Purpose of 3-winding transformer

[Pitt123]

What is the purpose behind using a 3-winding transformer? Is it simply to be able to provide two different output secondary voltages more economically in one transformer than having to purchase 2 different transofmers.

An exmple of one I saw was an 18kV primary and 4.16 and 13.8kV secondary windings.

 

[ron]

For solar PV, some inverter outputs require isolation in order to be paralelled, so I have two 500kW 480V inverters feed the (2) low voltage side inputs and I get (1) 1MVA medium voltage output that I can then transmit power to whereever I need.

 

[rcwilson]

18 kV to 13.8 kV, 4.16 kV sounds like a power plant auxiliary transformer fed from the 18 kV generator bus to supply in-plant generator loads.

The three winding transformer saves money and space with only one foundation, one HV input, one HV circuit breaker or tap.

Disadvantages: Since it is custom, it is hard to find a replacement if it fails. The voltage taps cannot be changed individually for the 13,8 and 4.16 kV. Any trip on one system trips the other.

Three winding transformers can also provide some benefits for short circuit issues by selecting different impedances for the H-X, H-Y and X-Y windings. For example, if there is a generator on the 13.8 kV winding and utility at 18 kV, the 18-13.8 kV impedance could be low for good MVAR capability and voltage control with the 13.8 kV-4.16 kV impedance high to limit 4.16 kV SC levels.

Biggest savings is when the high voltage connection is on a bus with high short circuit levels (>100kA) or high voltage (>69kV) and we can save the $1M cost of a second breaker.

 

[richxtlc]

In the utility industry, it is not uncommon to see a 3-winding transformer. Usually 345kV/138kv/13.8kv. The 345/138 is the normal power flow for a step-down transformer, the 13.8kv tertiary winding can be used for:
1. Input from a combustion turbine during peak load conditions
2. connecting a switchable power factor correction capacitor or reactor
3. to supply station light and power

 

[kingpb]

According to what Bob is describing the additional cost for the 3-winding Xfmr is around +15% over a 2-winding, but the increase in cost is more than offset by the reduction in other equipment.

You are putting all your eggs in one basket so to speak.

 

[rcwilson]

You are putting all your eggs in one basket so to speak.

That is why most of our customers don't want to do it. One utility lost a lot of money when their 500 kV/18kV/18kV, 3-winding transformer blew and shut down the power plant almost a year. Replacements are hard to find.

One approach is to use three single-phase, three-winding transformers and buy a 4th as a spare. That makes sense on the big units: >230 kV, > 150 MVA. It makes a lot of sense if you own several generating units and carry one spare for all of them. Bus duct in and out to connect three large transformers in delta on two different secondaries is an interesting design challenge.

One client's criteria is 3-winding is ok at 345 kV and less as long as both secondary windings are about the same size and voltage. Different MVA ratings, different ratios or high voltages, then go with two winding transformers, but it is OK to save money and have them on the same HV breaker.