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.
is actually ground and Neutral and ground are connected, so ship's system, when generators are supplying power, is TN-S.
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,
cools down, switches off and is in standby.
Please, look at this simplified diagram to understand the situation better:
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
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.
- 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
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
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!