6kVA autotransformer upstream of bidirectional grid tied inverter?

[cupoftea]

Hi,

I have just seen a grid tied inverter for connecting a battery to the 240VAC UK mains. Its bidirectional. Its 5.5kWh rated. The mains comes to it via an autotransformer, which knocks the mains down by just 15-20V or so. The reason for the autotransformer said to be because the GTI needs protection should the mains go too high.

This autotransformer was like a "ring donut" type of diameter approx. 20cm and depth 7.5cm

Would you agree this doesn’t seem big enough for 6Kva? (The one below is much bigger)

Also, it seems strange to have such a thing?...dropping the mains by so little?

(i wondered if the real reason for it might be too stop mains transients getting to the bidi GTI?)

6Kva Autotransformer

https://www.solaris-shop.com/content/PSX-240%20Specs.pdf

 

[cupoftea]

Thanks, i am referring to the fault seen at 8:47 of this video....where a broken PEN in the street gives 288VAC line to neutral in a house......and as you know, this could be up to 415vac depending on loading on the various phases.

 

[Easy peasy]

this is different to a broken neutral leading to a house -

in any event a failed neutral connection inside a Tx will not automatically give a high P-N result in a house, it can be any where from 0v to 230V depending on loading in the house and on the street ....

--- Updated Sep 26, 2021 ---

you can only get 415Vac if you connect one phase to neutral - if the neutral is broken leading to the house - this can never happen .....

--- Updated Sep 26, 2021 ---

you may well get higher voltages phase to earth - as a broken neutral at the supply Tx implies a loss of N - E connection - so all 3 phases can then float high - limited only by the P-E RFI caps in various connected loads - IF they are switched on at the wall .....

 

[Easy peasy]

I'm just gonna update this to say that if a house is fed with 3 phase mains to the domestic outlets in a 230/400 system - i.e. some are on differing phases - extremely unlikely as the code advises against this - then, if someone turned on a simple 3kW heater, P-N on one phase, AND the neutral connection was lost to the supply Tx - then it is possible you could see nearly 400Vac across another outlet ( different phase ) with the current limited by the resistance of the 3kW heater.
If you happen to turn on another 3kW heater - you may not notice anything as it would be in balance with 200Vac across each.

If you plugged in an high impedance device - like a phone charger - it would see all the 400V and likely go pop.

The other danger here is that because the N & E are tied at every switchboard and DB - the earthed metal of any appliance would rise to the new neutral voltage - causing a shock hazard - this is the main reason that double insulated equipments are safer to handle.

This is why it is so important that the Tx neutral be connected and earthed fully in an MEN system with oversized cables that are corrosion resistant.

 

[BradtheRad]

I read a story from a man familiar with mains wiring (in the US). Here a house has two branches (two opposite phases) each at 120 VAC. Hence three wires were suspended in the air carrying electric power from the transformer pole to his house. (Outlets carry a neutral and one or the other phase).

On a windy day he watched the wires swinging wildly, then got worried to see a gust send one wire flipping so it touched another wire (or a drip loop). There was a spark, and immediately he realized it exposed certain appliances to twice normal voltage. I think he said some appliances were shot, some were okay.

 

[FvM]

I'm just gonna update this to say that if a house is fed with 3 phase mains to the domestic outlets in a 230/400 system - i.e. some are on differing phases - extremely unlikely as the code advises against this

In Germany, the sketched scheme, 3 phase house supply and single phase loads distributed more or less equally is just standard. Neutral interruption between PE-N connection point and floor sub-distribution can definitely cause overvoltage up to 400 V. But it occurs very rarely due to good quality of work.

As previously mentioned, I'm completely missing a relation of this loss of neutral discussion to the original thread topic.

 

[cupoftea]

I'm just gonna update this to say that if a house is fed with 3 phase mains to the domestic outlets in a 230/400 system - i.e. some are on differing phases - extremely unlikely as the code advises against this - then, if someone turned on a simple 3kW heater, P-N on one phase, AND the neutral connection was lost to the supply Tx - then it is possible you could see nearly 400Vac across another outlet ( different phase ) with the current limited by the resistance of the 3kW heater.
If you happen to turn on another 3kW heater - you may not notice anything as it would be in balance with 200Vac across each.

If you plugged in an high impedance device - like a phone charger - it would see all the 400V and likely go pop.

Thankyou very much for this. I am wondering why it doesnt apply when say the heater was in one house (being on one phase), .....and then the P-N in the next house supplied by a different phase (of the same 3 phase) would see 400VAC ?

--- Updated Oct 2, 2021 ---

As previously mentioned, I'm completely missing a relation of this loss of neutral discussion to the original thread topic.

..its because i was wondering why a single phase inverter would need a 50Hz , step down autotransformer upstream of it...and wondered if ti was to step down the higness of voltage caused by a broken PEN.

..(though its only stepping down by some 5 to 18VAC or so.

I mean.....i cant understand why a single phase inverter would need an 50Hz autotransformer ahead of it.....so i am racking my brains to understand why it would ever be done....i mean, you can series two 450V 'lytics, so it cant be due to 'lytic voltage.

Is it common to see grid tied inverters with 50Hz autotransformers upstream of them?... this is what is bashing my brains up.

 

[FvM]

Quality inverters are designed for the rated mains voltage range, e.g +/- 10 %, and don't need transformers.

 

[Easy peasy]

it depends on where exactly the neutral break is, and if it is only one break or several.

 

[cupoftea]

Quality inverters are designed for the rated mains voltage range, e.g +/- 10 %, and don't need transformers.

Thanks, i am just wondering if an inverter (GTI) is easier to control if there is a big inductance seen at its output...ie the autotransformer?

 

[KlausST]

Hi,

As long as the autotransformer is connected to grid, there is no big inductance.

Klaus