BERTA Closed Loop Operating Mode: Islanded Grid

The islanded system simulation is the best means of adjusting properly the speed governor parameters and to validate the models of the speed governor and the turbine.

In the islanded grid (closed-loop) operating mode, a disturbance of the rated load or the triggering of virtual units allows simulating an imbalance between the mechanical and electrical couples that will translate into an unprogrammed dynamic behavior of the frequency.

The frequency signal calculated by the algorithm following this imbalance and the total rotational inertia of the islanded grid will be injected into the speed governor and cause chain reactions of the water, vapor or fuel intake devices, the turbines, the dynamic loads and again the frequency (loop closing).

The options are the following:

Step	Amplitude to be defined, unlimited duration.
Filtered step	Amplitude, natural frequency and dampening coefficient to be defined, unlimited duration.
Fixed frequency sinus	Peak amplitude and frequency to be defined, unlimited duration. Warning: There is a risk of resonance.
Pulse	Amplitude and duration to be defined.
Ramp	Value and duration to be defined.
Triggering of the virtual units selected

To simulate an islanded system disturbance, it is first necessary to ensure that the value of parameter H in the field Cst H (MJ/MVA) corresponds to the AC generator being tested. The disturbance is achieved by injecting a signal frequency error into the AC generator speed governor that mimics the real behavior of an islanded system if the latter were to experience a sudden variation in load on the AC generator terminals.

The procedure is the following:

Open the Controller panel (Controller button in the Control panel toolbar)
In the Settings tab, Generator data section, validate the parameter Cst H (Refer to General Introduction for details).
Select the Islanded system operating mode.
Select the type of Islanded system disturbance, such as Step” 5. Define the load amplitude variation dP0 (p.u.):

dP0 = load variation size in p.u. on the AC generator MVA base. The default value is set to 0.0 p.u.

Click on the desired graph panel button (Gr_1 to 5 or Gr_6 to 10) 7. Use the GO BERTA switch to select the servomotor command mode.
Click on GO Acquis. This starts recording the acquisition group 2 signals to the data file and increments the test number.
Click on GO Disturbance.
Examine the following graphs:
- Servomotor movement
- Speed variation
- Simulated mechanical power
- Simulated electrical power.

NOTE: Watch the Island Speed Deviation signal closely to instantly stop the disturbance should an instability occur during the test. This signal is limited to 0.05 p.u. (± 3 Hz) as a safety precaution.

Once the new parameters are established, or when you estimate that the test has run long enough, stop the simulation using the following procedure:

Click GO Disturb. This immediately freezes the frequency error signal injected into the speed governor and instantly stabilizes the servomotors.
Reset the operating mode that was in use before the test by clicking on the Reset dFre Sim button (this resets the simulated frequency error to zero and the islanded virtual units to their initial conditions).
Click GO Acquis. This stops the recording of acquisition group 2 signals and closes the file.

The figure below shows the results of a test with speed governor adjustments (proportional gain Gp = 4,8 and integral gain Gi = 0,4) that provide a stable behavior, but with small oscillations in the islanded system frequency and AC generator power.

The figure below shows the instability caused by an excessive proportional gain (Gp = 6).

Note: The Pelec_simul trace on the bottom graph clearly shows the islanded system load variation (in this case, 0.8 p.u. to 0.85 p.u.).

Once the test is done, click on GO Disturbance to stop the disturbance and reset the simulated frequency error to zero. The frequency deviation reset is not instantaneous so as to minimize the impact on the speed governor. As shown in Figure 41, the reset is gradual and follows the slope defined by the parameter Reset Duration on the Settings panel. The speed governor resumes its original position.

Click on GO Acquis to stop the acquisition and close the data recording file.