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Load Flow Bus

Identify and parameterize load flow bus

Library

Measurements

Description

The Load Flow Bus block is used in three-phase models to specify the bus locations and parameters in order to solve a load flow. The load flow is solved by the Powergui Load Flow tool. You normally connect this block on any phase (A, B, or C) of a three-phase load flow block. Load flow blocks are SimPowerSystems™ blocks where power and voltage can be specified. They are:

  • Asynchronous Machine

  • Simplified Synchronous Machine

  • Synchronous Machine

  • Three-Phase Dynamic Load

  • Three-Phase Parallel RLC Load

  • Three-Phase Programmable Voltage Source

  • Three-Phase Series RLC Load

  • Three-Phase Source

Once the load flow is solved, the bus voltage magnitude and phase angle are displayed below the Load Flow Bus blocks as block annotations. The Load Flow tool lists, for each bus, the active and reactive power transferred to other buses.

When several load flow blocks are connected together at the same bus, only one Load Flow Bus block is required. You can also connect the Load Flow Bus block at a location where you are interested in monitoring the load flow, even if no load flow blocks are connected at that location. In such a case, the Load Flow Bus block must be connected to any phase of a three-phase block (Three-Phase Breaker, Three-Phase PI Section Line, Three-Phase Transformer, and so on).

Implicit Buses

If you omit to connect a Load Flow Bus block to a load flow block, the Load Flow tool will automatically define an implicit (internal) load flow bus for that block. The bus base voltage of this implicit bus is set to the same value as the nominal voltage of the load flow block. When several load flow blocks are connected together, one nominal voltage is arbitrarily selected among the blocks.

Although the Load Flow tool can perform load flow on a model with no Load Flow Bus block in the model (working only with implicit buses), the recommended practice is to connect a Load Flow Bus block everywhere a load flow block exists.

Dialog Box and Parameters

The Load Flow Bus parameters are used for model initialization only. They have no impact on the simulation performance.

The block dialog box contains two tabs:

Parameters Tab

Bus Identification

Enter a meaningful alphanumeric string. The Bus Identification string appears below the block as a block annotation.

Base voltage (Vrms phase-phase)

The base voltage is usually the same as the nominal voltage of the load flow blocks connected to the Load flow Bus block. The Base voltage values appears below the block as a block annotation.

Swing bus or PV bus voltage (pu)

Specify the required bus voltage magnitude, in pu. Depending on the bus type of the load flow blocks connected at that bus, this voltage corresponds to the swing bus voltage or the PV bus voltage.

Swing bus voltage angle (degrees)

The swing bus voltage angle is used as a reference to compute voltage angles of all other buses in the model.

    Note   The bus type (PV, PQ, or swing) is not defined in the mask of the Load Flow Bus block. The reason is that you may have several machines and voltage source blocks with different generator type parameters connected at the same bus. The Load Flow tool determines the resulting bus type.

Load Flow Tab

Load flow solution: bus voltage V_LF (pu)

Displays the resulting bus voltage, in pu, once the load flow is solved. This parameter is intentionally grayed out because it is updated automatically by the Load Flow tool. The parameter value appears below the block as a block annotation.

Load flow solution: bus voltage Vangle_LF (degrees)

Displays the resulting bus voltage angle, in degrees, once the load flow is solved. This parameter is intentionally grayed out because it is updated automatically by the Load Flow tool. The parameter value appears below the block as a block annotation.

Example

The power_turbinepower_turbine example contains two Load Flow Bus blocks. The B1 block is connected to the 5 MW Three-Phase Parallel RLC Load and to the Synchronous Machine load flow blocks. The B2 block is connected to the 10 MW Three-Phase Parallel RLC Load and to the Three-Phase Source load flow blocks.

The two blocks display the bus voltages and angles resulting from a load flow solution previously saved with the model. The bus voltage at bus B1 displays 1 pu, -23.67 deg. Start the simulation, verify it starts in steady state, then zoom on the initial value of the measured voltage Va. You should read an initial voltage of about -0.4015 pu. At the MATLAB Command line, compute the angle of the measured voltage:

asin(-0.4015)*180/pi

ans =
      -23.672

You can also take a look at the power_LFnetwork_5buspower_LFnetwork_5bus model. In this model five Load Flow Bus blocks are used. The block labeled B120 is the swing bus.

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