SYNCHRONOUS GENERATOR CONNECTED TO THE GRID

6.1 Objective

This exercise presents the implementation of the synchronous machine operating in generator mode and connected to the grid (infinite bus) at constant frequency and voltage.
After completing the procedure to synchronize the generator to the grid, we study the control of active and reactive power flow with the grid and the operating stability limits.

6.2 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 voltage 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 "Oscilloscope" tab, disabled, see Table 4

6.3 Connecting the Generator to the Grid

In this exercise, the synchronous machine is operating in generator mode and is driven by the DC motor.
It will be connected to the AC grid at constant frequency and voltage.
In the basic tests on the synchronous machine recommended in this manual, we consider the AC grid as an infinite bus, i.e., a balanced, three-phase voltage source with constant frequency and voltage.

In a grid of this type, the balance of active and reactive power exchanged is always maintained by the appropriate automatic command from various power plants and voltage regulating devices (synchronous generator exciters, static compensators and synchronous compensators, to name just a few).

We use voltage source SC1 together with SC2, as in the procedure described in Section 4.3, to control the generator's speed and the frequency of the armature voltage before connecting it to the grid.

The user can then close switch K₁ to connect the synchronous machine to the grid, which supplies autotransformer SA1.
Once connected, the constant frequency of the infinite bus will force the drive speed.
By the same token, the constant voltage of the infinite bus will force the RMS voltage at the armature terminals of the synchronous machine.

Therefore, voltage source SC2 will control the torque provided by the DC motor, and, consequently, the active power exchanged between the synchronous machine and the infinite bus (under the assumption that conversion losses are small enough, therefore neglected).

Notice that on this virtual test bench, it is even possible to reverse the sign of the torque provided by the DC machine, and therefore, the power flow between the synchronous machine and the infinite bus (in other words, the DC machine is operating in generator mode and the synchronous machine is in motor mode).

After connecting the synchronous generator to the grid, the user has two inputs to control the active and reactive power exchanged between the synchronous machine and the infinite bus: (i) the torque available at the coupling shaft that can be adjusted using the DC motor control and (ii) the field current of the synchronous machine.

In this exercise, the nature, features and reversibility of these two control inputs are investigated, as well as the operating limits of the synchronous machine.

6.3.1 Setup Diagram

Figure 11: Synchronous machine connected to infinite grid

6.3.2 Exercise

To avoid harmful currents or transient torques when connecting the synchronous generator to the infinite bus, the three instantaneous voltages measured on both sides of the K₁ switch, i.e., at (i) the generator armature and (ii) the infinite bus must be approximately equal before closing the switch.
Consequently:

• The frequency and the RMS values of the voltages on both sides of the switch must be as close to identical
• The sequence of the phases must be the same

The following steps describe the connection process:

1. Select the "Oscilloscopes1" tab.
   The scope to the left displays two signals: (i) the line-neutral voltage of phase a of the generator's armature, in yellow, and (ii) the line-neutral voltage of phase a of the infinite bus, in red.
   The scope to the right displays the same entities but for phase b.
   Enable switch K₁₀ to activate the trigger for the Oscilloscopes 1 group.
   Buttons "xscale" and "yscale" can be used to adjust time and voltage scales, respectively.
2. Follow the procedure used to start the DC motor, described in Section 4.3, to set the synchronous generator no-load speed as close as possible to the synchronization speed (1798 rpm for example).
   In this way, the generator output voltage frequencies are very close to 60 Hz.
3. Gradually increase, starting from 0, current J_f supplied by the DC voltage source SC3, which is feeding the synchronous machine field winding, in such a way that the line-line RMS voltage of the generator armature is U_s = 460 V.
4. In the "AC Grid" tab, adjust the line-line RMS voltage of the autotransformer SA1 to U = 460 V.
5. Check that the sequence of voltage phases on both sides of switch K₁ are identical by observing instantaneous line-neutral voltages on both scopes.
   Notice, the slight difference in the frequency between the machine and the grid voltages, on both scopes.
   That is due to the difference between the rotating speed (almost 1798 rpm) and the synchronization speed (1800 rpm).
   If both instantaneous voltages, displayed on both scopes, are in phase at the same time, the sequence of phases is OK.
   If not, enable switch K₇, which inverse autotransformer SA1 phase sequences.
   To better understand the functioning of switch K₇, the user can observe the behavior of the phases for the two possible positions of that switch.
6. To achieve the generator to grid connection, close switch K₁ once the two instantaneous voltages are identical.
   If a fuse is blown after closing K₁, then the connection procedure has failed.
   In this case, it is necessary to wait until the speed decreases to 0 and then return to the initial setting described in Section 6.2.
   Then, and only then, the user can simulate the fuse replacement by resetting the protection system (Reset) using button K₈.
   After that, the entire connection procedure must be repeated from the previous step.
7. The synchronous generator is now connected to the grid and, the drive speed dictated by the infinite bus frequency must be constant and equal to 1800 rpm.
   Additionally, the line-line RMS voltage at the generator armature is also constant and equal to 460 V.

6.4 Study of the Regulation of the Active Power Flow Between the Synchronous Machine and the Grid, With Constant Field Current J_f - Operating Limits

6.4.1 Sign Convention for Measuring Devices

• The active sign convention is adopted for the armature (stator) current of the synchronous machine.
  Therefore, the algebraic value of the active power P is positive if the power is supplied to the grid and negative if it is absorbed by the synchronous machine.
  Consequently, if P is positive then the synchronous machine is operating in generator mode, if not, it is in motor mode.
• The passive sign convention is adopted for the armature (rotor) current of the brushed-DC machine.
  Therefore, the product V_a*I_a is positive when the machine is operating in motor mode and negative when it is in generator mode.
• The torque T_u at the DC-machine shaft is positive if the DC-machine is operating in motor mode and the direction of the rotation N is positive.
• The internal angle θ between (i) the electromotive force E phasors and (ii) the armature line-neutral voltage V_s of the synchronous machine is positive in generator mode and negative in motor mode.

For more details refer to Table 5.

6.4.2 Exercise

1. After synchronizing the synchronous generator to the grid do not modify its field current J_f.
   Observe the values and the signs of (i) active power P exchanged with the grid, (ii) torque T_u at the shaft, (iii) internal angle θ and the (iv) armature RMS current value I_s.
2. Using SC2, gradually decrease the DC machine field current I_f.
   By observing the values and signs of the four entities above, check that when I_f decreases, P and T_u increase.
   Therefore, the DC machine operates in motor mode and provides mechanical power to the shaft of the synchronous machine, which is operating in generator mode.
   The latter converts the mechanical power to electric power and supplies it to the grid, under the assumption that conversion losses are small enough, therefore neglected.
3. Continue gradually decreasing the DC machine field current I_f : P, I_s and θ increase.
   We are now close to the operating limits for this type of regulator.
   Two scenarios may occur: (i) the current I_s exceeds its value I_sn and the protection system triggers or (ii) the internal angle θ reaches its limit of static stability, starts oscillating, leading to the loss of synchronization.
   Save the value of θ corresponding to the limit of static stability.
   If the generator disconnects or the protection system triggers, open switch K₁ and repeat the complete synchronization procedure described in Section 6.3.2.
4. Return to the initial state after synchronization by decreasing I_f until it reaches the value obtained in step 1.
5. Using SC2, gradually increase I_f.
   Observe the values and the signs of the four entities enumerated in step 1.
   Check that when I_f increases, P and T_u decrease.
   Consequently, the mechanical power provided by the DC machine decreases progressively.
   Notice that there is a value of I_f for which the torque T_u vanishes.
   Save the values at this specific operating point, called the "floating point", where the power flow between the two machines vanishes.
   Also, notice that for this point, power P and the product V_a*I_a are not zero.
6. Continue to gradually increase I_f  starting from the floating point.
   The sign reverses for (i) the torque T_u at the shaft, (ii) the armature current I_a, (iii) the active power P and (iv) the internal angle θ.
   The DC machine is then operating in generator mode and the synchronous machine is in motor mode.
   The latter absorbs electric active power from the grid and converts it to mechanical power at the DC machine shaft.
7. Continue increasing I_f with caution: the absolute values of P, θ and I_s increase.
   We are now close to the operating limits for this type of regulator.
   Two scenarios may occur: (i) the current I_s exceeds its value I_sn and the protection system triggers or (ii) the internal angle θ reaches its limit of static stability, starts oscillating, leading to the loss of synchronization.
   Save the value of θ corresponding to the limit of static stability.
   If the generator disconnects or the protection system triggers, open switch K₁ and repeat the complete synchronization procedure described in Section 6.3.2
8. Return to the initial state after synchronization by decreasing I_f until it reaches the value obtained in step 1.

6.5 Study of the Regulation of the Reactive Power Flow Between the Synchronous Machine and the Grid, With Zero Active Power - Operating Limits

1. If the virtual test bench is in its initial state after synchronization, as in the last step 8 of the previous exercise, go to step 2.
   If the machines are stopped, return to the initial setup of Section 6.2 and repeat the complete synchronization procedure described in Section 6.3.2
2. Adjust I_f to retrieve the floating point, as described in step 5, where the active power flow vanishes between the two machines.
3. Using SC3, increase field current J_f of the synchronous machine, while maintaining the torque T_u at zero using the DC motor's current field I_f.
   The RMS armature current I_s increases.
   Keep increasing J_f until I_s is close to its nominal value I_sn. For this operating point: (i) save the values of active power P, reactive power Q, and I_s and (ii) compute the power factor of the synchronous machine.
   Does the machine produce or absorb reactive power?
4. Starting from the operating point of step 3 (I_s as close as possible to I_sn), decrease J_f using SC3, while maintaining T_u at zero using I_f.
   Notice that I_s and Q decrease.
   Keep decreasing J_f, while maintaining T_u at zero, until Q vanishes.
   At this operating point, there is no reactive power between the grid and the synchronous generator.
   What is the power factor of the synchronous machine?
5. Keep decreasing J_f using SC3 while maintaining T_u at zero using I_f.
   Notice that (i) I_s, which went through a minimum when Q = 0, increases again and (ii) that the sign of Q is reversed.
   Does the machine produce or absorb reactive power for this new operating point?
6. Keep decreasing J_f using SC3 while maintaining T_u at zero using I_f until I_s is close to its nominal value I_sn.
   If there is a loss of synchronization during this maneuver, i.e., while I_s is getting close to I_sn, return to the initial setup of Section 6.2 and repeat the complete synchronization procedure described in Section 6.3.2.
   Then repeat step 6 until I_s is as close as possible to I_sn without loss of synchronization.
   At this operating point: (i) save the values of P, Q, and I_s and (ii) compute the power factor of the synchronous machine.
   Does the machine produce or absorb reactive power?

6.6 Procedure to Stop the Drive

1. Bring the torque T_u as close as possible to zero using I_f.
2. Open switch K₁ to disconnect the synchronous machine from the grid.
3. Bring voltage source SA1 back to 0 V.
4. Bring voltage sources SC1, SC2 and SC3 back to 0 V.

6.7 Lab Report

1. Plot the phasor diagram for synchronous machine voltages E and V_s at the instant where the synchronization occurs.
2. Due to synchronization, the 60 Hz constant frequency of the grid forces the rotation speed to 1800 rpm.
   Using the steady-state model of the DC motor with independent excitation, show that for constant V_a = 460 V, I_f controls T_u and, therefore, the power flow between the synchronous machine and the grid.
3. Provide the two values of the internal angle θ, corresponding to the limit of static stability, recorded in steps 3 and 7 of Section 6.4.2, and corresponding to the synchronous machine operating in generator and motor mode, respectively.
   Compare them to the theoretical values of static stability of the synchronous machine connected to the grid.
4. What is the power factor of the synchronous machine obtained in steps 3 and 6 of Section 6.5?
   In which case does the machine supply reactive power to the grid?
   In which case does the machine absorb reactive power from the grid?
   Justify your answer.
5. In a hydraulic power plant connected to the grid and not equipped with a turbine speed governor, which control device is used instead of the DC motor field current I_f of this virtual test bench to control the torque at the shaft, and consequently, the active power exchanged between the synchronous machine and the grid?
6. When a private hydraulic plant connects to the grid to sell electrical energy, it is contractually obligated to maintain a minimum value of the power factor of its plant, seen from the infinite bus.
   If it cannot maintain a power factor greater than or equal to this minimum value, the sale price of its energy is penalized and reduced.
   Explain why?
   Which control device should they use to respect their contract?
   Should this device be adapted for the active power it supplies?
7. Is it possible to vary the active power produced by a synchronous generator connected to an infinite bus by controlling its field current J_f?
   Discuss.
8. When the mechanical power on a synchronous generator shaft connected to a grid varies and its field current J_f remains constant, does the reactive power exchanged with the grid vary?
   Discuss.