However, damping may be achieved with the utilization of other devices installed with the express purpose of stability augmentation, including energy storage technologies.
[1] Previously unaccounted for load growth, transmission lines operating closer to rated capacity, connecting two previously electrically isolated subsystems by a single transmission line, and increased renewable resource penetration increase the possibility of lightly-damped oscillations.
[2] While several causes of resonance exist in electrical grids, inter-area oscillations pose the greatest threat to wide-spread breakup leading to substantial power outages.
Both of these conditions continue to be imposed on most large interconnected systems transitioning to the smart grid architecture.
Rotor instability phenomena may be studied by considering two different disturbance types: small-signal and transient.
Note that to deliver additional power to a load with constant impedance with fixed sending and receiving end voltage, angular separation must increase.
[1] An ever increasing load with fixed power system equipment (e.g. transmission line impedances constant) force electrical grid to operate closer to unacceptable rotor angle ranges.
This has the effect of diminishing safety margins for the continuous operation of the system, warranting the implementation of a WADC.
The swing equation establishes a second-order differential relationship that may be solved analytically or using the equal-area criterion (EAC) for a single-machine infinite bus (SMIB) system.
Transient stability considers contingent events of substantial impact on the system that linearization fails to accurately represent the dynamics of interest, including generator trips and transmission line faults.
The result of transient analysis provides an indication of whether or not the generators, when perturbed substantially (i.e. allowed to accelerate/decelerate due to power imbalance), will eventually decelerate/accelerate back to an equilibrium point within reason.
When dealing with large multi-machine systems, analytical solution is intractable and stability assessment must be transitioned to a nonlinear numerical integration platform.
To enhance the rotor stability of a modern electrical grid, various methods to provide damping have been considered for WADCs.
[5] Since nominal PSS control design servos based on perturbations in rotational velocity, sensors in addition to a standard rotary encoder are required.
Modulation of active power between several coherent generator sets is a common approach to damping inter-area oscillations.
[6] By modulating the active power shared between converter stations, substantial positive impact may be realized by employing such equipment.
[7] While providing active power (and hence damping torque) in a similar fashion to HVDC, energy storage devices are limited by capacity and expense.
However, large scale aggregation and coordination of electric vehicles battery discharging in vehicle-for-grid (V4G) scheme can overcome such limitations.
Static VAR compensators (SVCs) and other reactive devices are also used as actuators in wide-area damping control.
While local caesium atomic clocks offer the highest accuracy time fidelity, GPS technology allows continued synchronicity of measured control feedback signals as they are sent to the aggregated WADC processing center.
These modern sensors provide sufficiently high reporting rate and minimal measurement error required for high-performance control systems.