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Synchronous Machines - 8. Exercise 5


Section Content

8.1 Objective

This exercise presents the implementation of two tests in the synchronous machine stator: (i) the standardized test of sudden three-phase bolted short-circuit, and (ii) the recovery voltage test after eliminating the short-circuit.
These tests highlight the dynamics of currents in the synchronous machine's armature and field.
Results are automatically saved, and the files are available to the user on their workstation in the following path:


The waveforms obtained through these tests allow the user to determine the transient and sub-transient time constants for the synchronous machine's dynamic model.
The methods for processing and calculating the parameters of the machine in this exercise are supplied in the references and can be programmed by the user in MATLAB.

8.2 Initialization of the Setup

When the simulator is started up, the initial settings on screen are as follows:

  1. Continuous voltage supplies SC1, SC2, SC3 set to 0 V.
  2. Continuous supply SC4, in the "Measurement Resistors" tab, set to 0 V.
  3. Alternative power SA1, in the "AC Grid" tab, set to 0 V.
  4. Switches K1, K2, K3, K4 opened: synchronous generator armature has no load, see Table 4.
  5. Switch K6 disabled: the synchronous machine rotor is driven by the DC motor, see Table 4.
  6. Switch K5 disabled: voltage source SC3 powers the synchronous machine field winding, see Table 4.
  7. Trigger switches K10, K11, K12, K13, K14, in the tab "Oscilloscope", disabled, see Table 4.

8.3 Dynamic Test: Three-phase Bolted Fault

8.3.1 Setup Diagram

Figure 13 : Synchronous machine short-circuit diagram

In this exercise, the synchronous machine is operating in generator mode and is driven by the DC motor.
The continuous supplies SC1 and SC2 are used to control the generator speed, as described in Section 4.3 of Exercise 1.
Make sure that switch K2 is open, and the synchronous machine armature windings have no load, i.e. the voltage measured is equal to the no load electromotive forces, as in the exercise from Section 4.4 before beginning the sudden, balanced, three-phase short-circuit test.
Close switch K2 to begin the sudden three-phase short-circuit test.
The waveforms for the currents in all three phases of the synchronous machine's armature and field are displayed on in the scope and saved in a standard file format on the user's computer.
Then, opens short-circuit switch K2. Momentary variations of the three phase voltages at the armature terminals are displayed on the scope and saved in standard file format.
The test data saved in the files can be used to determine the various time constants for the synchronous machine's dynamic model using the procedures described in the references.

8.3.2 Exercise

  1. Follow the procedure described in Section 4.3 of Exercise 1 to use the DC motor to drive the synchronous machine at its nominal speed of 1800 rpm.
  2. Set field current Jf provided by the variable voltage source SC3 to ensure that the armature's line-line RMS value is equal to 30% of nominal value Usn at 1800 rpm.
    Use the DC motor's settings to maintain the speed at 1800 rpm.
  3. Select the "Oscilloscopes3" tab.
    The first scope in this tab displays the waveforms of the short circuit currents in the three-phase armature and the current in the field winding of the synchronous machine.
    The second scope displays the waveforms for all three-phase voltages at armature terminals.
  4. During a sudden short-circuit, the contents of both scopes are saved as MATLAB data files on the user's computer.
    It is possible to save up to 11 separate short-circuit tests, each one generating "FileSCCurrents_X" and "FileRecoveryVoltages_X" mat files (where X can be anywhere from 0 to 10).
    The file names can be changed in the "Oscilloscopes3" tab, using one of three functions; "Display", "Saving_SC_Currents", or "Saving_Recovery_Voltages".
  5. Enable the oscilloscope's switch K12 to view waveforms and automatically save the short-circuit data for each waveform.
  6. Close switch K2 to short-circuit the synchronous machine's armature.
    The waveforms of the currents that circulate through the three-phase armature and the field current appear in the left-hand scope and are automatically saved in a file named "FileSCCurrents_X.mat".
  7. Open switch K2 to eliminate the short-circuit.
    The right-hand scope displays the recovery voltage at the terminals of the three-phase synchronous machine's armature.
    The waveforms are automatically saved in a file named "FileRecoveryVoltages_X.mat".
  8. Perform 3 consecutive tests without changing the value for Jf and the machine speed, and save each one with three different X numbers.
  9. Saved files can be viewed using any text editor on the computer.
  10. Stop the DC motor by bringing the SC1 voltage back to 0. Bring SC3 back to 0 and disable switch K12.

8.4 Lab report

  1. Using the 3 saved tests, compare the maximum instantaneous current values in the three phases of the synchronous machine during the first cycles of the short-circuit with the peak values measured when the currents reach the steady-state (we can also calculate these steady state values using Jf and the steady state short-circuit characteristic determined in Section 4.5 of Exercise 1).
  2. Why are the waveforms for the armature's short-circuit currents different for each phase?
    Why do these differences change with each test?
  3. Why is there an instantaneous variation of the synchronous machine field's current during the first cycles of the short-circuit if that winding is fed by an ideal continuous voltage source?
  4. The tests show that the armature reactance of the synchronous machine during the short-circuit is much lower than the constant reactance Xd, which was subtracted from the steady state short-circuit measurements obtained in the Section 4.5 of Exercise 1.
    Explain qualitatively the impact of the damping bar cage and the field winding on the armature's instantaneous reactance during the transient state.
  5. The armature reactance varies depending on the instant observed in the transient period.
    During the first cycles of the short-circuit, it is called the "subtransient" X''d, which has a smaller value than the transient reactance Xd.
    Then, it begins to increase and is called the "transient" reactance X'd.
    When the steady state of the short-circuit is reached, it becomes equal to Xd.
    If the machine does not have a damping bar cage and if the field winding was replaced by a permanent magnet with a high resistivity, would we see higher transient short-circuit currents in the armature of the synchronous machine with respect to the values reached in the steady state of the short-circuit?
    Would the armature reactance be constant and equal to Xd throughout the short-circuit test?
  6. Based on the knowledge acquired during this course and the two documents referenced below, develop your own procedure to determine the synchronous machine's dynamic model time constants using the data from the test described in Section 8.3.2.

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