Voltage Regulators Shipboard

[TraderJoe80]

I've read a lot on here about the use of voltage regulators droop vs. isochronous. Basically if I'm running in parallel I have to be in droop, and one generator in isochronous.

I work in the Merchant Marine and our tug boat fleet has been suffering from extensive power failures. I've found multiple things wrong, but one thing I just discovered yesterday is that our generators are running in parallel in isochronous mode. In such a small plant (150kW each generator, two generators) is this a big deal? The only time we parallel generators is when we are maneuvering in waters entering or leaving port.

Our marine electrician recommended that I remove the droop circuit and that all of sudden clicked on me that I think he's wrong. He wants to make all out boats operate in parallel only in isochronous. We are using as Basler 125-12 voltage regulator is that helps?

I understand the basics of the power triangle, reactive load, real load, droop in a governor vs. droop in a VR. I'm just having a hard time understanding:

1) Is he right that in a small grid we can parallel in isochronous?
2) If we operate in droop with one generator online what causes the voltage to continuously drop until the low voltage trip opens?
3) When we operate in Parallel with droop and the load increases how does the VR become "tricked" that the voltage is returned to its set value to ensure it doesn't increase too much in reactive load (VARS out)?
4) Is the droop and isochronous mode automatically built into the VR or do I need a selector switch on the front of the switchboard (I am looking this up in the manual now, but I wanted to hear what you had to say)?


OK I hope this all makes sense to you. I really look forward to hearing from you all!

 

[TheBlacksmith]

1) No. Causes circulating currents between the generators
2) Increase in VARS causes drop to lower regulation point.
3) Operator must increase setpoint to offset lower regulation point.
4) Should be built in.

 

[TraderJoe80]

I thought the VR should maintain the voltage if set in automatic? I do know sometimes now and again we have to manually adjust the voltage with the smaller fine tune knob on the front the switchboard, but for the most part should the voltage regulate itself?

 

[waross]

Possibly one point of misunderstanding is that droop is a property of the governor not the AVR.
The parallel/single switch n the switchboard is the quadrature circuit that helps share the VAR load between parallel generators.
ALL sets must be in droop for parallel service.
I recommend droop mode for all operations.
I'm short of time just now. Have to go.

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

 

[waross]

With two sets in isochronous one will "Hog" the load.

"I thought the VR should maintain the voltage if set in automatic? I do know sometimes now and again we have to manually adjust the voltage with the smaller fine tune knob on the front the switchboard, but for the most part should the voltage regulate itself?"
YES


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

 

[catserveng]

As mentioned above, you'll need voltage droop to parallel and share VAR's properly.

Try taking a look at this,

http://www.basler.com/downloads/VR_parallel.pdf

If you have a known load with some reactive content, say a large pump, use it to verify voltage on both units. Run one unit, apply the load, note the amount of voltage droop, calculate the droop percentage as described in the Basler paper, then do the same to the other unit, and compare, adjust them so they have as close to the same percetnage droop as possible. Most systems like your will work well with about 5% voltage droop. Make sure both units drop in voltage as load increases, if one goes up instead of down the droop CT is backwards, you can reverse the leads at the AVR for a quick fix to verify. At 5% droop your undervoltage protection should be ok, you may want to increase your no load voltage setpoint up a bit.

If you want a switchable system, you can wire a circuit to short the droop CT if only one unit is on line, this will defeat the droop function.

I ran into similar problems with a large West Coast fleet several years ago, biggest problem was that no one really did a proper start up on the system, we found all the droop setting different, varying stability settings, and a large number of droop CT's installed incorrectly or not wired in at all. The best way is to do testing with a resistive/reactive load bank, but most yards don't want to deal with the expense, especially on smaller systems that only parallel for short times.

Hope that helps.

 

[TraderJoe80]

I follow what you are all saying. What do you know about "cross current compensation". I'm told our Basler 125-12 VR oly has a droop setting. What we (our marine electrian)have been doing is installing a cross current compensation circuits.

He has been installing a CT on the B-Phase which has a small resistor in series with some current thing. Haha, I forget what the heck it was called. In addition there is a set of contacts between the two CTs on each gen. Anyway, with one generator online the CT senses the current into the VR and flows through a resistor. With one generator on-line the droop is dialed all the way down. Now when the other generator is on-line, breaker closed these contacts energize and basically causes the excessive VARS to circulate and cancel each other out. Does this sound right to you?

We are talkng small power grids here, so I imagine this makes a difference in relation to large utility

 

[catserveng]

Take a look at pages 14 and 15 of the Basler document I referenced above, Basler calls it Differential Reactive Current Compensation, but most of the industry calls it cross current compensation.

If you're going to use cross current compensation, you still have to set up the droop circuits as described, and you need to assure they both work properly with the same slope or the system won't balance the currents.

Why don't you have your electrician draw out the curcuits? Most of the problems with these type circuits are simple wiring/connections errors, or in setup.

Hope that helps.