When introducing the failure mode of the distributed power distribution network, we mentioned that the voltage of the distributed power grid connection point may drop, and the distributed power supply needs to have low voltage ride-through capability, so let’s analyze the low voltage ride-through characteristics.
Low Voltage Ride Through (LVRT) usually means that when the grid disturbance or fault causes the voltage drop at the grid connection point of the distributed power supply, within a certain voltage drop range, the distributed power supply can operate uninterruptedly connected to the grid. Operational experience shows that when the voltage drops due to a short-term fault in the system, the distributed power supply does not cut off immediately, but continues to connect to the power grid to output power, which is beneficial to the recovery of the power grid.
At present, the research on LVRT focuses more on wind turbines. Considering that the voltage drop will bring a series of transient processes to the motor, such as overvoltage, overcurrent or speed increase, etc., which seriously endanger the safe operation of the wind turbine itself and its control system. Under normal circumstances, if the power grid fails, passive self-protection will be implemented and the grid will be disconnected immediately, regardless of the duration and severity of the failure, which can ensure the safety of the wind turbine to the greatest extent, and it is acceptable when the grid penetration rate of wind power generation (ie, the proportion of wind power generation in the grid) is low. However, when wind power accounts for a large proportion of the power grid, if the wind turbine still adopts passive decoupling when the voltage drops, it will increase the difficulty of recovery of the entire system, and may even aggravate the failure, which will eventually lead to the decoupling of all other units in the system. Therefore, effective low-voltage ride-through measures must be taken to maintain the stability of the wind farm grid.
Usually, when the voltage of the power grid drops suddenly, it is difficult to establish the terminal voltage of the wind turbine. If the wind turbine continues to operate on the grid, the grid voltage will not be able to recover. Therefore, in this case, the wind turbine is generally cut off. As the percentage of the installed capacity of wind turbines in the total installed capacity of the power grid increases, the impact of the power cut on the safe operation of the power grid is becoming more and more serious. The experience of some countries shows that when this value reaches 3% to 5%, if a large area of power cuts occurs during high wind speeds, it will have a devastating impact on the power grid. The large-scale cutting of the wind turbine caused by the voltage drop of the power grid will not only have a serious impact on the power grid, but also cause great harm to the wind turbine itself. During this process, the mechanical input and electrical output power of the wind turbine will be unbalanced, and the transient process will lead to overcurrent in the generator, which may cause damage to electrical components. At the same time, the additional torque and stress caused by the unbalance may also damage the mechanical parts. This shows the importance of the low voltage ride-through capability of wind turbines.
Similar to wind power access, as the proportion of installed photovoltaic capacity in the system increases, the impact of its operation on the stability of the power system cannot be ignored. Different countries and regions have different LVRT requirements. Among the standards, only the German medium-voltage grid-connected standard clearly states that distributed power sources must have low-voltage ride-through capability and participate in the dynamic support of the power grid (applicable to various short-circuit types). The Canadian C22.3 No.9 standard also allows low-voltage ride-through for distributed power generation, subject to the agreement of the grid company. In this case, the time of the protection action is invalid, and after consultation between the grid company and the DG owner, the voltage response requirement is replaced by the low voltage ride-through requirement.
Among them, the low-voltage ride-through requirements of the German medium-voltage grid-connected standard are:
(1) When the power grid fails, it is not disconnected from the network;
(2) In the case of network failure, the network voltage is supported by injecting reactive current into the network;
(3) After the fault is cleared, no more inductive reactive current is absorbed into the medium voltage grid than before the fault occurs.
China pointed out in the GB/T 33593-2017 “Technical Requirements for Grid-connected Distributed Power Sources” that the distributed power supply that is directly connected to the public grid through the 10 (6) kV voltage level and the grid-connected through the 35 kV voltage level should have a certain low voltage ride-through capability. When the assessment voltage of the grid connection point is in the area above the voltage contour line in Figure 1, the distributed power supply should not be disconnected from the grid and run continuously; otherwise, the distributed power supply is allowed to be cut out.
Note: The assessment voltages under various types of power system faults are: three-phase short-circuit fault and two-phase short-circuit fault assessment of grid-connected point-to-line voltage, single-phase grounding short-circuit fault assessment of grid-connected point phase voltage.