3ph 400V/50Hz yacht can't connect to the Shore power

[gubavac12345]

I am working as an electrician on a yacht which normally gets power from 3 diesel generators which supply 3phase 400VAC/50Hz. Each generator's stator is connected in Y-connection, and neutral is used to produce 230VAC.

Steel hull is actually ground and Neutral and ground are connected, so ship's system, when generators are supplying power, is TN-S.

Now...
We have tried connecting to the shore power supply, also 3phase 400VAC/50Hz.

After connecting cables to the shore side and verifying correct phase rotation, there is an automatic procedure whereby:

1. our working generator's parameters are adjusted so they closely match the shore power supply parameters ( voltage, frequency and phase angle)

2. when the parameters closely resemble, the breaker on the ship (that is supplying shore power from the outside) is closed and shore power and working generator are now synchronized
3. shore power starts taking ship's load

4. cca 80% of the load is transferred to the shore power

5. working generator's breaker is disconnected, so that shore power takes all of the ship's load,

6. generator cools down, switches off and is in standby.

Please, look at this simplified diagram to understand the situation better:
https://ibb.co/qDSvN3z



We have a problem at step 4.

Once the shore power takes around 40% of the load (around 120kW), the breaker ashore gets tripped. It gets tripped because Earth leakage relay gets triggered and this, in turn, trips this breaker.



How does Earth leakage relay works (manual is here - https://download.schneider-electric....Ref=51005191AA):

A core balanced current transformer (CBCT) is measuring vector sum of all the phase currents. Once it detects a current of certain magnitude (depending on the setting - it can be set from 0.03A to 30A), the Earth leakage relay to which CBCT is connected, will get triggered.

When the Earth leakage relay gets triggered, it trips the breaker to which it is connected.



When the Earth leakage relay setting is 5A, the Earth leakage relay gets triggered as soon it takes 120kW of ship's load.
When we set it to 1A, the relay gets triggered as soon it takes 5kW of ship's load.

Btw, Earth leakage was replaced with the new one.
There is a small chance that core balanced current transformer is damaged, but we cannot check that now.




Some notes:

- Neutral is not used from the shore as it is provided by the secondary of the Isolation transformer

- I tried not connecting GND cable to the shore panel outside at all

- I tried connecting GND cable from the GND in the shore panel outside directly to the hull of the ship (instead to the earth screen of the transformer as suggested by the company that made ship's electrical installation)

- insulation test between the earth screen of the Insulation transformer and the ship's hull: around 80MOhms

- Note that right now when the ship's power is supplied by the generators, we don't have problem with earth faults, that is, none of the breakers are tripping


- tested insulation of the Insulation transformer:
PRIMARY: all phases to GND more than 200MOhms and between phases more than 200MOhms.
SECONDARY: all phases to GND more than 200MOhms and between phases couldn't test because I couldn't disconnect windings.

- tested continuity between Insulation transformer windings:
PRIMARY: between all windings 0,1Ohms.
SECONDARY: couldn't test because I couldn't disconnect windings.
- insulation test between the earth screen of the Insulation transformer and the ship's hull: around 80MOhms

Questions:

1. What can cause imbalance between vector sums of the phase currents?

2. Even if there is some earth fault on the ship, wouldn't it be seen only on the secondary of the Isolation transformer and not on the primary? And if the vector sum of all the currents through primary is 0, how could Earth leakage detect current imbalance?

3. I am pretty sure that increasing the Earth leakage relay setting to 30A would stop the breaker from tripping. What might be the consequences? Please note that when the ship is supplied by the generators, all the ship's consumers are working and not tripping their breakers.

4. Is it possible that we have multiple small earth faults on the ship (that add up) and this triggers the Earth leakage relay?


A bit long, but would really appreciate if somebody could help a bit.

Feel free to ask any questions. Thank you!

[smac999]

what Happens if you shut down the gens. Manually select shore power. And then plug it in. Without any sync functions. Does it still trip or stay on?

Is the earth trip before or after the isotransformer?

Edit I see pic before.

You should not be able to trip that Infront of a working isotransformer. It should only be tripping with an issue inside the transformer. But your readings look ok.

Have you checked over the cable And connections between the breaker and transformer? Maybe the cable is shorting out.

Note that a normal leakage breaker is 30ma. Did you mean ma? You certainly do not want to set it to 30a.

Have you put a clamp meter on the whole wire between breaker and transformer and seen leakage current as it switches?

[Dsanduril]

If a CBCT is used in a 3-phase, 3-wire system then it measures the overall imbalance in the phase currents. Is the loading aboard the ship even on all three phases? If not, can you make it even as a test?

When used for leakage current detection the CBCT is placed around 3 phases + neutral and so can measure the leakage instead of just phase imbalance. But, since you are not using the shore neutral, because of the isolation transformer, the neutral, even if in the CBCT toroid, is not carrying any current.

I suspect that your load is unbalanced, and because you are not using the neutral the VigiPac is actually acting as an unbalance relay rather than a leakage relay. [edit] one way to check this, do you have any current in the neutral aboard the ship? In that case you have an imbalance that may cause a circulating current on the shore side that may show up as an imbalance on the shore side.[/edit]

https://electengmaterials.com/core-b...nsformer-cbct/

[GordMay]

Greetings and welcome aboard the CF, gubavac.

That’s some technical challenge, you’ve posed!

[EngineerRetired]

"Steel hull is actually ground and Neutral and ground are connected, so ship's system, when generators are supplying power, is TN-S."


I believe your problem is right there. Even if you are not using the neutral from shore power (isolation transformer) it's an almost certainty the supply side to the dock is connected to an earth ground. Minimal leakage through the ship's steel hull is tripping the ground fault breaker.


In the US grounds and neutrals are never tied together on boats. Too many people have been electrocuted from it.

[CharlieJ]

Fundamentally, a properly wired and functioning isolation transformer will isolate the electrical system aboard from the shore electrical system and any ground to neutral bond or other fault aboard will not be sensed by the RCD upstream of the isolation transformer at the shore power source.

I am suspicious of the "low" reading of 80MOhms between the screen and hull and recommend that you contact the isolation transformer's manufacture to determine what the specification is. If there is insulation breakdown when loaded, the numerous electrical leaks that develop over time would not be blocked by the isolation xfmr and would be sensed by the shoreside RCD and shore power would trip.

[billgewater]

I'm no electrician but is it possible that on the jetty, all the single phase users are all using the same of the three single phases. Could this create the vector sum imbalance of the three phases you referred to?

[kish]

[QUOTE=gubavac12345;3483145]I am working as an electrician on a yacht which normally gets power from 3 diesel generators which supply 3phase 400VAC/50Hz. Each generator's stator is connected in Y-connection, and neutral is used to produce 230VAC.

;
;
;


Some notes:
;;
- Neutral is not used from the shore as it is provided by the secondary of the Isolation transformer

-A three phase shore connection without the shore neutral connection is only appropriate for a balanced load,

An example is connecting of a three phase motor, which is naturally balanced and will run satisfactorily without a neutral connection.

If a neutral is present it will have zero amps.


By connecting the 3 phase shore power without the shore neutral, a voltage imbalance will be present, as seen by the shore power, as the loads onboard are not equally distributed over the three phases causing a neutral current.

The generators run with a neutral connection, so the shore connection must see a neutral connection also.


If a 3 phase with neutral connection to shore is present, all should be well.

[gubavac12345]

Originally Posted by smac999

what Happens if you shut down the gens. Manually select shore power. And then plug it in. Without any sync functions. Does it still trip or stay on?

Is the earth trip before or after the isotransformer?

Edit I see pic before.

You should not be able to trip that Infront of a working isotransformer. It should only be tripping with an issue inside the transformer. But your readings look ok.

Have you checked over the cable And connections between the breaker and transformer? Maybe the cable is shorting out.

Note that a normal leakage breaker is 30ma. Did you mean ma? You certainly do not want to set it to 30a.

Have you put a clamp meter on the whole wire between breaker and transformer and seen leakage current as it switches?

We actually managed to connect to shore power after the blackout. All our load (cca 130kW at that moment) was take by shore power. However, it only lasted for 5 mins until a blackout occured again.

I do not know what caused it, but there is a strong probability that starting the HVAC compressor (cca 50kW) had something to do with that blackout because both occured at approximately the same time.

We could try to replicate the situation on the next occasion.



How can the cable short out?



I meant 30A,not 30mA. What would then potentially happen?



I couldn't measure leakage current really precisely as the whole thing happens in a matter of seconds.

[gubavac12345]

Originally Posted by Dsanduril

If a CBCT is used in a 3-phase, 3-wire system then it measures the overall imbalance in the phase currents. Is the loading aboard the ship even on all three phases? If not, can you make it even as a test?

When used for leakage current detection the CBCT is placed around 3 phases + neutral and so can measure the leakage instead of just phase imbalance. But, since you are not using the shore neutral, because of the isolation transformer, the neutral, even if in the CBCT toroid, is not carrying any current.

I suspect that your load is unbalanced, and because you are not using the neutral the VigiPac is actually acting as an unbalance relay rather than a leakage relay. [edit] one way to check this, do you have any current in the neutral aboard the ship? In that case you have an imbalance that may cause a circulating current on the shore side that may show up as an imbalance on the shore side.[/edit]

https://electengmaterials.com/core-b...nsformer-cbct/

CBCT is measuring 3phase 6wire (each phase has 2 wires). Ships load is isolated by transformer, so there shouldn't be any imbalances,correct? All the currents on primary side should be balanced?


There is 2.2A in neutral on the ship.

[gubavac12345]

Originally Posted by GordMay

Greetings and welcome aboard the CF, gubavac.

That’s some technical challenge, you’ve posed!

Thank you!

Forum members seem very knowledgeable, I am thrilled!

[gubavac12345]

Originally Posted by EngineerRetired

"Steel hull is actually ground and Neutral and ground are connected, so ship's system, when generators are supplying power, is TN-S."


I believe your problem is right there. Even if you are not using the neutral from shore power (isolation transformer) it's an almost certainty the supply side to the dock is connected to an earth ground. Minimal leakage through the ship's steel hull is tripping the ground fault breaker.


In the US grounds and neutrals are never tied together on boats. Too many people have been electrocuted from it.

You are saying: Minimal leakage through the ship's steel hull is tripping the ground fault breaker.

Are you refering to the leakage current on the secondary side? That shouldn't matter as there is an isolation transformer that will nullify any effect earth faults on the secondary side might have on the shore side.

There can only be some earth leakage on the primary side causing the earth leakage to trip...



How do people get electrocuted from neutral and ground being connected together on boats?

[gubavac12345]

Originally Posted by billgewater

I'm no electrician but is it possible that on the jetty, all the single phase users are all using the same of the three single phases. Could this create the vector sum imbalance of the three phases you referred to?

I don't think so because transformer's secondary is connected in wye meaning that there are 3 phases and neutral.

So, if there are imbalances in phases (which there definitely are), the result of that imbalance will be current passing through neutral.


And so, the currents on the primary side (delta connection) will be balanced.

[gubavac12345]

[QUOTE=kish;3483853]

Originally Posted by gubavac12345

I am working as an electrician on a yacht which normally gets power from 3 diesel generators which supply 3phase 400VAC/50Hz. Each generator's stator is connected in Y-connection, and neutral is used to produce 230VAC.

Originally Posted by gubavac12345

;
;
;


Some notes:
;;
- Neutral is not used from the shore as it is provided by the secondary of the Isolation transformer

-A three phase shore connection without the shore neutral connection is only appropriate for a balanced load,

An example is connecting of a three phase motor, which is naturally balanced and will run satisfactorily without a neutral connection.

If a neutral is present it will have zero amps.


By connecting the 3 phase shore power without the shore neutral, a voltage imbalance will be present, as seen by the shore power, as the loads onboard are not equally distributed over the three phases causing a neutral current.

The generators run with a neutral connection, so the shore connection must see a neutral connection also.


If a 3 phase with neutral connection to shore is present, all should be well.

Shore power is conencted to isolation trasnformer primary on the ship by means of 3 phases.

A secondary of the transformer is supplying ship's load with 3 phases and neutral.

So, basically, shore power is only supplying transformer primary with 3 phases, so I don't think any imbalances will occur there.

[s/v Jedi]

Gubavac,

Please know that very few members here have knowledge about hi power 3-phase electrical installations.

Step back: minimum required conductors from shore are L1, L2 and L3, powering the transformer and thus only creating a magnetic field. The shore ground is connected to the shield around the primary windings of the transformer.

Disconnect shore power and measure primary side of the transformer: between each of the primary phases and shield around primary winding. They should show infinite resistance.

Now do the same measurement between each of those phases and the GND terminal on the secondary side of the transformer. Often this is a lug to the housing. Should all be infinite as well.

Now measure between primary shield GND and secondary GND. This should be infinite resistance as well.

Can you measure current for each primary phase? They should all be equal.