When the load on the circuit increases, it is not possible to carry out work by an alternator in the power generation plant. As a result, in that case, two or more alternators are connected in parallel to increase the production capacity and meet the additional load demand. What is synchronizing Alternator? How does it work? That is, we will know everything about synchronization in this article.
What is Synchronizing Alternator
An alternator cannot meet the increased demand when the load on the circuit increases. In such a situation, two or more alternators are called synchronizing Alternators to meet the additional load demand by increasing the production capacity in parallel operation under certain conditions.
What is the purpose of synchronization in alternator?
Parallel operation or synchronizing is done for the following purposes. For example-
1. For getting maximum efficiency: The load demand of consumers is not the same at all times in 24 hours. Sometimes less and sometimes more. So using a higher-rated alternator during low load demand is not at all advisable and also not economical. By installing different alternators of different ratings in the power station, each alternator can be operated at its maximum efficiency by operating two or more alternators in parallel at lower ratings during low demand and during high demand, thereby reducing the production cost (Per unit cost). And efficiency also increases.
2. Service Continuity: It is also the responsibility of every power plant to ensure that there is always supply within 24 hours. Especially in emergency organizations such as hospitals, telephone, telegraphy, radio, television, fire service, airports, etc. Therefore, to achieve this objective, instead of having a single alternator in the power plant, several alternators of smaller ratings are kept. If one of them becomes useless for some reason, the other can be started immediately to continue the service and according to the demand, all the alternators can be operated in parallel to supply higher loads.
3. Repair and maintenance: Every piece of equipment has a lifespan and to increase this lifespan they need to be checked and overhauled after a certain period (Periodic check up). If the power plant has only one large size alternator, all supplies that are never idle must be switched off during operation. So if there are multiple small units instead of one, this task can be solved very easily with success.
4. Future extension of lead: Any plant or organization has some possibility of future extension. Therefore, the plant must always be prepared to carry the additional load that may come in the future. So if the plant has several alternators instead of one, they can be operated in parallel to carry the increased load.
5. Alternator size-shape advantage (Physical sizes of Alternator): If multiple alternators are operated in parallel to meet the specific load demand of a power system, their size-shape and the size of the prime mover are much smaller. However, if a single alternator is used, its size is very large. In many cases, it is not even possible to make alternators with larger ratings.
Alternators are mainly connected in parallel for the above-mentioned reasons. That is, synchronizing has to be done. Let’s know now, what are the conditions for synchronizing.
What are the conditions for synchronization of alternator?
Parallel operation or synchronizing of two or more alternators is possible if the following conditions are fulfilled.
1. The terminal voltage of the alternator must be equal to the busbar voltage.
2. The frequency of the voltage generated by the alternators must be the same.
3. Phase sequence of alternators must be the same. That is, the R.Y. B phase of alternators must be connected to busbar R, Y, and B, and at all times the difference between busbar voltage and alternator terminal voltage will be zero.
All these three are basically the condition of synchronizing alternator. Hope you understand the terms of synchronizing.
Synchronizing in Dark and Bright Lamp Method of Three Phase Alternator?
Synchronizing the dark and bright lamp (off and on) methods are described below. Note the figure below of synchronizing three-phase alternator.
Fig: (A) Dark and Bright Lamp Method of Three Phase Alternator
In the case of a three-phase alternator, only one phase needs to be synchronized. Synchronization of the remaining two phases will naturally occur with the first phase. But the first thing to note is whether the alternator to be synchronized is clearly marked or not. Three lamps are required for this work. How the lamps will be connected is shown in figure (a) above and figure (b) below. Alternator No. 1 above is supplying electrical power to the busbar when running. Its three phases are indicated by symbols R.Y and B.
- Also Read: Types of Torque in a Synchronous Motor
Fig: (B) Three bright lamp method
L1 lamp should be connected between R and R’ phase, L2 lamp between Y and B’ phase, and L3 lamp between B and Y’ phase. In such an arrangement the L1 lamp will be off and the L2 and L3 lamps will be burning brightly just before synchronization. This method of synchronization is called the two bright and dark lamp method.
But if all the three lamps are connected between two similar phases of the alternator i.e. L2 lamp is connected between Y and Y’ and the L3 lamp is connected between B and B’ phase, then all the lamps will turn off together at the moment of synchronizing. This method of synchronization is not widely used.
Fig: (C) Vector diagram of synchronizing alternator in dark and bright lamp method
The relative positions of the three phases of two alternators are shown in the figure above with the help of two vector diagrams. Here the three-phase voltage of alternator 2 is superimposed on the three-phase voltage of alternator 1 (Figure 1). If the frequency of these two alternators is not equal, their corresponding phase voltages will not be in the same direction. There will then be an angular distance between them.
If the frequency of alternator 2 is greater than the frequency of alternator 1, then the voltage vector of alternator 2 rotates in the anti-clockwise direction. This circuit is shown in the figure above. From this figure, it can be seen that the voltage (RR’) at the lamp terminal L1 is gradually increasing from zero value, the voltage at the lamp terminal L2 (YB’) is gradually decreasing from the maximum value and the voltage (BY’) at the lamp terminal L3 is gradually increasing towards the maximum value. in progress Therefore, three lamps L2, L3, and L1 will turn on and off sequentially.
On the other hand, if the frequency of alternator 2 is less than the frequency of alternator 1, the voltage vector of alternator 2 will continue to rotate in the clockwise direction. Its rotation is shown in the figure above. In this figure, it can be seen that L3 lamp terminal voltage (BY’) gradually decreases from the maximum value, L2 lamp terminal voltage, YB’ increases gradually towards maximum value, and L1 lamp terminal voltage RR’ gradually decreases and approaches zero.
Therefore, in this condition, the three lamps L3, L2, and L1 will turn on and off sequentially. The switchboard usually has three lamps mounted on the three corners of an equilateral triangle. Therefore, if the frequency of Alternator No. 2 is higher or lower than the frequency of the busbar, it can be understood only by observing the alternating switching on and off.
(R & R’), (Y & Y’) and (B & B’) will be superimposed on each other at the moment of proper synchronizing. That is, there will be no angular distance between them. As a result, the L1 lamp will be extinguished and the L2 and L3 lamps will continue to glow brightly. In this condition, turn off the main switch of alternator No. 2 and connect the alternator to the busbar.
Alternators generate electrical power at high voltage. The lamps cannot be connected directly to the led while synchronizing them. In all these cases the lamps are installed by reducing the voltage with the help of a potential transformer (PT). The lamps are supplied with electricity from the living room. It is called synchronizing busbar.
- Also Read: Transformer Differential Protection System
Synchronizing method using synchroscope
It is very difficult to determine the exact moment of synchronizing with the lamp. Most of the time it is not accurate and effective. To overcome this difficulty, most of the synchronizing panel boards use an improved device with the lamp. The name of this device is synchroscope. It consists of three fixed coils and a rotating iron spindle. A pair of three coils is connected to one phase of the busbar. Below is the vector diagram of the synchronizing method using a synchroscope.
Fig: (D) Synchroscope circuit diagram
The alternator to be synchronized, the third coil is connected to a phase of the alternator in the same phase as the first two coils are connected to the busbar phase. An iron spindle is fitted with an indicating fork, the fork rotates on a scale. Fast is written on one side of the scale and Slow on the other. Zero is written between these two writings.
The difference between the frequency of the alternator and the frequency of the busbar is the number of times the spindle rotates per second. If the frequency of the alternator is high, the needle of the synchroscope will turn in the direction where Fast is written and if the frequency of the busbar is high, the needle will turn in the direction where it is written Slow.
The synchroscope has a scale marked up to 360° degrees. The degree the indicator fork points are the degree of angular distance between the alternator and busbar voltage. The moment of synchronizing must be understood when the fork stops exactly at the zero value. A synchroscope is shown in figure (d) above.
What is synchronizing current and synchronizing power of Alternator?
When two alternators are running in parallel, they are naturally in synchronism with each other. If for some reason an alternator wants to deviate from the synchronism condition, a rotor is created between the two machines which restore the alternator. This rotor is called synchronizing torque.
For some reason, if the speed of alternator No. 2 (Figure – d) decreases a little, then the electric pressure of this alternator is not exactly opposite to the electric pressure of Alternator No. 1 but it forms an angle with each other. As a result, an integrated electric force is created. Due to this integrated voltage, some additional current is generated in addition to the armature load current of the 1st alternator. And that current enters the armature of the 2nd alternator through the busbar. This current is called synchronizing current.
As a result of the flow of synchronizing current, the 2nd alternator receives some electric power from the 1st alternator, increasing its speed. At the same time, the speed of machine No. 1 is reduced slightly to supply the electric power, and thus the voltages of the two machines are again opposite to each other. The electric power that flows from the 1st alternator to the 2nd alternator is called synchronizing power.
Effect of unequal voltages on synchronizing two alternators
The figure below shows two unequal voltages acting in phase.
Fig: (F) Unequal voltages on synchronizing Alternator
Suppose, E1 > E2 then gain voltage Er = E1 – E2. will be, which (Er) will act in phase with E1, and Er or Esy will generate the local synchronizing current (Isy). Isy will work at 90° lagging of Esy. The current will reduce I1 by causing the de-magnetizing effect of the first alternator.
As a result, the other alternator will act like a synchronous motor by taking a 90° leading current and due to the magnetizing effect of the armature reaction, its magnetic field will become stronger and E2 will increase. Due to these two effects (working together) the alternator returns to a stable condition at unequal voltages.
That is, the synchronizing current helps maintain synchronism by leading the leading alternator and the lagging alternator.
How can the load sharing between the alternators operating in parallel be changed?
By changing the field current or excitation of an alternator, its apparent power (KVA output) can be changed. But there is no change in the actual power (kW output) of the machine. As the excitation changes, the reactive component of the armature current changes. As a result, the power factor of the machine changes. When the field is overexcited, the power factor is lagging and when the field is under excited, the power factor is leading.
That is, in the first case the armature current will lag behind the terminal voltage, and in the second case ahead. But in this case, there will be no change in the effective part of the current.
Therefore, to increase or decrease the load of an alternator, its field tension cannot be increased or decreased. It is possible to meet the load demand by increasing or decreasing the power of the prime mover of the machine.
The machine that rotates the rotor of the alternator is called the prime mover. For example, steam engines, steam or gas turbines, water turbines, diesel engine,s etc. When two alternators are running in parallel, the load of one alternator on the other will decrease the prime mover power of the first and increase the prime mover power of the second.
Fig: (G) Load division of alternator
Assume, that Alternator No. 1 is alone carrying the full load while running. Now turn on Alternator No. 2 and connect it in parallel with No. 1. As a result some load of No. 1 can be carried by the No. 2 alternator. First No. 2 should be synchronized and connected to the busbar. But in this condition, the 2nd alternator will not supply any power to the busbar. In order to supply electricity with the help of No. 2, the power of its prime mover should be gradually increased.
That is, if it is a steam engine, more steam must be supplied. In this condition, Alternator No. 2 will continue to supply more and more power to the busbar.
But as the load of the entire generating station remains unchanged, the load on alternator 1 will gradually decrease. Thus the power supply to the prime mover controls which alternator can be engaged to carry a specific load.
Now suppose Alternator No. 1 is to be completely stopped and Alternator No. 2 alone carries the full load. In this system, the power of the No. 1 prime mover should be gradually reduced and at the same time, the power of the No. 2 prime mover should be gradually increased. When the ammeter reading of the 1st alternator reaches zero (0) its main switch should be opened. Then, if the power supply to the prime mover of the No. 1 alternator is completely stopped, the machine will slowly come to a standstill.
How is an alternator turned on? -Steps:
The alternator requires prime mover and exciter first. The alternator is rotated with the help of a prime mover. That is, when the rotor is rotated, kinetic energy is obtained. DC to supply DC through the field coil of the alternator. A shunt generator is used. This is called an exciter.
The operating steps of the alternator are described below—
1. First start the prime mover and check whether the prime mover rotates smoothly or not. For example, if the prime mover’s lubrication system, cooling system, fuel system, etc. are working properly if found correct, the speed of the prime mover is increased to synchronous speed and the alternator is coupled. In this condition, the alternator load is at zero.
2. DC supply is provided to the field coil with the help of an exciter driving the alternator at synchronous speed. For excitation, a DC shunt generator is usually driven by coupling to the shaft of the prime mover. When the prime mover starts running automatically the DC generator also starts generating electricity. A rheostat is connected to the field circuit to control the amount of power supplied to the field coil. As a result field current can be changed from zero to the maximum value.
3. If the alternator is operated in parallel, synchronizing must be done before connecting to the busbar. All the conditions of parallel operation of a three-phase alternator have to be properly fulfilled. There are various methods of synchronizing. But now it is possible to synchronize two alternators very easily with a synchroscope.
8. Now slowly operate the alternator for 15-20 minutes by providing a light load (20%) connection. If no difficulty is observed then the full load is slowly added.
Related Articles:
- What is Alternator? Working Principle of Alternator
- Excitation System of Alternator, Brush, Brushless
- V Curves of a Synchronous Motor: Fully Explained
- Types of Torque in a Synchronous Motor