Wide-area damping control (WADC) is a class of automatic control systems used to provide stability augmentation to modern electrical power systems known as smart grids. Actuation for the controller is provided via modulation of capable active or reactive power devices throughout the grid. Such actuators are most commonly previously-existing power system devices, such as high-voltage direct current (HVDC) transmission lines and static VAR compensators (SVCs) which serve primary purposes not directly related to the WADC application. However, damping may be achieved with the utilization of other devices installed with the express purpose of stability augmentation, including energy storage technologies. Wide-area instability of a large electrical grid unequipped with a WADC is the result of the loss of generator rotor synchronicity, and is typically envisioned as a generator (or group of generators) oscillating with an undamped exponential trajectory as the result of insufficient damping torque.
Rotor Instability Phenomena Large interconnected power systems are susceptible to generator rotor instability, particularly when disparate machine groups are connected to the system through high impedance transmission lines. 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. 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. Two main sources of inter-area modes are identified: 1.) two previously electrically isolated systems which are connected by a single (or several parallel) transmission lines or 2.) increased load and generation in an existing system without increased transmission capability. 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. Small-signal stability considers an electric grid subject to "normal" operating conditions, while transient stability studies the ability of the system to retain stability in the event of a large disturbance (e.g. transmission line fault). While many different features of the electrical grid impact rotor stability (e.g. transmission line congestion, power system stabilizer (PSS) settings, etc.), the WADC architecture introduces sufficient torque to quell the negative effects of resonant systems.
Small-Signal Stability Small-signal rotor stability is the ability of a system to retain synchronicity under ambient perturbation. The system is linearizable under such an assumption, facilitating the application of linear system theory for stability assessment and WADC design. The power transferred between two machines serially connected by impedance X {\displaystyle X} with sending voltage V s {\displaystyle V_{s}} and receiving voltage V r {\displaystyle V_{r}} is given by P = V s V r X sin δ {\displaystyle P={V_{s}V_{r} \over X}\sin \delta } , where δ {\displaystyle \delta } is the difference in internal rotor angle of the two machines. Note that to deliver additional power to a load with constant impedance with fixed sending and receiving end voltage, angular separation must increase. Maximum power is transferred between the machines when δ = 90 o {\displaystyle \delta =90^{o}} ; the two generators lose synchronicity for any angle greater than this value. Nominal operating conditions assume δ < 40 0 {\displaystyle \delta <40^{0}} to ensure a sufficient margin of stability. 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.
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