Nick Abi-Samra, EPRIsolutions
Recent transmission system blackouts have emphasized the importance of accurately representing electric power generator capabilities in system models. As a result, many domestic and international system operators and reliability councils are strengthening requirements to test generators on a regular basis to verify their capabilities, to confirm proper dynamic performance of their controls, and to update computer models.
The Planning Standards Subcommittee of the North American Electric Reliability Council (NERC) has developed a set of standards that includes requirements for periodic testing of main generator controls (excitation systems, power system stabilizers, governors) and validation of the mathematical models that are used to represent these systems. But, with the restructuring of the electric power industry, the responsibility for compliance with these requirements has been delegated to individual generation owners.
Specifically, the data to be verified include steady-state and dynamic data for both generator and control-system models. According to Sections IIA and IIB of the NERC Planning Standards: steady-state data include generator minimum and maximum ratings, regulated bus and voltage set-point; dynamic data include unit-specific data for generators, excitation systems, voltage regulators, turbine-governor systems, and power system stabilizers.
Benefits of generator testing and modeling
While generator manufacturers may provide “typical” design data and/or data that is valid within certain bands, generator testing provides real-world data to improve models for simulation studies, which can significantly improve transmission system operation and planning analyses. And further, generator testing and modeling benefit generating plant management by identifying equipment problems and by allowing operators to tune the control systems to respond to power system conditions.
As with generators, the development of accurate models for generator control systems is a key step in establishing operating-security limits and in simulating and understanding the operation of the system during disturbances. With better models for both generators and their control systems, system operators and planners can increase their accuracy in predicting system performance under various operating conditions, including such major cascading events as the 2003 Northeast blackout.
Generator tests include both steady-state and dynamic tests. The steady state tests include open circuit tests (the generator at rated speed and varying output voltage but not connected to the system) and online measurements, and they are used to derive steady-state generator model parameters. Dynamic tests involve applying controlled minor disturbances to the generator and its control system and recording the corresponding responses. These are used to derive dynamic model parameters. This process augments the component-by-component testing that is done routinely for plant protection and control systems. The most common transient response tests are load rejection tests and step changes to the reference voltage of the voltage regulator.
Because there are a variety of types of generators in the field, and their interactions with the power system involve many complex phenomena under various operating conditions, the testing and modeling of generators and their control systems is complex. As such, the testing team must establish a good test plan.
To prepare the test plan, a team that includes people from the plant, transmission system planning, and operation should be formed. The team prepares a test plan that includes description of each test, circuit and measurement location, plant conditions for each test step, and numerous “what-if” situations. Before performing the test, the team needs to calibrate the test equipment, which normally consists of off-the-shelf analog-digital recorders, specialized measurement sensors, signal-conditioning circuits, and laptop computers with software that processes the raw test data.
The testing is usually performed off peak and usually takes only few hours to complete for each generator, not counting equipment setup. Once the testing is completed, the test engineers analyze the data to determine generator performance or derive model parameters for the generator and its control systems. For generators this is done by choosing parameter values for industry standard circuit models. These parameters are tested by simulating the tests and are varied until the simulated output signals from the generator mathematical model match as closely as possible the actual (measurement) test measurements. EPRIsolutions’ software package for synchronous machine parameter derivation is a powerful tool that can help utilities derive model parameters based on the test data.
Abi-Samra, EPRIsolutions senior technical director, is responsible for generator testing and generator performance certification at the EPRI family of companies and is an expert in system planning. He can be contacted at 650-855-1022 or nabisamr@EPRIsolutions.com.