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EMTP | Load Flow

PV Type Node

Qmin and Qmax Parameters

Consider the following two situations in EMTP:

Situation 1:

Qmin or Qmax is exceeded

The load flow solution algorithm finds a solution with a constraint PQ with Q within the admissible range.

No warning message of the constraint change is issued.

Situation 2:

[Qmin, Qmax] too narrow

The load flow solution algorithm finds the solution with a constraint PQ with Q as one of the limits.

A warning message is issued to indicate that a limit has been reached and that the PV constraint has been changed to the PQ type.

In HYPERSIM, the PV constraint becomes a PQ constraint when one of its limit values is exceeded during the calculation of the solution. A warning message is issued to indicate that a limit has been reached and that the PV constraint has been changed to PQ.

In HYPERSIM the power flow solution will be different from that on EMTP when situation 1 is on EMTP (see the previous section). The user must adjust Qmin and Qmax limits to avoid situation 1 described above. Other options to make respect the power limits are: changing the voltage in the bus, changing the value of a inductance or capacitance in the network or changing the type of bus from PV to PQ. 

Remote Control

In EMTP there is an option to control the voltage (in PV mode) at a remote bus (different) from that of the Load Flow bus element.

In HYPERSIM, the PV constraint can only be applied to the local bus. In this case, the magnitude of the voltage can be obtained from the load flow solution calculated by EMTP. The user must adjust the voltage to control the local bus.

The following page contains more information about the load flow in HYPERSIM: Load Flow.

Nominal voltages in internal buses in subsystems

The nominal voltages can be defined in the Netlist on the buses where the power transformer are connected, see details in Load Flow (section 4.2). By specifying the transformer nominal voltages, it defines the voltage of all the buses connected to each side of the transformer. This information is necessary for solving the load flow. However, there is problem when the voltage values are automatically defined in multiple instances of subsystems that are of the same type but with different nominal voltage. In those cases there is a mismatch in the voltage level value. The following figures shows an example. 

In the windfarm1 and windfarm2 are two instances of the same type but with different nominal voltages. To address this problem, the nominal voltage of the subcircuit is defined as a parameter in the mask.

The Ubase parameter in the mask is used to define the nominal voltage of the internal buses.

Then, the nominal voltage parameter is entered in the internal bus.

Subsystem not considered in the load flow solution

In EMTP, the three-phase load flow allows to correctly represent the unbalanced circuits. In HYPERSIM, the three-phase load flow is not supported. A single-phase calculation represents the positive sequence of the three-phase circuits assuming that the system is balanced. Therefore, an EMTP model that does not have an equivalent three-phase model in HYPERSIM, is not considered in the load flow solution. This situation is illustrated in the following example.

In the following figure, the subsystem YD_1 is an ideal transformer built with single-phase components, which are not taken into account in the load flow solution.

The internal circuit that represents the ideal transformer is considered during the simulation of the model. However, since the transformer is built with single phase components it will be ignored in the calculation of the load flow.

To include the transformer in the load flow, the component must be replaced by the equivalent three-phase component available in the EMTP library. 

Note that the the subsystem containing single-phase components can be used in the simulation. However, the single-phase elements will not be considered in the load flow. If load flow is important for the study of the power system, the user should consider the following points for the cases where the EMTP model contains subcircuits.

  • Assure that all the subcircuits essential for the simulation are considered during the load flow
  • Replace subcircuits build with single-phase components by equivalent three-phase components in HYPERSIM when possible

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