7.2 Eigenvalue analysis and participation factors

Eigenvalue analysis and participation factors are crucial tools for understanding power system stability. They help engineers identify critical oscillatory modes, assess damping, and pinpoint which components contribute most to system behavior.

These techniques allow us to predict how changes in system parameters affect stability. By calculating eigenvalues, eigenvectors, and participation factors, we can design better control strategies and improve overall power system performance.

Eigenvalues and eigenvectors of power systems

Computing eigenvalues and eigenvectors

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• Eigenvalues and eigenvectors characterize the dynamic behavior of a matrix, such as a linearized power system model
• Eigenvalues provide information about the stability and damping of the system's oscillatory modes
• Eigenvalues are calculated by solving the characteristic equation $det(A - λI) = 0$, where $A$ is the state matrix, $λ$ represents the eigenvalues, and $I$ is the identity matrix
• Eigenvectors are non-zero vectors that, when multiplied by the state matrix $A$, result in a scalar multiple of themselves, i.e., $Av = λv$, where $v$ is an eigenvector and $λ$ is the corresponding eigenvalue

Interpreting eigenvalues and eigenvectors

• The right eigenvector represents the mode shape or relative activity of state variables in a particular mode
• The left eigenvector represents the contribution of each state variable to the mode
• Eigenvalues and eigenvectors can be computed using numerical methods (QR algorithm, Arnoldi iteration)

Eigenvalue significance for stability

Eigenvalue properties and stability

• The real part of an eigenvalue determines the damping of the corresponding oscillatory mode
  • Negative real parts indicate stable modes
  • Positive real parts indicate unstable modes
• The imaginary part of an eigenvalue determines the frequency of oscillation of the corresponding mode
• The damping ratio ($ζ$) of a mode can be calculated from the eigenvalue as $ζ = -σ / √(σ^2 + ω^2)$, where $σ$ is the real part and $ω$ is the imaginary part of the eigenvalue
  • Damping ratio of 0 indicates undamped oscillations
  • Damping ratio of 1 indicates critically damped behavior
  • Damping ratios between 0 and 1 result in underdamped oscillations
  • Damping ratios greater than 1 result in overdamped behavior

Time constants and stability margins

• The time constant ($τ$) of a mode represents the time required for the mode to decay to 37% of its initial amplitude and can be calculated as $τ = -1/σ$
• Eigenvalues close to the imaginary axis indicate poorly damped modes that may lead to sustained oscillations or instability in the power system

Participation factors for mode analysis

Calculating participation factors

• Participation factors measure the relative contribution of each state variable to a specific oscillatory mode and the influence of each mode on the state variables
• The participation factor $p_{ki}$ of the $k$-th state variable in the $i$-th mode is calculated as the product of the $k$-th element of the $i$-th right eigenvector ($v_{ki}$) and the $k$-th element of the $i$-th left eigenvector ($w_{ik}$), i.e., $p_{ki} = v_{ki} * w_{ik}$
• Participation factors are normalized such that the sum of the participation factors for each mode equals 1, i.e., $Σ_k p_{ki} = 1$

Interpreting participation factors

• State variables with high participation factors for a specific mode are more strongly associated with that mode and have a greater influence on its behavior
• Participation factors can identify the critical state variables and the corresponding components (generators, controllers) that significantly contribute to a particular oscillatory mode

Eigenvalue sensitivity to parameters

Calculating eigenvalue sensitivities

• Eigenvalue sensitivity analysis assesses the impact of changes in system parameters or operating conditions on the eigenvalues and the stability of the power system
• The sensitivity of an eigenvalue $λ_i$ to a parameter $α$ can be calculated as $∂λ_i / ∂α = (w_i^T * (∂A/∂α) * v_i) / (w_i^T * v_i)$, where $w_i$ and $v_i$ are the left and right eigenvectors corresponding to $λ_i$, and $A$ is the state matrix

Applying eigenvalue sensitivities

• Eigenvalue sensitivity can identify the most influential parameters or operating conditions that affect the stability of specific modes
• High sensitivity values indicate that small changes in the corresponding parameter or operating condition can significantly impact the eigenvalue and the associated mode's stability
• Sensitivity analysis can guide the design of control strategies (power system stabilizers, FACTS devices) to enhance the damping of critical modes and improve overall system stability
• Eigenvalue sensitivity can assess the robustness of the power system's stability under various operating scenarios and determine the stability margins with respect to key parameters