Synchronization Systems

Synchronization Systems

In recent years, in line with customer demands, it provides significant advantages such as ease of maintenance and fuel savings without interruption by saving the user in investment costs.

Dst power generator manufactures and commissions all synchronization systems by making special production according to the incoming demands, by designing them with its expert engineer staff.

Starting from the back-up operation of multiple generator systems in case of power cuts, forward-backward, smooth transitions, unconditional synchronization, fail-back synchronous, and notified synchronous applications can be made with the network. World-renowned brand switches and control panels such as Siemens, ABB, Schneider are used in the power and control panels of generator synchronization systems with DST power production.

How Synchronous Generators Work

Generators (also known as alternators) are machines that convert mechanical energy from sources into electrical energy, and they are basically divided into two groups as DC and AC. So what is the structure of synchronous generators? How to calculate the speed of a synchronous alternator and find the induced voltage, power and torque in a synchronous alternator?

If a DC current is applied to the rotor part of the synchronous generator, a rotor magnetic field is created. Then a rotating magnetic field is created inside the machine, which rotates the generator with a source of motion. Thus, a three-phase voltage is induced in the stator windings of the generator with the rotating magnetic field.

Now let's define the winding in the machines;

The first is the field windings (windings that produce the main magnetic field). They are also called rotor windings because they are located on the rotor. The second is the armature windings (windings in which the main voltage is induced). Since armature windings are on the stator, they are also called satator windings. There are magnetic poles on the rotor. These poles are salient or round poles. The salient pole means the pole outward from the rotor (usually they are produced with four or more poles). A round pole means the pole that is flush with the surface of the rotor (they are produced with two and four poles).

When DC current is passed through the field circuit in the rotor, the rotor rotates. Special methods are required to transfer DC current to the field windings located here. The first of these methods;

We can supply power to the rotor from an external DC source. We can do this with the help of a ring and brush. Second, we supply DC power from a DC source again, but the DC source here can be mounted on the rotor. So how do I apply the same DC voltage to all the field windings? If I connect the positive end of the DC voltage source to one brush and the negative end to the other brush, the job is done. In other words, DC voltage is applied to the field winding by keeping it separate from rotor speed or angular position.

We have just used the terms bracelet and brush. If we talk about these briefly, the ring is a metal ring that fully surrounds the shaft (rotor) but is isolated from the shaft. The DA rotor is connected to two rings on the shaft, and each ring is associated with the brush. Brushes are rods with carbon components that conduct electricity with low friction. Here, the low friction is due to the low friction between the structures forming the brush ring. Of course, no matter how little the friction is, after a while, abrasions occur on the brush. If a brush is severely worn, a lack of contact occurs and the motor does not operate, and voltage drops in the brushes can cause great power losses on the motor. However, brushes and collars are used in all small powerful synchronous machines. Because other methods that provide DC current are quite costly.

Brushless exciters are used to provide DC excitation current in large generators and motors. A brushless exciter is a small AC generator with the field winding on the stator and the armature circuit on the motor shaft. The three-phase output of the generator is converted to direct current with a three-phase rectifier circuit mounted on the shaft. And then the main DA feeds the field winding. With the small DC excitation current control of the excitation generator placed on the stator, it is possible to adjust the excitation current on the main machine without the collar and brush. The advantageous part of this system is; A brushless exciter requires less maintenance than a ring and brushed ones because there is no mechanical connection between its rotor and stator. Most of the synchronous generators that contain brushless exciters also have collars and brushes so that they can be used in case of any danger.

Speed of Synchronous Generator

The title of our article is Synchronous Generators. So what does the word synchronous mean here? The meaning is: means that the electrical frequency is locked (synchronous) with the mechanical rotational speed. In fact, the rotor of the synchronous generator is an electromagnet. According to the laws of physics, the magnetic field in the rotor determines which direction the rotor will turn. Now the rotation of the magnetic fields in the machine 

Let's examine the relationship between speed and stator electrical frequency;

Fe: Electrical frequency(Hz)
Fe=NmxP/120 Nm:Mechanical velocity of the magnetic field(rpm)
P : Number of poles

We can also make the following comment here; Since the rotor and magnetic field rotate at the same speed, this equation also gives the relationship between the rotor speed and the resulting electrical frequency. As we can see, the generator rotates at a constant speed depending on the number of poles on the machine.

Voltage Induced in Generator

The voltage induced in a stator phase;

Ea=K.Q.w ,     K= Nc/(1.4142)= NcxP/(1.4142)

In the equation, the induced voltage Ea is directly proportional to the current and the speed. But the current is dependent on the current flowing through the rotor field circuit. The excitation current If is directly proportional to the current. Since Ea is directly proportional to the current, the induced voltage depends on the excitation current. We call this change the idle running characteristic of the machine.

Power and Moment Calculation

Synchronous generators convert mechanical power into three-phase electrical power. Whatever source of motion is used, the only important point is that the speed should be kept approximately constant regardless of the power drawn. This point is very important for the frequency of the power system to remain constant.

At the beginning of our article, I said that the generators convert the mechanical power at the entrance to electrical power at the output. Not all mechanical power at the input is converted to electrical power at the output. The point I want to talk about here is machine losses. The difference between the input and output powers shows the machine losses. The input mechanical power is the power at the shaft of the generator. Power converted from mechanical to electrical;

P= 3 x Ea x Ia x cosq

Here q is the angle between Ea and Ia. The difference between the input power of the generator and the power transformed in the generator gives the mechanical core and dispersion losses of the machine. The real electrical output power of the synchronous generator, in terms of line sizes;

Pout= 1.7320 x Vt x I x cosQ

To summarize, the power obtained and the induced torque in a synchronous generator are exactly as follows;

P=(3 x Vq x Ea x sinw)/Xs  (w angle is the moment angle of the machine. Vq value is considered constant.)

Torque=(3 x Vq x Ea x sinw)/Wm x Xs

This expression defines the induced moment in terms of electrical quantities.