How do diodes isolate local circuits during power grid faults?

一, The physical mechanism of diode fault isolation
The PN junction structure of a diode endows it with natural current blocking ability. When a short circuit fault occurs in the power grid, the voltage at the fault point drops sharply, forming a reverse bias electric field. At this time, the diode enters the cutoff state, and the reverse resistance can reach the megaohm level. Taking the photovoltaic grid connected system as an example, when a pole to pole short circuit occurs on the DC side, the Schottky diode (such as SB560, with a forward voltage drop of 0.5V) connected in parallel to both ends of the photovoltaic module can withstand a reverse voltage of over 1000V and complete current blocking within 0.1 μ s, which is three orders of magnitude faster than traditional relay schemes.

In communication systems, the isolation characteristics of diodes are closely related to the type of fault. When a single-phase grounding fault occurs, the non fault phase voltage rises to the line voltage level. At this time, the fast recovery diode (such as FR307, reverse recovery time 100ns) connected in anti parallel to both ends of the switching device can effectively prevent capacitor overcharging. According to the data of Tennet's ± 500kV DC transmission project in Germany, after adopting this scheme, the fluctuation range of submodule capacitor voltage decreased from ± 15% to ± 3%, and the system efficiency improved by 1.2 percentage points.

二, Isolation application of typical fault scenarios
1. Fault zoning of DC distribution system
In a diode based DC distribution system, when a permanent two pole short circuit occurs in the line, the initial current of the faulty line quickly rises to 8.3kA, while the terminal current decays to 0 within 1ms due to the reverse cutoff characteristic of the diode. The research conducted by Li Bin's team at Tianjin University shows that this scheme can limit the impact range of faults between two converter stations, reducing it by 60% compared to traditional schemes, and shortening the voltage drop time from 200ms to 20ms, significantly improving power supply reliability.

In the specific implementation, each DC bus segment is equipped with an anti parallel diode module. When the fault current exceeds the threshold, the fast switching device cuts off the fault path within 100 μ s, and the diode automatically forms an isolation barrier. After adopting this technology, the Huawei SUN2000-125KTL photovoltaic inverter increased its power generation by 9.3% in partially obstructed scenarios, with a European efficiency of 98.8%.

2. Modular multilevel converter protection
In the MMC submodule, diodes and IGBTs form a bidirectional blocking structure. When the voltage imbalance of the submodule capacitor exceeds 10%, the series connected silicon carbide diode (such as C3D06060A) experiences a forward voltage drop 1.3V@10A )Can prevent capacitor overcharging. After adopting this scheme, the Siemens SICAM AIS grid stabilizer reduced submodule switching losses by 40% and shortened the system response time from 10ms to 3ms.

In engineering practice, the reverse recovery characteristics of diodes need to be considered. The use of fast recovery diodes (such as FR307) can reduce IGBT switching losses by 35% compared to ordinary rectifiers. ABB's Power Grid series intelligent isolation diodes monitor junction temperature, current and other parameters in real time through built-in sensors, warning potential faults 0.5ms in advance, and increasing the average time between failures of the system to 200000 hours.

3. Redundant design of distributed power sources
In string photovoltaic inverters, multiple MPPT channels achieve power redundancy through diodes or gate circuits. When the output power of a certain channel decreases due to shadow obstruction, the Schottky diode (such as MBR2045CT, with a forward voltage drop of 0.32V) automatically switches to the healthy channel. Tests have shown that this scheme can increase the power generation of photovoltaic arrays by 8% -12%, especially in partially obstructed scenarios where the advantages are significant.

The Tesla Megapack energy storage system adopts an integrated isolation scheme, and an ideal diode controller based on MOSFET (such as LM5050) achieves zero reverse recovery time. This scheme reduces the isolation loss between battery clusters from 2.5W to 0.3W, improves the system cycle efficiency by 0.2 percentage points, and reduces the conduction voltage drop of 0.05V by 90% compared to traditional diodes.

三, Engineering optimization and performance improvement strategies
1. Selection of low loss components
The conduction loss of traditional silicon diodes has become a bottleneck in high-frequency applications. The use of silicon carbide Schottky diodes can reduce conduction losses by 60%. In a 100kW photovoltaic inverter, this scheme reduces diode losses from 120W to 48W and improves system efficiency by 0.05 percentage points. The EPC2054 GaN diode launched by EPC company has a forward voltage drop of only 0.2V at 10A current, which is 85% lower than SiC devices.

2. Thermal management optimization
In high-power applications, diode junction temperature control is crucial. The composite heat dissipation scheme using thermal conductive silicone grease (thermal resistance 0.5 ℃/W) and aluminum substrate (thermal resistance 1 ℃/W) can reduce the junction temperature from 125 ℃ to 85 ℃ under 100A current, extending the device life by more than three times. Huawei inverters use liquid cooling technology to control the diode junction temperature within 105 ℃ and increase the power density to 1.2kW/kg.

3. Electromagnetic compatibility design
The di/dt noise generated by diode switches needs to be suppressed by an RC buffer circuit. In a 10kW inverter, a buffer circuit using 0.1 μ F film capacitors and 10 Ω resistors can reduce voltage overshoot from 50V to 5V, meeting the IEC 61000-4-5 electromagnetic compatibility standard. The Siemens SIRIUS series intelligent isolation diode suppresses switch noise below 20dB through a built-in RC network.

四, Frontier technology trends
1. Wide bandgap semiconductor applications
Gallium nitride diodes, with their ultra-low on resistance (0.1m Ω· cm ²) and high-frequency characteristics (fT>1GHz), are gradually replacing silicon devices in high-end fields such as 5G base station power supplies and aerospace power supplies. The EPC2054 GaN diode launched by EPC company has a forward voltage drop of only 0.2V at 10A current, which is 85% lower than SiC devices.

2. Integration of intelligent isolation technology
The intelligent diode module combined with digital control technology can achieve dynamic voltage drop compensation and fault prediction. The Power Grid series intelligent isolation diodes launched by ABB company monitor junction temperature, current and other parameters in real time through built-in sensors, and warn potential faults 0.5ms in advance, increasing the average fault free time of the system to 200000 hours.

五, Industry application cases
1. Tennet DC transmission project in Germany
In the ± 500kV DC transmission project, the MMC submodule using silicon carbide diode modules reduces the voltage fluctuation range of the submodule capacitor from ± 15% to ± 3%, and improves system efficiency by 1.2 percentage points. The annual transmission capacity of this project reaches 12 billion kilowatt hours, which is equivalent to reducing the consumption of standard coal by 3.6 million tons.

2. Tesla Megapack energy storage system
The battery cluster isolation scheme based on GaN diodes improves the system cycle efficiency by 0.2 percentage points, while reducing the conduction voltage drop by 90% compared to traditional diodes at 0.05V. The system has been deployed globally for over 10 GWh, supporting the consumption of renewable energy.