Paralleling dissimilar Generators

[NickParker]

What factors should be considered when operating generators in parallel when they have different pole configurations and different prime movers (high-speed vs. low-speed), but maintain the same electrical frequency? Specifically, what challenges might arise when running large diesel generators, small diesel generators, and steam generators in parallel?
are there any potential issues with prolonged parallel operation? Could there be tuning or synchronization difficulties? circulating currents would be there depending on the pitch?

 

[wcaseyharman]

Droop is your friend.

Use frequency droop. 3-5% is typical.

If the units connect to the same bus without transformers, assuming the units are in voltage control, use voltage droop to prevent the AVRs from fighting each other. You might use an external voltage setpoint once the units are online to make sure all the AVR setpoints are the same, otherwise you could get units absorbing and producing VARs unnecessarily.
If you have high resistance grounded generators connecting to the same bus without transformers be aware that all the machine neutral overvoltage protection (if equipped) will see a fault in any machine, so you will need to consider that in the protection.
If you have large and small machines with integral breakers carefully consider interrupting ratings - having multiple machines on the same bus could result in much higher available fault current.

That’s all I can think of at the moment. The grid is, by definition, a collection of all sizes of machines of different technologies, so I see no reason why your proposed system wouldn’t work provided it’s set up right.

 

[waross]

You may run without AVR droop.
In that case, the individual voltage settings influence the VAR sharing.
I have had issues with similar sets but with different governors picking up large motor starting loads.
I had two diesel sets in parallel, one with a hydraulic governor and one with an electronic governor.
We had one large motor that needed both sets on-line in order to start.
When the sets were started in the morning, if a start attempt was made with the large motor, the electronic governor would react quicker and hog the load.
The set would then trip off on over-load.
That would leave the starting load on the set with the hydraulic governor which would then also trip of on overload.
After 20 or 30 minutes of running, the oil in the hydraulic governor would be heated up enough that the governor would respond quick enough to start the large motor.
You may also experience block load sharing issues if one set has much more inertia than a parallel set.

 

[waross]

Use frequency droop. 3-5% is typical.

3% is typical for islanded sets.
5% is typical for grid connected sets.

 

[NickParker]

Droop is your friend.

Use frequency droop. 3-5% is typical.

If the units connect to the same bus without transformers, assuming the units are in voltage control, use voltage droop to prevent the AVRs from fighting each other. You might use an external voltage setpoint once the units are online to make sure all the AVR setpoints are the same, otherwise you could get units absorbing and producing VARs unnecessarily.
If you have high resistance grounded generators connecting to the same bus without transformers be aware that all the machine neutral overvoltage protection (if equipped) will see a fault in any machine, so you will need to consider that in the protection.
If you have large and small machines with integral breakers carefully consider interrupting ratings - having multiple machines on the same bus could result in much higher available fault current.

That’s all I can think of at the moment. The grid is, by definition, a collection of all sizes of machines of different technologies, so I see no reason why your proposed system wouldn’t work provided it’s set up right.

It's a combination of medium-voltage and low-voltage generators, with a step-down transformer (MV/LV) in between.

 

[waross]

The important thing is prime mover characteristics and governor characteristics.
Quadrature compensation will share the VARs.

circulating currents would be there depending on the pitch?

Different pitches will cause small circulating currents at no-load.
These currents are generally swamped out by load currents when under load.

 

[waross]

All sorts and sizes of generators are connected to the grid.
But, a block load such as starting a large motor, which may be insignificant on the grid, may become significant on an islanded group of generators.
That is why set inertia and governor response becomes a more important factor.

 

[wcaseyharman]

In my region, the western us, the regional planning entity (Western Electric Coordinating Council) has a guideline for frequency droop from 3-5%. We’ve set our larger hydros at 3% to improve the frequency response in our balancing area. Gas turbine droop typically is 4% and it’s trickier to change so we’ve left it there.

 

[waross]

Something to think about.
On islanded sets, block loading may be a significant percentage of the total capacity.
Apart from major faults, block loading on the grid may be a much smaller percentage of the grid capacity. 5% droop on the grid may result in less frequency deviation than 3% droop on an islanded plant.

 

[davidbeach]

How big a system? The Western Interconnect has hundreds of dissimilar, random, generators all running in parallel with no particular problems… The Eastern might well have more.

 

[crshears]

More problems, or more generators?

 

[NickParker]

More problems, or more generators?

We have three essential diesel generators, one non-essential steam generator, and one emergency diesel generator. All these generators should be capable of operating in parallel when required, either in response to specific situations or for periodic testing as per the specification.

 

[NickParker]

How big a system? The Western Interconnect has hundreds of dissimilar, random, generators all running in parallel with no particular problems… The Eastern might well have more.

This is a small ship board system. (3 Diesel generators + 1 steam generator + 1 Emergency generator)

 

[TugboatEng]

Are you still fully analog or do you have a digital control system such as Woodward EZ Gen? Analog systems require a lot of tuning while digital systems can command specific loads which simplifies operations.

 

[waross]

Long story short:
The sets will share steady loads depending on the individual speed and droop settings.
A set with a faster acting governor than the other sets may hog the load and trip out if the system is subject to a block load greater than the protection setting of the faster responding set.

 

[NickParker]

Long story short:
The sets will share steady loads depending on the individual speed and droop settings.
A set with a faster acting governor than the other sets may hog the load and trip out if the system is subject to a block load greater than the protection setting of the faster responding set.

But this risk can be eliminated by adopting the power factor mode (AVR) / Fixed power mode (Governor controller) for fast acting governor. Correct?

 

[waross]

But this risk can be eliminated by adopting the power factor mode (AVR) / Fixed power mode (Governor controller) for fast acting governor. Correct?

The AVR controls the field strength and thus the voltage set-point.
In a parallel set-up,, voltage adjustments on one set control the reactive current of that set and have negligible effect on real power.

Real power is controlled by the throttle setting of the prime mover which is controlled in turn by the governor.

I find it difficult to visualize how AVR setting will affect governor issues.

 

[wcaseyharman]

Generally you want fast acting governors and AVRs to maintain consistent voltage and frequency to the load. I wouldn’t advise block loading or PF mode.
(The US western grid tried something similar in the 1990s and dumped California in the dark. It was shortly after that PF and VAR modes were not allowed in the western interconnect)

 

[waross]

I wouldn’t advise block loading

Respectfully Casey, I am responding to a special case that I have encountered several times.
The case of islanded sets and motor starting:
It may not be possible to avoid block loading when a large motor is to be started on a relatively small plant.
I have experienced situations where at least one extra set needed to be online before starting a large motor.
Once the motor was running, the extra set could be dropped off.
The difference between this and a utility was that the motor starting load would trip the generator breaker if a start was attempted on only one gen-set.
An added factor was the case of a hydraulic governor paired with an electronic governor.
With islanded sets that sort of grew as a facility expanded over time there may be a mix of mechanical governors, hydraulic governors and electronic governors.
With droop, these governors generally work and play well together.
The exception is where a block load exceeds the capacity of a set with a faster acting governor.
The characteristics of the prime mover may be an additional factor:
eg; Naturally aspirated vs Turbo Aspirated vs Steam Driven.
The issue mostly arises when one large load exceeds the capacity of a set with a faster acting governor.
Other than that special case, droop will sort everything out.

 

[wcaseyharman]

I defer to your experience, I’ve done a few islands/Microgrids but most of my experience is with larger grid tied units.