AC synchronous generator (alternator) driven by a Pelton turbine : synchronization to grid

Intro

This page contains a simulation (Fig. 1) of a Pelton-type hydraulic turbine, a synchronous generator (alternator) with separate excitation, synchronization devices (synchroscope, indicator lights, voltmeters) and coupling switches. It is possible to view the phasor diagram of the grid voltages, the generator voltages and the voltages across the switches. Once the generator is grid-tied, you can see the plotted phasor diagram of the generator.

Turbine and generator

Water is admitted to a Pelton turbine by means of a nozzle, fitted inside with a movable needle valve (in red in Fig. 2 on the left), which allows the flow rate (and therefore the torque) to be adjusted. In the simulation, the adjustment is made with two blue sliders. The upper one allows a coarse adjustment, the second a finer adjustment making it easier to find the point of synchronism. The turbine is mechanically connected to a transmission which drives the generator. The speed N of the generator (in rotations per minute) is indicated in red.

The three-phase synchronous generator - or alternator - (Fig. 2 right) is driven by the turbine. It has an excitation (field) circuit shown in orange. The slider is used to adjust the DC excitation current.

• When the generator is not coupled to the grid, the excitation current is used to regulate the voltage;
• when the generator is coupled to the grid, the excitation current is used to adjust the reactive power Qtri exchanged. For a low excitation current, the generator absorbs reactive power. For a high excitation current, the generator provides reactive power.

On the simulation, two measuring devices give, on the one hand, the phase-to-neutral voltage of the generator and, on the other hand, the line current it supplies. There are also two power indicators above the generator. The exchanged active power Ptri is in red, the reactive power Qtri in green. The convention used is the generator convention: a positive sign means that the power is supplied by the generator to the grid. Animated arrows make it easy to visualize the transfer direction of the different powers.

Tie circuit breaker and grid phase sequence

Figure 3 (left) shows the circuit breaker that allow the generator to be connected to the grid. A click on one of the three switches closes the three switches at the same time (if they are open) or opens them if they are closed. The closing of the switches can only be considered if one is close to the optimal conditions of synchroniation (see below).

When the simulation is launched, the phase sequence is the same for the generator and for the grid (direct). It is possible to change the phase sequence of the grid by clicking on the "Reverse" button (Fig. 3 right). You can of course return to the direct sequence by clicking on the "Direct" button.

Frequency and voltage indicators

The measurements of the generator and grid frequencies are grouped on one device (Fig. 4 left) in order to be able to easily check one of the conditions of synchroniation, namely a small frequency difference between generator and grid. The blue color is used for the generator, and the red color for the grid. The numerical value of the frequencies is given, as well as a visual indication consisting of two triangular needles.

A similar indicator exists for phase-to-neutral voltages (Fig. 4 right). Here again, the two needles make it possible to visually ensure that the voltages of the generator and of the network are sufficiently close.

Synchroscope and lamps

Two measuring devices are used to determine the phase difference between the voltages of the generator and those of the grid:

• the synchroscope (Fig. 5a);
• the voltage indicators (lamps and voltmeters) at the terminals of the switches (Fig. 5b).

In addition, when the frequency difference between the generator and the network is sufficiently small (less than 5 Hz), the simulation draws the phasor diagram of the grid voltages (solid lines / Ⓐ in Fig. 5c), of the generator voltages (dashes / Ⓑ in Fig. 5) and voltages across the terminals of the breaker (dotted lines / Ⓒ in Fig. 5). It is easy to see that the voltages Ⓒ correspond to the differences of voltages Ⓐ and Ⓑ (for "dark lamp method"; for "bright lamp method", see the dedicated paragraph). In the simulation, at the bottom right, the instantaneous voltages of phase 1 are also plotted (Fig. 5d). The colors of the lines correspond to the colors of the conductors of the generator.

The synchronoscope pointer directly gives the phase difference between generator and grid. When this pointer is vertical and points upwards, the phase shift is zero. If the pointer is pointing down, the phase shift is 180°. When the pointer points rather to the right, the voltages of the generator are leading those of the grid. If the pointer points to the left, the grid is leading.

(a)

(b)

(c)

(d)

Conditions for synchronization

The conditions for synchronization of the generator to the grid under good conditions are:

1. same phase sequence for generator and grid;
2. same frequency (or at least a sufficiently small difference);
3. same voltage (or at least a sufficiently small difference);
4. a sufficiently small phase difference.

Conditions 2 and 3 can be checked using the indicators in figure 4. If the frequency (and therefore the speed of rotation) is too low, the flow must be increased. If the frequency is too high, the flow will be reduced. Voltage can be regulated with the excitation of the generator: one increases excitation current in order to increase armature voltage. It should be noted that the voltage also depends on the frequency (and therefore on the speed); it is even proportional to it.

Synchronization with "dark lamp method"

In this paragraph, the description of the synchronization conditions relates to "dark lamp method". For "two bright and one dark lamp" method, see dedicated paragraph.

Condition 4 can be checked with the instruments shown in Figure 5.

Provided that the voltage of the generator is equal (or at least close) to the voltage of the grid, it is possible to determine the moment when the phase difference between generator and grid is zero by measuring the voltages at the terminals of the breaker. Indeed, when the phase difference vanishes, the voltages at the terminals of the breaker (corresponding to the dotted lines Ⓒ in Fig. 5 right) also vanish. These voltages can be visualized in two ways in the simulation (Fig. 5b):

• with black voltmeters;
• with bulbs, represented by circles. When the voltage is low, the bulb is dark. When the voltage is high, the bulb glows (white).

Figure 6a corresponds to a case where we are close to condition for synchronization 4: the phase difference seen with the synchroscope is small. The voltage across the breaker terminals is low, and the indicator lamps are almost off.

Conversely, figure 6b illustrates a moment when synchronization is impossible:

• the phase difference is close to 180° (synchroscope pointer close to the bottom);
• the voltage across the terminals of the breaker is high (close to twice the phase-to-neutral voltage of the network) and the indicator lights are lit.

Finally, Fig. 6c represents a condition where synchronizing is impossible, because the phase sequences of the network and of the generator are not the same. In the simulation, this condition can be obtained by clicking on the "Reverse" button (Fig. 3).

In this configuration, it is impossible to obtain the simultaneous cancellation of the three voltages across the terminals of the breaker. One can in fact observe by looking at the phasor diagram of the voltages that when a voltage across the terminals of a switch (dotted lines) decreases, at least one other voltage increases. The visual effect on the lamps is noticeable, when we place ourselves at a low but not zero frequency difference (for example around 0.5Hz):

• if the phase sequence are the same for the generator and the mains ("Direct" position), the lights come on and go off at the same time (flashing lights);
• if the phase sequence are different ("Reverse" position), the lights come on and go out one after the other (rotating lights).

Synchronization with "two bright and one dark lamp method"

(a)

(b)

The search for synchronization using the coupling lamps, as described above, has a disadvantage: the lamps are almost extinguished while the voltage is still equal to 20% of the maximum voltage. The precise moment of synchronization can then hardly be determined. One solution is to modify the connection of the lamps (Fig. 7a) such that the synchronization time is indicated :

• by the extinction of the lamp L1;
• by the same brightness of lamps L2 and L3.

This condition is shown in Figure 7b. The moment when it is necessary to close the coupling circuit breaker is then indicated more precisely, because it is easier to determine the equality of luminosity of lamps L2 and L3. In the simulation, this lamp connection mode can be chosen using the drop-down menu located above the coupling indicators.

It should be noted that the appearance of the lamps is reversed when passing from a connection "at extinction" to a connection "at ignition". It is therefore necessary to know the actual connection type of the installation. The table below summarizes the different scenarios.

	Same phase sequence		Opposite phase sequences
	Apparence of lamps before synchronization	Synchronization conditions	Apparence of lamps	Synchronization conditions
Three dark method				Synchronization IMPOSSIBLE
One dark three bright method				Synchronization IMPOSSIBLE