CN108964044A - A kind of ocean micro-capacitance sensor group's equivalent modeling method interconnected by submarine cable - Google Patents

Abstract

The purpose of the present invention is the deficiencies for the research of existing ocean micro-capacitance sensor group's external network equivalent, a kind of ocean micro-capacitance sensor group's equivalent modeling method interconnected by submarine cable is provided, its main feature is that being directed to submarine cable Parameters variation situation and limited data exchange, the unified external network equivalent circuit of ocean micro-capacitance sensor is established, and establishes the parameter estimation equation of external network equivalent circuit based on covariance theory；Weighting function is established at a distance from current time further according to the sampling time of sampled point, weighting function is combined with equivalent parameters estimation equation, according to the measuring value of the voltage and current of tie point in a period of time, and then the equivalent parameters of equivalent circuit are solved, gained equivalent circuit is ocean micro-capacitance sensor external network equivalent circuit.The present invention provides a kind of analysis models that mathematics is rigorous, realize external network equivalent in limited data exchange and submarine cable Parameters variation, and to the adaptable of Intranet operation characteristic variation, equivalent effect is good.

Description

An Equivalent Modeling Method for Ocean Microgrid Groups Interconnected by Submarine Cables

technical field

The invention relates to the technical field of static equivalence of power systems, in particular to an equivalent modeling method of marine micro-grid groups interconnected through submarine cables.

Background technique

At present, more and more marine microgrids, such as marine renewable energy fields, all-electric ships, and offshore oil and gas platforms, are interconnected to improve energy efficiency and better control emissions. The interconnection of different types of marine microgrids with “plug and play” functionality is also becoming more popular. Therefore, in order to realize the functional evaluation and the analysis of the operating characteristics of the connected marine microgrid system, it is necessary to establish a complete marine microgrid model. However, because these marine microgrids are far apart and the marine environment is harsh and changeable, it is difficult to collect detailed data of each local microgrid to establish a complete marine microgrid model. When the local microgrid model and state information are unknown, for a specific marine microgrid or the internal grid of key research, the external system or external network is a "black box". Therefore, in the case of only internal network data, how to realize the effective equivalent of the "black box" external network has become a key issue to ensure the accuracy of internal network characteristic analysis.

The existing research on equivalent modeling of external networks only focuses on land power grids, and the equivalent modeling of land external networks is more complicated, requiring more measurement points, and land power grids are connected by transmission lines. In the analysis of external network equivalents, it is considered that The parameters of the line are constant, that is, the influence of the change of the parameters of the transmission line on the equivalent effect is not considered. Yan Wei, Li Shiming, Chen Jun, Lu Jiangang, Guo Lin, Li Qin, Zhao Xia. Estimation method of static equivalent parameters of two-port external network based on measured information of internal network[J]. Chinese Journal of Electrical Engineering, 2011,31(13) :101-106; Liu Zhiwen, Liu Mingbo. Multi-area reactive power optimization decomposition coordination algorithm based on Ward equivalent[J]. Electric Power System Automation, 2010,34(14):63-69. When using long-distance submarine cables to transmit power, it will not only consume a certain amount of active power to cause network loss like ordinary land transmission lines, but also cause the reactive power passing through the submarine cables to change in voltage due to the large capacitive effect of the submarine cables. It is particularly sensitive, but more importantly, the parameters of the submarine cable will change with aging. When the parameters of the submarine cable change, it may affect the performance of the entire offshore system. M.Runde, R.Hegerberg, N.Magnusson, E.Ildstad and T.Ytrehus, "Cavity formation in mass-impregnated HVDC subsea cables-mechanisms and critical parameters," in IEEE Electrical Insulation Magazine, vol.30, no.2, pp.22-33; Kolstad ML, Atle Rygg, Sharifabadi K, et al. Integrating Offshore Wind Power and Multiple Oil and Gas Platforms to the Onshore Power Grid Using VSC-HVDC Technology [J]. Marine Technology Society Journal, 2014, 48(48): 31-44(14). Therefore, it is particularly important to propose an equivalent modeling method for marine microgrid groups that considers the variation of submarine cable parameters.

Contents of the invention

The purpose of the present invention is to provide an equivalent modeling method for marine micro-grid groups interconnected through submarine cables in view of the deficiency of the existing research on the equivalent value of the external network of marine micro-grid groups. In the case of limited data exchange and submarine cable parameter changes, the method estimates the complete equivalent model of the submarine cable and marine microgrid based on the statistical algorithm of the point of common connection (PCC) measurement data to meet the "plug and play" function, Based on the equivalent model, a unified external network equivalent circuit of the marine microgrid is established, and its equivalent impedance and equivalent current source are estimated based on the covariance theory. In addition, the equivalent impedance can reflect the response characteristics of the marine microgrid, so it can be used for real-time control.

The equivalent modeling method of the marine microgrid group interconnected by submarine cables proposed by the present invention includes the following implementation methods and steps:

(1) Classification of marine microgrid groups

The marine microgrid connected to the ocean group grid control center is defined as a master microgrid, and other marine microgrids connected to it are defined as slave microgrids. Different marine microgrids are connected to each other through submarine cables, as shown in Figure 1. The connection point between the main microgrid and the submarine cable is set as PCC, and the currents of the slave microgrid and the main microgrid are I _MG and I _PCC , respectively. Similar to the terrestrial power grid, the online calculation of the marine microgrid group usually requires that the uninteresting or unobservable secondary microgrid, that is, the external network part, be equivalent; the main microgrid is the research power grid, that is, the internal network part.

(2) Marine microgrid group slave microgrid equivalent circuit

An ocean microgrid can be an AES, a power plant, or a platform with distributed generators, and they are connected to the main microgrid through submarine cables. In the case of parameter changes, a unified secondary microgrid equivalent circuit of the marine microgrid is established. The equivalent parameters of the secondary microgrid equivalent circuit are composed of the ideal voltage source U _L of the boundary node and the equivalent impedance Z _mg . Considering that the parameters of the slave microgrid and the cable are unknown, the slave microgrid and the cable are taken as a whole, and the system in Fig. 1 is transformed into that shown in Fig. 2. At this time, the equivalent parameters of the slave microgrid equivalent circuit are Ideal current source I _L and equivalent impedance Z _L , based on covariance theory to estimate its equivalent impedance and ideal current source.

(3) Establish weight function

According to the sampling time of the sampling point, the time weight function is established as follows. The closer the time of the sample point is to the current time, the greater the weight coefficient obtained.

In the formula, t represents the current time; t _i represents the collection time of the i-th sample point; t ₀ represents the sampling start time.

(4) Solve the equivalent parameters of the equivalent circuit from the microgrid

According to the measured value of the voltage amplitude and phase angle, current amplitude and phase angle of the connection point PCC within a period of time, and based on the covariance theory, the parameter estimation equation of the equivalent circuit from the microgrid is established, and the equivalent impedance is estimated The equation is combined with a time weighting function.

In the formula, U _PCC and I _PCC are the voltage and current values of the connection point PCC respectively; D represents the variance, and D(I _PCC ) is the calculation of the variance of the current I _PCC ; COV represents the covariance, COV(U _PCC , I _PCC ) That is, the covariance calculation of the connection point voltage U _PCC and current I _PCC ; both voltage and current are vectors.

From the weight function (1) and the parameter estimation equation (2), the equivalent impedance Z _L is solved:

Find the equivalent current source I _L from the equivalent impedance:

Compared with the prior art, the inventive method has the following advantages:

(1) The application of the external network equivalent modeling method proposed by the present invention is a marine microgrid group. Compared with the land external network equivalent method, it is more concise and reasonable, and does not need to know more detailed information about the external network. , based on the characteristics of the marine microgrid group and the covariance theory, it can be used to solve the static equivalence problem of the "black box external network".

(2) The equivalent modeling method proposed by the present invention fully considers the parameter change of the submarine cable, eliminates the influence of the submarine cable parameter change on the equivalent effect, and makes the equivalent result more accurate.

(3) The equivalent modeling method proposed by the present invention performs time-weighted processing on the measurement data, which ensures the accuracy of the equivalent impedance and equivalent power supply, and makes the obtained external network equivalent parameters adapt to changes in the internal network operating characteristics Strong ability and good equivalent effect.

Description of drawings

Figure 1 is the structure diagram of marine microgrid group

Figure 2 is a structure diagram of the marine microgrid group considering the change of cable parameters.

Fig. 3 is a wiring diagram of the marine microgrid group system applied to the method of the present invention in the embodiment, and the numbers in the figure represent the node numbers. In this system, nodes 19-23 are external network nodes (slave micro-grid nodes), node 18 is a border node, and the rest are internal network nodes (main micro-grid nodes), that is, the virtual frame part in the figure is the external network that needs to be equivalent.

Fig. 4 is the system wiring diagram after considering the equivalent value of the external network of the submarine cable parameter change in the embodiment, is the external network (from the micro-grid) equivalent circuit in the dotted line frame, and its equivalent parameter is equivalent impedance Z _L and equivalent Current source I _L .

Figure 5 is the change curve of the VAE value and node voltage value of each node of the main micro-grid of the system under the static power flow calculation under the three equivalent methods when the load of the internal network (main micro-grid) changes, and the VAE value and node voltage Values are per unit.

Detailed ways

In the following, the present invention will be further described in detail in conjunction with the accompanying drawings and embodiments, so as to make the purpose, technical solutions and advantages of the present invention more clear. It should be understood that the specific embodiments described here are only used to explain the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the contents of the present invention above.

Figure 3 shows the connection structure diagram of an actual large-scale offshore platform power grid and offshore mobile platform power grid in the Bohai Sea area. The single-line diagram in the dotted box in the figure depicts the electrical wiring diagram of the mobile offshore platform, and the rest is the grid structure of a large offshore platform in Bohai Sea. The two microgrids are connected by a 10-kilometer submarine cable between the fourth and eighteenth nodes. Nodes 1, 10, and 19 are respectively connected to gas turbine generators, and the two offshore microgrids are radial structures. Apply the method of the present invention to perform equivalent value to the external network in the dotted line box in Fig. 3 . The steps are as follows:

(1) Classification of marine microgrid groups

The marine microgrid connected to the control center is defined as the master microgrid, and the rest connected to it are slave microgrids, that is, the mobile offshore platform in the dotted line box in Figure 3 is the slave microgrid, and the one outside the dotted line box is the master microgrid. After the marine microgrid group is classified, the master and slave microgrids are connected by submarine cables. The connection between nodes 4 and 18 in Figure 3 is the submarine cable.

(2) Establish the equivalent circuit of the external network of the marine microgrid

Based on a mathematically rigorous analysis model, the equivalent circuit of the marine microgrid is represented as a circuit composed of an ideal voltage source and equivalent impedance by performing an equivalent value on the mobile offshore platform in the dotted line box in Figure 3. Considering that the parameters of the slave microgrid and the cable are unknown, the equivalent system in Figure 3 is transformed into the equivalent model shown in Figure 4 by taking the slave microgrid and the cable as a whole, and the equivalent value of the external network to be obtained The parameters of the circuit are ideal current source I _L and equivalent impedance Z _L .

(3) Solve the equivalent parameters of the equivalent circuit of the external network

According to a set of measured values of the PCC voltage and current at the connection point within a period of time, based on the covariance theory, the parameter estimation equation of the equivalent circuit of the external network is established and solved, as follows:

The status information of the boundary nodes over a period of time, that is, voltage, current amplitude and phase angle, is obtained through measurement devices, such as micro-phase measurement units or smart meters. Through the sampling time of the collected sampling points, the time weight coefficient of each measurement value is obtained from the formula (1) in the claims. Substituting the measured voltage, current value and weight coefficient into the formula (3) of the claim, that is, calculating the equivalent impedance, and then calculating the equivalent current source by formula (4).

In order to verify the effectiveness of the method of the present invention, the equivalent accuracy of the method of the present invention is verified by taking the connection structure diagram of an actual large-scale offshore platform power grid in the Bohai Sea region and the power grid of the offshore mobile platform shown in Figure 3 as an example. There are two methods to participate in the comparison: (1) PQ equivalent method; (2) PV equivalent method.

What needs to be specially explained is: the PV equivalent and PQ equivalent methods, that is, according to the parameters such as the short-circuit capacity and generator power of the microgrid obtained during interconnection, they are equivalent to PV nodes or PQ nodes.

In order to quantitatively illustrate the accuracy of the three equivalent methods, two evaluation indexes are defined: the average value of absolute error of node voltage (VAE) and the value of node voltage. The larger the VAE, the greater the error of the equivalent model under the condition of load change during the test period, and vice versa, the higher the accuracy of the equivalent model. The mean value of the absolute error of the node voltage is defined as follows:

In the formula, X _n is the voltage value of node n of the equivalent model obtained through power flow calculation; is the voltage value of node n in the complete model, which can be used as the voltage reference value of node n in each equivalent model; N is the number of sampling points in this time period.

The load of the internal network (main micro-grid) changes. Figure 5 shows the VAE value and node voltage value of each node of the main micro-grid of the system under the three equivalent methods obtained through static power flow calculation. The VAE value and node voltage value are per unit values .

Shown by the simulation result of Fig. 5: (1) the main microgrid node VAE value calculated after the equivalent method of the present invention is all smaller than the VAE value calculated after the PV, PQ equivalent method, indicating that the equal value proposed by the present invention The value method is more accurate than the other two equivalent methods. (2) From the voltage fluctuation curve in the figure, it can be seen that the change trend of the voltage value calculated by the three equivalent methods is basically the same as that of the actual value, and the node voltage value calculated by the PV equivalent and PQ equivalent deviates from the actual The larger the value, the larger the error. However, the node voltage value calculated after equivalent value by the method of the present invention is closer to the actual value, and the error is the smallest.

Claims (1)

1. A method for modeling equivalents of marine microgrid groups interconnected by submarine cables, characterized in that, the method may further comprise the steps: (1) Classification of marine microgrid groups The marine microgrid connected to the ocean group grid control center is defined as a master microgrid, and other marine microgrids connected to it are defined as slave microgrids. Different marine microgrids are connected to each other through submarine cables, as shown in Figure 1. The connection point between the main microgrid and the submarine cable is set as PCC, and the currents of the slave microgrid and the main microgrid are I _MG and I _PCC , respectively. Similar to the terrestrial power grid, the online calculation of the marine microgrid group usually requires that the uninteresting or unobservable secondary microgrid, that is, the external network part, be equivalent; the main microgrid is the research power grid, that is, the internal network part. (2) Establish the equivalent circuit of the marine microgrid group from the microgrid An ocean microgrid can be an AES, a power plant, or a platform with distributed generators, and they are connected to the main microgrid through submarine cables. In the case of parameter changes, a unified secondary microgrid equivalent circuit of the marine microgrid is established. The equivalent parameters of the secondary microgrid equivalent circuit are composed of the ideal voltage source U _L of the boundary node and the equivalent impedance Z _mg . Considering that the parameters of the slave microgrid and the cable are unknown, the slave microgrid and the cable are taken as a whole, and the system in Figure 1 is transformed into that shown in Figure 2. At this time, the equivalent parameters of the slave microgrid equivalent circuit are Ideal current source I _L and equivalent impedance Z _L , based on covariance theory to estimate its equivalent impedance and ideal current source. (3) Establish weight function According to the sampling time of the sampling point, the time weight function is established as follows. The closer the time of the sample point is to the current time, the greater the weight coefficient obtained. In the formula, t represents the current time; t _i represents the collection time of the i-th sample point; t ₀ represents the sampling start time. (4) Solve the equivalent parameters of the equivalent circuit from the microgrid According to the measured value of the voltage amplitude and phase angle, current amplitude and phase angle of the connection point PCC within a period of time, and based on the covariance theory, the parameter estimation equation of the equivalent circuit from the microgrid is established, and the equivalent impedance is estimated The equation is combined with a time weighting function. In the formula, U _PCC and I _PCC are the voltage and current values of the connection point PCC respectively; D represents the variance, and D(I _PCC ) is the calculation of the variance of the current I _PCC ; COV represents the covariance, COV(U _PCC , I _PCC ) That is, the covariance calculation of the connection point voltage U _PCC and current I _PCC ; both voltage and current are vectors. From the weight function (1) and the parameter estimation equation (2), the equivalent impedance Z _L is solved: Find the equivalent current source I _L from the equivalent impedance: