How to integrate bypass diodes in wind power controllers?

一, The core function of bypass diode: active defense against hot spot effect
The core function of bypass diodes in wind power generation systems is to prevent hot spot effects. When the blades of a wind turbine are obstructed (such as snow, bird droppings, or local faults), the battery cells in the obstructed area cannot generate electricity and instead become a load that consumes the current generated by other normal battery cells, resulting in a sharp increase in local temperature (up to 200 ℃ or above) and the formation of hot spots. Hot spots not only permanently damage battery cells, reduce power generation efficiency, but may even cause fires.

The bypass diode is connected in reverse parallel at both ends of the battery pack and is in a reverse cutoff state under normal operating conditions, without affecting the operation of the circuit; When a certain battery cell is obstructed, the voltage at both ends rises to the forward conduction threshold of the diode, and the current bypasses the fault area to avoid the formation of hot spots. For example, a 5MW offshore wind turbine unit adopts a scheme of parallel connection of a bypass diode for each series of battery cells. In the local shading test, the hot spot temperature decreased from 185 ℃ to 45 ℃, and the component life was extended by more than three times.

二, Integrated scenario: technological extension from photovoltaic modules to wind power controllers
Although bypass diodes were first applied in photovoltaic systems, their technical principles have high adaptability in wind power generation. In wind power controllers, the integration of bypass diodes is mainly reflected in the following scenarios:

1. Rotor side protection of doubly fed wind turbines
When a doubly fed induction generator experiences a voltage drop in the power grid, overcurrent may occur on the rotor side, which may damage the inverter. The active Crowbar protection circuit releases rotor energy to the bypass resistor through a diode rectifier bridge, where each bridge arm of the rectifier bridge is composed of two diodes connected in series, ensuring energy release within 10ms. For example, a 10MW offshore doubly fed unit adopts IGBT type Crowbar circuit, combined with fast diodes, to maintain stable system operation even when the voltage drops to 20%.

2. Optimization of rectification process for direct drive permanent magnet unit
The direct drive permanent magnet wind turbine adopts an uncontrollable diode rectifier circuit to convert three-phase AC power into DC power. Although the structure is simple, the conduction loss of the diode directly affects the system efficiency. By replacing traditional silicon diodes with silicon carbide (SiC) Schottky diodes, the conduction voltage drop can be reduced from 0.7V to 0.3V. At a current of 1000A, a single unit can save up to 200000 kWh of electricity per year.

3. Battery management of off grid wind power controllers
In off grid systems, the controller needs to prevent the battery from discharging in reverse to the wind turbine during nighttime or cloudy weather. By connecting bypass diodes in series in the charging circuit, a unidirectional current path is formed. For example, a microgrid project in a remote area uses a controller with anti reverse diodes, which extends the battery life from 3 years to 6 years and reduces the overall system cost by 40%.

三, Selection criteria: from parameter matching to reliability verification
The performance of bypass diodes directly affects the stability of wind power controllers, and their selection needs to comprehensively consider the following core parameters:

1. Working current and reverse breakdown voltage
Working current: It needs to be greater than the short-circuit current of the battery cell string. For example, if the short-circuit current of a 210mm sized battery cell is 18A, the rated current of the bypass diode needs to be ≥ 20A.
Reverse breakdown voltage: It should be 1.2 times higher than the open circuit voltage of the battery cell. Taking the 1500V system as an example, the reverse withstand voltage of the diode needs to be ≥ 1800V.
2. Thermal performance and heat dissipation design
Junction temperature: It must meet the maximum operating temperature requirements of the system. The sea environment temperature can reach 55 ℃, and the diode junction temperature needs to be ≤ 175 ℃.
Thermal resistance: Low thermal resistance can accelerate heat conduction. For example, diodes packaged on copper substrates can have a thermal resistance as low as 0.5K/W, which is 60% lower than traditional plastic packaging.
3. Dynamic response and reliability testing
Switching speed: It needs to adapt to sudden changes in wind speed. For example, a certain onshore unit requires diodes to complete conduction/cutoff switching within 10 μ s.
Life test: It is required to pass the thermal runaway test in IEC 62979 standard, which means that the surface temperature rises by ≤ 15 ℃ when the rated current is applied for 1 hour in a 75 ℃ environment.