Electrical Load Analysis Example

[Eri Pfaff]

Youcan use the data to determine ideal operating conditions or estimate the response of your system to hypothetical situations. For example, if you know the active and reactive power in each transmission line, you can determine if the remaining lines can handle the extra load that occurs when one or more transmission lines go offline.

You can also use the data to calculate transmission line or system losses and examine the overall voltage profile of the network. Investigating these attributes can help you determine if the system needs reactive power compensation to overcome low voltage levels.

Enabled for Simscape data logging. For complex models or long simulation runs, you can improve simulation performance by enabling data logging for selected blocks by using local solver settings. For a load-flow analysis, data logging is required only for Busbar blocks. For more information, see Enable Simscape Data Logging for the Whole Model and Log Data for Selected Blocks Only.

In an electrical transmission system, a bus bar connector, or bus, is a vertical line that connects power components such as generators, loads, and transformers. To represent buses, the Simscape > Electrical > Connectors & References library provides the Busbar and Busbar (DC) blocks.

You need to select the right three-phase voltage source for your model to conduct a load-flow analysis. Which source you choose depends on whether you want to prioritize simulation accuracy or performance. The balance between simulation accuracy and performance depends, in part, on the blocks that you use to represent the voltage sources in your analysis model. Simulation accuracy is a measure of model fidelity, that is, how closely the simulation results agree with mathematical and empirical models. As model fidelity increases, so does the computational cost of simulation. As computational cost increases, simulation speed, decreases. Conversely, as model fidelity decreases, simulation speed increases.

Prioritize Model Fidelity by Using Machine Blocks. To prioritize model fidelity over simulation speed, represent voltage sources by using induction or synchronous machine blocks. For modeling induction machines, the Simscape > Electrical > Electromechanical > Asynchronous Machines library provides both the Induction Machine Squirrel Cage and Induction Machine Wound Rotor blocks. For modeling synchronous machines, the Simscape > Electrical > Electromechanical > Synchronous Machines library provides the Synchronous Machine Model 2.1, Synchronous Machine Round Rotor, and Synchronous Machine Salient Pole blocks.

Prioritize Simulation Speed by Using Load Flow Source Blocks. For a faster simulating, but lower fidelity model, represent the voltage sources in your analysis model by using a Load Flow Source block from the Simscape > Electrical > Sources library. The Load Flow Source block supplies either an idealized or a current-dependent voltage source. The voltage can contain series impedance or can act as a source for a swing, PV, or PQ bus.

At the beginning of a load-flow analysis, the equation variables for transmission line losses are unknown. While the unknown variables are being solved, the buses balance the losses by providing or absorbing active and reactive power. For each bus there are four variables:

Two of the variables are known and two are unknown. Which variables are known and which are unknown depends on the actively controlled three-phase sources and loads that are connected to the bus bar. The voltage source block configurations determine which bus types are used in load-flow analysis. You can include more than one bus type in your model. Bus type options are:

To avoid a simulation issue due to a nonoptimal minimum for PV or PQ buses, in the Expected Ranges settings, specify minimum and maximum values for the Internal source phase search range parameter.

To fully specify the initial condition, you must include an initialization constraint in the form of a high-priority target value. For example, if your induction machine is connected to an Inertia block, the initial condition for the induction machine is completely specified if, in the Variables settings of the Inertia block, the Priority for Rotational velocity is set to High. Alternatively, you could set the Priority to None for the Inertia block Rotational velocity, and instead set the Priority for the induction machine block Slip, Real power generated, or Mechanical power consumed to High.

If your model is configured for a power-flow analysis, you can also use the Load-Flow Analyzer to perform a power-flow, or load-flow, analysis for a three-phase AC or DC electrical power transmission system. The app generates three tables. The AC Nodes table contains data for the AC network nodes, as represented by AC busbar, load flow source, synchronous machine, induction machine, and three-phase load blocks. The AC & DC Busbars table contains data for all busbars. The AC Connections table contains data for the network connections, as represented by transmission line and transformer blocks. When you open the app, the tables are preloaded with the specified parameter values for the relevant blocks in the current or specified model. After you run the power-flow analysis, the tables also display the steady-state voltage magnitudes, voltage phase angles, active power, and reactive power for the node and connection blocks.

If you encounter issues when simulating a load-flow model, apply these troubleshooting measures. Testing your load-flow model incrementally can help you avoid specifying nonphysical load-flow requirements.

Including internal source impedance for a Load Flow Source block when the Source type parameter of the block is set to Swing bus, PV bus, or PQ bus can prevent initialization convergence. To resolve any convergence issues, use one of these methods:

If you initialize a synchronous machine block for a load-flow analysis, the block solves all Park-transformed flux variables and mechanical variables for steady state. However, incorrect initialization of an automatic voltage regulator (AVR) or governor can result in a field-circuit transient or an initial rotor acceleration. To resolve these issues:

For the synchronous machine block, in the Initial Conditions settings, set the Initialization option parameter to Set real power, reactive power, terminal voltage, and terminal phase.

Print the required initial conditions for the AVR and governor to the MATLAB workspace. Right-click the machine block and, from the context menu, select Electrical > Display Associated Initial Conditions. The relevant data are the field circuit voltage, si_efd0, and the mechanical torque, si_torque0.

For example, the table shows the annotated data for the Busbar block that is next to the Synchronous Machine Salient Pole block in LoadflowSMInitialization, the model for the Synchronous Machine Initialization with Loadflow example. If you open the Synchronous Machine Salient Pole block, click the Initial Conditions settings, and set the Initialization option parameter to Set real power, reactive power, terminal voltage, and terminal phase, you can observe that the specified parameter values are equal to the load-flow simulation values.

To initialize correctly, specify 85.4468 V as the value for the field voltage source, and 828709 Nm as the value for the Shaft torque Constant block that is connected to the Ideal Torque Source block.

There are often multiple solutions to the set of load-flow targets specified when initializing an AC electrical network. For example, for a PV bus source where you specify the active power and voltage, there are two solutions for the reactive power. For the desired solution, the magnitude of the reactive power is typically less than the specified active power magnitude. For the undesired solution, the reactive power magnitude is much larger than the active power magnitude.

If the initialization returns the undesired solution, reconfigure the Load Flow Source or synchronous machine block and increase the value for the minimum boundary of the Internal source range search range parameter. For the Load Flow Source block, the parameter is in the Expected Ranges settings. For synchronous machine blocks, the parameter is in the Initial Conditions settings.

The simulation can stop and generate an error if, to satisfy the active and reactive power demands, the optimization decreases the Busbar block voltage, to the point where the solution is closer to an undesired local minimum around zero busbar voltage than to the desired load flow solution. To prevent this type of issue, reconfigure the Load Flow Source or synchronous machine blocks and increase the value of the Minimum voltage (pu) parameter. For the Load Flow Source block, the parameter is in the Expected Ranges settings. For synchronous machine blocks, the parameter is in the Initial Conditions settings.

You can only perform a load-flow analysis by using the frequency and time simulation mode. Replace any blocks that are not compatible with the frequency and time simulation mode. For more information, see Frequency and Time Simulation Mode.

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Every change to an aircraft electrical loads requires a thorough analysis of the impact the configuration / component change / additional load will have on all systems as they flow up to the power source. Operators that outsource STC or OEM modifications to vendors can have the vendor revise the ELA but the operator always remains accountable.

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