The alternator is an energy conversion device, converting mechanical energy into electrical energy. The Output of an alternator which is the electrical energy supplies its load consumers. Although, it’s quite rare to find an alternator independently supplying its own load. Meanwhile, the process of generating and transmitting is done in a power generating station. These plant stations are interconnected to a national grid, and as such, a very large amount of three(3) phase alternators are connected in parallel.
Parallel Operation of Alternators
The parallel connection of all alternators results in the less significant of a single alternator. Assuming that, the total capacity of the National grid could be up to 50,000MW. But the single alternator connected in parallel to the grid could be contributing just 500 MW. Making its effect less significant. Therefore, it’s quite impossible to see that a single alternator affecting the stability of the voltage and frequency of the grid.
Again, these alternators are connected to infinite busbars. Irrespective of a single alternator effect, its output characteristics still remain constant. Theses characteristics involve constant frequency and voltage.
The necessity of a power station is to supply power to its different load at the various load center. These loads are connected to an infinite busbar at various load centers, supplied by alternators connected in parallel. However, an alternator can be connected or disconnected from the infinite busbar depending on the power load demand. Meanwhile, when an alternator is connected to an infinite busbar, any other incoming alternator won’t cause a change in the voltage. As well as the frequency of the system remains the same.
It’s expected to know that the operation connection of alternator to an infinite busbar is known as Paralleling with the infinite busbar. Although, before an alternator is connected to an infinite busbar, certain conditions have to be met.
Conditions for Paralleling Alternator with Infinite Busbars
The method of connecting an alternator with an infinite busbar is known as synchronizing. While, during the synchronization of an alternator, certain conditions must be considered. Meanwhile, one should note that a stationary alternator( alternator with induced EMF as Zero) shouldn’t be connected to the live busbar. Doing so could result in a short circuit and such, before connecting, the following conditions must be met.
- The voltage generated by the incoming alternator must be the same as that of the voltage on the busbar. That’s the terminal voltage in its r.m.s value. Let’s say, the voltage on the busbar is 330KV. Then the terminal voltage of the incoming alternator should be 330KV, so it could be connected in parallel. Remember that in parallel connection, the voltage is the same but the current is not. However, the voltage can be indicated using a voltmeter.
- The operating frequency of incoming alternator must be the same as that of the busbar. The frequency can be indicated using a synchronizing lamp or a synchroscope. However, the frequency can be adjusted by changing the speed of the prime mover. The prime mover help to convert heat energy into mechanical energy.
- Both the incoming alternator and the busbar must be in phase. This implies that the phase of the incoming alternator should be the same as that of the busbar. Again, the phase can be indicated using a synchronizing lamp or a synchroscope. Meanwhile, the magnitude of the voltage can be varied by varying the field excitation.
- The phase sequence of the voltage of both the incoming alternator and that of the busbar must be the same. The phase sequence can be indicated using a phase sequence indicator.
Advantages of Parallel Operation of Alternators
- Continuity of supply: The priority of a power generating station is to continually meet to load demand. Here, alternators are connected in parallel, which implies that the voltage present is the same no matter if any alternator is cut off. That’s when an alternator gets faulty, the system will still be supplied by the other units.
2. Maintenance and repair: when the alternator needs to be maintained or repair, such alternator can be cut off from the grid without disrupting the power supply. That is, during maintenance or repair, there’s still a continuity of supply.
3. Efficiency: The load connected to the busbar fluctuates. It becomes maximum during the day time and minimum during the night. Meanwhile, alternators operate efficiently at full load. Thereby creating a better efficiency for the system.
4. Load growth: The load demand increases as each day passes. Although, this load increment doesn’t pose any effect to the power station. The reason is that there’s always room for extendibility.
We earlier saw that the method of safely connecting an alternator to a live busbar is known as synchronization. Also, to note that a stationary alternator shouldn’t be connected to a live busbar due to zero emf. Moving further, to see the conditions to be met and how to indicate them. Like that of the terminal voltage that can be indicated using a voltmeter. The phase sequence of the voltage can be indicated using a phase sequence indicator. Meanwhile, the difference between the frequency, phase of the voltage of both the incoming alternator and busbar can be indicated using the following:
- A synchroscope
- Using Three Lamp method
So, let’s take a look at these methods.
As shown in the diagram below, the synchroscope is sort of a motor with a rotating part, housing a pointer. This device has part of its terminal connected to the incoming alternator and the other part of it connected to the busbar. The pointer can either move clockwise or anti-clockwise direction. Meanwhile, when the incoming alternator is running fast. Implying that the frequency of the incoming alternator is higher than that of the busbar. When this occurs, the pointer moves clockwise.
Again, when the incoming alternator is running slow. Meaning, the frequency of the incoming alternator is lower than that of the busbar. Then the device will indicate by moving the pointer in the anti-clockwise direction. Moreover, when the frequency of the incoming alternator is the same as that of the busbar. Then the pointer of the device will be pointing vertically upward, which is at 12 o’clock. The switch of the incoming alternator can then be closed to connect with the busbar.
This method of synchronization of the frequency and the phase voltage is better than the three lamp method. The reason being that it gives an accurate indication as to when to close the switch. Also, it indicates what adjustments should be made should there be any difference in the phase and frequency of the voltage.
Three Lamp Method
From the name, you can easily deduce that this method comprises of three lamps. All lamps have one end of its terminal connected to the connection going to the busbar. While the other end of its terminal connected to the other connection going the busbar. Meanwhile, both connections are separated by a switch, which will be closed when the necessary condition is met.
However, one of the lamps is straight connected to its corresponding phase. While the other two lamps are cross-connected to the other phase. This method as earlier mentioned is used to indicate the frequency difference and the phase difference of the voltage. Meanwhile, when the frequency and phase of the incoming alternator are the same as that of the busbar. When this happens, the straight connected lamp turns dark, with the other cross-connected lamps turn bright. This indicates when the switch of the incoming alternator can then be closed to connect with the busbar.
Although, this method doesn’t tell the adjustments to be made should there be any difference in the phase and frequency of the voltage. Making the synchroscope more superior in usage.
The synchronization of alternators as we earlier saw, is the operating method of connecting an alternator in parallel to the busbar. So, when two alternators are connected in parallel under normal conditions, both remind in stable operation. This implies that both voltages, frequency, speed, and phase are the same.
We can also say that when an alternator is connected properly at normal conditions. Then the alternators will run in synchronism with each other. Although, when an alternator tries to fall out of synchronism. At this point, a synchronizing torque is being produced to immediately bring it back into synchronism. However, this process is done automatically. Therefore, the automatic action of bringing back alternators into synchronism is known as Synchronizing Action.
Conclusion on Parallel Operation of Alternators
In every parallel circuit connection, the output voltage at each point is the same, just that the current isn’t. So, with this principle, in parallel operation of alternators, the voltage should also be the same. The necessity for this action is to create continuity in supply to load demand. Although, before this process is to be met, necessary consideration should be put in place. Meanwhile, it’s rare to actually find a three(3) phase of this device to supply its own load independently except under test conditions.
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