Can dirty input power trip small thermal magnetic circuit breakers ? 3

[bdn2004]

A new rectifier system is installed on an existing unit substation as the simplified sketch shows below. The 45A thermal magnetic circuit breakers that feed the small rectifiers trip shortly after the system is powered up. They heat up to 150 degrees F per infrared testing. We measured the THD at the Mains as shown. The results show a total harmonic distortions at the connection point is 1% Voltage and 18% current.

The manufacturer of the rectifier equipment is claiming the 45A circuit breakers are tripping due to "dirty" input power and of no fault of their own. Note there are fairly large VFDS installed and HVAC equipment off the same substation bus. There are other small loads in the HVAC equipment that have operated for years without tripping. Is the manufacturer right ? Are there other things that could be causing this that we should check for?

 

[bdn2004]

Here's the same file uploaded.

 

[waross]

Look for loose or dirty connections.
Verify that there is no current through the breakers.
If there is current through the breakers, measure the voltage drop across the breakers.
1% harmonic distortion is not generally considered "dirty".
Look for sources of heat:

Magnetic encirclement.
Is the filter generating heat that is conducted to the breakers via the conductors or bus?
If possible, remove the breakers and check for heating of the bus bar.​

The source of the heat may be external or internal.
If the rectifiers are drawing current, despite no load applied, there may be failed rectifier components.
This could happen if the wrong components were installed in both rectifiers.

You have not indicated that the breakers were checked for current flow.
You have not indicated the manner of tripping;

Dirty power will typically cause instantaneous trips.
Overloads will typically cause inverse time trips.​

The results show a total harmonic distortions at the connection point is 1% Voltage and 18% current.

Voltage harmonic distortion is reflected throughout the system affected by:

Non linear loads.
The impedances of the conductors, buses and transformers feeding the non linear loads.
The wave form of the current drawn by the non-linear loads.
The source of voltage distortion is often external to the equipment under test.​

Current harmonic distortion may be cause primarily by non linear loads.
Voltage harmonic distortion may cause some current harmonic distortion, but it is unusual for 1% voltage distortion to cause 18% current harmonic distortion.

I mentioned possible wrong components.
If the rectifiers have internal surge arresters, and the applied voltage is above the rating of the surge arresters, that would cause both breaker heating and tripping and current harmonic distortion.
eg: 240 Volt arresters installed in 480 Volt equipment.
Please let us know what the outcome is.
Thanks.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Another possibility is that arresters were installed line-to-line instead of line to ground/neutral.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bdn2004]

Thanks for the detailed reply. There is 20A load current measured at the main breaker. They had a thermal camera on it and they watched the temperature rise to 150 degrees. And that's when the circuit breakers tripped. The schematics don't show surge arresters.

 

[waross]

20 Amps tripping a 45 Amp breaker?
I would first look for dirty or loose connections close to the breakers.
The breakers may be faulty. (Too long in bad storage conditions prior to use?)
I would measure the voltage drop across the breakers and multiply this by the current to find the internal loss in Watts.
Use your best judgment as to whether the internal Watts generated may be enough to raise the breaker temp to 150C.
This test will go a long way to either identifying or ruling out faulty breakers.

Anecdote:
I have had poor contact between breakers and panel bus bars generating heat and tripping breakers at far below their ratings.
I have had a poor connection to a switch develop heat. The heat was conducted by the conductors and softened the insulation.
The conductor pushed through the insulation and contacted the metal bushing on the end of the conduit.
There was a flash and a bang.
A critical drainage pump went down on Friday afternoon, going into a long weekend.
I could go on with a long list of heat related issues with poor or dirty connections.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

PS: take lots of pictures and keep notes.
Attach the pictures and notes to the invoice that you will send to the equipment manufacturer for time and expenses spent finding that the mistake was an equipment issue and not a power quality issue.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Arentsch]

I'd like to add that these rectifier cabinets were brand new from Factory and were being installed by the manufacturer in a week long outage. The new rectifier cabinets were to replace a working but obsolete system. The system was powered up after installation but never commissioned due to the intermittent breaker overloads. There are over 40 breakers within theses three cabinets. As part of the troubleshooting, the breakers were replaced 2 times with similar breakers of different manufacturers, all brand new out of the box. Manufacturer claims Plant power system is "dirty" and is causing breaker overloads (overheating). The commissioning of the new installation was called off at the last minute and the old system was re-connected and fortunately is back online. The cabinets remain in place and another attempt to commission will occur in the winter if a resolution can be found.

 

[waross]

The bus bars may have been stored in poor conditions for too long and have surface corrosion.
Depending on the style of breaker connection to the bus, the problem (with a dirty bus) may remain despite changing breakers.
A poor connection from the supply cable to the bus may be heating the bus.Try bypassing the bus with a similar breaker connected by a foot or so of adequately sized jumper conductors.
The jumpers should be long enough to isolate the breakers from any heat generated at the bus bars.

My first experience with the expert blaming "dirty power". His PLC was hanging up and he was fixated on a scope trace of the voltage output of his power supply.
The trace was not a perfectly straight line, there was a small amount of ripple.

The actual problem?
An input limit switch was faulty.​

Another time: An expert was setting up a data link.
He reported that the power wiring done by our company was to blame.
When I arrived, followed shortly by my boss,the expert was lounging in a chair watching data hash scroll past on his monitor.
I asked what the problem was.
"The circuits are bad.
These two devices must be on two different circuits."
I replied;
"Well, I see a total of 7 devices.
We have given you 7 separate circuits.
I don't know where we can go from here."
My boss and I both turned and left.

The list goes on.
One of the most expensive experts caused a series of failures during the start-up of a very large walking dragline.
His culmination was several tens of thousands of dollars damage to the machine due to his refusal to reverse one trace when comparing two phases of a delta circuit on a dual trace scope. That effectively reversed the incoming signal.
The circuit was from a Selsyn transmitter in a circuit that kept both sides of the machine in sync when it was walking.
His mistake resulted in one side trying to walk west while the other side tried to walk east.
With 2100 HP fighting 2100 Hp, the damage was expensive.

Find the source of the 150C heat!!!
Use my suggestions or your own method,but;
Find the source of the heat!

Don't worry too much about what the expert says.

But, are the breakers GFCIs? You havent said.
Some GFCIs are sensitive to very dirty waveforms.
While dirty power may still be the source, I doubt it.
There are a lot of things to check first, and your 1% voltage distortion makes dirty power about a 1/100,000 possibility.
The 18% current distortion is telling us that the rectifiers are a non-linear load, and the 18% current distortion may or may not be contributing to the voltage distortion.
And, by the way, when dirty power tripped the GFCIs, they did not get hot. They tripped instantly.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Parchie]

@Arentsch,
Proper commissioning involves a lot of things to be done, not just the connection of the power supply and output lines of the equipment. What did the commissioning engineer do before putting the breakers ON?

 

[TugboatEng]

Since I get the most poorly designed systems to work with, let me offer my "it couldn't possibly be that wrong" idea. Are the breakers even rated for DC? Interrupt rating? A single trip can damage a breaker (burned contacts) if the interrupt rating is exceeded and most breakers have very low interrupt ratings in DC. Do you have blowout coils? Do you need them?

 

[waross]

As I understand the system, these breakers are on the AC input to the rectifiers.

The thermal trip is normally heated by the load current.
The heater element rejects heat to the ambient.
When the heat generated by the current exceeds the ability of the breaker to reject the heat fast enough, the bi-metal bends and the breaker trips.
At 150F they will mostly trip due to ambient heat.
Unless the breakers are specially rated, when 90C rated conductors are used the maximum continuous load on a breaker may be 80% of the breaker rating.
150F? Off the chart.
Whatever is generating the heat is most likely the root cause.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

I don't see blowout coils much anymore in the small sizes.
De-ion plates are much more common.
Some devices use convection.
The arc heats the air, which rises and carries the arc into the de-ion stack.
Some are arranged so that the magnetic field of the current through the arc pushes the arc into the de-ion stack.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

@ Mr. waross (Electrical)25 Jul 23 05:16
1. Please advise why you mention 150 [sup]o[/sup]C at all ? Mr. bbdn2004 stated the temperature 150 [sup]o[/sup] F, which is about 65.55 [sup]o[/sup] C.
2. I agreed with your learned advice
2.1. tripping is unlikely due to
a) "dirty" power source as it was operating fine previously. And proven fine when the original breaker was put back again.
b) over-load as the load is only 20A while the breaker is 45 A.
c) magnetic trip as it trips shortly after the system was powered up.
2.2. tripping is likely due to
a) heart, over-temperature in some location/connection.
3. Likely fault is high contact resistance/bad connection that result to over-heating due to i[sup]2[/sup] R , Joule effect.
Che Kuan Yau (Singapore)

 

[waross]

1. Please advise why you mention 150 oC at all ?

Thanks for catching my silly mistake.
I hope that you will agree that even 150F is too hot with 20 Amps on a 45 Amp breaker.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

@ Mr. waross (Electrical)26 Jul 23 05:43
"...I hope that you will agree that even 150F is too hot with 20 Amps on a 45 Amp breaker..."
That depend on .....
1. See UL 489:
a) para 7.1.4.2.2 for standard rated breaker max temperature at wiring terminal < 90 [sup]o[/sup]C.
b) para 7.1.4.2.3 for 100% rated breaker max temperature at wiring terminal < 100 [sup]o[/sup]C.
c)non-metallic handle < 85 [sup]o[/sup] C.
d) temperature at the contact point or the housing sides/bottom is likely > terminal temperature.
e) standard breakers are calibrated in 40 [sup]o[/sup] C.
2. With this guidelines, the temperature measured 65.55 [sup]o[/sup] C is permissible.
3. But, the breaker is tripping shortly after the system was powered up.
a) thermal scan temperature is incorrect due to wrong setting of e-factor?
b) the current measured was 20 A on a 45 A breaker. There was No over-current.

Che Kuan Yau (Singapore)

 

[LionelHutz]

The source can only supply distorted voltage and voltage alone doesn't heat anything. Distorted current flow does. If the 20A has a decent amount of higher frequency harmonics then it could be causing the breakers to heat up. 20A of 60hz should not.

 

[che12345]

@ Mr. LionelHutz (Electrical)26 Jul 23 12:32
"...The source can only supply distorted voltage and voltage alone doesn't heat anything. Distorted current flow does. If the 20A has a decent amount of higher frequency harmonics then it could be causing the breakers to heat up. 20A of 60hz should not".
1. Agree with your learned advice. The bi-metal thermal element is badly affected by high frequency current, but NOT at say within 3,6 x 50-60Hz. Note: the harmonic content are "usually" very much lower at say 9... x power frequency.
2. The tripping seems? to be due to heat (temperature) by high contact resistance some where i.e. i[sup]2[/sup] R....Joule effect.
Che Kuan Yau (Singapore)

 

[TugboatEng]

Even at 150°F, the trip derate isn't going to be more than 50%. Here is an example from a Square D t/m breaker.

 

[waross]

That's for external ambient temperature Tug.
That chart is not valid for internally generated heat or heat conducted from a hot bus bar connection.
If you stay on the tugboats long enough, you will eventually see a good example of a bad connection causing premature trips.
I can remember the first time that I saw a heat related trip.
I can also remember much further back than that, when I had not seen it yet.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!