3 ms·

To summarize: * Multiple contingencies occurred simultaneously (loss of generation from two major generators and lost of distributed generation totalling 1,400

To summarize: * Multiple contingencies occurred simultaneously (loss of generation from two major generators and lost of distributed generation totalling 1,400 MW) resulting in a drop in system frequency to 49.1 Hz * Standby generation (frequency response reserve) was deployed, totaling 1,000 MW or the largest single generation contingency and began to arrest the system frequency decline * Just as system frequency began to recover, a third contingency occurred resulting in the loss of a further 210 MW of generation. This caused system frequency to decline again to 48.8 Hz * Load shedding kicked in as designed and dropped 5% of load to stabilize the system The largest loss of generation was from Hornsea offshore wind farm. The wind farm should have rode through the system disturbance, but instead its control and protection systems rapidly curtailed active power generation in response to an undamped oscillation in the response of its voltage regulator through the disturbance. Basically, the internal voltage of the Hornsea wind farm collector system dropped due to the voltage regulator oscillations (from 35 kV nominal to 20 kV), while active power generation remained the same. Power = current * voltage, so an overcurrent condition occurred and protection systems operated to prevent overload of the wind turbine generators. Subsynchronous oscillations (SSO), i.e. oscillations at below power frequency (50 Hz), are a known issue in power system controls that can lead to unstable or unexpected consequences during system disturbances. The reduction in system inertia caused by the replacement of large synchronous machines with asynchronous generators as wind and solar replace conventional generators exacerbates the possibility of problematic SSO because there is less damping. Nowadays, in North America, very specific modelling is done in design stage to identify the possibility for such behaviour and ensure that if present it is adequately damped. Some system operators, such as ERCOT (Texas), require this for new wind projects. I imagine that this major occurrence will led to revisions to modelling and grid code testing standards in the UK to protect against future incidents. All in all, kudos to Ofgem, National Grid and all other participants for producing a thorough, public technical report in just about one month.

ID1452319 7y ago
Was the fact that the Hornsea site is a wind farm a contributing factor or is that merely a coincidence?
- radiowave 7y ago
  Without knowing much about the specifics of regulating wind generation, I'd say it adds some complexity, and we don't have the same number of decades of experience of operating it at scale that we do for some other generation techniques. So I don't see that wind (in the sense of weather patterns) was a contributing factor, but that complexity of regulation probably did contribute. But then again, as the Little Barford CCGT station showed, it's still perfectly possible to have unexpected failure modes on more conventional generating equipment. (Little Barford enter service in 1996, and is presumably fairly typical of the kind of CCGT stations that were built in large numbers in the UK through the 90s.)
  - ID1452319 7y ago
    Interesting thanks. A lot of people are using this power outage as an argument against wind power.
- trebligdivad 7y ago
  Not directly from my reading; but it was a very windy day so it was contributing quite a lot, so when it failed it made a bigger impact than normal. They also take about sources that provide 'inertia' - and I don't understand if a wind farm counts or not; I don't think it does
- pjc50 7y ago
  Sort of: what matters is that it was not an AC synchronous generator, like traditional thermal power plants. Wind and solar systems these days have fully digital AC-AC conversion systems which take the variable frequency multi phase output of the wind turbine(s) and turn it into standard three phase. Turbine-generator systems have nice, simple behaviour in reponse to frequency drop: they act to maintain the frequency by transferring more energy from the shaft rotation to the generator. In the long run this slows the turbine down or triggers a throttle response, but over the few second period we're talking about the shaft speed is basically constant due to its own inertia. The wind farm "saw" the rapid fluctuations in connection voltage, tried to compensate, and instead went into oscillation. This appears to have been a software bug: > "During the incident, the turbine controllers reacted incorrectly due to an insufficiently damped electrical resonance in the subsynchronous frequency range, so that the local Hornsea voltage dropped and the turbines shut themselves down. Orsted have since updated the control system software for the wind turbines and have observed that the behaviour of the turbines now demonstrates a stable control system that will withstand any future events in line with Grid Code and CUSC requirements" (Oscillation damping is "control theory 101", but in a complex system like this it's not so easy!) Good news is that, while more renewables do potentially have this kind of vulnerability, battery systems are the perfect counter. Some are already being deployed for "fast frequency response". Being a DC-AC system, they can deploy power with any frequency and phase angle required to compensate for problems.
benj111 7y ago
"The reduction in system inertia caused by the replacement of large synchronous machines with asynchronous generators" So would a large fly wheel, or something similar be of value?
- pjc50 7y ago
  Potentially, yes. Or a battery: https://arstechnica.com/information-technology/2018/05/teslas-new-battery-in-belgium-shows-value-is-in-dispatch-speed/ https://arstechnica.com/information-technology/2018/05/tesla...
  - benj111 7y ago
    Yes I was thinking its basically a big battery/capacitor. I wasn't sure for the intended usecase (compensating for 1000s of MW of powerloss) whether a battery would necessarily be the best thing.

ID1452319 7y ago

Was the fact that the Hornsea site is a wind farm a contributing factor or is that merely a coincidence?

radiowave 7y ago
Without knowing much about the specifics of regulating wind generation, I'd say it adds some complexity, and we don't have the same number of decades of experience of operating it at scale that we do for some other generation techniques. So I don't see that wind (in the sense of weather patterns) was a contributing factor, but that complexity of regulation probably did contribute. But then again, as the Little Barford CCGT station showed, it's still perfectly possible to have unexpected failure modes on more conventional generating equipment. (Little Barford enter service in 1996, and is presumably fairly typical of the kind of CCGT stations that were built in large numbers in the UK through the 90s.)
- ID1452319 7y ago
  Interesting thanks. A lot of people are using this power outage as an argument against wind power.
trebligdivad 7y ago
Not directly from my reading; but it was a very windy day so it was contributing quite a lot, so when it failed it made a bigger impact than normal. They also take about sources that provide 'inertia' - and I don't understand if a wind farm counts or not; I don't think it does
pjc50 7y ago
Sort of: what matters is that it was not an AC synchronous generator, like traditional thermal power plants. Wind and solar systems these days have fully digital AC-AC conversion systems which take the variable frequency multi phase output of the wind turbine(s) and turn it into standard three phase. Turbine-generator systems have nice, simple behaviour in reponse to frequency drop: they act to maintain the frequency by transferring more energy from the shaft rotation to the generator. In the long run this slows the turbine down or triggers a throttle response, but over the few second period we're talking about the shaft speed is basically constant due to its own inertia. The wind farm "saw" the rapid fluctuations in connection voltage, tried to compensate, and instead went into oscillation. This appears to have been a software bug: > "During the incident, the turbine controllers reacted incorrectly due to an insufficiently damped electrical resonance in the subsynchronous frequency range, so that the local Hornsea voltage dropped and the turbines shut themselves down. Orsted have since updated the control system software for the wind turbines and have observed that the behaviour of the turbines now demonstrates a stable control system that will withstand any future events in line with Grid Code and CUSC requirements" (Oscillation damping is "control theory 101", but in a complex system like this it's not so easy!) Good news is that, while more renewables do potentially have this kind of vulnerability, battery systems are the perfect counter. Some are already being deployed for "fast frequency response". Being a DC-AC system, they can deploy power with any frequency and phase angle required to compensate for problems.

benj111 7y ago

"The reduction in system inertia caused by the replacement of large synchronous machines with asynchronous generators" So would a large fly wheel, or something similar be of value?

pjc50 7y ago
Potentially, yes. Or a battery: https://arstechnica.com/information-technology/2018/05/teslas-new-battery-in-belgium-shows-value-is-in-dispatch-speed/ https://arstechnica.com/information-technology/2018/05/tesla...
- benj111 7y ago
  Yes I was thinking its basically a big battery/capacitor. I wasn't sure for the intended usecase (compensating for 1000s of MW of powerloss) whether a battery would necessarily be the best thing.