"Can you interpret that for those of us here who are not electrical engineers? Are Greg's problems with wind and solar real, and if so, how bad?"
Sorry, I forgot not everyone is a protection engineer.
In a nutshell, for the last 100 years or so, electricity has been generated using synchronous generators. These are 3 phase machines that produce electricity with control over the voltage as well as the reactive power. Without getting in the weeds, the important thing is that these are generally enormous machines which naturally have a lot of inertia. Consider a large thermal turbine with a shaft 8-18" in diameter, 20+ feet long, spinning at 3000 or 3600 RPM.
These same machines have very predictable behavior. The power generated by a 3 phase machine is balanced, meaning each phase is producing the same quantities when the system is health. Various faults occur on the system all of the time. Think lightning, trees blowing into conductors, etc. When these faults occur, the generators and power system change, instantaneously and become unbalanced. Sustained unbalanced conditions cause other problems for both the customers and other components on the system. It is very important for a number of reasons to detect the fault and quickly isolate the faulted circuit from the rest of the system. If a fault on a HV system persists for too long (think 5 seconds), the rest of the system can become unstable, leading to blackouts, as well as expensive equipment damage. The known behavior of synchronous machines allows protection engineers, who design the protection systems and tune the protection via settings and logic, to accurately model the system, thus being able to accomplish fast and selective tripping of only the faulted circuit and nothing else. The mathematical models used rely on the use of symmetrical components. "Physically, in a three phase system, a positive sequence set of currents produces a normal rotating field, a negative sequence set produces a field with the opposite rotation, and the zero sequence set produces a field that oscillates but does not rotate between phase windings. Since these effects can be detected physically with sequence filters, the mathematical tool became the basis for the design of protective relays, which used negative-sequence voltages and currents as a reliable indicator of fault conditions. Such relays may be used to trip circuit breakers or take other steps to protect electrical systems.
The analytical technique was adopted and advanced by engineers at General Electric and Westinghouse, and after World War II it became an accepted method for asymmetric fault analysis."
Because the protection devices can measure the symmetrical component quantities, they are used to determine the settings of the protection devices. Inverter Based Resources (IBR), which include solar and most modern wind turbines, do not generate electricity in the traditional sense like synchronous machines, but via large inverters. The inverters do not produce the same symmetrical component quantities during fault conditions, thus the protection devices can't see the fault and clear it from the system.
As well during fault conditions, the inertia of all the connected synchronous machines helps the rest of the system ride through faults. The ride through period is sufficient for the generator controls to catch up and respond. IBRs have zero inertia.
Everything happens very quickly. The fastest HV circuit breakers can clear a fault in 2-3 cycles (~33-60 ms). Most modern protection devices can sense a fault and tell the circuit breaker to trip in ~17-20 ms. One leading protection device manufacturer has recently decreased the tripping time to micro seconds vs. milli seconds. This huge decrease in speed is still obviously limited by the circuit breaker time. As we continue to load up the transmission grid and push power across larger distances, the stability of the system decreases.
So IBR's are a real challenge as their penetration grows. If its less than 20% of the total system generation it can be tolerated. As it increases, the problems listed above will become more apparent.
"Build More Transmission Lines" they say. NIMBY is said back. My company and our sister companies design, build and commission HV transmission lines and substations. A typical 345kV line can take between 7-10 years to be complete. 5-8 years is the siting, permitting, legal challenges, etc. A 174 mile we built a few years back took 8 years and over $800M for a single line.
None of this includes the obvious problem of intermittency.
