Documentation

Load Flow Bus

Identify and parameterize load flow bus

Library

Measurements

Description

You use the Load Flow Bus block in models to specify the bus locations and parameters to solve a load flow. The Powergui Load Flow tool solves the load flow. Simscape™ Power Systems™ allows you to perform two types of load flows:

  • Positive-sequence load flow applied to a three-phase system. Positive-sequence voltages as well as active power (P) and reactive power (Q) flows are computed at each three-phase bus.

  • Unbalanced load flow applied to a mix of three-phase, two-phase, and single-phase systems. Individual phase voltage and PQ flow are computed for each phase.

To perform a positive-sequence load flow, you must define all of the Load Flow Bus blocks in your model with the Connectors parameter set to single. For this type of load flow, the load flow blocks 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

To perform an unbalanced load flow, you must define all of the Load Flow Bus blocks in your model with the Connectors parameter set to one of the following values: ABC, AB, AC, BC, A, B, or C. For an unbalanced load flow, the load flow blocks are:

  • AC Voltage Source

  • Asynchronous Machine

  • Parallel RLC Load

  • Series RLC Load

  • Synchronous Machine

  • Three-Phase Dynamic Load

  • Three-Phase Parallel RLC Load

  • Three-Phase Series RLC Load

  • Three-Phase Source

The Powergui Load Flow tool reports an error if a model contains a mix of Load Flow Bus blocks set to perform a positive-sequence load flow and Load Flow Bus blocks set to perform an unbalanced load flow.

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.

Implicit Buses (for Positive-Sequence Load Flow Only)

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

Connectors

When you perform a positive-sequence load flow, you connect a Load Flow Bus block with the Connectors parameter specified as single to any phase (A, B, or C) of every load flow block in the model. The icon of the block displays the number 3, indicating that the block is defined to perform a positive-sequence load flow.

When you perform an unbalanced load flow, you connect a Load Flow Bus block to all phases of every load flow block in the model. Depending on the number of phases, you need to specify the appropriate Connectors parameter by selecting one of these connector combinations:

  • Three connectors: ABC

  • Two connectors: AB, AC, or BC

  • A single connector : A, B, or C

The figure shows examples of Load Flow Bus blocks set to define different phases of a given model, to perform an unbalanced load flow.

Configuration

When the Configuration parameter is set to on one side, the block displays the block phase ports only on one side of the block. You can connect the Load Flow Bus block anywhere in the model to identify a connection line as a load flow bus:

When the Configuration parameter is set to on both sides, the block displays the block phase ports on both sides of the block. You can connect the Load Flow Bus block in series with other Simscape Power Systems blocks to identify a connection line as a load flow bus.

Both options perform the same action, and you can seamlessly use either method to connect the Load Flow Bus blocks.

Bus Identification

Enter a meaningful alphanumeric label. The Bus Identification label 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. This parameter is visible only when the Connectors parameter is set to single.

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. This parameter is visible only when the Connectors parameter is set to single.

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. This parameter is visible only when the Connectors parameter is set to single.

Base voltage (Vrms phase-ground)

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. This parameter is visible only when the Connectors parameter is not set to single.

Swing bus or PV bus voltage, per phase (pu)

Specify the required bus voltage magnitude, in pu, for every phase specified by the Connectors parameter. 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. This parameter is visible only when the Connectors parameter is not set to single.

Swing bus voltage angle, per phase (degrees)

Specify the required bus voltage angle, in pu, for every phase specified by the Connectors parameter. The swing bus voltage angles are used as a reference to compute voltage angles of all other buses in the model. This parameter is visible only when the Connectors parameter is not set to single.

    Note   The bus type (PV, PQ, or swing) is not defined in the mask of the Load Flow Bus block. You can 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

Bus voltage section

Phase A

Displays the resulting bus voltage of phase A, in pu, once the load flow is solved. This parameter appears dimmed because it is updated by the Load Flow tool. The parameter value appears below the block as a block annotation.

The value is set to NaN by the Load Flow tool when the load flow solution is not computed for this phase (that is, when the Connectors parameter is set to BC, B, or C).

Phase B

Displays the resulting bus voltage of phase B, in pu, once the load flow is solved. This parameter appears dimmed because it is updated by the Load Flow tool. The parameter value appears below the block as a block annotation.

This parameter is visible only when the Connectors parameter is not set to single. The value is set to NaN by the Load flow tool when the load flow solution is not computed for this phase (that is, when the Connectors parameter is set to AC, A, or C).

Phase C

Displays the resulting bus voltage of phase C, in pu, once the load flow is solved. This parameter appears dimmed because it is updated by the Load Flow tool. The parameter value appears below the block as a block annotation.

This parameter is visible only when the Connectors parameter is not set to single. The value is set to NaN by the Load flow tool when the load flow solution is not computed for this phase (that is, when the Connectors parameter is set to AB, A, or B).

Bus angle section

Phase A

Displays the resulting bus voltage angle of phase A, in pu, once the load flow is solved. This parameter appears dimmed because it is updated by the Load Flow tool. The parameter value appears below the block as a block annotation.

The value is set to NaN by the Load Flow tool when the load flow solution is not computed for this phase (that is, when the Connectors parameter is set to BC, B, or C).

Phase B

Displays the resulting bus voltage angle of phase B, in pu, once the load flow is solved. This parameter appears dimmed because it is updated by the Load Flow tool. The parameter value appears below the block as a block annotation.

This parameter is visible only when the Connectors parameter is not set to single. The value is set to NaN by the Load flow tool when the load flow solution is not computed for this phase (that is, when the Connectors parameter is set to AC, A, or C).

Phase C

Displays the resulting bus voltage angle of phase C, in pu, once the load flow is solved. This parameter appears dimmed because it is updated by the Load Flow tool. The parameter value appears below the block as a block annotation.

This parameter is visible only when the Connectors parameter is not set to single. The value is set to NaN by the Load flow tool when the load flow solution is not computed for this phase (that is, when the Connectors parameter is set to AB, A, or B).

Examples

The power_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 prompt, 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_5bus model. In this model five Load Flow Bus blocks are used. The block labeled B120 is the swing bus.

The power_13NodeTestFeeder example contains Load Flow Bus blocks set to perform an unbalanced load flow for a 13-node test feeder IEEE benchmark model.

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