How to use diodes to achieve unidirectional energy transmission in microgrids?

1, The physical basis of unidirectional conductivity of diodes
The core structure of a diode is a PN junction, formed by the combination of a P-type semiconductor (with high hole concentration) and an N-type semiconductor (with high electron concentration). At the PN junction interface, electrons diffuse from the N region to the P region, and holes diffuse from the P region to the N region, causing the P region to be negatively charged near the junction and the N region to be positively charged near the junction, forming an internal electric field (space charge region). This electric field has two key characteristics:

Positive conduction: When the P region is connected to the positive pole of the power supply and the N region is connected to the negative pole, the external electric field weakens the built-in electric field, the space charge region narrows, and most carriers (electrons and holes) can cross the junction region to form a current, resulting in a low resistance state of the diode.
Reverse cutoff: When the P region is connected to the negative electrode and the N region is connected to the positive electrode, the external electric field enhances the built-in electric field, the space charge region widens, most charge carriers are blocked, and only a few charge carriers form a small reverse current (leakage current), resulting in a high resistance state of the diode.
This characteristic makes diodes an ideal component for achieving unidirectional energy flow. Taking silicon diodes as an example, their forward conduction voltage drop is about 0.6-0.7V, and their reverse breakdown voltage can reach several hundred volts, which can meet the isolation requirements of low voltage DC (such as 48V) to medium voltage DC (such as 400V) in microgrids.

2, The core requirement for unidirectional energy transmission in microgrids
The energy flow of microgrids has the characteristics of multi-source, bidirectional, and dynamic, and its energy management needs to address three core issues:

Isolation between power sources: to prevent different power sources (such as photovoltaics, energy storage, diesel generators) from affecting each other due to voltage fluctuations or faults.
Energy feedback control: To prevent energy from flowing back into the weak grid and causing voltage rise during motor braking or photovoltaic over generation.
Quick fault isolation: When a power supply or load experiences a short circuit, the fault path is cut off to prevent the fault from spreading.
Traditional solutions rely on contactors or circuit breakers, but suffer from slow response times (in milliseconds), mechanical wear, and other issues. The diode, with its nanosecond response speed and no mechanical contact characteristics, has become a key component for achieving fast and reliable energy isolation.

3, Typical application scenarios of diodes in microgrids
(1) Unidirectional transmission of DC bus energy
In DC microgrids, diodes are commonly used to construct unidirectional conductive links, enabling energy flow control between busbars of different voltage levels. For example:

Photovoltaic energy storage system: The photovoltaic array supplies power to the 48V DC bus through diodes, and the energy storage battery is connected to the same bus through a DC/DC converter. When the photovoltaic output power exceeds the load demand, the diode prevents energy from flowing back into the photovoltaic panel, avoiding damage to the panel due to reverse bias heating; Meanwhile, the energy storage system absorbs excess energy through bidirectional DC/DC converters.
Parallel connection of multiple power sources: In a wind solar energy storage complementary microgrid, different power sources are connected in parallel to the DC bus through diodes. When a power supply shuts down due to a fault, the diode automatically cuts off its connection to the bus to prevent the fault voltage from affecting other power sources.
(2) Energy feedback suppression on the communication side
In the communication microgrid, the combination of diodes with thyristors or IGBTs can construct energy feedback suppression circuits. For example:

Motor drive system: When the motor is in the braking state, the regenerated energy is fed back to the DC bus through reverse parallel diodes. If the bus voltage is too high, the diode is connected in series with the braking resistor to convert excess energy into thermal energy consumption, preventing overvoltage of the DC bus.
Distributed generation grid connection: At the output end of the inverter, diodes can prevent energy from flowing back into the inverter in the event of grid faults (such as voltage surges), protecting power devices from overcurrent damage.
(3) Quick fault isolation and protection
Diodes have unique advantages in microgrid fault protection. For example:

DC short-circuit protection: In a DC microgrid, if a short circuit occurs in a branch, the short-circuit current will form a low impedance circuit through a diode. At this time, the fast fuse or circuit breaker can detect the overcurrent signal and cut off the faulty branch, while the diode can prevent the short-circuit current from flowing back to other healthy branches.
Grounding fault isolation: In IT grounding systems, diodes can be used to construct insulation monitoring circuits. When a grounding fault occurs in a certain phase, the diode conducts to form a small current, and the monitoring device locates the fault point by detecting this current. At the same time, the diode limits the amplitude of the fault current to prevent equipment damage.
4, Key technical points in engineering practice
(1) Diode selection and parameter matching
In microgrid applications, the selection of diodes should take into account the following parameters:

Rated voltage: It should be greater than the maximum operating voltage of the system and leave a margin of 20% -50%. For example, in a 400V DC bus, diodes with a withstand voltage of 600V or higher should be selected.
Rated current: It needs to be selected based on the maximum load current and overload capacity. For example, in a photovoltaic system, the rated current of the diode should be greater than the short-circuit current of the photovoltaic array.
Reverse recovery time: In high-frequency switching applications (such as PWM modulation), fast recovery diodes with short reverse recovery time (<50ns) should be selected to reduce switching losses.
Thermal resistance and heat dissipation: The junction temperature of the diode should be controlled below 150 ℃, and the appropriate heat dissipation method (such as natural cooling, air cooling, or liquid cooling) should be selected according to power consumption.
(2) System topology optimization
The topology structure of diodes in microgrids needs to be designed according to specific requirements. For example:

Series diode: used to improve the withstand voltage level, but attention should be paid to voltage equalization to prevent overvoltage breakdown of a diode due to uneven voltage distribution.
Parallel diode: used to improve current carrying capacity, but attention should be paid to current sharing to prevent overheating and damage of a diode due to uneven current distribution.
Diode MOSFET/IGBT hybrid topology: In scenarios where bidirectional energy flow is required, a hybrid topology of diode and MOSFET/IGBT can be used. For example, in bidirectional DC/DC converters, diodes are used for unidirectional conduction and MOSFETs are used for reverse conduction, achieving bidirectional flow control of energy.
(3) Collaborative control strategy
The energy management of diodes in microgrids needs to be coordinated with control strategies. For example:

Energy management algorithm based on diodes: By monitoring the DC bus voltage and the output power of various power sources, dynamically adjusting the conduction state of diodes to achieve optimal energy allocation.
Fault protection strategy: Design fast and reliable fault detection and isolation algorithms based on the conduction characteristics of diodes. For example, when an abnormal current is detected in a certain branch, the diode of that branch is immediately cut off to prevent the fault from spreading.
5, Case study: Application of diodes in island microgrids
The microgrid project on a certain island adopts a DC bus architecture, integrating photovoltaic, energy storage, diesel generators, and loads. The energy management plan is as follows:

Photovoltaic system: The photovoltaic array supplies power to the 48V DC bus through diodes, which prevent energy from flowing back into the photovoltaic panel at night or during faults.
Energy storage system: Lithium batteries are connected to the bus through a bidirectional DC/DC converter to achieve energy charging and discharging control.
Diesel generator: As a backup power source, it is connected to the busbar through diodes to prevent energy backflow from the busbar when the generator is shut down.
Load management: DC loads are directly connected to the bus, while AC loads are connected through an inverter. The output terminal of the inverter is equipped with diodes to prevent energy from flowing back into the inverter in case of grid faults.
This scheme achieves safe isolation and unidirectional energy flow between photovoltaic, energy storage, and diesel generators through diodes, improving system efficiency to 92% and shortening fault response time to within 10 μ s.