Capacitor Bank Sizing 1

[BoyNaruto]

Dear Gentlemen,

Anyone with experience in capacitor bank sizing calculations? I have a 280KW Vane Type Compressor and it is to be powered by a 400KVA Cos=.80 Diesel Generator, however problems occured during our test run. Initial Star-to-Delta start up is fine until the pump reaches 11Bars and automatically stops. But during the auto start again, when the tank is still filled with 8Bars, then the starting really gets too much high current from the generator at times almost "killing" the genset, and so therefore, my control voltages for the relays is affected. Voltage errors occurs and everytime I have to re-start the compressor manually.

And for this, I suggested to use a capacitor bank having a 300KVAr capacity. From my calculation, I used:

Step1:
For Maximum Power Factor Correction = 100%
Required KVar to improve from .80 to Unity
Capacitor Bank Rating (KVar) = 300KW x .750 (Factor) = 225KVar.

Step2:
Required KVar to improve from 0.80 to .95
Capacitor Bank Rating (KVar) = 300KW x .0.421 (Factor) = 126.3KVar

Step3:
Required KVar to improve from 0.80 to .90
Capacitor Bank Rating (KVar) = 300KW x .0.266 (Factor) = 79.8KVar

From these values, I choosed the higher rating, 300KVar from ABB.

Can someone here has some other calculations? Suggestions?

Regards.....

 

[LionelHutz]

Can you unload this compressor until the motor is running at full speed?

From your description, the compressor is pumping as soon as the motor begins to rotate. This is a much higher load than a compressor with an unloaded.

 

[rbulsara]

Agree with what LionelHutz said. Caps does not sound like a solution to your problem. You need to review your starting methods and conditions. Plus generators do not like capacitors, in general.

Also your PF calcs are for kvar required during a 'steady' state for PF improvement. It has little to do with holding up voltage during the start.

Also a 400kVA/320kW genset sounds too small to start a 280kW motor on 'load'. You must start he motor unloaded.

Rafiq Bulsara

http://www.srengineersct.com

 

[BoyNaruto]

There are few minutes delay before the compressor pumps. But this is I'm a bit confuse, how come during initial run when the tank is not being filled with air, the motor runs normal and the generator runs also normal.

When the pressure reaches to about 11bars, then the whole compressor stops and wait until the minimum pressure is reached back.

When the pressure in the tank reaches 8bars (due to air consumption), then the motor runs back, but this time, during star to delta transition, I can see hesitations on the generator, the load voltage drops from 380V down to 330V. This low voltage drop causes my relays to malfunction thereby not switching the air solenoid automatically.

I was thinking of lowering the pressure limit switch down to 6Bars, but I'm not sure if this will help me up. Also another solution that I was thinking is to put UPS on the control supply, so even though the power goes down, the UPS can still supply my relays and contactors.

Appreciate your help. Thanks

 

[rbulsara]

In the beginning there is no back pressure as the air in the tank is not pressurized. Once pressurised and trying to compress against already compressed air (8 bar), the motor/compressor sees a lot more load due to the back pressure. Its like starting a car from the rest halfway on a steep hill.

Check the compressor manual and consult your mechanical engineer, there would be a unloading valve. It will unload the motor during the start (bypassing the tank). And re-close it slowly once the motor has reached the full speed.

Also consider a soft starter vs. the star-delta one, they are obsolete or at least use a closed transition star-delta starter. The transient during star to delta transition is one of the drawback of Star-D starters and a big one at this size motor.

Other points are already made.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Fix your unloader mechanism. Even on the grid you will have problems starting a 280 kW motor against 8 bar.
Most of us would have sized the generator at 700 kW to 800 kW.
You are fortunate to be able to start the compressor even unloaded on a 400 kW gen-set.
But you may want to brush up on power factor calculations.
I get 168 KVAR to improve the power factor from 0.8 to unity.
But back to motor starting on a generator.
There are several issues.
The first issue is generator stalling;
The real load on the generator is the energy required to accelerate the motor and compressor to running speed and supply the heating losses of the starting current. These factors are what slows or stalls the gen-set and are not affected by capacitors.
The next issue is the starting current. The high starting current of a motor causes a severe transient voltage drop on the generator, much more severe than on a transformer.
The Automatic Voltage Regulator quickly increases the excitation to return the voltage to the set point. The voltage regulator is fast in relation to motor starting time.
This current may be reduced by the application of capacitors, BUT:-
Back in 1961, the Electric Machinery MFG. Co. (The holders of the original patent on the Polarized Field Frequency Relay) published calculations showing that the starting current may be reduced to about 50% of normal starting current by the application of capacitors to supply part of the reactive component of the highly reactive starting current.
BUT:-
The the amount of capacity required is about 290% to 300% of motor capacity, or over 800 KVAR of capacity.
And this must be switched out in steps as the motor is accelerating.
And:- Any failure of the control gear that leaves some or all of this excess capacity connected to the generator when the motor is not starting may result in generator instability, over voltages and possible damage.
Furthermore:- I have never seen or heard of this starting method applied in the field, even on a grid installation where it may have fewer issues than a generator installation.
Forget Capacitors for your problem.
Another issue may be Under Frequency Roll Off. This is a feature of most modern AVRs that reduces the voltage set point if the frequency drops. UFRO normally allows 3 Hz drop in frequency and then reduces the voltage in direct proportion to the frequency.
This may be working to your advantage. It maintains the V/Hz ratio to the motor and gives starting characteristics somewhat similar to a VFD. This feature may be working to your advantage.
WHAT CAN YOU DO?
First, fix your unloader. If need be, add a large dump valve and downstream check valve to the compressor discharge. I recommend power to close so as to avoid low voltage issues when the motor is starting.
If low voltage is causing mis-operation of relays, consider a small UPS to supply control voltage.
All of this will be much less expensive that adding capacitors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BoyNaruto]

Hi Waross and Rbulsara,

Thanks for the informative response. I will try it sooner and hope I can give you some feedback to my problem. As of now the generator combo with the compressor is unusable because of this auto starting problem.

I hope I can resolve this by not buying the capacitor banks, its gonna be very expensive since my boss plans to buy this from ABB Switzerland.

Attached are some of the pages from the Manual, please help me identify where would be the UNLOADING valve. Thanks in advance, appreciate it too much to be here...

 

[rbulsara]

Firstly, do not buy the capacitors as it is not the solution.

You have more than one deficiencies in your installation. You really need to consult the supplier or the designer of the original package and see what they say. Failing that you need to hire some professional, perhaps both mechanical and electrical.

I would hope that some mechanical folks here would chime in, if they see this.

Having said that, from the description it appears that the controls open both the inlet and regulation valve upon transition from star to delta. Is there a time delay between the starter transition and the opening of the valves? A time delay could help, try that manually. Failing that you may need to add/replace the valve with motorized controls to open the valves slowly, if at all possible. (This is where my mechanical expertise begin to fade......)

I also believe that your generator is seeing a significant step load (block loading) and it cannot keep up its voltage and frequency. The kVA capacity of the alternator may be too small as well even if the kW rating may be sufficient.

The bottom line, consult the mfr and/or a mechanical engineer to sort out your unloading issue. Failing that you need to upgrade your generator capacity.








Rafiq Bulsara

http://www.srengineersct.com

 

[jcarrr]

Read the manual for the generator. Make the adjustments for stability and voltage roll off. If the roll off is set wrong the voltage regulator will try to maintain rated voltage as the frequency dips while starting loads. The high voltage will cause current to greatly increase as the load impedance decreases as the result of reduced frequency, further bogging down the engine. And so on. If the voltage will drop faster than the speed decreases the system will have a much greater chance of starting the load. Start the roll off at about 59.5 Hz.

 

[waross]

Forget capacitors, forget UFRO, leave it at the normal factory setting, 57Hz. Many AVRs have a jumper, 50Hz or 60 Hz. The actual operating points are 47 Hz and 57 Hz respectively.
Large compressors are expected to start unloaded.
This compressor does start properly when it is unloaded. The problem is that it is not unloading properly.
This is mostly a mechanical problem. There may be a wire broken or a solenoid or relay may have failed, but this is not a generator or AVR problem. Find and fix the unloader.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BoyNaruto]

Hi Bill,

I've checked the pressure release valve, and it is properly working. In the machine, you can press a pneumatic button to check the back pressure on the pump, and it is down to zero before the pump, rolls back again.

Since all the relays are 110V AC, and there are no varistor/capacitor between A1 & A2, I am pretty convinced as well that the voltage drop during start up (when the big contactors has been energized) gives the relay not enough voltage to stay in contacts. I might be doing a test using a UPS to supply my control line.

Thanks guys, appreciate your help, will update you as soon as I can get a UPS and have performed my tests.

 

[LionelHutz]

Check for this occuring. The system is probably build so the pump begins to pump air and pressurize the pump chambers and piping to the tank the moment it begins to rotate. You are on a soft sytem so the motor takes longer to accelerate and also does not have as much power to accelerate. The pump is putting enough load on the motor that it can't accelerate to full speed.

 

[BoyNaruto]

Hi Bill,

I tried isolating the 110V control supply for the relays and timers. I used a fixed normal line from the shop and hooked it up on the control side.

I've also modified the circuit and put one relay to delay the pumping action while the machine performs star-to-delta transition.

The automatic start and shutdown works perfectly. But there is still some delay on Engine reaction, still voltage (Line to neutral) drops down to 172V (from 230). I am still lucky that at this rate, the engine can still recover and give the compressor push to move on the delta mode.

Thanks

 

[waross]

That is a very good point, Lionel.

Bill
--------------------
"Why not the best?"
Jimmy Carter