Imagine if the national power grid failed? How would power plant owners with no access to electricity from the grid re-start their generators? It is an unlikely scenario but one that National Grid needs to be prepared for. Some of the UK’s power plants have what is called ‘black start’ capability – a function that allows the grid to recover from complete system failure. In the UK, black start capability was typically provided by coal fired plants built in the 1960s and 70s. However, the continuing retirement of these coal fired units due to commercial and environmental pressures, means the country’s black start capacity is steadily being lost. At the same time, the intermittent nature of wind and solar means the probability of the need for a black start is higher than it has ever been. With old coal plant largely being replaced by renewables, the existing gas fired generation is the obvious place to look for new black start capability.
A real challenge
Commenting on the situation, Steve Read Managing Director, Siemens Power Generation Services, UK said: “National Grid has calculated that it needs a certain number of black start stations across the UK to be able to re-start the grid in the event of a complete system shutdown. As the coal fired power plants with black start capability are closed, the amount of time it would take to re-start the grid is increased, they are now looking to replace those stations they have lost with gas fired power plants that have the capacity to allow a faster re-start.”
This, however, is not without challenges. Combined cycle gas turbine (CCGT) power plants are highly efficient, but they were never designed to carry out that function. CCGT plants are started up by loading the gas and steam turbines at a steady, smooth, specified rate (ramp rate) until they reach full load. Black start, however, calls for a plant to respond to instantaneous block loads from the system. Finding a way to make these CCGT plants operate under black start conditions is a real technical challenge.
Although difficult, fortunately the task is not insurmountable. Siemens has worked with generating companies to develop full turnkey solutions to enable existing CCGT power stations to become black start capable.
As an original equipment manufacturer (OEM) of all the major equipment in a power plant – gas turbine, steam turbine, electrical systems, protection systems, control system, excitation systems, etc. – Siemens has the ability to modify all of the equipment that has to be adapted to make a plant black start capable.
In addition to the technical expertise, it also has the industry experience to design a system that has minimal impact on the existing power plant. At the same time, it can plan implementation of black start ability around outage times so the effect on plant operation and downtime is reduced.
Typically, a diesel or gas genset, or small gas turbine facility, along with the electrical connections to control it, have to be installed. At the same time, a number of modifications have to be made to the existing plant so it can operate in black start mode.
One of the key modifications is to mitigate the physical impact of black starts on the gas turbine itself. Under a black start, the gas turbine may have to endure three times its normal per minute loading rate within one second. The effect of this on a spinning turbine is significant. For example blade fatigue caused by the massive thermal stresses could soon become a problem.
Here Siemens is able to use its expertise in gas turbine design to determine the long and short term effects of this instant loading operation.
At the same time, protection schemes not designed to operate under black start have to be modified, as do the excitation systems that are not designed to cope with a dead grid.
While the chance of ever having to conduct a black start is very small, if the grid does fail plant owners have to be ready to perform a black start within two hours of getting the call from National Grid. So in addition to making the plant technically capable, Siemens makes the task easier by ensuring the whole black start process looks familiar to the operator.
Although two hours may seem like a long time, it is actually quite a short window to bring a CCGT power plant back on line when considering what is involved. A completely independent power source such as a diesel generator or small gas turbine using an uninterruptible power supply, first has to be started up to provide the power plant’s electrical needs. The main gas turbine(s) then has to be run up to speed in black start mode, ready to synchronise and receive the block loads required for grid restoration.
Work is under way
Siemens is currently working with a number of generators on engineering designs to see if the plants can be retrofitted to perform black start, and if so what the solution might look like and what modifications would be necessary.
The first of these potential retrofits has now been implemented and commissioned. This marks a significant milestone – being one of the first existing CCGT plants modified for black start in the UK.
As coal fired plants continue to be retired, no doubt there will be an ongoing programme to ensure the UK’s gas fired power stations are ready to come to the rescue in the unlikely event of a total system shutdown.
“A lot of the coal plants that have black start are closed, so National Grid really has a depleted black start capability. I think there will be a number of projects over the next few years to replace that lost capability,” noted Read.
As a manufacturer of all the major components and systems that make up a power station, with the ability to provide a complete turnkey solution for black start modification, Siemens is well equipped to play a major part in the programme. National Grid can rest assured that Siemens is an experienced, reliable partner when it comes to preparing for the worst.