What is the delay effect of diodes in automatic switching of distribution networks?

一, The physical basis of diode delay characteristics
The delay characteristics of a diode stem from the dynamic processes of its internal charge carriers. When a diode switches from a conducting state to a cutoff state, the non-equilibrium carriers accumulated in the PN junction (such as electrons in the P region and holes in the N region) do not disappear instantly, but gradually decrease through two paths: drift motion under the action of a reverse electric field, and recombination with most carriers. This process causes the reverse current to maintain a high value in the initial stage, and then gradually decay to a steady-state value, forming a reverse recovery time (Trr). The length of reverse recovery time directly affects the switching speed of the diode, which in turn determines its delay characteristics in automatic switching.

Experimental data shows that the reverse recovery time of diodes is closely related to junction capacitance and stored charge. The larger the PN junction area, the more stored charges and the longer the delay time; The larger the forward current, the greater the amount of stored charge and the longer the turn off time; The larger the reverse current, the faster the charge disappears and the shorter the turn off time. For example, in the rectifier circuit of automatic switch in distribution network, if ordinary rectifier diodes are used, the reverse recovery time can reach hundreds of nanoseconds to microseconds, while Schottky diodes can shorten the reverse recovery time to nanoseconds through the majority carrier conduction mechanism, significantly improving the switch response speed.

二, Circuit Implementation of Diode Delay Control in Automatic Switching
In automatic switching of distribution networks, the delay control of diodes is mainly achieved through two circuit forms: one is a delay circuit based on RC charging and discharging, and the other is a transient suppression circuit based on the reverse recovery characteristics of diodes.

1. Application of diodes in RC delay circuits
The RC delay circuit achieves time delay through the charging and discharging process of capacitors, and the diode plays a role in controlling the charging and discharging path in this circuit. For example, in the closing control circuit of an automatic switch, when the input signal is high, the diode conducts in the forward direction, and the capacitor charges quickly through a small resistor, shortening the closing time; When the input signal is low, the diode is turned off in reverse, and the capacitor slowly discharges through a large resistor, extending the opening time. This design can achieve delay control ranging from tens of microseconds to milliseconds by adjusting the forward and reverse resistance ratio of the diode, meeting the time requirements for fault isolation and recovery in the distribution network.

2. Application of diodes in transient suppression circuits
In the overvoltage protection circuit of automatic switches, diodes (such as TVS diodes) absorb transient overvoltages through their reverse breakdown characteristics, and their reverse recovery time directly affects the protection response speed. For example, when lightning strikes or switch operations generate overvoltage in the distribution network, the TVS diode conducts in nanoseconds, clamping the overvoltage to a safe level, and then restoring the cutoff state through a reverse recovery process. If the reverse recovery time is too long, it may cause secondary overvoltage surge, so ultra fast recovery diodes (such as UF4007, Trr<50ns) need to be selected to optimize the protection effect.

三, Typical application of diode delay characteristics in distribution network automation
1. Timing control for fault isolation and recovery
In the automation of distribution network feeders, automatic switches need to quickly isolate faulty sections based on fault signals and restore power supply to non faulty sections. The delay characteristics of diodes can achieve timing coordination of switch actions. For example, in a reclosing circuit, the charging time of the capacitor is controlled by a diode to ensure a delay of several seconds before reclosing after fault isolation, avoiding the repeated impact of transient faults. After adopting this scheme in a 10kV distribution network project, the fault isolation time was shortened to less than 200ms, and the success rate of reclosing was increased to 98%.

2. Anti backflow protection for distributed power access
With the popularization of distributed photovoltaic and energy storage systems, the distribution network needs to prevent distributed power sources from sending electricity back to the grid in the event of a fault. The diode is connected in series with the output terminal of the inverter, using its unidirectional conductivity to block reverse current, while controlling the response speed of anti backflow protection through reverse recovery time. For example, when a certain photovoltaic power station adopts Schottky diodes (such as SS14, Trr<10ns), the anti backflow protection action time is shortened from milliseconds to microseconds, effectively avoiding the expansion of power grid faults.

3. Synchronous control of switches in DC distribution systems
In the DC distribution network, the synchronous opening and closing of automatic switches needs to solve the problem of arc reignition. The diode achieves phase synchronization of the switching action by delaying the control of capacitor charging and discharging. For example, in a DC circuit breaker, a diode forms a resonant circuit with an inductor and a capacitor. By adjusting the conduction angle of the diode to control the resonant frequency, the switch can break at the zero crossing of the current, reducing the arc energy to below 1% and significantly improving the switch life.