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Topic Title: Dyn11 transformer as a step-up transformer.
Topic Summary: using a Dyn11 as a step-up transformer
Created On: 25 November 2012 07:51 PM
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 25 November 2012 07:51 PM
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Joined: 27 April 2008

a customer would like to install a single circuit MV system, it has a Dyn11 as a step-up transformer and and another Dyn11 as step-down transformer. (to avoid investing for a YNd11 as step-up).

Since i haven't had any practical experience with this sort of configurations, i kindly request your inputs to this.

as there's no grounding point in the MV side of Transformers,what are the possible problems (if any).

kind regards.
 26 November 2012 12:36 AM
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I assume that your use of the term MV relates to an ac voltage >1kV, <36kV.

The above link takes you to a relevant section of the Electricity, Safety, Quality & Continuity Regulations which apply to generators, distributors and meter operators in the UK. In these Regs, voltages >1kV are classed as "HV". If your customer belongs to such a category, then operating an unearthed HV circuit would be a breach of these Regs.

Otherwise, provided that you can demonstrate that the system can be operated safely (eg with a Design Risk Assessment), I'm not aware of any reason why this shouldn't be done. I would expect "safe operation" to include a means of interupting the circuit in the event of an earth fault. Such protection would need a VT configured either as a 5-limb 3-phase type, or 3 x 1-phase units supplying a neutral Voltage Displacement protection relay. The VT HV star point must be earthed for satisfactory operation.

The additional cost of the VT may well cause a reconsideration of the step-up transformer to a YNd configuration, with the N earthed (either directly, or via an impedance). This is the approach I'd commend, as this allows "standard" current-operated earth fault protection to be employed on the MV side with either/ or both Balanced EF/ Standby EF. In general, I'd expect to be able to achieve faster fault clearance, whilst avoiding spurious operation, with current-operated EF protection, especially if any of the MV circuit is run as overhead lines.

In any event, I'd recommend that an experienced HV design engineer be employed to sign-off on the final design.

 26 November 2012 07:57 PM
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I second Jonathan's approach.

The use of a YNd transformer with NE resistor to limit earth fault current (to say full load transformer current) with the use of standard IDMT protection and standby earth fault (on the NE resistor) is 'typical' for LV (400V) to MV (11000V) installations.

There are other options available, which I can forward on a paper (please PM me if you would like a copy) that discusses the pro's and cons of each such as using the Dyn11 transformer. The use of a Dyn11 has inherent problems, which any DNO will tell you that an embedded CHP or other device backfeeding into their MV network via a Dyn11 can bring. This can be overcome, by using a Neutral Voltage Displacement relay and the appropriate VT as Jonathan has mentioned, however it does depend on the MV network that you are paralleling with (if you indeed are?) and the reliability of the MV network under fault conditions that you have created. To ensure that the MV network is reliable and has good voltage stability under fault conditions it will need to be referenced to earth at some point.

Also, using a Dyn11 you have a NE reference on the LV side of the Dyn11, however, the source voltage you are stepping up/ back feeding from, say a CHP can only generate when the NE (via the transformer) is in circuit. If the NE link is removed, via opening the LV ACB (4 pole) the CHP should shutdown and only start again if it's own NE resistor/ contactor is utilised or the NE link is reinstated.

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