Parralleling of generators. 4

[elvey]

Can anyone advise on the problems with regard to the parralleling of two or more 3 phase generators. Must they all be of the same output? how do you get them all to "lock " into the same phase sequence. I realise that if the phase sequence is not synchronised it would be quite dangerous.

 

[peterb]

The generators don't have to be the same output - there are several options for adjusting their governors and voltage regulators to share real and reactive load equitably once they have been paralleled.
The phase rotation (sequence) of the units must be the same to allow them to parallel. You get them to lock in together by synchronizing them. This consists of adjusting the voltages to be close together and then adjusting the speed of the incoming machine (the one being synchronized to the running machine) to be equal or slightly faster than that of the running supply. When the vectors of the two machines are in phase, the circuit breaker is closed to parallel them.
Synchronizing instrumentation that facilitates the process would include a minimum of incoming and running voltmeters and a pair of "dark lamp" indicators. The dark lamp indication uses two lamps connected in series between one phase of each of the two supplies; when the two sources are in synch, voltage is a minimum and the lamps are dark; when the sources are out of phase, the lamps are on with variable brightness (depending on the phase difference). Note that this is a very crude indication but effective indication that would only be used for small generators. Larger units would require incoming and running voltmeters and frequency meters as well as a synchroscope, which is a 360 degree instrument that shows the relative phase angle between the incoming and running supplies - traditionally this is an electromechanical movement instrument, but solid state indicators are also available. Automatic synchronizers are also available which take any guesswork out of the process.

Synchronizing out of phase is a real hazard, as the rotors will lock into the same phase position when the breaker is closed. If there is a large phase difference, the resultant current can exceed the fault current capability of the generator and the shock loading can damage both the rotor and stator, as well as the prime mover and/or gearbox. On the other hand, a small voltage difference when synchronizing in-phase will not be so severe, as this results in a reactive current inrush only that does not stress the generator windings.

 

[jbartos]

Generators should have approximately same voltage magnitudes, the same phase rotation sequence (ABC or ACB), frequency, time interval for synchronization, phase voltage displacement angle between phase A of generator A and phase A' of generator A' must be small, e.g. 45 degrees or less. The automatic synchronizers have various settings and overrides, see Beckwith Electric Co., Inc. (813-535-3408, in Largo, Florida, USA)
1. Automatic synchronizer "Synchrocloser Check Relay M-0188,"
2. Synchrocloser Line M-0388 Sync-Check Relay which comes with Voltage Verifier M-0399
3. Etc.
for example.

 

[Dan76]

Running two generators in parallell is ok but 3 or more can be difficult especially if loads are cycling (at least some cross-over circuitry is needed between the regulators)

One way to get generators to share load is to introduce a slight (1-5%) Voltage and Frequency droop from No Load to Full Load. Equal droop will cause the machines to share load in proportion to their ratings (Output).

Voltage droop can be adjusted by a feed back circuit in the Voltage regulator, which senses current and reduces excitation accordingly.

Speed droop can be adjusted by a similar feedback circuit or by adjusting the mechanical linkage to the actuator on the prime mover (Steam or Fuel throttle) I'm not sure how to do it for a Gas Turbine but it will work on Diesels and Steam Turbines.

Another note: If a Steam turbine and a Diesel are in Parallell they dont always "play well" together since diesels have resistance due to compression and turbines only have Counter EMF (Load) as a resistance (Mechanical Resistance and windage etc.. are neglgible since most condenser are around 28" Hg Vacum) they also have a different response time making control somewhat difficult.
This also leads to poor speed control at light loading for turbines.

Also, In regards to jbartos' response: 45* Phase difference seems a little high I Think around 10* would be better much more and you get voltage and frequency transients (not to mention the machines make one heckuva noise!)If Your using a synchroscope shut the breaker when the needle is at about the 11:45 position this accounts for the breaker taking that fraction of a second to shut. The needle should be rotating clockwise (Fast direction) around 4-8 RPM This allows the incoming machine to Immediately assume some load and reduces the risk of reverse powering a generator.

Hope this Helps, Dan76

 

[jbartos]

Suggestion to Dan76 Nov. 30:
Please, would you convey your suggestion about the 45° phase voltage displacement angle to the Beckwith manufacturer to make the scale on their automatic synchronizing equipment according to your experience since the scale is up to 45°. Else, I would go by the size of generators and their prime movers, loads, etc. at the moment of synchronization. I am sure that some generator synchronizing cases would accept the voltage phase displacement angle more than 10° (may be two unloaded small (1kW) diesel generators, if one happens to use the automatic synchronizer at all).

 

[jack6238]

A couple of comments concerning paralleling generators and the angle between the vectors at the instant of closure.

Let's take a system which is operating at 13,200 volts and it is desired to bring another generator on line with the existing unit. As mention by several others, the incoming machine should be running at a slightly higher speed and a slightly higher terminal voltage than the machine which is on the bus. The higher speed is indicated by the synchroscope rotating in the fast direction and the voltage is indicated on their respective maters. Two posts have indicated different closure angles.

The higher speed of the incoming speed is desired because the incoming machine will immediately assume some of the bus KW load. The slighly higher incoming voltage will cause this machine to assume some of the KVAR bus loading.

Let's assume you have a bus voltage of 1 per unit voltage(13,200 volts) and the incoming machine has a voltage of 1.02 per unit ( 13,464 volts). If the paralleling circuit breaker is closed when the closure angle is 10 degrees, the voltage across the breaker contacts is .1772 per unit. When the closure angle is 45 degrees, the voltage across the circuit breaker contacts is .773 per unit-a very large value. Converting this information into actual voltages for a system operating at 13,200 volts phase to phase results in the following voltages on this system.

10 degrees closure. The line to neutral voltage is 1,350 volts across the breaker contacts.

45 degrees closure. The line to neutral voltage is 5,891 volts across the breaker contacts.

The instant the breaker contacts are closed the voltage difference must instantaneously become zero. This is accomplished by a rapid change of vars between the generators. This change of vars should be controlled to avoid upsets on the electrical system. The smaller the closing angle, the less the effects on the system. The voltage across the breaker contacts can be determined by using the law of cosines. It can be seen that the voltage across the breaker contacts is a function of the cosine of angle between the two vectors. It can be demonstrated that the closing angle has much more impact on the voltage across the contacts than the magnitudes of the inoming and bus voltages.

In summary the smaller the angle of closure, the less the effects on the system. I only discussed the effects on the systems voltages and there is another part of the story and that is the mechanical effects associated by lagre closure angles. The mechanical shock is also proportional to the closing angle and the smaller the angle, the less the shock. If the closure angle is excessively large and the prime movers are steam turbines, you had better have spare thrust bearings available and an alternate source of power.

One other note concerning power flow into a machine. Power flow into a generator can create a serious condition for the prime mover (steam and diesel) but generally this is only a concern when the steam rack or fuel rack is in a closed position. A closed steam rack deprives the turbine from the cooling effects of the steam and the unit tries to act as an air pump which will overheat the turbine blading and causes distortion. In a diesel unit there is a risk that some unburned fuel remains in the diesel and this creats an explosion hazard. To prevent these conditions for extended periods of time ( ~ > 10 seconds), a directional power relay is installed on each unit capable of being operated in parallel. Some refer to this device as an anti-motoring relay.

 

[Dan76]

Reply to jbartos 30Nov00: Sorry if I came off wrong, I was just relating my experiences as a Navy Electrician where I often parallelled generators and was speaking from an operators viewpoint. 45* just seemed high compared to how I was trained. Manual Synchronizing on 480V 60Hz 3-Phase Machines (Diesel and Steam Driven)in the low (single digit) MW range, and NO reverse power relays (Due to system design, based on how external power was recieved). 10* was an arbitrarty number based on observed scope position and would probably be low for a small lower voltage generator and high (as shown in jack6238's calculations for a large higher voltage generator). The angle would also change in value depending on how often you parallelled the machines, based on the effects on the circuit breakers mainline and arcing contacts, less voltage difference = less wear and tear -> less work for the electricians. Your careful when you fix your own mistakes.
-Dan76