NERC PRC-019-2: Coordination of Generator Capabilities, Voltage Regulating Controls, and Protection
Keeping protection out of the generator's normal operating range
What Is PRC-019-2?
A generator's protection system is designed to trip the unit when something is wrong. The problem is that without careful coordination, a relay can be set in a way that also trips the unit when nothing is wrong: when the generator is operating perfectly normally near the edge of its reactive capability. PRC-019-2 exists to prevent that. It requires generator owners to verify that the three layers governing generator reactive operation (equipment capability, the control limiters, and the protective relay settings) are consistent with each other and operate in the right order.
When these three layers aren't coordinated, a protection element such as a loss-of-field relay can operate before the generator's under excitation limiter. That means tripping a generator that is providing reactive support to the grid at exactly the time the system needs it. PRC-019-2 addresses this directly.
The standard applies to both the leading (underexcited) and lagging (overexcited) ends of the capability curve, and covers multiple relay functions that operate based on the generator's reactive output.
It doesn't require a facility has specific limiters or protection, but if they do, limiters must operate before protection, and protection must operate before capability.
Who Must Comply?
PRC-019-2 applies to Generator Owners (GO) of synchronous generating units that are connected to the BES and have an automatic voltage regulator (AVR) in service. The standard also extends to certain inverter-based resources that have plant-level volt/var control functions with capability limits. Generally, generators and synchronous condensers are applicable if above 20MVA and directly connected to the BES, or if a plant aggregate capacity directly connected to the BES is above 75MVA.
The study must be updated whenever any of the three layers changes, including AVR replacements, applicable relay/control setting changes, or changes to equipment capability.
- Synchronous generators (steam, gas, hydro, nuclear) with AVR in service
- Synchronous condensers used for reactive support
- IBRs with plant-level volt/var control and defined reactive capability limits
Key Requirements
Capability Curve Documentation
The generator's actual reactive capability curve must be documented at the generator terminal voltage. The curve is the foundation of the coordination: relay and control limits must be coordinated with it.
AVR Limit Coordination
The underexcitation limiter (UEL) and overexcitation limiter (OEL) settings in the AVR must be set to operate before protection, keeping the control system from driving the generator into a region where protection would operate.
Protective Relay Coordination
Loss-of-field, underexcitation, and overexcitation protection elements must be set to operate before equipment capability limits.
Compliance Documentation
A compliance study must be produced showing the three-layer coordination graphically and numerically.
Common Compliance Challenges
Outdated or Missing Capability Curve Data
Many generators don't have a current, tested capability curve. Original OEM curves may date back to commissioning and don't reflect decades of wear on the rotor, stator, and cooling system. Without accurate curve data, the coordination study is built on a shaky foundation.
Compaison of different manufacturer's protection and control equipment
Performing a PRC-019 assessment requires that you develop and compare curves from multiple different suppliers, often requiring extensive review of documentation from a number of manufacturers.
AVR Replacements That Reset the Coordination
When a new AVR is installed, the default settings from the OEM almost never satisfy PRC-019-2. The UEL and OEL need to be tuned against the specific machine's capability curve and relay settings before the unit goes back into service. This step is frequently missed.
Voltage Basis Differences Between Relay and AVR
The AVR operates on generator terminal voltage, but some relay settings are referenced to a different point. Coordination comparisons must be performed on a consistent voltage basis, and translating between them requires knowledge of the transformer configuration and CT/VT ratios.
How TWC Can Help
TWC performs PRC-019-2 coordination studies for generator owners across North America, from capability curve development through audit-ready documentation.
Three-Layer Coordination Analysis
We overlay the capability curve, AVR limits, and protective relay characteristics on a consistent basis to identify any regions where coordination doesn't hold, and recommend specific setting changes to correct them.
AVR and Relay Setting Recommendations
Where settings need to change, we develop specific recommendations for UEL, OEL, LOF, and overexcitation protection settings that satisfy the coordination requirements while maintaining adequate machine protection.
Post-Upgrade Compliance Verification
After an AVR replacement or relay upgrade, we perform a fresh PRC-019-2 coordination review to confirm the new installation meets the standard before the unit returns to service.
Audit-Ready Documentation
We document the assessment above in a clear, audit-ready report with references to the data sources used. Our workflow requires that each data source we use has a highlight in our records in the case of an audit.
Need a PRC-019-2 Assessment?
Contact us to discuss your facility's coordination requirements.
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