A distribution network refers to a network for distributing electric power in power systems. With a great number of distributed generators and diverse loads and the introduction of novel AC/DC hybrid distribution network frames, in a distribution network, there are both generators and loads, rather than only loads. To be an active distribution network in which the power flows can be two directions, higher requirements are proposed on the coordination control level and service quality of distribution networks. A transmission network is connected with multiple distribution networks. Since there are a large number of elements such as loads, branches, distribution generators and sections in the distribution networks and the transmission network itself is large in size, it is difficult to intensively and collectively analyze the transmission network and the distribution networks. Therefore, it is necessary to perform equivalence on the distribution networks. In the equivalent method of the distribution networks, the influences of the complex distribution networks on the transmission network are retained in a simple equivalent network, so that important characteristics of the influences of the distribution networks on the transmission network are reserved while decreasing the calculation complexity. Accordingly, the accuracy of steady-state analysis of power systems is improved.
Existing static equivalent methods for an active distribution network mainly include a PQ equivalent method, and an equivalent method of constant impedance, current and load (ZIP equivalent method). In the PQ equivalent method, distribution networks are replaced with their equivalent power injections/extractions. The ZIP equivalent method further considers the nonlinear characteristics of loads with respect to voltage changes. However, the existing methods mainly have the following disadvantages: 1) voltage and power support characteristics of distributed generators are ignored, and the power injection direction from a transmission network to a distribution network cannot be changed, so that a large error is brought to the steady-state analysis of power systems; and 2) only the consistency of power flow states before and after equivalence is taken into consideration, but the consistency of sensitivity and power loss before and after equivalence are ignored, so the accuracy of equivalence of the active distribution networks is low.
USPTO Backgrounds
USPTO Backgrounds