When faults such as short circuits or lightning strikes occur in the power grid, causing voltage drops, the low voltage ride through (LVRT) capability of energy storage inverters becomes the key to ensuring grid stability. This technology requires the inverter to remain connected to the grid and provide reactive power support even when the voltage drops to a certain level (such as 20% of the rated voltage), avoiding a chain reaction caused by large-scale disconnection. It is the "admission certificate" for energy storage systems to participate in grid peak shaving and frequency regulation, and is also one of the core indicators for measuring inverter performance.
1 Graded response to voltage drop: precise response from mild to deep
The core of LVRT technology is to adopt different strategies based on the degree of voltage drop. When the voltage drops to 50% -90% (mild drop), the inverter prioritizes maintaining stable active output while injecting a small amount of reactive power (approximately 20% of rated power) to help restore the grid voltage. When a transformer failure caused a voltage drop of 70% in an energy storage system in an industrial park, the inverter adjusted the reactive power output within 0.1 seconds, allowing the voltage to return to normal levels within 2 seconds without affecting factory production.
When the voltage drops to 20% -50% (moderate drop), the inverter reduces active output (to below 50%), increases reactive power injection (up to 50% of rated power), and quickly compensates for grid reactive power shortage through SVG (Static Var Generator) mode. According to the GB/T 36547-2018 standard in China, energy storage inverters must maintain a voltage drop of at least 625ms without disconnecting from the grid when the voltage drops to 20%. A certain brand of product can operate continuously for 2 seconds in this state through optimized control algorithms, far exceeding the standard requirements.
In the face of extreme situations where the voltage drops to 0-20% (deep drop), the inverter enters the "islanding prevention" mode, cutting off most of the active output and only retaining the minimum current to detect the grid status, while preparing to reconnect to the grid. During the voltage recovery process, the inverter adopts a "soft start" strategy, and the active power gradually increases at a rate of 5%/ms to avoid causing secondary impact on the power grid.
2 Collaboration of Hardware and Software: Technical Support for Traverse Capability
Hardware design is the fundamental guarantee of LVRT. The power devices of the inverter are selected as high-voltage resistant IGBTs (such as 1200V/600A specifications), with an avalanche energy (EAS) of up to 500mJ, which can withstand voltage spikes during faults; The DC side is equipped with a large capacity electrolytic capacitor (above 1000 μ F), which releases energy to maintain the stability of the DC bus during voltage drops. The bus voltage fluctuation of a certain product can be controlled within ± 10%.
Software algorithms determine the accuracy and speed of crossing. Dq axis decoupling control based on instantaneous power theory can separate active and reactive currents within 100 μ s and achieve independent regulation; The predictive control algorithm can predict the voltage recovery trend 5ms in advance and adjust the output strategy. The actual measurement of a power grid side energy storage project shows that the inverter using this algorithm has a reactive power response time of only 20ms when the voltage drops to 30%, which is three times faster than traditional PI control.
The fault detection circuit needs to have a "zero delay" characteristic. By combining hardware comparator with software filtering, the inverter can identify voltage drop faults within 2ms, avoiding misjudgment. In a wind storage combined power station, the inverter successfully distinguished between the real voltage drop caused by lightning strikes and the transient voltage fluctuation caused by motor start-up, without any misoperation.
3 Scenario based verification: rigorous testing from laboratory to on-site
In the laboratory, LVRT performance was validated through a "voltage drop simulator". The simulator can generate voltage waveforms with different drop depths (0-100%) and durations (0.1s-2s) to test the output characteristics of the inverter under various operating conditions. According to tests conducted by a certification agency, inverters certified by LVRT showed no significant aging of power devices after 1000 cycles of testing, with a performance degradation rate of less than 5%.
On site applications face more complex challenges. In the distribution network, voltage drop is often accompanied by harmonic distortion, and inverters need to have the ability to resist harmonic interference; In areas rich in new energy, when multiple inverters are simultaneously responding to LVRT, communication coordination is required to avoid overvoltage caused by reactive power superposition. A certain photovoltaic energy storage power station controlled the reactive power output deviation of 20 inverters within ± 5% through cluster control, ensuring a smooth voltage recovery process.
With the increasing dependence of the power grid on new energy, LVRT technology requirements are constantly upgrading. The latest EU standard requires inverters to maintain a voltage drop of 0% for 150ms without disconnecting from the grid, and some regions in China have also extended the LVRT time to 1.5 seconds. The low voltage ride through capability of energy storage inverters has changed from an "optional function" to an "essential performance". It not only ensures the safety of the energy storage system itself, but also becomes an important support for the power grid to cope with faults and maintain stability, making clean energy more resilient in the process of integrating into the power grid.