MV Cable Sizing - Power Transformer Tertiary to Auxiliary Services Transformer 1

[mikeangel]

Hi!

I need help to sizing the MV cable suitable for the following situation:

I have a 220/60/15kV Power Transformer. The tertiary of the Power Transformer will be connect to a 15/0.4kV (150kVA) Transformer for supplying the Substation Auxiliary Services.

The maximum tertiary power of the Power Transformer is 20MVA. By my calculations the maximum current in the tertiary will be 20MVA/(1.73x 15kVA)=769,8A. The maximum current in primary of the 15/0.4kV (150kVA) Transformer is 5,8A.

In this case what will be the criterion to be applied for the dimensioning of the MV Cable that connects the tertiary of the Power Transformer to the Auxiliary Services Transformer.

Best regards,

 

[RRaghunath]

Hope you will have 15kV fuses at the aux transformer primary.
You need to calculate expected 3-phase fault current on 15kV side and size the fuses and cables accordingly.
Cable size shall be good enough to carry the load current. The fuses shall be good enough to carry the load current as well as transformer inrush and also adequately protect the cable/transformer against 15kV short circuit currents.

Rompicherla Raghunath

 

[mikeangel]

RRaghunath, thany you for your advice.

My client does not have considered any protection in primary side (as usual in is Substations) of the auxiliary transformer primary...the connection is made directly without any protection device.

The nominal current to be considered should be the 769,8A (independent from the short circuit currents fault calculations)?

 

[RRaghunath]

Doesn't make sense to me.
If there is a problem in 150kVA transformer what is expected to sense the fault and how is it cleared??

Rompicherla Raghunath

 

[mikeangel]

I really do not know. What I really liked to know is what should be the criterion for dimensioning the cable.

 

[RRaghunath]

In your case, the cable should be able to withstand through fault current (15kV current for a fault in 400V system) for a min of 2s, apart from the continuous rating for load current of ~6A.
2s duration comes from the IEEE for transformer withstand capability.

Rompicherla Raghunath

 

[bacon4life]

Wow, I am amazed someone would connect a distribution class transformer to a substation transformer tertiary without fuses. A fault on the tertiary has extremely high fault current.

 

[prc]

In such applications normally care will be taken to see that chances of 3L or 2L faults are eliminated at both ends of 15 kV cable connections to transformers. But in case a primary fault occurs inside the transformer at line end (chances are rare but cannot be ruled out) then cable will see fault current corresponding to 2or 3L fault on tertiary terminals ie 15-20 times 770A. Otherwise what raghunath said is ok ie 25-30 times 6A.

 

[mikeangel]

This is an example (see in the picture) from what we do for our major clients. As you can see the connection is made directly from the tertiary to the 20kV side of the auxiliary transformer, without any protection in the 20kV side.

For instance in that particular installation that you see in the picture we have:

+ Power Transformer 400(450MVA)/150(450MVA)/20(60MVA)kV, so in the tertiary we have a maximum corrent of 1728 A.
+ Auxiliary Transformer 20/0.4kV (400kVA)
+ Cable: XHIOV 18/30 (36kV) 95 mm2

 

[RRaghunath]

The 400kVA transformer, 20kV cable has to be considered as part of the power transformer.
In case of any fault in cable or transformer, power transformer differential protection will operate.
This means the quality of cable/terminations as well as the auxiliary transformer need to be as good as main power transformer so as not to lower the reliability of the system.

Rompicherla Raghunath

 

[prc]

Hope it is not an auto transformer but three winding transformer with OLTC. With 95 mm2 cable you are taking care of 400V side fault. But with an inside 2L fault at primary terminal side of auxiliary transformer, the cable will get damaged. May be Ok as you will be replacing the cable too along with transformer. But do regulatory agencies allow this?

 

[mikeangel]

prc in fact, is this example it is an Auto-transformer.

All of these situations I describe are for substations whose projects were developed by the entity responsible for the energy transport network. I do not really know how the faults on the primary side of the Power Transformer and its corresponding propagation to the tertiary of the auxiliary transformer are guarded. I have never seen any protection devices in any substations on the primary side of the auxiliary transformer. Also to my knowledge there has never been any incident that has led to the disruption of the tertiary circuit. But I have always wondered what the criteria for cable selection are, since the tender specifications are not very explicit and do not indicate how the calculation was done and how the circuit is protected.

 

[cuky2000]

See if the threads below can help.

thread238-437143
thread238-173137
thread238-216022