposed based on the following biological principles: cuckoos typically lay eggs in nests of other birds, brooding by parasitic means. This behavior is likely to be discovered, and once the host bird finds the intruder, the bird will discard the cuckoo egg or directly discard the original nest. The cuckoo search algorithm continuously searches for the optimal nest by simulating the cuckoo, and performs an idealized assumption:
firstly, only one egg can be produced by one cuckoo at a time, and only one egg can be produced in one bird nest;
secondly, the best nest in each generation is reserved to the next generation;
③ host birds with a certain probability PaWhen finding the invader, the host bird abandons the whole nest and immediately searches a new nest;
the new nest is created by means of Levy flight:
in the formula:the position of the ith bird nest in the t iteration is represented, a step scale factor alpha is larger than 0 and is directly related to the severity of the problem, wherein alpha is generally selected to be L/100, and L is a characteristic range of the severity of the problem;characterizing point-to-point multiplication, Levy (λ) is a value generated by Levy random process, obeying Levy distribution:
L(λ):u=t-λ,(1<λ<3) (16)
the Levy flight consists of a high frequency short-distance flight and a low frequency long-distance flight, and the flight path of the Levy flight is shown in fig. 1 after 1000 times of continuous flight. It can be seen that the flight process can suddenly generate long-distance flight at a short-distance flight gathering place, and the flight direction is not fixed, so that the local optimal solution is easy to jump out, and the global optimal solution can be searched in the whole feasible region more effectively.
In the standard cuckoo algorithm, the step-size scaling factor α is a fixed value and is directly related to the severity of the problem, and is generally selected to be L/100, where L is a characteristic range of the severity of the problem. When the step size scale factor alpha takes a larger value, the global search capability of the standard cuckoo algorithm can be enhanced, and the global optimal solution area can be quickly determined; when the step size scale factor alpha is a small value, the local search capability of the standard cuckoo algorithm can be enhanced, and the convergence speed of the algorithm is improved;
the cuckoo algorithm sets the step-size scale factor alpha as a variable, takes a large value at the initial stage of iteration, accelerates the positioning of a globally optimal region, takes a small value at the later stage of iteration, and accelerates the convergence of the algorithm;
by taking the improvement of the inertia coefficient in the improved particle swarm optimization as reference, the step-size scale factor α is set as:
wherein alpha ismaxAnd alphaminIs the maximum and minimum of the step-size scale factor, NmaxThe maximum iteration number of the cuckoo algorithm is shown, and N is the current iteration number of the cuckoo algorithm.
As shown in fig. 2, the specific calculation flow of the cuckoo algorithm in step 2 further includes the following steps:
step 2.1, reading in system data, and deciding variables and upper and lower limits; collecting an energy value of the storage battery and an actual output value of the distributed power supply at the last optimization time, and forming closed-loop feedback of model predictive control by using a known variable during current optimization; initializing parameters of a cuckoo algorithm;
step 2.2, initializing the iteration number k to be 0, randomly generating n nests in the feasible region, and setting the position as x (x)1,x2,...,xn)TEach bird nest contains m state variables;
step 2.3, calculating the fitness of each nest based on the power flow analysis of the fundamental frequency and the harmonic frequency; penalizing the fitness function if the power flow is not aggregated;
step 2.4, sorting the individual advantages and disadvantages, selecting the optimal nest position, and reserving the nest position to the next generation;
step 2.5, updating other nest positions by utilizing Levy flight, combining with a parent generation, searching for the best and reserving to the next generation;
step 2.6, judging whether the bird is found by the host bird, if so, selecting a abandoned bird nest, and updating the position by utilizing Levy flight;
step 2.7, selecting the current global optimal bird nest and judging whether the algorithm ending condition is met; if not, the step returns to the step 2.3.
As shown in fig. 3, in step 3, when each optimization time t comes, the energy value of the storage battery and the actual output force value of the distributed power supply at the end of the previous optimization time t-1, namely the beginning of the optimization time t, are collected and substituted into the relevant constraint as a known variable during the optimization at the current optimization time t; the optimization program takes the minimum network loss in the predicted time domain T as an objective function, adopts a cuckoo algorithm for optimization, only issues a scheduling plan of an optimization time T, and temporarily repeats the optimization process when the next optimization time T +1 comes; the real storage battery energy value of the system and the actual output value of the distributed power supply are acquired during each optimization, so that effective closed-loop feedback is formed, and the uncertainty influence brought by renewable energy is reduced.
In the present invention:
1. meanwhile, an optimal power distribution network operation model is established in consideration of harmonic pollution and voltage unbalance level;
2. an improved cuckoo search algorithm is provided for solving an optimal power distribution network operation model;
3. the optimal power distribution network operation model is matched with the model prediction control to form closed-loop feedback, so that the influence caused by the uncertainty of renewable energy sources is reduced.
The foregoing detailed description of the preferred embodiments of the invention has been presented. It should be understood that numerous modifications and variations could be devised by those skilled in the art in light of the present teachings without departing from the inventive concept. Therefore, the technical solutions available to those skilled in the art through logic analysis, reasoning and limited experiments based on the prior art according to the concept of the present invention should be within the scope of protection defined by the claims.
Claims (7)
1. An optimal management method for the power quality of an active power distribution network is characterized by comprising the following steps:
step 1, establishing an optimization management model of the electric energy quality of an active power distribution network in a prediction time domain, wherein the optimization management model takes minimization of network power loss in the prediction time domain as an optimization target, takes conditions related to the electric energy quality and safe system operation as constraint conditions, and the constraint conditions comprise electric energy quality constraint and system safety constraint, wherein the electric energy quality constraint comprises integral voltage harmonic distortion constraint, single harmonic distortion constraint and voltage unbalance factor constraint; the system safety constraint comprises a voltage effective value constraint and a current effective value constraint; the objective function of the optimization management model is as follows:
wherein the content of the first and second substances,is the net active loss associated with the fundamental frequency at time t,is the active power loss associated with the harmonic frequency h at time t; Ω is the set of relevant harmonic frequencies; t is a prediction time domain related to model prediction control, where the scheduling interval is 1h or 0.5h, that is, the optimal management model will take the minimum active power loss in the prediction time domain T as an objective function, execute once for 1h or 0.5h, but only issue a scheduling plan at the next moment, and repeat the process until a boundary point at the next moment comes;
step 2, solving the optimized management model by adopting a cuckoo algorithm to obtain an optimal management model result;
the cuckoo algorithm is an algorithm obtained by improving a standard cuckoo algorithm;
the standard cuckoo algorithm continuously searches the optimal nest process by simulating cuckoos, and performs an idealized assumption:
firstly, only one egg can be produced by one cuckoo at a time, and only one egg can be produced in one bird nest;
the best bird nest in each generation can be reserved to the next generation;
③ host birds with a certain probability PaWhen finding an invader, the host bird abandons the whole nest and immediately searches a new nest;
the new nest is generated by means of Levy flight:
in the formula:the position of the ith bird nest in the t iteration is represented, a step scale factor alpha is larger than 0, the step scale factor alpha is directly related to the severity of the problem, alpha is selected to be L/100, and L is a characteristic range of the severity of the problem;characterizing point-to-point multiplication, Levy (λ) is a value generated by Levy random process, obeying Levy distribution:
L(λ)~u=t-λ,1<λ<3 (16)
the Levy flight consists of high-frequency short-distance flight and low-frequency long-distance flight;
when the step size scale factor alpha takes a larger value, the global search capability of the standard cuckoo algorithm can be enhanced, and a global optimal solution area can be quickly determined; when the step size scale factor alpha is a small value, the local searching capability of the standard cuckoo algorithm can be enhanced, and the convergence speed of the algorithm is improved;
the cuckoo algorithm sets the step size scale factor alpha as a variable, takes a larger value at the initial stage of iteration, accelerates the positioning of a globally optimal region, takes a smaller value at the later stage of iteration, and accelerates the convergence of the algorithm;
the step size scaling factor α is set to:
wherein alpha ismaxAnd alphaminFor the maximum and minimum values of the step-size scale factor, NmaxThe maximum iteration number of the cuckoo algorithm is obtained, and N is the current iteration number of the cuckoo algorithm;
the step 2 further comprises the following steps:
step 2.1, reading in system data, and deciding variables and upper and lower limits; collecting an energy value of a storage battery at the last optimization moment and an actual output value of a distributed power supply, and forming closed-loop feedback of model predictive control by using the energy value and the actual output value as known variables during optimization at the current moment; initializing parameters of the cuckoo algorithm;
step 2.2, initializing the iteration number k to 0, and randomly generating n in the feasible region0A bird nest in the positionEach bird nest contains m state variables;
2.3, calculating the fitness of each nest based on the power flow analysis of the fundamental frequency and the harmonic frequency; penalizing a fitness function if the power flow is not aggregated;
step 2.4, sorting the individual advantages and disadvantages, selecting the optimal nest position, and reserving the nest position to the next generation;
step 2.5, updating other nest positions by utilizing the Levy flight, combining with a parent generation, searching for the best and reserving to the next generation;
step 2.6, judging whether the bird is found by the host bird, if so, selecting a abandoned bird nest, and updating the position by utilizing the Levy flight;
step 2.7, selecting the current global optimal bird nest and judging whether the algorithm ending condition is met; if not, returning to the step 2.3;
and 3, on the basis of the optimal management model result, reducing the influence caused by the prediction uncertainty problem by adopting model prediction control with a feedback characteristic in a matching way, wherein the prediction uncertainty problem is the uncertainty caused when renewable energy is accessed into the active power distribution network.
2. The method for optimally managing the power quality of an active power distribution network as recited in claim 1 wherein said overall voltage harmonic distortion constraint can be used to reflect the overall harmonic level of node i as follows:
wherein THD is an abbreviation for Total voltage harmonic distortion, referring to the overall voltage harmonic distortion constraint; vi p,t,Fundamental frequency and harmonic frequency h voltage value, THD of p phase at time t node ii,maxIs the upper limit of THD;
the individual harmonic distortion constraint is an index that evaluates the harmonic level of each relevant frequency, and the individual harmonic distortion constraint for each node i satisfies the following constraint:
wherein IHD is an abbreviation of Industrial Harmonic Distortion Constraints, which refers to single Harmonic Distortion Constraints;is the upper limit of the individual voltage harmonic distortion for the harmonic frequency h;
the voltage imbalance factor constraint may be used to reflect the voltage imbalance level, and to ensure a relatively good voltage imbalance level, the following constraints should be satisfied:
wherein VUF is an abbreviation of Voltage Unbalance Factor Constraints and refers to Voltage imbalance Factor Constraints; vi 1,t,Vi 2,tPositive sequence voltage values and negative sequence voltage values of a node i at the moment t respectively; UM% is the upper limit in the form of percentage VUF; since the definition of the voltage imbalance factor is for a sine wave, only the voltage at the fundamental frequency is used to calculate VUF.
3. The method for optimizing and managing the power quality of the active power distribution network according to claim 1, wherein the voltage effective value constraint is as follows:
wherein the content of the first and second substances,is the effective value of the voltage of the p phase of the node i at the moment t;the upper limit and the lower limit of the voltage effective value are respectively;
the current effective value constraint is as follows:
wherein the content of the first and second substances,is the p-th phase of branch ij at time tAn effective value of current of;the upper limit and the lower limit of the current effective value are respectively;is the current value of the p-th phase fundamental frequency and harmonic frequency h of the branch ij at the time t.
4. The method for optimizing and managing the power quality of the active power distribution network according to claim 1, wherein the constraints of the optimization management model further include battery constraints, and the battery constraints include energy value constraints and power value constraints;
the energy value constraint is:
wherein the content of the first and second substances,is the energy stored in the battery at the end of time t; ei,max,Ei,minAre respectivelyUpper and lower limits of (P)i ES,tIs the active power of the accumulator at time t, epsilonin,εoutThe charging and discharging efficiency of the storage battery respectively; equation (7) describes the relationship between two continuous-time energy values and the limit that the energy value of the storage battery satisfies the upper and lower limits, respectively;
in order to ensure that the active power, the reactive power and the apparent power of the battery are within limits, the power value constraints should satisfy the following constraints:
5. The method for optimizing and managing the power quality of the active power distribution network according to claim 1, wherein the constraints of the optimization management model further include controllable distributed power supply constraints:
wherein, Pi DDG,tIs the active power of the controllable distributed power supply at time t,are respectively Pi DDG,tThe upper and lower limit values of (a),the upper limit value and the lower limit value of the gradient rate of the distributed power supply, wherein the controllable distributed power supply is a micro gas turbine or a fuel cell;is the power factor angle of the distributed power supply at time t,is thatUpper and lower limit values of (d);
the constraints of the optimal management model further include capacitor constraints:
wherein the content of the first and second substances,is the capacitor capacity available at node i at time t,respectively, the limit of the capacity; for operation of the distribution network, since the number of available capacitors is discrete, the distribution network is provided with a plurality of capacitorsIs a discrete decision variable;is the number of times the capacitor state is changed,is the maximum number of transitions, the purpose of which is to ensure that the capacitor has a good life cycle.
6. The method for optimizing and managing the power quality of the active power distribution network according to claim 1, wherein the constraints of the optimization management model further include balance constraints; the balance constraint is a power flow balance formula of the fundamental frequency and the harmonic frequency at each moment;
for power flow at the fundamental frequency, the node current injection equation is written as follows:
V1 abc,t=[Vi a,t Vi b,t Vi c,t]T (12)
wherein the content of the first and second substances,is a sub-matrix of the fundamental frequency node admittance matrix at time t, Ii abc,t,Vi abc,tA node injection current vector and a voltage vector of the fundamental frequency are respectively at a time t node i; the injected node current comes from a load, a renewable power supply, a controllable distributed power supply and a storage battery; i isi a,t,Ii b,t,Ii c,tThe node injection current values V of A, B and C phases of the fundamental frequency at the time t node ii a,t,Vi b,t,Vi c,tVoltage values of the fundamental wave frequency A, B and C phases at a time t node i respectively;
similarly, the power flow equation for the harmonic frequency h is expressed as:
wherein the content of the first and second substances,the node injection current values of A, B and C phases of the harmonic frequency h of the node i at the time t are respectively,voltage values of A, B and C phases of i harmonic frequency h of a node t at the moment are respectively;is the node admittance matrix of the harmonic frequency h at time t; harmonic components of distributed power sources and batteries are modeled as current sources with one or more branch circuits.
7. The method for optimizing and managing the power quality of the active power distribution network according to claim 1, wherein in the step 3, when each optimization time t comes, the energy value of the storage battery and the actual output value of the distributed power supply at the end of the last optimization time t-1, which is the initial time of the optimization time t, are collected and substituted into the relevant constraint as a known variable during the optimization at the current optimization time t; the optimization program takes the minimum network loss in the predicted time domain T as a target function, adopts the cuckoo algorithm for optimization, only issues the scheduling plan of the optimization time T, and temporarily repeats the optimization process when the next optimization time T +1 comes; and the real storage battery energy value of the system and the actual output value of the distributed power supply are acquired during each optimization, so that effective closed-loop feedback is formed, and the uncertainty influence brought by the renewable energy is reduced.
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