Good morning readers! As promised, here is a short piece on getting the power out to the Transmission system from the generator. This piece was requested by Gene Nelson after he discussed the current output of the Diablo Canyon units with one of the staff. I am hoping Kenneth Kaminski will chime in with some of his personal knowledge from Diablo Canyon in the comments.
So it might seem like a simple matter, the generator produces power and we use a transformer to step-up the voltage to put it out on the transmission system, simple right? Actually in it’s simplest form, it is just that simple but like everything, it get’s complicated from here. So let’s discuss what matters.
Size of the generator and its output voltage matter. The generally accepted practical limit for circuit breaker size is 5000 amps, regardless of voltage. While you can special order circuit breakers with higher current ratings, most of the time custom equipment is avoided. So once generator current exceeds the practical circuit breaker value, the generator and the transformer are hard wired together without any breaker. This is called a “unit connected transformer”. The parallel connection to the power system is made with the Transmission voltage breaker, in the case of Diablo Canyon it is a 500kV breaker.
Where each generator has it’s own step-up transformer, you will often hear them referred to as a GSU (Generator Step Up). These transformers usually have a delta winding connection on the generator side, and a wye connection on the Transmission side. Every three-phase transformer actually has three windings. In some cases three single phase transformers are used with a fourth single phase as a spare (I believe this is the Diablo Canyon configuration), (Ken I was unable to find any Internet images of the Diablo Canyon GSUs). The delta-wye connections are made up external to the transformers. This provides redundancy is case of a failure. The delta-wye winding helps protect the generator from certain line faults and prevents third order harmonic from being transferred.
On unit connected transformer set-ups, the generator bus is usually also tapped to feed a station service transformer. Station service powers all the pumps and equipment needed to make the power plant run. Usually there is also a station service off the Transmission bus, and in the case of nuclear, there is a third station service feed from an offsite substation providing a redundant station service feed. The station service is designed to transfer between energized sources to assure there is power to station equipment. Nuclear stations also have large backup diesel generation to maintain station service. This is all to make sure there is cooling flow to the reactor.
For unit-connected transformers, you will always find the transformers placed very close to the generator to reduce the length of the bus work connecting the two. Here you can see the GSUs at Shasta Dam sitting over the afterbay.
Shasta Powerhouse GSUs on deck above afterbay This bus work can get incredibly large and complex with large units like Diablo Canyon. Each Diablo Canyon Unit is rated at 1164MW @ .9PF, or 1293MVA. Output voltage is 25kV, which is about the upper voltage limit for a generator winding. At full MVA output each unit can produce a shocking 29,861 amps. That is a massive amount of current, which produces some extremely powerful magnetic fields, and requires extensive cooling. To achieve this they use Isolated Phase Bus (IPB), tubular bus bar inside a tubular isolation housing that contains the electromatic fields produced by the high current. The tubular bus allows for forced air cooling both through and around the bus.
IPB ready to install The Transmission breakers are often located a fair distance away from the generators and the GSU transformers in the Transmission substation. The breaker layout can get complex involving more than one circuit breaker. Transmission substation layouts are a whole different conversation.
Shasta Transmission Substation across river from powerhouse.
As you can see, connecting a large generator to the power system may appear easier than it actually is. These special GSU transformers often take three to four years lead time to have manufactured, so it takes some logistical planning. Once they are in, they are expected to be in service for fifty years or more.
This completes a little more light on another specialized area, I hope you enjoyed it. Let me know what else you would like me to cover next.
Thank you for another interesting and informative article.
I was once an IT consultant doing communications architecture at Commonwealth Edison in the Chicago area. We were told that there was no breaker on the high voltage transmission line from the nuclear plant, and that a fault at the far end could not be fixed with a circuit breaker because the voltage was too high to open. Our job was to design reliable communication from afar to a circuit breaker on the low voltage step-up transformers at the nuclear power plants. No managers nor engineers would give us a spec for an affordable comm failure rate. They all insisted on impossible 100% reliability. We eventually do the best we could with fiber optic loop comms.
Ironically, Comed.com was down as I was writing this, because of storms in the area.