Fuse Blowing on Step-down Transformer

[TwoBlocked]

Correct. There are a few different types of wind turbine converters, but in this case, the stator is in direct line to the PMT and the converter is bi-directional to accommodate sub-sync and super-sync frequencies at the rotor. The rectifier test checks the switch capabilities of the IGBTs before a pole test for phase location.

So, is the actual "moderate load" during the "rectifier test" a resistor bank? Please don't feel insulted if I ask if you have swapped leads to the resistor bank, or whatever the actual "moderate load" is, for troubleshooting purposes.

 

[synchro]

The downstream side of the 690V is a power converter consisting of a 1000+Vdc bus, IGBTs on either side of it, and a filtration circuit, connected to an asynchronous generator. ...

How well balanced are the input Line-to-Line and Line-to-Ground or Neutral voltages?
With a diode based rectifier, if it has no load on it then the current drawn from its line inputs should be negligible even if the line voltages are very unbalanced. Also, the current can only flow one way through the rectifier because of the diode's current vs. voltage characteristic.

However, with a synchronous pulse-width-modulated (PWM) rectifier using IGBTs like you are describing, I'm thinking that a voltage unbalance on the line inputs could result in significant current flow between the line inputs even without any loading. That's because IGBTs incorporating a diode shunt will allow a bidirectional current flow when they are turned ON.

With an IGBT rectifier, the PWM waveform will determine the ratio between the AC input voltage and the DC output voltage, and vice-versa (similar to what happens in a VFD). Consider the case where one of the 3-phase input voltages is lower than the other two. Then the DC bus voltage which that low voltage input would produce is lower than the higher DC bus voltage produced by the other two inputs. As a result, that higher DC bus voltage will cause the bus to feed current back into the IGBTs with the lower AC voltage input, and then drive AC current into that low voltage line. That's because those particular IGBTs would be acting as an inverter in the reverse direction instead of a rectifier in the forward direction.And since the current on the low voltage line is being powered from the other two lines, it would likely have around twice the current as these other two lines, and so its fuse would blow first.

The same situation would arise if one AC input voltage was higher than the other two, just that the currents would be flowing in the opposite direction.

It's possible that an IGBT rectifier could selectively modify the PWM waveform for each of the 3 phases so that the currents will be balanced with unbalanced input voltages, but I don't know if your rectifier would do this.

 

[electrofelon]

electrofelon said.....even if SBJ...........

The bonding jumper should not be missing. It is vital to the grounding and bonding scheme for voltage stabilizing as well as transient voltage.

I am certainly not saying to leave out the SBJ, but everything will in all likelihood be fine without it. The phase to phase and phase to neutral voltages will be perfect. In In my experience, even with the neutral floating, it will be essentially at ground potential. I just dont like putting excess emphasis on grounding and bonding being the solution to all problems, particularly things like fuses blowing and equipment failing.

it is crucial that the system bonding jumper (SBJ) be properly and appropriately located at one place on the secondary side of the transformer or at the PB/Switchgear, but not both.

generally yes, but dot forget about the exception which is commonly used for outside transformers.

is this a three wire three phase service/feeder (no neutral) or a three phase four wire with a neutral? If it is a four wire service/feeder and a 600 "Y" volt four wire with a neutral, then the neutral voltage would be 347 volts, 600 / 1.732 = 347 volts, as per 220.5 Calculations. Thus, 600 Y/347 or 600 volts.

So, I am curious about the 400 volt and the 690 volt.

sounds to me like a 690Y/400 system. Not real common in the states, but a research vessel I was on had 690V generators.

 

[synchro]

Another thing to check is to put a clampmeter on the line inputs to the IGBT rectifier and see if there are any significant DC currents.

 

[Errorsfordays]

sounds to me like a 690Y/400 system. Not real common in the states, but a research vessel I was on had 690V generators.

Yes, it is a 690Y/400 system.

that higher DC bus voltage will cause the bus to feed current back into the IGBTs with the lower AC voltage input, and then drive AC current into that low voltage line.

The input voltages read the same on all three phases, both before and after the DC bus is charged. I had not considered that the other two phases could be compensating for a faulted phase and will look into that.

Another thing to check is to put a clampmeter on the line inputs to the IGBT rectifier and see if there are any significant DC currents.

We did this recently, and there is significant amperage on all 3 phases. Usually the rectifier pulls 30-40A, and it is pulling 350-480A. Since then we have been doublechecking the converter, and moved away from the upstream side of the fuse.

 

[ptonsparky]

How many fuses do you replace before looking for a faulted phase?
N+1, or 1 more than the # of fuses available.

 

[Errorsfordays]

How many fuses do you replace before looking for a faulted phase?
N+1, or 1 more than the # of fuses available.

Haha, just once! My partner and I walked into this about three months after the fault first occurred. There are dozens of troubleshooting reports we sift through from multiple crews. Everything has been “checked”, but we still go back and verify results. My original post was an outside-the-box route suggested by my superiors, and I needed clarification on if it could be a possibility.

 

[synchro]

Another thing to check is to put a clampmeter on the line inputs to the IGBT rectifier and see if there are any significant DC currents.

The input voltages read the same on all three phases, both before and after the DC bus is charged. I had not considered that the other two phases could be compensating for a faulted phase and will look into that.

We did this recently, and there is significant amperage on all 3 phases. Usually the rectifier pulls 30-40A, and it is pulling 350-480A. Since then we have been doublechecking the converter, and moved away from the upstream side of the fuse.

A suggestion is to measure current with a meter having a large enough flexible coil to encompass all of the phase conductors.

One measurement would be around all of the phase conductors after the fuses. If there is a ground fault you should read a current, because the fault current returning through ground is not going back through the coil to cancel the magnetic fields from the other conductors. If there is a significant ground fault current, you could go downstream with the same measurment to help isolate where the fault is.

Another set of measurements would have the coil around each pair of the phase conductors before the filter that feeds the IGBTs (i.e., U&V, V&W, U&W in IEC parlance). In other words, measuring around all of the conductors except those associated with one of the phases, and doing this for all three pairs of phases.
If the line currents are balanced and in-phase with their voltages, then all the pairs should measure the same current.

I mentioned in an earlier post measuring the DC current (instead of AC) on each of the phases. You'd need to have a meter that can measure DC (by using an internal Hall effect device). If the IGBTs on each phase are switching properly then there should be very little DC current. If you have parallel conductors and can't get the clamp of a DC reading meter around all conductors of a given phase, then one or more of them is OK for this test.

 

[ruxton.stanislaw]

Yes, a loose connection in the pad mount transformer or 12kV switch gear upstream from the secondary transformer could potentially be causing the fuse to blow on the single phase of the 690V side, even if the voltage readings appear normal. Here's why:

• Loose Connections and Increased Resistance: A loose connection can act like a partial resistance in the circuit. This increased resistance can cause localised heating at the connection point, leading to arcing and sparking.
• Arcing and Voltage Transients: The arcing and sparking caused by a loose connection can create voltage transients or spikes. These voltage spikes can exceed the normal operating voltage and trip the fuse on the 690V side, even if the overall voltage reading appears normal.
• Damage to Downstream Components: The heat and voltage transients generated by a loose connection upstream can also damage downstream components, explaining why you're seeing some sensitive components burning up intermittently.

Even though you're not observing voltage drops, loose connections can still cause issues due to the localised heating and voltage spikes they create. Here's why checking upstream might be worthwhile:

• Eliminating Potential Causes: Ruling out a loose connection upstream can help you narrow down the troubleshooting process and focus on other potential causes within the 690V circuit itself.
• Potential for More Serious Issues: Loose connections left unattended can worsen over time, leading to more extensive damage and potential safety hazards.

Here are some additional troubleshooting steps to consider:

• Thermal Scan: Perform a thermal scan of the pad mount transformer and switchgear to identify any hot spots that might indicate loose connections.
• Review Maintenance History: Check the maintenance records for the pad mount transformer and switchgear to see if any loose connections have been identified or addressed in the past.
• Consult a Qualified Electrician: If you're not comfortable troubleshooting medium-voltage equipment yourself, consider consulting a qualified electrician with experience in substation/switch gear maintenance. They can perform a more thorough inspection and diagnose the problem accurately.

Remember, safety is paramount when dealing with medium-voltage electrical systems. If you're not qualified to work on these systems, it's crucial to seek help from a qualified professional.

 

[TwoBlocked]

Yes, a loose connection in the pad mount transformer or 12kV switch gear upstream from the secondary transformer could potentially be causing the fuse to blow on the single phase of the 690V side, even if the voltage readings appear normal. Here's why:
...

I also thought that could be a possibility until later info was given. There is a steady current draw 10 times greater than what would be expected. There is something seriously wrong downstream from the fuses. I stopped commenting when I realized I don't understand the system well enough to be of any help. I do believe the OP will find the true cause of the fault. He seems to be methodical.

 

[synchro]

The downstream side of the 690V is a power converter consisting of a 1000+Vdc bus, IGBTs on either side of it, and a filtration circuit, connected to an asynchronous generator.

Assuming this filtration circuit is between the 690V line inputs and the IGBTs of the rectifier, are there any wires connecting the 690V line inputs directly to the power converter? Also, are there any current transformers on these line inputs that have wires going to the power converter? Just speculating, but if such connections are present maybe there's an improper swap between the phases such that the converter is not monitoring the right phases. Or a current transformer is not oriented over a wire in the right direction to have the correct polarity.

 

[Jpflex]

Fuses blow based on downstream/load issues, not upstream ones.

Have you checked the wiring and loads? The drop in voltage could be indicative of a short circuit.

Actually for primary only OCPD transformers, a short from loads supplied by secondary causes the upstream primary breaker to blow

The op didn’t give the transformer KVA sizes but only primary and secondary voltages. Also what are load amperes?

 

[jim dungar]

Actually for primary only OCPD transformers, a short from loads supplied by secondary causes the upstream primary breaker to blow

Which is a downstream problem, not a upstream/source issue.

 

[Jpflex]

Which is a downstream problem, not an upstream/source issue.

A downstream problem “seen” on the upstream source

 

[jim dungar]

A downstream problem “seen” on the upstream source

I hope protective devices see downstream problems, if not they don't provide much protection.

A fuse does not care what faults are happening upstream, if it did we would have lots of branch fuses opening whenever there was a fault on a feeder.

 

[Jpflex]

I hope protective devices see downstream problems, if not they don't provide much protection.

A fuse does not care what faults are happening upstream, if it did we would have lots of branch fuses opening whenever there was a fault on a feeder.

You may need to review transformer rules that allows primary only protection. The reason, a short circuit on the secondary will trip the breaker on the upstream primary is because a short on the secondary causes voltage on secondary to drop while current to rise. With lower voltage there is not enough CEMF counter electromotive force to limit primary current and therefore the primary breaker trips

 

[jim dungar]

You may need to review transformer rules that allows primary only protection.

That's okay, I am fine with my current understanding of power systems.

The reason, a short circuit on the secondary will trip the breaker on the upstream primary is because a short on the secondary causes voltage on secondary to drop while current to rise. With lower voltage there is not enough CEMF counter electromotive force to limit primary current and therefore the primary breaker trips

You keep talking about a fault on the downstream/secondary being seen by the upstream/primary, which is how typical over current protection works.

My point has been that a fuse on the secondary does not react to a fault on the primary.

 

[Jpflex]

That's okay, I am fine with my current understanding of power systems.


You keep talking about a fault on the downstream/secondary being seen by the upstream/primary, which is how typical over current protection works.

My point has been that a fuse on the secondary does not react to a fault on the primary.

I was never talking about a fuse on the secondary reacting to a primary fault

 

[jim dungar]

I was never talking about a fuse on the secondary reacting to a primary fault

But I have been saying a downstream fuse will not react to an upstream fault. Did you read my first sentence in the quote you cited in post #32?

 

[synchro]

This circuit consists of a pad mount transformer stepping down 34.5kV to 12kV, a 12kV switch-gear, and a secondary transformer stepping down 12kV to 400V and 690V. One fuse keeps blowing on a single phase of the 690V side every time a moderate load is put on it. ...

Is the 12kV to 690/400V transformer a wye-wye with a 3 leg core? If so, maybe the scenario below could be happening. If not, you can ignore the rest of this post.

With a Yg-Yg three-leg transformer, if there's a blown fuse on one of the line inputs then the missing phase will be regenerated on the secondary of the transformer, and the voltage on the missing phase will be close to the proper value if it's lightly loaded. But the voltage on that regenerated leg will start to drop as the transformer becomes more heavily loaded because of the higher impedance from that leg of the secondary winding. This is described in the paper at the link below.

The front end of the power converter using IGBTs that was described in the OP's post #11 will synchronously rectify the line inputs, keeping the DC bus at a voltage proportional to the L-L input voltages. If there is no output load connected to the DC bus, then there should be a relatively small amount of current drawn by the line inputs to the converter. But if there is some other significant AC load current on the regenerated output leg, then I'd expect the IGBTs connected to that leg to start supplying current to that load to maintain its proper voltage. This is because the conversion between AC and DC using PWM of IGBTs is bidirectional.
The two legs of the transformer that have primary fuses intact would supply the necessary rectified current to the DC bus for powering the DC-to-AC conversion on the regenerated phase to maintain its proper voltage while under load.

https://selinc.com/api/download/124320

And so you might want to check for a blown fuse on one of the 12kV lines to the transformer. Just measuring the voltage may not be adequate without further measures, because in the scenario above the voltage on the open leg of both the primary and secondary would be regenerated. This problem is mentioned in the paper linked above. If the power converter is disconnected or shut down from operating, and a relatively heavy load on the transformer is applied, then a voltage drop on regenerated phase should be detectable. Alternatively, if there's a significant load on the transformer, then you can see if having the converter operating actually increases the voltage on one of the phases vs. it not operating (this would be indicative of a regenerated phase).

An open phase on one of the transformer inputs might also lead to ferro-resonance happening, which might create transient voltages that could damage more sensitive equipment, as the OP has mentioned is occurring.