Sequential Load Flow Solution Method for HVDC Links Embedded in AC Power Systems — Version 2.1

This demo illustrates a sequential AC-DC load flow method involving iteration between DC and AC solutions. The AC load flow is performed by the load flow tool of the Powergui. The DC load flow solution is computed running a simulation (see Link_A model) using the HVDC Link Steady-State block that models the HVDC Link in steady-state operation. It is now possible to include a second HVDC link in the network (see Link_B model).

The AC network used in this demo is based on the 29-bus system of the power_LFnetwork_29bus model. A DC link has been added between LG27 bus (rectifier) and MTL7 bus (inverter).  The DC network is based on the power_hvdc12pulse_average model.

 A script LF_ACDC_solve_V2_1 controls the whole process and iterates until the convergence criteria for the AC and DC load flows have been reached simultaneously. The parameters used by the script are defined in the block Sequential Load Flow Param embedded in the network model. The script is called by the block’s ‘Compute’ button.  The load flows results are printed in report file(s) specified by the user.

Once the AC-DC network has been initialized with the sequential solution method, you can run a simulation and observe that the specified power flow settings are respected in steady-state.

Silvano Casoria, Gilbert Sybille (Hydro-Quebec)

 

Contents

                 

Method Description

 

Demonstration

 

Method Description

Open the LF_AC29bus_HVDCdemo_V2_1 model. The HVDC system transmitting 1000 MW at 500 kV through a 800 km line consists of two converter units (YYD transformer and 12-pulse thyristor bridge) represented by their average model and 600 Mvar AC filters. The 29-bus AC system uses a 735-kV series and shunt compensated transmission system and seven 13.8-kV power plants (total generation capacity of 27 000 MW) including detailed modeling of turbines, speed regulation, excitation systems and PSS. Please, refer to these demos for a detailed description.

The block Sequential Load Flow Param is embedded in the network model. The block specifies the parameters used by the sequential AC-DC load flow algorithm having the form of a script (LF_ACDC_solve_V2) called by the block. Click Help for more details.

 

For the AC load flow solution the converter units loads are represented by PQ-type loads modeled by the block Three-Phase Dynamic Load (see magenta blocks LG27_DC and MTL7_DC).

 

For the DC load flow solution, the HVDC link (Link A) is defined by its steady-state model (block HVDC Link steady-State) embedded in the Simulink model Link_A. The solution provides the converters active and reactive power to update the PQ-type loads connected at the converters buses. The converters AC voltages, inputs to the block, are updated by the AC load flow solution. Click Help for a description of the model and its parameters.

The script calls in sequence the DC and the AC load flow programs printing their intermediate results in report files named by the user (ACDC_report_A.txt). The DC and AC load flows will converge simultaneously when the converter tap changer ratios remain unchanged during two consecutive DC solutions. The PQ loads representing the converter units loads during load flow computation are disabled by setting their P and Q values to practically zero.  Finally, the adequate HVDC model variables (e.g. the transformers tap ratios) are updated to their steady-state values obtained from the solution.

 

Demonstration

In this example, the load flow solution takes in consideration a typical tap changer control and the DC power as the rectifier main control mode The DC power is set at 1000 MW to be received at the inverter (the rectifier is chosen to be the slack station). The auxiliary mode, when there is a dip in AC voltage would be the alpha minimum mode, set to 5°. At the inverter, the main control mode is the DC voltage, set at 500 kV. The auxiliary mode, when there is a dip in AC voltage would be the minimum extinction angle gamma set to 17°. The tap changer control is set to hold α angle between 14° and 17° at the rectifier and the gamma angle between 20° and 23° at the inverter. The initial tap ratios are set to 1.0 (pu) and the initial AC voltages to 735 kV.

Start solving the sequential load flow by opening the Sequential Load Flow Param block and click Compute. Once solved, open the report file ACDC_report_A.txt to see the results. Three DC load flows and two AC load flows were necessary to obtain the solution. Note that, the used HVDC average model control system lacs a tap changer control and only the current control mode is present. Thereafter, the resulting DC current will be transcribed as the DC current reference value in the Master control model. The final Tap ratios (tap_pu_R, tap_pu_I) values are transcribed in both converter unit models.

The resulting AC and DC load flow values are printed below:


--> Simultaneous convergence after 3 DC load flow(s) and 2 AC load flow(s)

Rectifier AC voltage:  740.89 kV

Inverter  AC voltage:  758.83 kV

Rectifier: Tap = 0.9375 (pu), alpha = 16.15 (deg), Udi0 = 290.41 (kV)

Inverter:  Tap = 0.9750 (pu), gamma = 22.80 (deg), Udi0 = 286.00 (kV)

 

Rectifier: Vdc = 524.63 (kV), Idc = 2.0024 (kA), Pdc = +1050.52 (MW)

Inverter:  Vdc = 499.40 (kV), alpha = 144.58 (deg), Pdc = -1000.00 (MW)

 

Bus LG27: P = +1055.87 MW, Q =  479.88 Mvar

Bus MTL7: P = -994.65 MW, Q =  563.34 Mvar

 

Start simulation. At start-up the DC link is not in operation and the power is ramped up to its steady state value at 0.04 (s). Consequently the AC network initial power flow must readjust since the load flow solution was solved with the DC link in service. Once the DC link power is established the AC power flow should readjust again to the desired power flow.

Check the DC waveforms (voltages, powers, and angles) and verify that the steady-state results correspond to the load flow solution. Note that the observed DC voltages are measured at DC line ends (i.e. VdL = 524.0 kV at rectifier) and that the DC voltages given by the load flow solution are those at the converter terminals (i.e. Vdc = 524.63 kV at rectifier).

Finally, you can apply a 6 cycles three-phase fault at the inverter bus (MTL7) to observe the dynamic behaviour of the AC and DC systems. You will need to set the fault application time to 2 (s).

MATLAB release MATLAB 9.1 (R2016b)