| Symbol | Variable |
|---|
| Network Parameters | |
| ik, Ik | Current injection at node k |
| vk, Vk | Voltage at a node or a bus k |
| d, q (subscript) | Direct and quadrature axes components of a variable, respectively |
|
| Internal generator EMF in magnitude and angle of generator i (with respect to a synchronous reference frame) |
| P, Q | Real and reactive powers, respectively |
| Y | Short-circuit admittance matrix (usually expressed to the internal generator nodes) |
| Yij | The ijth element of Y |
| Gij, Bij | Real and imaginary components of Yij |
| Cij, Dij | EiEjBij, EiEjGij |
| δij |
|
| ω |
|
| ωR | Reference or base value of ω |
| * (superscript) | Conjugate of a variable |
| Center of Inertia Variables | |
| COI | Center of inertia |
| δ0 | Center of inertia angle |
| ω0 |
|
| θi | Angle of generator i with respect to the COI
|
|
|
|
| PCOI | COI accelerating power |
| Generator Parameters | |
|
| d–and q–axes transient reactances |
| H | Stored kinetic energy, per MVA, at rated speed |
| M |
|
| xd, xq | Synchronous d–and q–axes reactances |
|
| Open circuit d–and q–axes time constants |
| E′ | Internal generator EMF (two-axis model) |
|
| d–and q–axes components of E′ |
| EFD | Field voltage converted to an equivalent stator EMF |
| Pm | Mechanical power |
| Pe | Electrical power |
| IGL | Current injection at generator node due to nonlinear loads |
| D | Damping factor in per unit (pu) |
| Transient Energy Function Variables |
θcl,
| Angle and speed at fault clearing (COI frame) |
| SEP, θs | Stable equilibrium point (usually for the postdisturbance network) |
| UEP, θu | Unstable equilibrium point |
| V | Transient energy function (TEF) |
| VPE, VKE | Transient potential energy, transient kinetic energy |
| cr, sys (subscripts) | Refer to the critical group of generators and the rest of the generators in the system respectively |
| corr (subscripts) | Corrected (usually used with VKE) |
| ΔV, ΔVPE | Transient energy margin, potential energy margin |
| ΔVn | Normalized energy margin (ΔV / VKEcorr) |
| MOD | Mode of disturbance; identifies the severely disturbed generators, usually their angles in the UEP are advanced (>90°) |
| Iij | A term in V obtained from a linear trajectory approximation of a path–dependent integral |
