Design Of AC Current Injection

I don't see how you can inject current. You can impose a volatge onto a line, component, squirrel or whatever. But I don't see how you can force current to flow unless there is the correct load.
An AC source and some suitably rated capacitors would probably be a good start.
Maybe search for AC signal injection or EMC pulse testing.
Sorry if I'm on totally the wrong track, not my field of expertise really... just trying to get the ball rolling.

First step is to write a spec'.
What do you want to inject?

Into what?

E.G
A while back I made up a rig to inject approx 1kv pulses, about 1mS duration switched onto the live neutral or earth of the incomming 240v ac mains supply of a small control circuit.

Without that information you/we have no chance.

Del

This what my supervisor told me and to be honest I'm clueless.You will built a small portable test set called AC Current Injection Test Set. There is already an existing one, but I want to build an improved version. Materials used will be autotransformer, regulated power supply, relays, timers, ammeter, voltmeter, etc. You will do the layout design and assembling the components. You'll need to do drilling of metal casing, cutting rectangular holes, soldering, etc.

Go back to the Supervisor and report that more information is required before the task can be completed, and that the specification given to date is insufficient to devise such an arrangement without the risk of ambiguity leading to re-work.

Also add the magic word 'safety'.

It is possible to copy an existing item without understanding it.
It is downright dangerous to try and 'improve it' if you don't know what you are doing.
Del

Quite.

This device is to test CT, circuit breaker etc. I know the specifications but all I need the design layout even it's a sample of PCB to get me started

What you are describing is a professional AC current injector

Assuming it is not one of these professional units (which have complex propriatory circuits you are unlikely to be able to get any circuit diagrams for let alone build without complex multilayer SMD circuit board technology) but something smaller or home-made, you mention an existing unit, can you reverse-engineer it and upgrade it to meet your supervisor's requirements?

What current levels (and accuracy) are you trying to generate and what can the existing unit you mention generate?

Can you provide a picture of the existing unit and the inside showing the control electronics?

I'm waiting for the reply from the supervisor

That's good: the <...supervisor...> has gone away to think about it following the discussion. This indicates that the incoming specification is inadequate.

One hopes that there are other things to progress before the <...supervisor...> comes back.

I once tested circuit breakers by connecting a current transformer to a variac. A heavy gauge wire was run through the transformer core and connected across the circuit breaker under test. The current transformer induces current through the wire and circuit breaker. A clamp around ammeter was used to monitor the current in the loop. The voltage was slowly increased on the variac to determine the amount of current needed to trip the breaker. The advantage of this scheme is that you don't have to deal with high voltages.

I don't know if this helps you. Please be careful.

Any information is helpful. Thank you

Bye the existing item. That will save face for you.

It will be much faster and will not belie the fact that you are clueless.

Being a "newly graduate EE", it seems your supervisor is just trying to find out more as to what kind of an "engineer" you really are? He may want to learn more on your personal as well as technical capabilities?

Given the scope, specifications and components to be used, your supervisor pretty much has defined and set your "boundary conditions"! All you need is to familiarize yourself what /which those components safe characteristics and functions are? Be able to put them together (wired as a working circuit) to meet your assigned task . As an example, come up with a circuitry that will safely enable you to accomplish the following:

1) generate variable current, (minimum to maximum)

2) rig up a coupling mechanism, (test jig)

3) parameter measurements, etc.

Prior to the above tasks, your problem lies on creating a circuit diagram, and bread boarding them. Creating a functional PC board, and packaging them into box in my opinion should be the least of your problems.

Good luck!

It's just I never designed a circuit board on PCB. I agree with you totally and I appreciate your help alot

It is not highly technical! You can start practicing by laying out your schematic on a piece of copper clad plate using a pentel pen! Remember wherever the pentel pen passes (markings) will be the current carrying conductors, after soaking it in the etching solution. Be sure you don't accidentally cross lines or pentel marks that are not electrically connected!

That's amazing I didn't think about it this way before. I'm so thankful

AC current injector.

I do hope you are an Electrical Engineer.

If you are then you must try to analyse, use of components (auto transformer, meters, switches, timer etc. you have recognized and named in your comments). Please do it.

You will learn better by applying yourself - will understand - will be able think improve the existing and best of all will generate in you self confidence to do more difficult designs. A Secondary Current Injection kit is very simple device.

If still you can not understand - go and buy an AC Arc Welding Set - fix CT, power contactor and meters and timers - this will be Your Primary and Secondary AC Current Injection Kit -

Do not cultivate habit of spoon feeding.

I'm fresh graduate in electrical engineering, this is my first assignment. I need guidance not spoon feeding. I appreciate your advice and will do it. I just wanted to understand how I can design the layout that's all

I would like to know what you learned in school.

Maybe you have the skills, and don't know it.

Control systems, power transmission and distribution, power insulation, power electronic, electrical power, industrial electronics, AC DC machines, electromagnetic, analog and digital communication...

Sorry, I did not knew that you are a fresher. I feel that we Seniors must help you to the extent possible and guide you to be a good professional in field of Engineering.

Coming to your subject,

1. First decide basic functions, out of following, you should take up in your project work:

1.1 Simple AC Primary injection or secondary injection Test Set.

Primary injection is to test Ratio of CTs. Secondary injection is to test protection relays. Rating of Primary injection kit is say 1000A, 5V (remember output voltage is always low maximum 5V AC, because the burden of CT to be tested is a low burden or low impedance. That means when doing Primary injection an ammeter is always connected at secondary of CT. If you do not, the current will not pass and it will develop high voltage at secondary which can be fettle). The input rating is 1000x5/.95 (95% efficiency of auto-transformer (Variac).

Similarly in secondary current injection it is say up to 50A or 100A maximum, 5V. With Secondary Injection kit described above you can maximum do calibration of 1 Phase overload and short circuit relays and ammeters.

1.2 Calibration of directional relays, reverse power relay and kW meters.

In this case you also require additional voltage source and phase angle control between current injected and voltage applied.

1.3 Three phase calibration.

For this case you need 3phase current and voltage source along with phase angle control.

Best for you is to select Kit based on 1.1 for project, but also study other two types of kits to answer external examiner.

Hence for 1 Phase, 50A current injection test kit, you require:

- 250VA (250V, 1A) autotransformer (Variac), Single pole 2A mcb, fuse holders with HRC fuses for 2A, 5A, 20A and 63A.

- 5 position 5A, 2 pole Selector switch Off/1/5/20/50A positions. Each position to have 2 set of contacts (one each for connection to high impedance isolating transformer and ammeter/CT secondary).

A digital ammeter of accuracy class 0.5 or better and up to 3 1/2 counts.

CTs of 5:1. 20:1 and 50:1a ratio, class 0.5M.

An isolating Transformer, Core Type with Primary and Secondary windings on separate limbs. Reason for using separate limbs of yoke for primary and secondary is to reduce mutual reactance between two windings so that self reactance is very high. High impedance is necessary to limit the current to maximum values for each range.

Primary of the transformer shall be rated for 230V, 1A rating.

There will be total 4 secondary windings, one each for 1A, 5A, 20A and 50A respectively.

You can also limit to two secondary coils each with taps. That means one coil of 1.5sq mm Cu wire for 1A and 5A with suitable tap and another of 6 sq mm Cu for 20/50A with 20A tap. You can select a core size (x-section area) to limit flux density to 1.5 Tesla or lesser. This is to avoid saturation in core, hence true Sin wave with practically nil harmonics in test voltage/ current. You must have done transformer design in 3^rd year engineering and I assume shall not be a problem for you.

Applying your studies, of designed transformer:

From dimensions of each winding, Calculate leakage reactance of each winding.

Next from equivalent diagram calculate total impedance for each current rating (1A, 5A, 20A and 50A).

Calculate maximum fault currents for each, considering dead short at output for each rating.

Above shall work out equal to test currents. If not adjust turns to achieve the result.

Use super enamelled conduction for primary. Use super enamelled conductor and sleeve it with woven fibre glass sleeves.

Hint: While winding the transformer, wind few extra turns for each current rating and also take out some taps. During actual testing of Test Kit, if you find that you can correct any deviation from specified values of Amps by changing tap - selecting one most matching with the rating.

Additionally you will require:

- 5A, 1 pole isolating switch.

- 5A Power Contactor.

- Timer with least count of 0.01 and up to 100 Secs.

- Test start and stop switch.

- Emergency stop switch.

- Set of terminals (0 to 5A)

- Set of terminals (0 to 50A).

- Set of terminals to connect timer through trip contact of Protection Relay under secondary injection test.

Circuit shall be (I am not good in AUTOCAD to make the drawing) supply to autotransformer via 2A, 1P MCB to 5A isolating switch to auto-transformer.

Output of autotransformer through power contactor and start push button to common Terminal of 5 position selector switch.

Output of each position of Selector switch to respective tapping of high impedance isolating transformer and via CT to output terminals.

Connect Ammeter in secondary of CT. Select CT ratio according to position of selector switch.

Besides, you can also use above kit to test AC Over and Under Voltage relays and Voltmeter. For this buy Variac with +10% tap. Connect input to normal tap and output at variable tap and common. Provide two output voltage test terminals (labelled 0 - 250V) connect via start switch and contactor and timer to test voltage relays. Also you can add a digital voltmeter of class 0.5 or better.

My purpose of writing these comments was to show you the write track so that you can get started on your project work. I am sure if you surf the net and search for suppliers of Secondary Current Injection Kit - you can lay hand on O & M manual of the manufacturer with circuit diagram. This will give you further clarity.

I started keying this note in the afternoon after I read your reply to my comment. Finding it too long and having other urgent work at hand left in to complete later. I do hope it is of some help. In case of further clarification you can ask.

Ramesh

You need to start asking questions. The questions we can't answer. We can't do this for you. What was the original piece of equipment for? Why was current injected? What type of circuit was it used on? What was the parameters of the old equipment, i.e. what was the Voltage and current and impeadance of the circuit it was used on? Why is the old device not suitable anymore? Many more questions will be needed before you complete the task. It is not a test of what your EE skills are, it is a task of your investigative skills. Think of it as one of your study assignments about something you knew nothing about before you started. Start asking question about what, why, when and anything else you can. Don't be cowed. Ask boldly, but ask you supervisor first and then anyone else your investigation leads to in you workgroup or any group using the device and find out everything you can about the subject before you start. Be curious.

Thanks alot for the help, you are right in everything you said and I will follow your advice and update with my findings.

Another good question is what the purpose is to inject the current. Is it to test a device, if so what are the current requirements? Is it to provide an indication to a test circuit to locate a fault (one of the many uses of current injection), if so a pulsed current is usually used. There are so many questions that come to mine when the purpose is not known.

Take a look at this site, it should help. Compensated current injection circuit, theory and applications. - arXivarxiv.org/pdf/physics/0209038‎

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Thanks alot for your help

Has the <...supervisor...> come back to discuss yet? What was the outcome?