Integrating Protection with Transient Stability
The CAPE-TS Link™ is a simulation tool that gives electric power grid engineers the ability to design and evaluate the fast, wide-area control needed to avoid cascading outages and blackouts. This next-generation program integrates CAPE as the premier existing protection simulation program with conventional transient stability programs using new control and communication models to help analyze and design the smart grid of tomorrow.
Blackouts in the United States like the September 2011 and August 2003 disturbances were exacerbated by protection system relays that tripped electrical facilities. The US North American Electric Reliability Corporation (NERC) and the Federal Energy Regulatory Commission (FERC) concluded that overly conservative relays settings, combined with relay systems operating too quickly, led to large scale outages with millions of people losing power. For the August 2003 event, NERC also concluded that “the relay protection settings for the transmission lines (zone 3 impedance), generator and under-frequency load-shedding in the northeast may not be entirely appropriate and are certainly not coordinated and integrated to reduce the likelihood and consequence of a cascade – nor were they intended to do so.”
We need a means to simulate the bulk electric power system in a way that models the effect of protective relay operations on the dynamic behavior of the system.
To ensure the reliable operation of the electric grid when subject to disturbances, design and analysis of wide-area control and protection algorithms needs tools which use sensor inputs not just locally, like conventional protective relays do, but from multiple locations in the network.
There has not been such a unified tool available until now. It is difficult to model the thousands of local protection relays in a typical transient stability program. Often a stability studies reflect only the action of a few selected relays, and sometimes only manually. On the other hand, the typical software to study protection systems can model thousands of relays but cannot model the dynamic behavior of the power system.
The interdependence of system dynamics and relay actions must be captured if some of these cascading failures are to be accurately predicted and studied. Moreover, the increasing use of special protection schemes and wide-area controls makes it imperative that the available analytical tools be able to adequately model them. The CAPE-TS Link brings these two approaches together.
Solution: how the CAPE-TS Link works
- Siemens PSS®E provides the dynamically changing voltages to CAPE at each time step.
- The CAPE model contains detailed knowledge of the protection system.
- CAPE calculates branch currents using the PSS®E voltages and CAPE's own zero- and negative-sequence networks, and evaluates relay operation. If breakers operate, that information is passed on to PSS®E in that time step.
- The calculation proceeds until the simulation time elapses or the user decides to halt the simulations.
CAPE-TS Link features and results
Combines the detailed protection modeling and simulation environment of CAPE with the transient stability modeling environment of PSS®E.
- Gives planning engineers access to a ready-made protection model, and the ability to use that protection model in their planning studies.
- Obviates the need to model relays and enter their settings in the planning environment.
- Removes the need for stability program vendors to get into the detailed algorithms of relays–the protection model is used for that purpose.
- CAPE’s graphical interface provides updates on the one-line diagram as the simulation progresses
- Creates time-varying plots of voltage magnitudes and angles, frequency, relay apparent impedance, generator rotor angles, etc.
- Includes new (for CAPE) types of relays: Under/Over frequency, out-of-step relaying, loss-of-field, V/Hz, etc.
The CAPE-TS Link provides a simulation platform that allows engineers to study the impact of protective relay switching on the overall dynamic behavior of the system in an integrated and accurate manner. It combines the electromechanical transient stability function with a detailed protection system simulation to yield a result that is indeed greater than the sum of its parts.
CAPE-TS Link allows the study of different contingencies and scenarios, some of which may lead to cascading outages, while others may not. For the ones that do, the engineers will be able to tune the relay settings to confine their effect to a small area. CAPE-TS Link also offers tools to design, model, analyze, and test a large variety of sophisticated next-generation protection and control systems. Such systems will help make the electric grid “smart” – one that almost anticipates developing problems, and takes steps to mitigate them.
CAPE In Action
CAPE-TS Link creates a plot of the voltage magnitude and angle at a bus, showing when the disturbance was applied and when breakers opened. The oscillation in the voltage shows instability after the fault is cleared.
Typical CAPE-TS Link applications
- Conventional planning studies, with full consideration of protective relay behavior.
- Post-mortem analysis of events where protective relays played a part in spreading the blackout.
- Operations planning studies – for example, if transmission facilities (line, generator or transformer) are out of service for maintenance or other reason, and if a fault occurs, are the protective relay settings still able to maintain dependability and security? Furthermore, do relay operations create stability problems for the system?
- Relay settings adjustments to help prevent stability problems while maintaining the ability to operate quickly and securely for faults.
Proven in real-world testing
The U.S. Department of Energy supported the development of the CAPE-TS Link. In its first iteration, we established a link between the Electrocon’s CAPE commercial protective relay simulation program and Siemens PTI’s PSS®E transient stability analysis program.
Using the link, we simulated the effect of protective relay operation on the dynamic behavior of the electric grid. Tests have shown that by modifying relay settings or introducing additional protection components, we can alter system behavior.
Validation testing has been performed to confirm the simulation with actual events, using existing real-world stability and protection models. Both the Eastern and Western US system models have been utilized in CAPE-TS Link testing.