Load Flow Analyzer

Positive-Sequence Load Flow

Open the Initializing a 5-Bus Network with the Load Flow Tool of Powergui example to open a model containing five Load Flow Bus blocks and six load flow blocks.

The Load Flow Bus blocks are shown in orange and the load flow blocks are shown in yellow.

The Load Flow Bus blocks specify the bus base voltages (nominal phase-to-phase rms voltage). They also specify the voltage at PV buses or the voltage and angle of the swing buses. Once the load flow is solved, the Load Flow Bus block displays the bus positive-sequence voltage magnitude and phase angle as block annotations.

The bus type (PV, PQ, or swing) is determined by the load flow blocks connected to the bus. If you have several load flow blocks with different types (specified in the Generator type parameter or in the Load type parameter) connected to the same bus, the Load Flow tool determines the resulting bus type (swing, PQ, or PV).

In the Initializing a 5-Bus Network with the Load Flow Tool of Powergui example, the bus types are determined as follows:

Some restrictions apply when you connect several source blocks and synchronous machines at the same bus:

For more information on how to use the Load Flow Bus block in your model, see the Load Flow Bus page.

Using the Load Flow Tool to Perform Load Flow Analysis

Once you have entered the load flow parameters in the Load Flow Bus blocks and in the various load flow blocks, open the Load Flow Analyzer by clicking the Load Flow Analyzer button of the powergui block. The tool displays a summary of the load flow data of the model. The table below shows the data found in the Initializing a 5-Bus Network with the Load Flow Tool of Powergui example.

Note that the table contains seven lines, but there are only six load flow blocks in the model. This is because the bus B25_2 is not connected to any load flow block. Line 5 is added in the table for that particular bus, so that you can see all buses listed together with their bus voltages. This bus will be considered in the load flow analysis as a PQ bus with zero P and Q.

The Block name column identifies the block type. The Block type column displays the bus type of the load flow blocks. The next four columns give the bus identification label, the bus base voltage, the reference voltage (in pu of base voltage), and the voltage angle of the load flow bus where the block is connected. The following columns are the P and Q values specified in the Load Flow tab of the blocks.

The last four columns display the current load flow solution. Because the load flow has not been performed, the columns display zero values.

The load flow parameters in the Preferences tab of the powergui block are used to build the Ybus network admittance matrix and to solve the load flow. The base power is used to specify units of the normalized Ybus matrix in pu/Pbase and bus base voltages. The Initializing a 5-Bus Network with the Load Flow Tool of Powergui model contains five buses; consequently, the Ybus matrix will be a 5-by-5 complex matrix evaluated at the frequency specified by the Frequency (Hz) parameter.

The load flow algorithm uses an iterative solution based on the Newton-Raphson method. The Max iterations parameter defines the maximum number of iterations. The load flow algorithm will iterate until the P and Q mismatch at each bus is lower than the PQ tolerance parameter (in pu/Pbase). The power mismatch is defined as the difference between the net power injected into the bus by generators and PQ loads and the power transmitted on all links leaving that bus.

To avoid a badly conditioned Ybus matrix, you should select a Base power parameter value in the range of nominal powers and loads connected to the network. For a transmission network with voltages ranging from 120 kV to 765 kV, a 100 MVA base is usually selected. For a distribution network or for a small plant consisting of generators, motors, and loads that have a nominal power in the range of hundreds of kilowatts, a 1 MVA power base is better adapted.

To solve the load flow, click the Compute button. The load flow solution is displayed in the last five columns of the table.

To display the load flow report showing power flowing at each bus, click the Report button. Save this report in a file by specifying the file name at the prompt.

The report displays the summary of active and reactive powers, including the total PQ sharing between generators (SM- and Vsrc-type blocks), PQ loads (PQ-type RLC loads and DYN loads), shunt constant Z loads (Z-type RLC loads and magnetizing branches of transformers), and asynchronous machine loads (ASM):

The Load Flow converged in 2 iterations !        
                                                 
SUMMARY for subnetwork No 1                      
                                                 
Total generation :    P=  5.61 MW   Q= 25.51 Mvar
Total PQ load :       P= 13.00 MW   Q=  5.00 Mvar
Total Zshunt load :   P=  0.68 MW   Q= -0.51 Mvar
Total ASM load :      P= -8.90 MW   Q=  4.38 Mvar
Total losses :        P=  0.83 MW   Q= 16.64 Mvar

The Total losses line represents the difference between generation and loads (PQ type + Z type +ASM) and represents series losses. After this summary, a voltage and power report is presented for each bus:

1 : B120  V= 1.020 pu/120kV 0.00 deg  ; Swing bus 
        Generation : P= -114.39 MW Q=   62.76 Mvar
        PQ_load    : P=    0.00 MW Q=    0.00 Mvar
        Z_shunt    : P=    0.25 MW Q=    0.23 Mvar
   -->  B13.8      : P= -116.47 MW Q=   53.89 Mvar
   -->  B25_1      : P=    1.84 MW Q=    8.63 Mvar
                                                  
2 : B13.8  V= 0.980 pu/13.8kV -23.81 deg          
        Generation : P=  120.00 MW Q=  -37.25 Mvar
        PQ_load    : P=    3.00 MW Q=    2.00 Mvar
        Z_shunt    : P=    0.17 MW Q=    0.17 Mvar
   -->  B120       : P=  116.83 MW Q=  -39.42 Mvar
                                                  
3 : B25_1  V= 0.998 pu/25kV -30.22 deg            
        Generation : P=    0.00 MW Q=    0.00 Mvar
        PQ_load    : P=   10.00 MW Q=    3.00 Mvar
        Z_shunt    : P=    0.25 MW Q=    0.21 Mvar
   -->  B120       : P=   -1.83 MW Q=   -8.44 Mvar
   -->  B25_2      : P=   -8.41 MW Q=    5.23 Mvar
                                                  
4 : B25_2  V= 0.967 pu/25kV -20.85 deg            
        Generation : P=    0.00 MW Q=    0.00 Mvar
        PQ_load    : P=   -0.00 MW Q=   -0.00 Mvar
        Z_shunt    : P=    0.01 MW Q=   -0.03 Mvar
   -->  B25_1      : P=    8.87 MW Q=   -3.67 Mvar
   -->  B575       : P=   -8.88 MW Q=    3.70 Mvar
                                                  
5 : B575  V= 0.953 pu/0.575kV -18.51 deg          
        Generation : P=    0.00 MW Q=    0.00 Mvar
        PQ_load    : P=   -0.00 MW Q=   -0.00 Mvar
        Z_shunt    : P=    0.01 MW Q=   -1.09 Mvar
   -->  ASM        : P=   -8.90 MW Q=    4.38 Mvar
   -->  B25_2      : P=    8.89 MW Q=   -3.29 Mvar

For every bus, the bus voltage and angle are listed on the first line. The next three lines give the PQ generated at the bus (all SM and voltage sources), the PQ absorbed by the PQ type loads, and the PQ absorbed by the Z-type loads.

The last lines, preceded by an arrow (-->), list the PQ transmitted to neighbor buses connected through lines, series impedances, and transformers, as well power absorbed by ASM.

Apply the Load Flow Solution to Your Model

When performing a load flow analysis, you may need to iterate on P, Q, and V values until you find satisfactory voltages at all buses. This may require, for example, changing generated power, load powers, or reactive shunt compensation.

To change the load flow setup, you need to edit the parameters of the load flow blocks and the Load Flow Bus blocks. Then click the Update button to refresh the load flow data displayed by the table in the Load Flow Analyzer. The previous load flow solution is then deleted from the table. Click the Compute button to obtain a new load flow solution that corresponds to the changes you made.

Once you have obtained a satisfactory load flow, update the model initial conditions according to the load flow solution. Click the Apply to Model button to initialize the machine blocks of the model, and as the initial conditions of regulators connected to the machines.

Open the Three-Phase Parallel RLC Load block connected at the B13.8 bus. Because the Load type specified in the Load Flow tab is constant PQ, the nominal voltage of this block has been changed to the corresponding bus voltage of 0.98 pu. The Nominal phase-to-phase voltage Vn (Vrms) parameter is set to (13800)*0.98.

Open the Three-Phase Dynamic Load block connected at the B25_1bus. The Initial positive-sequence voltage Vo [Mag(pu) Phase (deg.) is set to [0.998241 -30.2228].

Note that the voltage magnitudes and angles obtained at each bus are written as block annotations under the Load Flow Bus blocks.

Open the Scope block and start the simulation.

The Three-Phase Fault block applies a six-cycle fault at the B120 bus.

Observe the waveforms of the SM active power, SM and ASM speeds, and PQ of DYN load, and notice that simulation starts in steady state.

Example of Unbalanced Load Flow

Open the IEEE 13 Node Test Feeder to open a model containing 12 Load Flow Bus blocks and 13 load flow blocks. This model is a benchmark network taken from the Radial Distribution Test Feeder in the Distribution System Analysis Subcommittee Report by the Power Engineering Society in pages 908–912 written in 2001.

The original benchmark system contains 13 nodes. However, because the IEEE 13 Node Test Feeder model does not include the regulating transformer, it contains only 12 nodes.

The Load Flow Bus blocks are shown in orange and the Load Flow blocks are shown in yellow.

The Load Flow Bus blocks specify the bus base voltages (nominal phase-to-ground rms voltage). They specify the voltage at PV buses or the voltage and angle of the swing buses. Once the load flow is solved, the Load Flow Bus block displays the bus voltage magnitude and phase angle as block annotations.

The bus type (PV, PQ, or swing) is determined by the load flow blocks connected to the bus. If you have several load flow blocks with different types (specified in the Generator type parameter or in the Load type parameter) connected to the same bus, the Load Flow tool determines the resulting bus type (swing, PQ, or PV). The table shows how the bus types are determined for some of the model buses of the IEEE 13 Node Test Feeder example.

Some restrictions apply when you have several source blocks and synchronous machines connected to the same load flow bus:

For more information on how to use the Load Flow Bus block in your model, see Load Flow Bus block.

SUMMARY for subnetwork No 1                                 
                                                            
  Total generation  : P=   3518.74 kW   Q=   1540.14 kvar   
  Total PQ load     : P=   3101.90 kW   Q=   1880.42 kvar   
  Total Zshunt load : P=    363.47 kW   Q=   -479.42 kvar   
  Total losses      : P=     53.36 kW   Q=    139.14 kvar