| Chinese existing low voltage ride through(LVRT) techniques and standards are based on outlet of wind generators(WGs). Constrained by some factors such as geography, it is difficult to test LVRT of offshore WGs. For example, some installation site can afford draught depth only when it is high tide, besides, wind generators require to be tested in both situations when they output heavy(more than 0.9.p.u.) and light power(0.1-0.3p.u.). LVRT detection on offshore WGs wastes a lot of manpower and other resources. Thus, a method for remote detection on offshore WGs is provided, i.e., the testing devices are located the point that is easy to arrive such as coast, then connect the devices and WGs through submarine cable to test the LVRT of WGs. However, under the influence of the cable, some new problems occur, in engineering after the devices are put into the circuit the WGs disconnect from the power system or the output of the power of WGs can’t meet requirements during voltage recovery. Because of this, this paper emphasize on researching what impact the cable brings to remote detection, analyzing why the cable lead the WGs to disconnect from power system, which can provide thoughts for the development of the devices which is used for remote detection.This paper firstly analyzes the advantage and disadvantage of LVRT devices based on transformer, power electronic switches and impedance voltage divider type in terms of economy, applicability and reliability. Then it focuses on the configuration of the testing device based on impedance voltage divider type and its relevant flowsheet. Secondly, according to this flowsheet, it analyzes the difference between remote and near LVRT detection under the influence of cable during no-load conditions. Following the same idea, setting doubly fed induction generator(DFIG) as research object, it analyzes the influence of cable on load test, and summarizes the reasons for the disconnection of DFIG from power system in remote detection. Finally, on the basis of these reasons, flowsheet is improved. Main contributions and innovations are described as follows:1)The mathematical expression of the terminal voltage is derived according to every step of the testing process in no-load state, which can conclude that the mutual influence between the current-limiting reactance and cable and the capacitance effect of cable heightens the terminal voltage. If the voltage is beyond its permitted range, DFIG may disconnect from power system. When no-load transient analysis is done, the cable is equivalent to muti-π circuit. The expression in complex domain of the terminal voltage is derived. The relationship between current limiting reactance and transient process is obtained by pole analysis method. With the increase of current limiting reactance, the real part of the main pole tends to 0, the voltage transient process of the machine is prolonged, and the harmonic content is increased. When load LVRT test is analyzed, the paper emphasizes on making different equivalent circuits according to whether the crowbar resistance is triggered or not. Through calculation and simulation, no matter whether crowbar is triggered, the influence of current limiting reactance and cable on load LVRT test is the same to no-load test. The extension of the transient process will weaken the function of the crowbar protection circuit, which will cause DFIG to lose power support to power system during voltage recovery. DFIG may be wrongly judged to be unqualified.2) After analyzing the characteristics of the remote detection, the test equipment and process are improved, that is, the switches in the original detection device are replaced by the power electronic switches, to improve the control performance of the test equipment. The operation sequence of the impedance in the detection device is revised, to avoid the mutual influence of the current-limiting current and cable, then to make the device based on impedance divider type adapt to remote LVRT detection. The simulation validates its effectiveness. |
PDF Full Text Request