RLC-Elements
EMT Equations and Modified Nodal Analysis
Inductance
An inductance is described by
vj(t)−vk(t)=vL(t)=LdtdiL(t) Integration results in an equation to compute the current at time t from a previous state at t−Δt.
iL(t)=iL(t−Δt)+L1 ∫t−ΔttvL(τ) dτ There are various methods to discretize this equation in order to solve it numerically.
The trapezoidal rule, an implicit second-order method, is commonly applied for circuit simulation:
∫t−Δttf(τ) dτ≈2Δt(f(t)+f(t−Δt)) Applying the trapezoidal rule to leads to
iL(t)=iL(t−Δt)+2LΔt(vL(t)+vL(t−Δt)) This can be rewritten in terms of an equivalent conductance and current source and the number of time steps k with size Δt.
iL(k)=gLvL(k)+iL,equiv(k−1) iL,equiv(k−1)=iL(k−1)+2LΔtvL(k−1) gL=2LΔt Hence, components described by differential equations are transformed into a DC equivalent circuit as depicted in the figure below.

Capacitance
The same procedure can be applied to a capacitance.
Integration on both side yields
iC(t)=Cdtd vC(t) vC(t)=vC(t−Δt)+C1∫t−ΔttiC(τ)dτ Finally, the equivalent circuit is described by a current source and a conductance.
iC(k)=gCvC(k)+iC,equiv(k−1) iC,equiv(k−1)=−iC(k−1)−gCvC(k−1) gC=Δt2C This equation set is visualized in the figure below.

Hence, the vector of unknowns x and the source vector b become time dependent and this leads to the system description:
Ax(t)=b(t) To simulate the transient behavior of circuits, this linear equation has to be solved repeatedly.
As long as the system topology and the time step is fixed, the system matrix is constant.
Extension with Dynamic Phasors
The dynamic phasor concept can be integrated with nodal analysis.
The overall procedure does not change but the system equations are rewritten using complex numbers and all variables need to be expressed in terms of dynamic phasors.
Therefore, the resistive companion representations of inductances and capacitances have to be adapted as well.
Inductance
In dynamic phasors the integration of the inductance equation yields
⟨vL⟩(t)=⟨LdtdiL⟩(t)=Ldtd⟨iL⟩(t)+jωL ⟨iL⟩(t) ⟨iL⟩(t)=⟨iL⟩(t−Δt)+∫t−ΔttL1⟨vL⟩(τ)−jω ⟨iL⟩(τ)dτ Applying the trapezoidal method leads to the finite difference equation:
⟨iL⟩(k)=⟨iL⟩(k−1)+2Δt[L1(⟨vL⟩(k)+⟨vL⟩(k−1))−jω(⟨iL⟩(t)+⟨iL⟩(k−1)] Solving this for ⟨iL⟩(k) results in the \ac{DP} equivalent circuit model:
⟨iL⟩(k)=1+b2a−jab⟨vL⟩(k)+⟨iL,equiv⟩(k−1) with
a=2LΔt,b=2Δtω ⟨iL,equiv⟩(k−1)=1+b21−b2−j2b⟨iL⟩(k−1)+1+b2a−jab⟨vL⟩(k−1) Capacitance
Similarly, a capacitance is described by as follows
⟨iC⟩(k)=C dtd⟨vC⟩+jωC ⟨vC⟩(t) vC(t)=vC(t−Δt)+∫t−ΔttC1 iC(τ)−jω vC(τ) dτ Applying the trapezoidal rule for the capacitance equation leads to the finite difference equation:
⟨vC⟩(k)=⟨vC⟩(k−1)+2Δt[C1 ⟨iC⟩(k)−jω ⟨vC⟩(k)+C1 ⟨iC⟩(k−1)−jω ⟨vC⟩(k−1)] The DP model for the capacitance is defined by
⟨iC⟩(k)=a1+jb ⟨vC⟩(k)+⟨iC,equiv⟩(k−1) with
a=2CΔt,b=2Δtω ⟨iC,equiv⟩(k−1)=−a1−jb ⟨vC⟩(k−1)−⟨iC⟩(k−1) RL-series element
In dynamic phasors the integration of the inductance equation yields
⟨v⟩(t)=Ldtd⟨i⟩(t)+jωL ⟨i⟩(t)+R ⟨i⟩(t) ⟨i⟩(t)=⟨i⟩(t−Δt)+∫t−ΔttL1⟨v⟩(τ)−jω ⟨i⟩(τ)−LR ⟨i⟩(τ)dτ Applying the trapezoidal method leads to the finite difference equation:
⟨i⟩(k)=⟨i⟩(k−1)+2Δt[L1(⟨v⟩(k)+⟨v⟩(k−1))−(jω+LR)(⟨i⟩(k)+⟨i⟩(k−1))] Solving this for ⟨i⟩(k) results in the \ac{DP} equivalent circuit model:
⟨i⟩(k)=(1+Ra)2+b2a+Ra2−jab⟨v⟩(k)+⟨iequiv⟩(k−1) with
a=2LΔt,b=2Δtω ⟨iequiv⟩(k−1)=(1+Ra2)+b21−b2−j2b+2Ra+(Ra)2−j2Rab⟨i⟩(k−1)+(1+Ra)2+b2a+Ra2−jab⟨v⟩(k−1)