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Adding an ZTH_MP row in the Probes tab to the excel file allows the calculation of the Thevenin impedance between two specific points of the network. Impedance calculation can be done :

1) between a bus and reference (usually ground).
2) between two buses. 

The results of the calculation can be in SI values or in p.u. depending on how the system has been defined. For transmission systems modeled only from the positive sequence, the data is given in p.u. so the impedance calculation will also be in p.u. In the case of distribution systems, these are usually modeled in SI values per phase which will produce a calculation of Thevenin impedance in SI values. 


Probe Parameters

Symbol

Description

Instruction

Type

Calculation type to be made by the probe.
ZTH_MP is used to specify Thevenin impedance.

ZTH_MP

ID

Identifier for the Thevenin impedance calculation.

Probe ID

Bus

List of bus identifiers of the model where impedance calculation will be performed.

Bus ID

Available I/O pins

No

Pin Description

Pin Type

Value/Unit

Instruction

1

Real part of Thevenin impedance

O

ohms or p.u.

ProbeID_zBusIndex1_BusIndex2/resistance

2

Imaginary part of Thevenin impedance

O

ohms or p.u.

ProbeID_zBusIndex1_BusIndex2/reactance

In the previous table,

  • ProbeID  : is the probe ID that does the Thevenin impedance calculation.
  • BusIndex1 : is the index of the bus #1 defined in the list of buses of the Probes page
  • BusIndex2 : is the index of the bus #2 defined in the list of buses of the Probes page. When BusIndex1 is equal to BusIndex2, the Thevenin impedance is calculated between BusIndex1 and ground.

As an example, to define the calculation of the impedance between bus having index 2 and the ground for any system, the pin entry should be written as follows: ProbeID_z2_2/resistance and ProbeID_z2_2/reactance. In a similar way, for the calculation of the impedance between buses having indexes 2 and 3, the pin entry should be: ProbeID_z2_3/resistance and ProbeID_z2_3/reactance. To better understand the way of indexing the buses, see the following 2 examples: 

Examples

Example #1

Let's say we want to measure the Thevenin equivalent impedance between buses of the 39-bus transmission power system defined in the PHASOR-01 demo. As a reminder, the PHASOR-01 is a positive sequence model.

  • The first step is to add a new Thevenin impedance ZTH_MP probe on the Probes page. In the below figure, a new measurement is defined having calc as the ProbeID. Moreover, the Thevenin equivalent impedances will therefore be calculated between all buses having the identifiers 4, 5, 6, 7 and 8 and the ground reference.
  • The figure also shows the mapping between the bus indexes in red and the bus identifiers in black where the impedances will be calculated on the grid.

  • Then, to output impedance values on the ePHASORSIM block outgoing pin, a new I/O pin entry needs to be added on the Pins page. In the below figure, three impedance values has been added on the outgoing pins Zth_R and Zth_X of the block.
    • The first entry of the first row calc_z2_3/resistance defined that the Thevenin resistance between buses having index numbers 2 and 3 will be output.
    • The second entry of the first row calc_z2_5/resistance defined that the Thevenin resistance between buses having index numbers 2 and 5 will be output.
    • The third entry of the first row calc_z2_2/resistance defined that the Thevenin resistance between bus having index number 2 and the ground will be output.
    • And the last row, the same calculations for the reactances of thevenin's impedances are presented.

  • Important : the indexes in the above example refers to the indexes of the buses on the Probes page. For example, bus having index 2, is the bus identifier 5 in the power system network. In the same way, index 3 is the bus identifier 6, an index 5 is the bus identifier 8. The next figure show the mapping between bus indexes and bus identifiers. 

Example #2

  • Now, let's say we want to measure Thevenin equivalent impedance on the 4-bus distribution power system defined in PHASOR-07 demo. In this three-phase case, the bus identifiers have been defined by number and phase. For example, bus 1 would be: 1_a, 1_b and 1_c.  The figure below shows in black how to define impedance calculation on buses having identifiers 1 to 4 in the Probes page.  The bus indexes are also shown in red. 

  • The following figure now shows how to output resistance and reactance values between buses having indexes 2,3 and 5. Output will be available on the Zth outgoing pin of the ePHASORSIM block.

  • Important : the indexes in the above example refers to the indexes of the buses on the Probes page. Bus having index 2, is the bus identifier 1_b in the power system network. In the same way,  index 3 is the bus identifier 1_c, and index 5 is the bus identifier 2_b. 
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