Grid oscillation

The grid oscillations are oscillations in an electric grid manifesting themselves in low-frequency (mostly below 1 Hz) periodic changes of the power flow. These oscillations are a natural effect of negative feedback used in the power system control algorithms. During the normal operation of the power grid, these oscillations, triggered by some change in the system, decay with time (are "damped" within few tens of seconds), and are mostly not noticeable. If the damping in the system is not sufficient, the amplitude of oscillations can grow eventually leading to a blackout.^[1]

Oscillation on the graph of a power flow of one transmission line during the 10 August 1996 blackout. Undampened oscillations around the 750 seconds mark are leading to system collapse, while a similar oscillation triggered around the 400 second mark had decayed as it should under normal conditions

For example, shortly before the 1996 Western North America blackouts the grid after each disturbance was oscillating with a frequency of 0.26 Hz for about 30 seconds. At some point a sequence of faults and operations of automatic protection relays caused loss of damping, eventually breaking the system into disconnected "islands" with many customers losing power.^[2] The other notable events involving oscillations were the Northeast blackout of 2003 and the 2009 subsynchronous oscillations in Texas.^[3]

While the theory and calculations tools for analyzing oscillations are available, pinpointing the source of instability in a real grid is frequently difficult as of the early 2020s. The oscillations are a normal occurrence, yet the difference in a flow as small as 10 MW is known to occasionally push the system from the stable mode with decaying oscillations into a situation where their amplitudes grow with time. The system operator frequently gets no warning that the grid is close to its damping limit.^[4]

Terminology

North American Electric Reliability Corporation suggested the following classification for the grid oscillations:^[5]

• System (Natural): low-frequency changes in the rotor angle triggered by power imbalance:
  • Local: oscillations of a generator caused by heavy load interacting with the generator control
  • Intra-plant: oscillations between units within a power plant caused by unit control interactions and poor control settings
  • Inter-area: oscillations between few coherent parts of the system due to weak inter-area tie lines (0.15–1.00 Hz)
  • Torsional: relatively high (but still below line frequency, "subsynchronous") due to resonance between transmission lines with high level of voltage compensation and the mechanical resonances of turbine generators (also known as subsynchronous resonance, 5.00–50.00 Hz)
• Forced: oscillations due to externally injected forced, like faulty equipment.

History

The oscillations are inherent in a synchronous electrical power system. Oversimplified, a synchronous generator behaves like a pendulum, with synchronizing torque playing the role of gravity by pulling the machine into synchronicity, and the inertia of the rotor causing the generator to overshoot the ideal synchronized rotor angle.^[6] The oscillations were therefore immediately discovered once multiple generators were connected in parallel to increase power and reliability. At the time, the involved generators were located close together, the oscillation frequencies were on the order of 1-2 Hz, and a damper winding was added to the generator design in order to absorb the energy of oscillations.^[2]

As the power systems grew in size, rapid automatic voltage control was introduced. The fast feedback of these systems had a side effect of lower damping, so power system stabilizers (PSS) were added to damp the oscillations. In the 1950 and 1960, the electric power industry consolidated the grids into larger and larger ones for reliability and savings of scale. However, oscillations became a major issue, and some attempted interconnections were actually abandoned until asynchronous means of connecting systems arrived in the form of HVDC links.^[2]