Black start Generator Versus Emergency Generator

[kssschsekhar]

Hi all

In general, In a power plant (capacity 100 MW to 4000 MW) the following equipment exists. Here power plants are Gas based or coal based.

Emergency Diesel Generator and Black start Diesel Generator.

Now I would like to know Why can not we club together and install a Black start diesel generator (In general larger capacity than Emergency Diesel Generator)?

Assume, Black start diesel generator will be operated as both purpose (Emergency and Black start).

Please provide any theoritical knowledge or Articles or our discussion forum thread.

Thank you

 

[LSpark]

Most of the older power stations that have black start capability in the UK just have the open cycle gas turbines that can be used for black start or emergency power (connect on to an 11kV board) not two seperate engines. Most of the newer gas plants don't have black start capability (not ecomonic to install) so have smaller diesel gens that are connected at a lower voltage (415V).

 

[PHovnanian]

Its a question of providing the two functions adequately (black start vs emergency power). If the two functions can be combined without interference, absent some regulatory prohibitions against using the emergency source for other loads, then it should be possible.

As LSpark points out, there are economic considerations that may work against combining these functions. One may not want to be committed to running a machine sized for black start any time EP (emergency power) is required. In addition, a higher reliability requirement (including maintenance costs) for an EP source might make the combined option even more expensive.

 

[Mbrooke]

I would Imagine that at has to do with emergency codes set in place, similar to the way many buildings (like hospitals) have separate risers, panelboards and ATS switches for life safty, critical and non essential loads. The black start diesel Gensets must be available load for pick up within 10 seconds, if not that would be one reason.
NFPA 70 and the NEC are two of several codes that I think could be settting restrictions based on emergency requirements.


Also as a side note, if the black start units have to run emergency load for an extended period of time, which I would think load wise of it being much, much less than what is needed for a cold start, the issue of running the gensets underloaded comes up. A 2,000kw genset running only 100kw of load for hours is killer on the engine.

 

[mesutphen]

It might just be a naming convention. Like byrdj said, "black start" just means that I can start it up without utility grid power. I would consider all diesel generators to be "black start capable" because I haven't met one yet that required AC power to be started.

But federal regulations in the US (specifically NERC) specify that a "black start generator" can supply power to the grid. In a roundabout way, this means that if I have a "black start generator" my transmission authority can call me up and tell me to start it and supply power to the grid. The diesel generator at my power plant is an "emergency" generator because it will only supply power to the local site. It may be "black start capable" but it is not a "black start generator" and I don't want it to be.

 

[redlinej]

Black start genset are use to start up auxiliary equpiment when grid power is not there.
emergency genset could mean couple of things like quick response to a grid failure to put power back to the grid or use to recover frequency when one main generator fail and u need to maintain the cycle .

 

[waross]

Not all generators are driven by diesel engines. Steam plants need a lot of power for some time to run pumps, boilers, controls and other systems before they are able to start generating.
Large gas turbine plants typically need much more power to start than is feasible with a battery bank. These plants often use grid power to start up.
A plant with black start capability will have an onsite generator to supply start-up power. This may be a diesel set, or an aero derivative turbine.
In a large plant the power needed for a black start may be many times the power needed for emergency services.
There may be plants where the difference in capacity may not be that great, but regulations will generally demand that the set be capable of supplying emergency power as well as black start power simultaneously, so you are still looking at the sum of emergency power plus black start power. That's a lot of excess capacity that regulations may require to be run every month.


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

 

[LionelHutz]

I'd say Mbrooke hit on the most likely answer. You would end up running a large genset at low load during any times you're running on EP.

There might be another reason as well. The BS genset likely feeds into the HV switchgear so you couldn't shut down the plant and do any HV swithgear maintenance or repairs if EP requires the BS/EP genset running which energizes the HV switchgear.

 

[ScottyUK]

Most emergency sets are LV machines so they tend to be located fairly close to the critical load, e.g. the plant control room, in order to keep cable size manageable and volt-drop reasonable. It is unusual for a gas turbine black start generator to be located close up to the plant control room, so this type of black start machine usually has a local diesel set capable of running the auxiliary systems; somewhere in the 500kVA - 1500kVA range is typical for this duty, and at that power level long cable runs quickly become impractical and expensive at low voltage. The GT is typically an aero-derivative type and drives an MV alternator to provide power to the balance of plant in order to allow a larger machine to spin up.

 

[bacon4life]

In addition to sizing the diesel generator large enough to supply plant auxiliary loads, the plant itself must be capable of islanded operation before being called a blackstart generator. Blackstart resources usually require:
1. Governors/exciters responsive to load variations,
2. Nearby loads sufficient in size and number of blocks to allow operation within the safe turn-down limits of the prime mover,
3. Breakers & relays at a remote substation capable of synchronizing islands.