NERC PRC-024-3: Understanding “Ride Through” Capability and “No Trip Zones”

“Ride through” capability in power systems has become increasingly important in recent years, because it contributes to maintaining grid stability during system disturbances. In this video excerpt from the new Interconnection of Utility-Scale Solar PV to Transmission course, HeatSpring instructor Tim Taylor breaks down the basics of the NERC PRC standard that governs “ride through” requirements, explains the concept of “no trip zones” for both frequency and voltage, and discusses how these standards apply to modern inverter-based generation facilities. 

“ Ride through” is the ability of a gen [generator] facility to stay connected to and synchronized with the transmission system during system disturbances.

So it’s referencing disturbances, such as faults or some other conditions in which system frequency or the voltage has gone well outside of normal tolerances, and there’s a chance without some type of mitigation actions that that disturbance would become even worse.  

The standard defines a “no trip zone”, one for frequency  and one for voltage.

In these phrases, the important words are “does not cause the generating resource to trip or cease injecting current.” You know with inverters, they can not only trip offline, but they also can enter a condition known as momentary cessation. In that state, it will cease injecting current. It’s still connected to the system, but it will cease injecting any type of current into the system.

So if the generating facility enters this “no trip zone” with respect to frequency, then it must stay online. 

It’s the same thing with voltage, as you can see in the second phrase here. Now voltage is a local phenomenon, meaning that it can have different values at different buses or different points on the system that are relatively close together.

Frequency tends – and frequency does – stay the same up and down, say the western interconnection in the U.S. or the eastern connection. Frequency is pretty much constant. Whether you’re looking at southern California or you’re looking at northern Washington, the frequency is the same. 

But with voltage, that is very much a local phenomenon, and as a result, they had to define where the voltage excursion actually occurs. In this definition as you see in the bottom line, the no trip zone during a voltage excursion at the high side of the generator step up (GSU) or the main power transformer (MPT).

This is what the “no trip zone” looks like for voltage. We’ve got time on the horizontal axis, and we’ve got voltage on the vertical axis. You can see that we’ve got this window of voltages as a function of time for which the inverter and the generation may not trip. 

So starting off at very small time periods in the order of cycles up to say a 10th of a cycle or so, if the voltage should stave less than 1.20 – as we can see up here – and greater than, well, practically zero, then it is not allowed to trip and that it must stay connected to the system and injecting current. 

Then as you see with the longer times the “no trip zone” gets more narrow. We can see, for example, at 3 seconds, if the voltage goes below 0.75, then the inverter is allowed to trip. 

This standard is saying that the inverters must have settings programmed into it to follow this behavior. Inverters can be programmed in order to follow these curves.

Looking to learn more? Enroll in the Electric Transmission and Solar PV Interconnection Bundle to gain a better understanding of electric power transmission in the United States and interconnection considerations for utility-scale solar.