Synchronous Generator vs. Induction Generator

Right from commissioning, these units never generated the targeted annual energy generation of 2 MU each

2 mini hydel units of 350kW and 400kW capacity with induction generator

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What is 2 MU EACH ? IS IT SOME Energy Unit? In what Time?

Questions: 1 Location Same?Head same?

2 Identical flow-through both-- ALL THE TIME?

3 Did the one labelled 350KW - show generating 350KW ( in the Power Metering?)

4 Did the one labelled 400KW - show generating 400KW ( In the power metering?)

5 why 350 and 400 for identical parallel installations?

If answers to 3&4 are YES -- how can Synchronous Generators produce moreKW?

The 350kW unit is installed in the Ganekal powerhouse and the operating head is 3.81m. The 400kW unit is installed in the Kalmala powerhouse and the operating head is 7.62m. Both these powerhouses have been erected along the Tungabhadra left bank canal.The distance between these powerhouses is around 6 to 8km. When these units are running at full load, the kW meters indicates 350kW and 400kW. Both these units are connected to the 11kV grid. The low annual energy generation is due to the powercuts on these lines for about 12 hours a day. Since induction generators have been provided for these units, they cannot generate in isolation but can only generate when the 11kV line is charged. However, if the 11kV line is charged and there is no powercut, at least on paper, the annual generation of both these units should be much higher

My question is, if these induction generators are replaced with synchronous generators, will the annual energy generation be much higher (assuming that there are no powercuts). If there is no significant energy increase (other than efficiency) will it be better to continue with the induction generators instead of replacing them with synchronous generators?

SAMUEL CHELLIAH

The out put as pointed out should be same for any machines of same rating if their efficiencies are also same.Induction generator has certain advantages in terms of maintenance .If u replace the induction generator by synchronous generator continuous power will be ensured .However maintenace and DC source for excitaion.Normally these power houses being in remote areas excitation has to be inependent.Wheter ur turbines can additionally deliver reqiured to generate excitation power has to be looked into.

I too had problems with your units of measurement, they are unfamiliar in my neck-of-the-woods...

Be that as it may, I think you are mixing up some issues here. Synchronous generators do have a higher efficiency coefficient, so yes they can produce more energy for a given PHYSICAL size, but a 350kW rated generator will provide 350kW, regardless of the design. It's just that an asynchronous generator will be larger for the same output. That is an important issue when designing wind generators where the unit size makes a big difference.

But synchronous generators run at very low speeds, which typically means a gearbox is necessary and that increases the wear parts and maintenance.

The biggest issue from your stated scenario however is not the efficiency, it is power reliability. Asynchronous (induction) generators DO NOT generate unless connected to a grid (network) because they must derive their excitation current from somewhere else, i.e. from the grid. So the generally accepted "rule of thumb" is that a facility should not rely upon asynchronous generation for more than 1/3 of their total power needs. The reasoning is that if you try to get more, you run the risk that any significant load change or short circuit can cause the excitation current to drop, and that creates a cascade of energy output loss which quickly ends in total failure.

Synchronous generators are separately excited and can get that excitation power from any DC source, even batteries in order to get them up to full output, at which time they can sustain themselves. This makes it possible to have output (power) in what is called "island mode" when the normal utility power is off-line. You cannot have that with asynchronous generators.

Good reply . Very educative.

JRaef,

You are right with regard to sizes, efficiencies, etc. The only item that I am not OK with is the fact that you state that an asynchronous (induction) generator cannot work unless connected to a grid. If connecting a capacitor bank to the output terminals of the induction machine (parallel to the load), that bank provides the reactive power necessary for its magnetization and it (the machine) becomes a generator. Definitely when connecting an induction generator to a grid, the voltage & requency are imposed by the grid and basically are considered constant. In case of a standalone generator the regulation of the output voltage & frequency is more difficult, since it has to be correllated with the rotation speed of the drive motor (rpm) to keep frequency constant (taking into account also the slip); changing the capacity of the condenser bank (which usually cannot be done continuously, but only in steps) keeps the output voltage variation within the prescribed limits.