Distributed Parameters Line

Implement N-phase distributed parameter transmission line model with lumped losses

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Description

The Distributed Parameters Line block implements an N-phase distributed parameter line model with lumped losses. The model is based on the Bergeron's traveling wave method used by the Electromagnetic Transient Program (EMTP) [1]. In this model, the lossless distributed LC line is characterized by two values (for a single-phase line): the surge impedance $Z_{c} = \sqrt{l / c}$ and the wave propagation speed $v = 1 / \sqrt{l c}$ . l and c are the per-unit length inductance and capacitance.

The figure shows the two-port model of a single-phase line.

For a lossless line (r = 0), the quantity e + Z_ci, where e is the line voltage at one end and i is the line current entering the same end, must arrive unchanged at the other end after a transport delay τ.

$τ = {\frac{d}{v}}_{}$

where d is the line length and v is the propagation speed.

The model equations for a lossless line are:

$e_{r} (t) - Z_{c} i_{r} (t) = e_{s} (t - τ) + Z_{c} i_{s} (t - τ)$

$e_{s} (t) - Z_{c} i_{s} (t) = e_{r} (t - τ) + Z_{c} i_{r} (t - τ)$

knowing that

$i_{s} (t) = \frac{e_{s} (t)}{Z} - I_{s h} (t)$

$i_{r} (t) = \frac{e_{r} (t)}{Z} - I_{r h} (t)$

In a lossless line, the two current sources I_sh and I_rh are computed as:

$I_{s}_{h} (t) = \frac{2}{Z_{c}} e_{r} (t - τ) - I_{r h} (t - τ)$

$I_{r}_{h} (t) = \frac{2}{Z_{c}} e_{s} (t - τ) - I_{s h} (t - τ)$

When losses are taken into account, new equations for I_sh and I_rh are obtained by lumping R/4 at both ends of the line and R/2 in the middle of the line:

R = total resistance = r × d

The current sources I_sh and I_rh are then computed as follows:

$I_{s}_{h} (t) = (\frac{1 + h}{2}) (\frac{1 + h}{Z} e_{r} (t - τ) - h I_{r h} (t - τ)) + (\frac{1 - h}{2}) (\frac{1 + h}{Z} e_{s} (t - τ) - h I_{s h} (t - τ))$

$I_{r}_{h} (t) = (\frac{1 + h}{2}) (\frac{1 + h}{Z} e_{s} (t - τ) - h I_{s h} (t - τ)) + (\frac{1 - h}{2}) (\frac{1 + h}{Z} e_{r} (t - τ) - h I_{r h} (t - τ))$

where

$\begin{matrix} Z = Z_{C} + \frac{r}{4} \\ h = \frac{Z_{C} - \frac{r}{4}}{Z_{C} + \frac{r}{4}} \\ Z_{C} = \sqrt{\frac{l}{c}} \\ τ = d \sqrt{l c} \end{matrix}$

r, l, c are the per unit length parameters, and d is the line length. For a lossless line, r = 0, h = 1, and Z = Z_c.

For multiphase line models, modal transformation is used to convert line quantities from phase values (line currents and voltages) into modal values independent of each other. The previous calculations are made in the modal domain before being converted back to phase values.

In comparison to the PI section line model, the distributed line represents wave propagation phenomena and line end reflections with much better accuracy.

Parameters

Number of phases N

Specifies the number of phases, N, of the model. The block icon dynamically changes according to the number of phases that you specify. When you apply the parameters or close the dialog box, the number of inputs and outputs is updated. Default is 3.

Frequency used for rlc specifications

Specifies the frequency used to compute the per unit length resistance r, inductance l, and capacitance c matrices of the line model. Default is 60.

Resistance per unit length

The resistance r per unit length, as an N-by-N matrix in ohms/km (Ω/km). Default is [0.01273 0.3864].

For a symmetrical line, you can either specify the N-by-N matrix or the sequence parameters. For a two-phase or three-phase continuously transposed line, you can enter the positive and zero-sequence resistances [r1 r0]. For a symmetrical six-phase line you can enter the sequence parameters plus the zero-sequence mutual resistance [r1 r0 r0m].

For asymmetrical lines, you must specify the complete N-by-N resistance matrix.

Inductance per unit length

The inductance l per unit length, as an N-by-N matrix in henries/km (H/km). Default is [0.9337e-3 4.1264e-3].

For a symmetrical line, you can either specify the N-by-N matrix or the sequence parameters. For a two-phase or three-phase continuously transposed line, you can enter the positive and zero-sequence inductances [l1 l0]. For a symmetrical six-phase line, you can enter the sequence parameters plus the zero-sequence mutual inductance [l1 l0 l0m].

For asymmetrical lines, you must specify the complete N-by-N inductance matrix.

Capacitance per unit length

The capacitance c per unit length, as an N-by-N matrix in farads/km (F/km). Default is [12.74e-9 7.751e-9].

For a symmetrical line, you can either specify the N-by-N matrix or the sequence parameters. For a two-phase or three-phase continuously transposed line, you can enter the positive and zero-sequence capacitances [c1 c0]. For a symmetrical six-phase line you can enter the sequence parameters plus the zero-sequence mutual capacitance [c1 c0 c0m].

For asymmetrical lines, you must specify the complete N-by-N capacitance matrix.

Note

The powergui block provides the RLC Line Parameters tool, which calculates resistance, inductance, and capacitance per unit of length based on the line geometry and the conductor characteristics.

Line length

The line length, in km. Default is 100.

Measurements

Select Phase-to-ground voltages to measure the sending end and receiving end voltages for each phase of the line model. Default is None.

Place a Multimeter block in your model to display the selected measurements during the simulation.

In the Available Measurements list box of the Multimeter block, the measurement is identified by a label followed by the block name:

Measurement	Label
Phase-to-ground voltages, sending end	`Us_ph1_gnd:`
Phase-to-ground voltages, receiving end	`Ur_ph1_gnd:`

Limitations

This model does not represent accurately the frequency dependence of RLC parameters of real power lines. Indeed, because of the skin effects in the conductors and ground, the R and L matrices exhibit strong frequency dependence, causing an attenuation of the high frequencies.

Examples

The power_monophaseline example illustrates a 200-km line connected on a 1 kV, 60-Hz infinite source.

References

[1] Dommel, H., “Digital Computer Solution of Electromagnetic Transients in Single and Multiple Networks,” IEEE^® Transactions on Power Apparatus and Systems, Vol. PAS-88, No. 4, April, 1969.

Documentation