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# Generator

#### How an induction motor works as a generator during braking?

##### Friday, November 3, 2017

Induction motors produce torque based on the DIFFERENCE between the rotor speed and synchronous speed. The applied line voltage produces a rotating magnetic field whose lines of flux are cut by the squirrel cage rotor windings. When those windings cut flux lines, a current is induced that produces a rotating magnetic field. Thus you have 2 magnetic fields attracting each other. If the induction motor is sped up by an overhauling load, the difference can fall to 0 and thus no current in the ro ...

#### Generator Short Circuit Current

##### Tuesday, May 2, 2017

Assumption: Generator Xd''=0.20, Xs=1.5 pu.
Now, Generally Speaking if we speak of bus fault at generator terminals we say fault current =1 (pu voltage)/0.20= 5 pu. Similarly fault current after 5 seconds or so =1/(1.5)=0.66 pu.

1) The concern is that 1 pu voltage assumed is generator open circuit terminal voltage. So internal voltage generated by machine is 1+(armature current(phasor)*reactance(phasor)). This value reaches up to 1.5-2 times the terminal voltage. All the reac ...

#### Understanding Load Commutated Inverter (LCI) for Gas Turbine Startup

##### Thursday, February 9, 2017

Load Commutated Inverter (LCI) is a system with an input thyristor bridge and an output thyristor bridge. The input bridge is naturally commutated by the AC supply and supplies DC current with harmonics into a large inductor. The output bridge operates at low speed by turning on a set of 2 thyristors to pass the DC current into the generator's stator to cause it to rotate. As it rotate the first set has to be switched off and another pair turned on to keep the generator turning.

Ab ...

#### Can a 3-phase motor be converted into generator without changing its speed?

##### Thursday, December 29, 2016

The answer depends upon the type of motor. A synchronous motor can usually be driven as an alternator by simply substituting a prime mover for the mechanical load. Things are less clear for an asynchronous/induction motor since it will depend upon whether it is attached to a three-phase supply or not.

If there is a three phase source then the prime mover has to operate at greater than synchronous speed, however its output frequency will be locked to the line frequency, assuming th ...

#### How alternator is selected for diesel generator with 10 kW output?

##### Monday, December 12, 2016

From first principle, the diesel engine can run very hard for a short period of time. This is called the STANDBY time. The PRIME POWER is when the engine runs at load factor not exceeding 80% power for any 24 hour period. Then there is the continuous rating which is BASE LOAD. This causes problems when you order a STANDBY generator at say Engine Gross Power of 1437kWm Net Engine power of 1395kWm which gives 1325kWe. Where m = mechanical and e = electrical. For the same set up, PRIME POWER Gro ...

#### Under Frequency Operation of Power System

##### Saturday, October 29, 2016

An established grid operating at a certain frequency (say 60 Hz) would have a certain inertia associated with this particular frequency. Given that it's possible to instantaneously drop the system frequency without losing sync, then, if we operate at a lower frequency, can we get some extra power injected to the system?

The grid standard for power quality dictates the frequency deviations allowable. Keep in mind that some electrical equipment are frequency sensitive. On the other h ...

#### Can I increase the generator output power by increasing its speed?

##### Saturday, October 15, 2016

IF you are connected to STIFF grid, yes. If your generator is connected to a soft grid, it's different.

By definition, if this generator can impact the Hz or V of the grid, it is a soft grid to this generator's control system. For example, two 10MW generators and no grid connection. One generator can "push" the other around up to some point. It is NOT size dependent: you can parallel a 10MW with a 10kW and to the 10kw, it is a stiff grid. To the 10MW, it is a soft grid.

...

#### Over Frequency Protection of Grid Connected Generator

##### Monday, October 3, 2016

Most generator designers use either IEC or NEMA as guidelines; the specific choice of which is applicable is often made by the end user (or at least the one purchasing the equipment). Both of these entities specify "normal" operation as falling within a set range of voltage, frequency, and a combination of both.

Normal operation occurs when ALL of the following are true.
(0.90 PU Volts < operating line voltage < 1.10 PU volts)
(0.95 PU Hertz < operating line freq ...

#### Generators in parallel with high current on Neutral

##### Monday, May 2, 2016

I have two generators 125kW that when they are in parallel and the power is perfectly shared with 60+60kW @PF=1 (resist load) I have 10Amps on neutral. When I unbalance the load (I want remove from grid one Generator) I saw current over 100Amps on neutral, and when the generator is close to have 0kW, current on neutral is over 300Amp.
What's the cause of this problem?

If you have 10kVAR, you have reactive power. Circulating currents have nothing to do with load power factor. Is t ...

#### Choosing Vector Group of Generator between YNd11 & YNd1

##### Tuesday, April 26, 2016

The power plant project is 163MW CCPP and the owner want to select Generator Converter vector group between YNd11 & YNd1. Which vector group should be chosen?

The following will tell you the suggestions with reasons.

Basically this vector group arrangement will define the physical displacement of the generator shaft and the electrical phase of the network. Both solutions can work, the decision depends on the environment in which the generator and the converte will work. I ...

#### Voltage Drop Control

##### Thursday, April 14, 2016

While reading a generator AVR operation description: "When a transformer is used to boost the generator voltage, the transformer impedance provides the DROOP characteristic (voltage drop) and so compensates for any reactive cross current flow. In these circumstances, the AVR does not provide a DROOP characteristic".

AVR of generator has a circuitry which senses reactive component & hence called quadrature droop compensation. When we operate multiple generators in parallel exci ...

#### Forward synchronization and reverse synchronization

##### Sunday, March 27, 2016

Q: I know what is forward synchronization and reverse synchronization. But in the article I read its mentioned that reverse synchronization is dangerous.

1) Why is reverse synchronization dangerous?
2) What are the differences in parameters in auto synchronization for both types of synchronization?
3) If there is no auto sync relay, is there any difference when synchronized manually?

A: In the old days when you had mechanical governors with a lot of droop (the ...

#### Are you oversizing generators?

##### Thursday, March 17, 2016

Over sizing generators is guzzling energy (higher fuel), money (higher cost) and larger space (high footprint). By applying Harmonic Filter, that depending upon load types, say metal, rolling mills, or much easier loads like buildings, textile plants etc., one can load the Generator and Engine by nearly to 100% of its capacity sustainably. In the past few decades, we have done over several hundreds, if not thousands, of jobs applying Harmonic Filter with generators, both LT (415/ 433/ 440V) a ...

#### Non uniform load sharing by generators

##### Saturday, January 16, 2016

Q: We have 02 motor generator sets each of same KW rating and speed etc running in parallel and having pure inductive load. Thrice it happened that 01 set got tripped on reverse power protection. 01 set operates with 0.65PF lagging while other runs with <0.5PF leading. After tripping of MG set, remaining set's current decreases from 42A to 35A. If we want to synch again two MG sets while plant is running at 100% power, is there any danger of tripping?(if remaining set got tripped, plant wi ...

#### How is wind generator treated in power flow calculation?

##### Thursday, January 14, 2016

As with all such questions, it depends (on the purpose of the studies, and the type of wind generator). What is modeled should be what is there. Induction generator turbines should use an induction motor model with negative real power (so that their fault current contribution, and reactive demand increase when (re) accelerating are captured). 100% inverted turbines are usually represented as a static P (possibly Q) load. Double fed turbines...may require careful thought. Obviously, dispatch/s ...

#### Relationship between grid frequency and load in power system

##### Thursday, December 17, 2015

Higher output power of generator (hundreds of MWs) has bigger impact on the system frequencies than low power generators, because of its massive spin momentum (torque). The active (R) load acts directly on generator speed because it creates strong magnetic field in stator that is perpendicular rotor's spinning magnetic field thus forcing an opposite momentum(torque) which slows down the turbine, this is fixed with a speed governor which controls turbine speed by manipulating its spin momentum ...

#### How frequency mismatch leads to grid failure?

##### Saturday, September 5, 2015

We know that the entire grid is at the exact same frequency (50Hz or 60Hz). This is because the grid is made up of synchronous machines. The difference is that of the phase angles. If you go back to the fundamentals of synchronous machines, you will see how important the frequency is to its operation. In fact, power transfer between two nodes depends largely on the angular difference between the nodes.

Load is inversely proportional to frequency. When load increases, that is curren ...

#### How can the vibration be produced on end winding?

##### Monday, August 24, 2015

Vibration is produced by the electromagnetic forces between the current-carrying conductors that make up the end-winding. The forces are proportional to the square of the current, with currents in the same direction causing attraction, and current in opposite directions causing repulsion.

The vibration can be reduced by improved support and bracing the end-winding, and particularly ensuring that the end-winding does not have natural / resonant vibration frequencies at or close to s ...

#### DC Excitation Voltage in Practical 3-phase Generator

##### Thursday, August 6, 2015

Depending on the size of the main synchronous generator (alternator), there can be a number of excitation elements.

For "separately excited" systems, the main synchronous field circuit is fed by DC which is passed to the winding through a mechanical (brush-and-collector) interface. The source feeding the interface can be an old-school DC generator or a dedicated power electronics supply.

For "self-excited" systems, there are typically one (or more) rotating elements invo ...

#### The main causes of voltage drop in generator

##### Sunday, July 19, 2015

Some common reasons are :
1). Load applied to generator exceeds capacity, typically would cause machine to slow down, drop frequency and voltage typically cause exhaust black stacking and depending on the overload and protection settings could cause machine to stall.

2). AVR Voltage gain set too low to respond to load being applied.

3). If AVR feedback is from generator bus VT and voltage dips too far, all excitation could be lost and would typically result in an u ...