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  • Mark Craig

Wednesday Write-up: Diodes Part 3

Updated: Mar 4, 2021

How do diodes protect electrical power systems?

Welcome to Writeup Wednesday - we’re continuing our previous discussion about diodes. You can check out our first post about What is a Diode if you’d like to learn more about the basics. Today we’ll talk about what diodes can protect a system against, what they can’t, and what technologies can be used in combination with diodes to better protect electrical systems.

What can diodes protect against?

In the system diagram below there are 2 power supplies (PS), 3 loads, 2 unknown quantity loads, several miniature circuit breakers (MCB), and 2 diodes.

Diagram of a healthy dual power supply system

Figure 1 Diagram of a healthy dual power supply system

The diodes can protect the system in multiple ways. First, the diodes can protect against transient overvoltage on either power supply from propagating to the opposite side. For example, if the 24 V DC voltage on the output of PS2 was to rise to 36V, load 2-1 & load 2-N would both see a rise to 36V. However, diode D1 will block the overvoltage from propagating to PS1, and loads 1-1 & 1-N would remain unaffected (seen in black).

Diagram of overvoltage at power supply #2

Figure 2 Diagram of overvoltage at power supply #2

Diagram of overvoltage at diode

Figure 3 Diagram of overvoltage at diode

This protection extends to an overvoltage fault at load D. In this case, if for some reason there was a high voltage spike induced on load D it would not propagate to the other loads or to either of the power supplies. In this case, both diode 1 and diode 2 will block the voltage from propagating to PS1 and PS2.

The diodes will also prevent a voltage collapse on both power supplies if a short circuit is experienced on the adjacent system. An example of this would be if load 2-1 on PS2 were to suffer a short circuit the voltage output from PS2 would be reduced to zero due to the high current flow to the fault (seen in red). The full voltage will remain on load D and all other loads connected to PS1. This protection functions by diode 2 preventing current flow from PS1 to the fault located on the PS2 side of the system.

Diagram of power supply #2 short circuit

Figure 4 Diagram of power supply #2 short circuit

While these examples are not comprehensive they do exemplify why diodes are critical components of these electrical systems - especially those vessels that aim to or currently function on closed bus-tie operation. Coupled with the correct technology these systems can be made to operate safely and with minimal opportunity for electrical failure and no loss of position when experiencing the faults described.

What can’t diodes protect against?

Using the same system diagram we can derive the two most probable situations in which the diodes do not protect the system. Those situations being a short circuit or an overload on the load side of either diode (seen in red).

Diagram of diode-load short circuit

Figure 5 Diagram of diode-load short circuit

The reason that short circuits or overload on the load side of the diodes can not be protected against is that the diodes do not function to limit the amount of current passing through them. In the case of the current being sufficiently high, the diodes would fail, though with modern switch-mode power supplies (SMPS) this is not likely; SMPS are built with current limits and shutdown features to prevent damaging equipment in the instances of overload and short circuits.

To remedy the limitation of diodes, other technology must be implemented in conjunction with diodes to ensure that short circuit and overload faults do not propagate to the rest of the system.

What technology can be used in conjunction with diodes to protect the system?

As diodes are limited in their ability to regulate current, a more active component needs to be selected to prevent the propagation of faults to other parts of the system. For isolation of overcurrent events, components such as DC:DC converters, multiple SMPSs or electronic circuit protection devices should be used.

At OneStep we have successfully trialed the replacement of a traditional MCB with an electronic circuit protection device. This device simply replaces MCB1, MCB2, MCBn etc. It is a direct replacement for most standard-sized MCBs and will limit the current that can be drawn from the connected equipment and prevents a voltage dip on the supply side. More information can be provided on request.

It is also recommended that SMPSs with output diode protection are selected when the design of the 24V power system is being considered. This is to ensure that any back feed or overvoltage that may be encountered are prevented from damaging the power supply units themselves.

Once the power supply and distribution systems are optimally designed to prevent fault propagation, how is it proved?

Stay tuned for next week’s Write-up Wednesday when we will discuss OneStep Power's new technology that has been delivered to prove these very complex systems, the DCShortCUT!

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