There are two Numbers DG sets are in service. The name plate details are
Make: Cummins
Model: 4BT3 - G3
Make: Newage Stamford
Model: UCI 22-4F

In one Generator the speed hunting problem is persisting. While carrying out EFC calibration the following abnormalities are observed.

1. Idle speed correctly set at 1500 RPM. No hunting. (EFC voltage 6.5 V)

2. With Droop setting at 30% (3% Droop) and gain at 50% when we changed over to Run mode the speed started to hunting. Increasing or decreasing the gain have no effect.

3. Once we increase the droop to more than 70% than the speed is constant.

4. After that the Circuit breaker closed and machine loaded up to 75% load. Then speed started to hunting but stabilized after some time.

5. If again the circuit was opened the speed is again started hunting and tripping at either low or high frequency.

What may be the problem. We changed governor control board (P/N: 3044196) and EFC solenoid. No change in machine behavior.

Compare to other machine the only abnormality noticed is the No load EFC voltage is 6.5V in this machine and the other machine which is stable at no load the EFC voltage is 4.5V only.

1. Kindly advice what could be the problem.

2. Whether the problem is related with tuning. If it is because of tuning how to stabilize the machine in step-2. (That is while changing from idle mode to run mode why the machine is not stable).

3. Why the machine is stable in idle mode and why it is hunting in no load run mode.

4. What is the difference between idle and run. Is it like open loop & close loop control.

5. What are the other parameters to be checked.

 

You need to give more information.

What type of controller? Has the unit ever been stable?

Is this unit synching to a grid or sharing load with other units or is it a stand-alone unit? If synching to some type of grid, how is it "speed hunting"?

Does your controller have online (breaker closed) and offline (breaker open) adjustments?

 

Pmmeeranl... I suggest you search Control.Com archives for similar threads.

Regards, Phil Corso

 

Yes, there is one more thing you can check.

I have seen this where the governor feedback cable was run too close to the output wires for over 50 ft through an unshielded troth in the floor from generator to the secondary switchgear. The feedback was PWM and it was so noisy that it had 30 volts of noise peak-to-peak confusing the governor on the Cat generator.

We solved this by connecting a PULL-DOWN resistor between both feedback wires and ground. One 500 ohm 1 watt resistor connected to each feedback wire and connected the other ends to ground.

The noise no longer showed up on the scope and the generator ran problem free.

I hope this helps,

William Hinton

 

1. It is a woodwards controller for Stanadyne Pumps Using DC-70025 Integrated Actuators. The controller having Magnetic Pickup speed input and idle, run, gain and droop adjustment potentiometers.

2. It is sharing with other generator at the time of paralleling only. Once it is parallel the other generator will be stopped and this generator alone feed the full load. Some time during paralleling it is hunting for few cycles and then stabilize. Some times even at no load or at steady full load it is hunting.

3. The online breaker open and closing are controlled by Deep sea PLC DSE 550 module. This speed governor only controls the EFC only.

 

My first question. Is the problem occurs with the same DG or can be any?

 

The problem occurs in this particular generator only. Other Generator is working fine.

 

Let me try...

> There are two Numbers DG sets are in service. The name plate details are
> Make: Cummins
> Model: 4BT3 - G3
> Make: Newage Stamford
> Model: UCI 22-4F

> In one Generator the speed hunting problem is persisting. While carrying
> out EFC calibration the following abnormalities are observed.

> 1. Idle speed correctly set at 1500 RPM. No hunting. (EFC voltage 6.5 V)

The control mechanism uses EFC 6.5V to supply no load fuel flow to keep your DG at no load RPM.

> 2. With Droop setting at 30% (3% Droop) and gain at 50% when we changed over to
> Run mode the speed started to hunting. Increasing or decreasing the gain have no effect.

Question-If you repeat the same steps with the other (healthy) unit what will happen?

> 3. Once we increase the droop to more than 70% than the speed is constant.

Need to know the answer to the above question

> 4. After that the Circuit breaker closed and machine loaded up to 75%
> load. Then speed started to hunting but stabilized after some time.

Your droop responding to the demand.

I may add later.

Question-What is the droop percentage set point for this part?

> 5. If again the circuit was opened the speed is again started hunting and
> tripping at either low or highfrequency.

I may answer later.

> What may be the problem. We changed governor control board (P/N: 3044196)
> and EFC solenoid. No change in machine behavior.

> Compare to other machine the only abnormality noticed is the No load EFC
> voltage is 6.5V in this machine and the other machine which is stable at no load
> the EFC voltage is 4.5V only.

I reserve my conclusion pending to you answers to my above questions. I may or may not able to conclude. It depends on the given answers.

> 1. Kindly advice what could be the problem.

> 2. Whether the problem is related with tuning. If it is because of tuning how
> to stabilize the machine in step-2. (That is while changing from idle mode
> to run mode why the machine is not stable).

> 3. Why the machine is stable in idle mode and why it is hunting in no load run mode.

The controller starts to define the operating range at no load fuel supply. Starting point can hardly go wrong. When you put the load on, the controller has to seek the correct position between the minimum to the maximum as a function of applied load by trial an error. Note that your DG does not know the load being applied to it. Load/frequency oscillation is expected but it has to be within the tolerable range. Finding the balance between applied load and corresponding fuel flow is not as simple as defining no load position.

> 4. What is the difference between idle and run. Is it like open loop & close loop control.

Idle mean your DG supplies power just enough to rotate its shaft at 1500RPM but produces zero output. Run means you apply load the DG.

> 5. What are the other parameters to be checked.

I may answer it later.

 

Thanks for the reply,

In the healthy machine if gain is increased it starts hunting.

One more information. In Governor calibration procedure the first step is setting the idle speed. Even though the rated RPM is 1500 the manual recommends to set the idle speed between 600 to 650 RPM by adjusting idle speed potentiometer. Then in the run mode the run potentiometer is to be adjusted to 1500 RPM. I want to clarify the difference between idle and run speed. When I checked the healthy machine the idle speed is 1200 RPM. So first we adjusted idle speed to 1200 RPM then to 600 RPM. But till the problem is persisting.

Regards
Pmmeeran1

 

When you increased gain for a healthy unit, what was the droop percentage set point? Was it 30% too?

In your case, idle is you have yet synchronized to an electrical system. You can decide at what RPM you want to keep. Normally the GS supplier will recommend the lowest but stable RPM to reduce wear tear and fuel cost. In run mode the GS is ready to synchronize. You can't synchronize at any RPM. In has to be at a synchronizing RPM. In this case 1500RPM I suppose.

 

In short load/frequency hunting can occur because of the droop percentage set point is too low (percentage value) to a point that the inertia (of rotating mass) of the DS is to small to absorb transient load change. Since the inertia is too small to absorb transient load change, its shaft will be accelerating too fast. If the rate of speed change is faster than response time of your governor, the final result is load/frequency hunting.

That explains why load/frequency hunting becomes negligibly small when you set your droop to 7%. When you increase your gain, then your DS starts hunting again due to same mechanic but diff root cause.

 

When we increased gain for the healthy unit the droop was maintained constant at 30%.

Regards
Pmmeeran1

 

Most likely speed to fuel ratio for the problematic GS has already been disturbed.

From your information these are a few things I can construct.

For the healthy GS:

Idling EFC =1.8V corresponding to 600RPM.

Nominal speed at no load EFC = 4.5V (1500RPM)

Load up to the minimum stable generation just after synchronizing EFC = 10V (Made up number).

For the problematic GS:

Idling EFC =3.8V corresponding to 1200RPM

No load EFC = 6.5V corresponding to say 1800 RPM but it will never make to 1800RPM because the syncronizer (or speed control) will take over the control after the RPM reaches 90% of 1500RPM.

Just after this GS is synchronized I'm expecting the EFC will increase by 6.2V (10-3.8V) as per the healthy unit. Thus the new EFC voltage becomes 12.7V instead of 10V as per the healthy GS. Therefore its load increases above the minimum stable generation. I will explain mechanic of the hunting if this is really the case.

I'll stop here for this possibility to allow us to make a reality check. If I'm right the load for problematic GS will increase a lot higher then the healthy GS just after it breaker is closed. Otherwise, forget about it.

If this is not the case try to look whether or not both droop and gain for problematic GS using the same value of speed reference. They may have their own RPM sensors. The RPM readings being fed by their sensors shall almost be the same. Error as small 0.1Hz can cause hunting.