Fault Induced Delayed Voltage Recovery, or FIDVR, occurs when thousands of air conditioners simultaneously reaccelerate after a momentary voltage depression caused by a three phase fault. In worse case conditions, an FIDVR could create a wide area blackout.
This important issue needs to be resolved before renewable energy, with minimal excitation energy production, becomes the workhorse of the electric energy industry. There are several steps that need to be taken to resolve this issue. Read on to learn more about FIDVR parameters and analysis.
FIDVR Basic Parameters
Air conditioner compressors are positive displacement pumps that force evaporated gas into a condenser to change the state of the refrigerant from gas to liquid. Fans and pumps, on the other hand, are variable displacement devices that load and unload in response to voltage variations. While fans and pumps simply slow down when voltage dips occur, air conditioners are subject to stall, which leads to FIDVR events.
Typical air conditioner compressor motors rotate at 1750 RPM when connected to a 60 Hertz power supply. This means that an air conditioner motor completes one revolution every 34 milliseconds.
When faults occur, voltage is depressed until high speed protective relays actuate to open circuit breakers, recovering full voltage. The result is that electric energy transferred to a compressor motor can be interrupted for 66 milliseconds or more, until the fault is cleared, and voltage recovers. During the 66 milliseconds, air conditioners stall, and then require additional energy to reaccelerate. The simultaneous reenergization of many air conditioning units across the area impacted by the fault could lead to a wide area blackout.
3 Important Variables
The three variables that increase the possibility of FIDVR are:
Proximity of excitation energy sources to air conditioners.
Number of air conditioners that are operating.
Geographic spread of depressed voltage during a fault.
Air conditioners will stall when these three variables negatively coincide and a three phase fault occurs anywhere on a 230 KV, 345 KV, 500 KV, or 765 KV facility. This can lead to FIDVR, and potentially to a wide area blackout if excitation energy is insufficient for system conditions. Let’s explore these three variables in more detail.
1. Proximity of Excitation Energy Sources
Excitation energy, produced by generators and measured in vars, is needed for transformers, transmission lines, distribution lines, and motors to perform properly. Unlike electric energy, measured in watts, excitation energy is not transportable across wide areas.
This is complicated by the fact that excitation energy sources have been minimized as electric systems grew. For example, a 100 MVA generator installed in the 1950s could produce 80 MW and 60 MVAR. A 100 MVA generator installed in the 1980s could produce 90 MW and 43 MVAR. A 100 MVA nuclear powered generator that was uprated in the 1990s could produce 95 MW and 31 MVAR.
A wind turbine generator installed in 2022 may only produce MW, and not produce any excitation energy. This is problematic because the system needs dynamic vars to recover after a three phase fault.
2. Number of Air Conditioners Operating
FIDVR is a seasonal concern. In the spring, only a minimal number of air conditioners are operating. During the summer, almost all air conditioners are operating; peak load days typically occur in June, July, and August.
If peak hourly load is 50,000 MW in April and 100,000 MW in August, it is reasonable to theorize that air conditioners represent 50% of peak load. If a three phase fault occurs on a 230 KV, 345 KV, 500 KV, or 765 KV facility during a peak load day, 50% of the air conditioners located in the area with less than 70% voltage will stall (see Required Action 1, below, for more information on 70% voltage area calculations).
This will instantaneously increase watt load in the study area by 25% and var load by 500% until air conditioners reaccelerate or trip offline. In contrast, when a three phase fault occurs on a 230 KV, 345 KV, 500 KV, or 765 KV facility during light load days, few air conditioners will be operating and FIDVR will not occur.
3. Geographic Spread of Depressed Voltage
This variable is harder to ascertain because voltage dips are calculated using short circuit models that are not linked to load flow models. Though not common practice, electrical engineers could determine the geographic spread of depressed voltage by overlaying short circuit voltage calculations on satellite maps that show the location of customer loads.
In metropolitan areas, short, parallel transmission lines amplify the extent of the depressed voltage because the system impedance is reduced every time a transmission line is added.
The location of every substation, generating station, and transmission line in the United States is easily available online. The impedance of transmission lines can be easily calculated since it is a function line length. This online data is quite helpful when determining the geographic spread of depressed voltage.
Required Actions to Mitigate FIDVR Events
It’s important to remember that our efforts to expedite recovery from fault induced depressed voltage will be inexact, and that finely tuned calculations with three digits after the decimal point are overkill. Nevertheless, the following actions are appropriate to mitigate FIDVR events:
Develop bifurcated calculations to understand FIDVR. Traditional load flow calculations have been prepared since computer-based models have been in use. The bifurcated calculation should use the same impedance model but include reaccelerating air conditioners in the geographic area where voltage is less than 70%.
During peak load conditions, dispatch generators are based on FIDVR calculations, rather than traditional least cost calculations.
Install power factor biased, undervoltage load shedding schemes that actuate to shed accelerating air conditioners during FIDVR conditions.
Install solenoid series reactors (SSRX) to minimize the duration of the voltage dips during three phase faults to less than 16 milliseconds.
By implementing these actions, the risk of FIDVR events will be eliminated.
Tune in next week, and for the first quarter of 2022, when we’ll be discussing next generation concepts often overlooked by utilities as they focus on traditional topics.
Visit Prescient’s website for more information on Prescient’s wide area blackout risk assessment service, or to learn more from our blackout blog collection. Contact us to continue to discussion on next generation power system concepts.