Background
The current power system contains various unbalance factors, the unbalance problem of normality is caused by three-phase asymmetry existing in a power grid, such as an arc furnace in a smelting system and a traction load in an electrified track, and the unbalance state of normality can exist or persist for a long time. The loss on a phase line is increased due to unbalanced three phases in a distribution network, the damage is caused to electric equipment and the normal operation of a transformer in a distribution station area, the temperature of local metal parts of the transformer is increased, the output is reduced, the active output of an induction motor is reduced, and the service life of the motor is shortened. In addition, the power supply voltage deviation of the power distribution network is also an important index for measuring the power supply quality of the power distribution network, the power distribution network is long in line and thin in line diameter, and the power supply voltage is very low when the load is heavier, so that the normal use of user equipment is influenced. The problems of three-phase imbalance and power supply voltage deviation of the power distribution network are treated urgently and are paid high attention.
The access of a large number of distributed power supplies is the trend of a future power distribution network, the voltage regulation, the power control and the three-phase imbalance compensation of the power distribution network are researched by utilizing the distributed power supplies, but the comprehensive treatment research of the three-phase imbalance and the voltage deviation of the power distribution network based on the distributed power supplies is not common at present. On the other hand, most of the existing control systems utilizing the distributed power supply are centralized control modes, the system is crashed due to single-point failure of the centralized controller, the communication and calculation pressure of the centralized controller is high, and the expandability of the system is poor and cannot meet the plug and play requirements of the distributed power supply. The distributed control system cancels an integrated controller, has high system expandability and control robustness, and has been widely applied to a plurality of engineering fields.
According to the method for improving the voltage quality of the power distribution network based on the single-phase photovoltaic inverter, the single-phase photovoltaic inverters distributed in the power distribution network are comprehensively applied, and the single-phase photovoltaic inverters are applied to form a plurality of distributed triangular compensation groups and a plurality of distributed star compensation groups. Each distributed triangular compensation group carries out local compensation of three-phase voltage unbalance, a plurality of distributed star compensation groups are cooperatively matched to carry out voltage deviation compensation of the power distribution network, the single-phase photovoltaic inverter exchanges reactive capacity utilization rate information through a local communication network, a reactive power reference value is calculated by adopting a distributed consistency algorithm, and fairness on the same meaning of the reactive capacity utilization rate is realized while voltage deviation compensation is carried out.
According to the method for improving the voltage quality of the power distribution network based on the single-phase photovoltaic inverter, the available reactive capacity of the single-phase photovoltaic inverter is fully utilized to comprehensively treat the three-phase imbalance and the voltage deviation of the power distribution network, and the system adopts a distributed control mode, so that the expandability and the robustness of the control system are improved, and the method has important practical application values for improving the power supply voltage quality of the power distribution network, playing the role of a distributed power supply and the like.
Disclosure of Invention
The invention aims to utilize the reactive capacities of a plurality of single-phase photovoltaic inverters distributed in a power distribution network to cooperatively treat three-phase unbalance and power supply voltage deviation of the power distribution network. The method effectively utilizes the regulation and control functions of the photovoltaic inverter, realizes comprehensive treatment of power supply voltage treatment of the power distribution network in a distributed mode, and has the characteristics of high expandability and high robustness of the control system.
The technical scheme of the invention is a method for improving the voltage quality of a power distribution network based on a single-phase photovoltaic inverter, which specifically comprises the following steps:
step 1: the method comprises the following steps that single-phase photovoltaic inverters distributed in a power distribution network form a plurality of distributed triangular compensation groups and distributed star compensation groups;
step 2: compensating the unbalance of the power distribution network by combining a control strategy of the distributed triangular compensation group with the distributed triangular compensation group, and reducing three-phase negative sequence voltage;
preferably, the control strategy of the distributed triangle compensation group in step 2 is as follows:
step 2.1: calculating the unbalance degree of the three-phase voltage at the kth control moment;
step 2.2: determining the maximum value of the available reactive capacity of each PV inverter of the triangular compensation group at the moment k;
step 2.3: determining the reactive power compensation degree of each PV inverter of the triangular compensation group at the moment k;
step 2.4: calculating reactive power reference values output by each PV inverter of the triangular compensation group at the moment k;
and step 3: and for the power distribution network with the improved three-phase imbalance, each phase voltage is independently adjusted within an allowable range through the distributed star compensation group according to the control strategy of the distributed star compensation group, so that the voltage of the power distribution network is improved.
Preferably, the control strategy of the distributed star compensation group in step 3 is as follows:
step 3.1: judging whether to start compensation;
step 3.2: calculating the available reactive capacity of the PV inverter;
step 3.3: calculating the reactive power compensation degree of each PN inverter;
step 3.4: calculating reactive power reference values output by each PV inverter of the star compensation group;
the invention has the beneficial effects that: according to the method for improving the voltage quality of the power distribution network based on the single-phase photovoltaic inverter, the available reactive capacity of the single-phase photovoltaic inverter is fully utilized to comprehensively treat the three-phase imbalance and the voltage deviation of the power distribution network, and the system adopts a distributed control mode, so that the expandability and the robustness of the control system are improved, and the method has important practical application values for improving the power supply voltage quality of the power distribution network, playing the role of a distributed power supply and the like.
Detailed Description
In order to more clearly illustrate the present invention and/or the technical solutions in the prior art, the following will describe embodiments of the present invention with reference to the accompanying drawings. It is obvious that the drawings in the following description are only some examples of the invention, and that for a person skilled in the art, other drawings and embodiments can be derived from them without inventive effort.
As shown in fig. 1, the present invention designs an effective voltage regulation method for a three-phase four-wire unbalanced distribution network by utilizing the reactive power available from single-phase PV inverters randomly distributed in the distribution network. In the figure, A, B and C are phase lines, and N is a neutral line. Available single-phase PV inverters are connected to the network in two ways.
Step 1: the method comprises the following steps that single-phase photovoltaic inverters distributed in a power distribution network form a plurality of distributed triangular compensation groups and distributed star compensation groups;
in step 1, the single-phase photovoltaic inverters distributed in the power distribution network form a plurality of distributed triangular compensation groups and distributed star compensation groups, specifically:
as shown in fig. 1, one of the star compensation groups is connected between the phase (a, B or C) and the neutral line (N) to form a distributed star compensation group, and there may be a plurality of such star compensation groups; the other is connected at intervals (AB, BC or CA) to form a distributed triangle compensation group.
Step 2: compensating the unbalance of the power distribution network by the distributed triangular compensation group according to the control strategy of the distributed triangular compensation group, and reducing the three-phase negative sequence voltage;
as shown in fig. 2, the operations of the plurality of distributed triangular compensation groups are independent from each other, and the control methods are the same, and the control method will be described below with respect to one triangular compensation group.
Step 2.1: calculating the unbalance degree of the three-phase voltage at the kth control moment;
the triangular compensation group acquires the relative ground voltage of the access point, and calculates the unbalance degree of the three-phase voltage according to the following method:
in the formula, VAVG(k) The average value of the inter-phase voltage effective values of the access points is acquired by the triangular compensation group at the moment k, and the calculation method comprises the following steps:
in the formula, VAB(k)、VBC(k) And VAB(k) Effective values of A phase and B phase, B phase and C phase and A phase voltage of the triangular compensation group access point are respectively set; Δ VAB(k)=VAB(k)-VAVG(k)|、ΔVBC(k)=|VBC(k)-VAVG(k) I and Δ VCA(k)=VCA(k)-VAVG(k) The deviation of the effective value of the voltage of the access point from the average value of the effective values is the difference of the effective values of the voltage of the access point from phase B, phase B from phase C and phase C from phase A.
Step 2.2: determining the maximum value of the available reactive capacity of each PV inverter of the triangular compensation group at the moment k;
the calculation method comprises the following steps:
in the formula, PAB(k)、PBC(k) And PCA(k) The active power output by the PV inverter k at the moment is respectively accessed between the phase A and the phase B, between the phase B and the phase C and between the phase C and the phase A, and is usually determined by the photovoltaic inverter according to the maximum power point tracking technology; sAB、SBCAnd SCAThe rated capacities of the PV inverters connected between the A phase and the B phase, between the B phase and the C phase and between the C phase and the A phase are respectively. Qmax AB(k)、Qmax BC(k) And Qmax CA(k) Maximum values of the available reactive capacity of the PV inverter k moment accessed between the A phase and the B phase, between the B phase and the C phase and between the C phase and the A phase are respectively.
Step 2.3: determining the reactive power compensation degree of each PV inverter of the triangular compensation group at the moment k;
the calculation method of the reactive power compensation degree of the PV inverter at the moment k comprises the following steps:
in the formula etaΔAB(k)、ηΔBC(k) And ηΔCA(k) Reactive power compensation degrees of PV inverters at the k moment accessed between the phase A and the phase B, between the phase B and the phase C and between the phase C and the phase A are respectively; etaΔAB(k-1)、ηΔBC(k-1) and ηΔCA(k-1) reactive power compensation degrees of the PV inverter at k-1 moment accessed between the phase A and the phase B, between the phase B and the phase C and between the phase C and the phase A respectively; beta is aAB、βBCAnd betaCAPV inverter reactive power regulation constants which are respectively connected between the phase A and the phase B, between the phase B and the phase C and between the phase C and the phase A.
Step 2.4: calculating reactive power reference values output by each PV inverter of the triangular compensation group at the moment k;
in the formula, Qref AB(k)、Qref BC(k) And Qref CA(k) And outputting reactive power reference values at the moment of accessing the PV inverter k between the A phase and the B phase, between the B phase and the C phase and between the C phase and the A phase respectively. Each PV inverter of the triangular compensation group outputs corresponding reactive power according to each reactive power reference value, and local compensation is carried out on the three-phase unbalance of the voltage at the access point.
And step 3: and for the power distribution network with the improved three-phase imbalance, each phase voltage is independently adjusted within an allowable range through the distributed star compensation group according to the control strategy of the distributed star compensation group, so that the voltage of the power distribution network is improved.
Through the in-situ compensation of each distributed triangular compensation group, the three-phase voltage unbalance can be preliminarily compensated, and the three-phase unbalance is further compensated by utilizing a plurality of distributed star compensation groups and adopting a consistency algorithm. As shown in fig. 3, m distributed star compensation groups are provided, each distributed star compensation group is connected between PV inverters between a phase and N phase, between B phase and N phase, and between C phase and N phase, and local communication networks are provided, and the operation of the PV inverters connected between a phase and N phase, between B phase and N phase, and between C phase and N phase are independent from each other and the control manner is the same, and the control method is described below by taking the PV inverter connected between a phase and N phase as an example.
Step 3.1: judging whether to start compensation;
the judgment condition for judging whether to start compensation is as follows:
VAN,i(k)<Vmin or VAN,i(k)>Vmax (6)
wherein VAN,i(k) The voltage effective value at the moment k between the phase A and the phase N of the ith star compensation group connecting point is represented as i ═ 1,2, …, m; vminAnd VmaxAnd respectively providing the lower limit and the upper limit of the effective value of the node voltage of the power distribution network according to corresponding standards. And (4) carrying out the next step when the formula (6) is satisfied, and otherwise, keeping the current state and not acting.
Step 3.2: calculating the available reactive capacity of the PV inverter;
the calculation method for calculating the available reactive capacity of the PV inverter comprises the following steps:
in the formula
The method comprises the following steps that the available reactive capacity of a PV inverter k moment between an A phase and an N phase is accessed to an ith star compensation group; p
AN,i(k) The active power output by the PV inverter k between the A phase and the N phase is accessed to the ith star compensation group, and is usually determined by the PV inverter according to the maximum power point tracking technology; s
AN,iAnd accessing the rated capacity of the PV inverter between the A phase and the N phase for the ith star compensation group.
Step 3.3: calculating the reactive power compensation degree of each PN inverter;
the calculation method for calculating the reactive power compensation degree of each PN inverter comprises the following steps:
in the formula etaY AN,i(k) Accessing reactive power compensation degree of a PV inverter k moment between the phase A and the phase N to the ith star compensation group; etaY AN,l(k-1) the reactive power compensation degree of the first star compensation group at the moment of accessing the PV inverter k-1 between the phase A and the phase N; beta is aAN,iThe reactive power regulation constant of the PV inverter between the phase A and the phase N is accessed to the ith star compensation group; omegai,lThe PV inverter connected between the A phase and the N phase for the ith star compensation group is etaY AN,l(k-1) the weight coefficient is set by the following calculation method:
in the formula (d)i,lFor the ith and the l-th star compensation groupsIdentification of whether a communication link exists between PV inverters connected between the A phase and the N phase, and if the communication link exists, di,l1, otherwise di,l=0;di,pIdentification of whether communication links exist between PV inverters connected between A phase and N phase for ith and pth star compensation groups, and if communication links exist, di,p1, otherwise di,p0; in addition, di,i=1。
Step 3.4: calculating reactive power reference values output by each PV inverter of the star compensation group;
the calculation method for calculating the reactive power reference value output by each PV inverter of the star compensation group comprises the following steps:
in the formula (I), the compound is shown in the specification,
and accessing an output reactive power reference value of the PV inverter at the k moment between the A phase and the N phase for the ith star compensation group. PV inverters connected between the A phase and the N phase of all the star-shaped compensation groups output corresponding reactive power according to respective reactive power reference values, the voltage of the power distribution network is subjected to cooperative compensation, and the voltage along the line is adjusted to be within an allowable range.
The method for improving the voltage quality of the power distribution network based on the single-phase photovoltaic inverter can be seen in the invention, on one hand, the single-phase photovoltaic inverters distributed in the power distribution network are utilized for local compensation, so that the three-phase unbalance degree of the power distribution network is reduced, on the other hand, the single-phase photovoltaic inverters distributed in the power distribution network are utilized for cooperative matching, so that the power supply voltage deviation of each node of the power distribution network is reduced, the reactive capacity utilization rate of the single-phase photovoltaic inverters in the voltage compensation process is the same, and the fairness of the single-phase photovoltaic inverters is.
It should be understood that parts of the specification not set forth in detail are well within the prior art.
It should be understood that the above description of the preferred embodiments is given for clarity and not for any purpose of limitation, and that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.