How can I implament a current driven circuit breaker?

Typical circuit breakers "sense" current and operate based on that. I've only found circuit breakers externally controled. Essentially the existing circuit breaker models in Simulink and specifically Simscape Electrical seem to operate as switches. Am I using these block incorrectly or is there a better way to represent a current driven circuit breaker (preferably 3 phase AC).

Answer 1

[Shivam Gothi]

Hello @Shane,

I understand that you're working on operating the three-phase circuit breaker based on sensing currents. However, it seems there's some confusion because the circuit breaker is designed to accept voltage as an input.

To resolve your query, let us take a brief look at the construction and operation of circuit breaker.

There are three components involved in the operation of Circuit breaker:

• Current Transformer
• Relay
• Circuit breaker main winding.

The circuit breaker is equipped with a "main winding" and an electromagnet, which are responsible for opening and closing the contacts. When the main winding is energized by a voltage, the contacts close. Conversely, when the main winding is de-energized, the contacts open. It's important to note that the circuit breaker does not have a feature to "sense" currents. This is why Simulink provides a "voltage port" as the input signal instead of a current source. The main winding of Circuit breaker is connected with the excitation voltage source through “NC” (Normally closed) contacts of the relay

Refer to the below links to explore more:

https://www.ksinstruments.net/current-transformers-function-usage-electrical-panels/

https://electricalbaba.com/basic-principle-of-relay-operation/

The current sensing part is done by current transformers (CTs). (conventionally called as current sensor). The CT has primary winding and secondary winding. The line current is allowed to flow through the primary winding and a relay coil is connected at the secondary winding of CT. When the line current flowing through the primary winding exceeds its rated value, the relay activates and opens the "main winding" of the circuit breaker. This de-energizes the main winding, which in turn opens the circuit breaker contacts.

Now, coming to the implementation, the “current driven circuit breaker” can be implemented in following approaches:

Approach-1: simplified approach:

The current sensor and relay are modelled in a very simple way as shown below:

Approach-2: (Detailed approach) By using “current sensor”, “relay” and “circuit breaker” block from Simulink library browser:

This model simulates a three phase circuit breaker. Whenever the current in a particular phase exceeds the threshold value, the relay is operated and opens the contacts of Circuit breaker for all the phases. It then waits for some time (determined by “Auto_reclosing_logic” ) and again closes the contacts of circuit breaker (this is called auto-reclosing). You can code your own auto-reclosing logic inside it.

The screenshot of Simulink model is attached below for you reference

Note: Here, the “modelling option” property of “Circuit breaker (three phase)” was configured as “Expanded Three-phase Port | Electrical control Port”

I hope you find this information useful!

[Shane]

Shivam thank you!

This is very helpful and explained very well. I'm going to pursue your Approach-2. Could you explain the "Auto_reclosing_logic" this seems like a custom logic block? I'm unfamiliar.

Also, I plan to introduce Short and Long term tripping over time. For instance, if the current remains at a specified current for a few seconds the circuit breaker should trip. The time measurement portion of this function is tricky because you cannot just measure only at the beginning and end of the period (this could give false positives). Any suggestions for how to approach this?

Answer 2

[Shivam Gothi]

Hello @Shane,

As I understand, you want to implement a circuit breaker which should trip if the current stays above its threshold value for more than few seconds say ("To").

This task is generally performed by digital relays. Digital relays are much more advanced that can sense current and also have an embedded microcontroller reciding inside it.

In Simulink, you can simulate such a relay using state machine. I have used the "Approach-2" and replaced the conventional relays with a state machine (that models digital relay). I have attached the simulink model with this answer.

The description of the state machine is as follows:

It senses three phase currents. As soon as any of the phase currents increases above its threshold value ("I_threshold"), the relay will wait for some time (as determined by input "" to the state machine), and then issue the trip command to the breaker, as desired by you.

It has 6 inputs:

• Three of the inputs are currents ("ia, ib,ic").
• "counter_threshold" : this determines the "wait period" of the relay. ("To" as mentioned in above explanation)

Note: Here, the sampling time of state machine is configured as 1e-6 by specifying in the "block parameters" of "chart" block (also called state machine here)

• "I_threshold": Threshold value of current
• "close_breaker" : when you give a pulse at this port, it will reclose the breaker again once it is tripped. (Remember: give a pulse narrower than the "wait period"). After reclosing, if the current is still above the threshold value, the breaker will trip again.

I have simulated the above attached simulink model and attaching the screenshot of data inspector. (Here, I have tested the algorithm by simply exciting all the phases by same current source. You can modify the connections as per your need.)

Conclusions: At time t=0.1 seconds, the phase A current went above its threshold value of 100A. Therefore, the relay waited for some time and then tripped the breaker. You can see that current comes to zero after the breaker trips. The "blue" graph is the trip signal issued by the digital relay algorithm.

This is exactly as per your expectations.

Below given screenshot zooms at the transients. (notice that relay waited for 500 microseconds before generating trip command).

I hope this simulink model helped you !

[Shivam Gothi]

Just a small update,

I have edited the above answer and attached the modefied file "Digital_relay_algorithm_R2023a.slx" file

[Shane]

If you can set the sampling time for the chart (via settings), how do you get the entire function to run at soft real-time? The idea is to run the program in real-time or nearly as possible but set the sample time of the relay logic to sample faster. Eventually this will be part of a larger system so coordinating the time and sampling rates will be imperative.

ALSO is there a way to improve the counter using native functions?