Gas Turbine Generators - Loading Rate 2

[RRaghunath]

Normally, the loading ramp for GTGs is stipulated by the manufacturers, such as 20 minutes from no load to full load.

My doubt is when a machine is functioning as "Spinning Reserve" in the power system, the machine is expected to pick up load pretty quick in case of tripping of one of the loaded generators (otherwise the grid may be in jeopardy) and with ramp rate stipulation as above, how is this achieved?

Could some one throw light on the subject. Thanks in anticipation.

 

[xabproject]

Dear RRN,

Some conceptual clarifications - various definitions:

1) Spinning Reserve-
Reliability is also of great concern to the system dispatcher, who must plan for the possibility that one or more generators may fail to produce power at any time. This is done by ensuring that other generators are available on short notice. But, most generating units require a 12 to 18 hour warm-up period. Large utility systems operate several units partially loaded at all times, so they can be brought up to full load quickly if and when they are needed.
THIS ROLLING RESERVE or spinning reserve is the remaining capacity that the generator could produce should the need for power arise. It is common to have a 15 to 20 percent spinning reserve to provide reliable service in case some generating units drop off line, break down, or customer demand suddenly surges.

2) spinning reserve, non-spinning reserve
Spinning reserve is any back-up energy production capacity which is can be made available to a transmission system with ten (10) minutes' notice and can operate continuously for at least two hours once it is brought online.
Non-spinning reserve is generating capacity which is capable of being brought online within 10 minutes if it is offline, or interrupted within 10 minutes if it is online, and which is capable of either being operated or interrupted for at least two hours.
Spinning is derived from hydroelectric and combustion turbine terminology. Reserve generator turbines can literally be kept spinning without producing any energy as a way to reduce the length of time required to bring them online when needed.

3)When Generation is lost the immediate power requirement comes from the rotating masses and not by ramping up the input to the prime-mover. Severe frequency deviations and commensurate corrective load shedding occupy the scene. A GT with 20min time to load fully does not qualify to be a good spinning reserve!

4)If the reserve generator turbine is in a state of readiness (for example, as in the case of the steam/gas turbine driven generator: casing temps at 50% depth and 90%depth being within specific TSE(Turbine Stress Evaluator) limits) the load pickup is fast as CVs open up rapidly. Survival mainly assisted by load shedding / islanding arrangements.

Hope this clarifies some part of your query

Best regards

 

[pablo02]

Depends upon how your Power Pool determines spinning reserve... Ours permitted a CT to serve as spinning reserve if it would start and be at base load within a designated time frame (? I don't remember the time frame?) -- also, on the smaller CT's there were different starting ramps -- normal, fast, and emergency and each had a life cost to them... As I recalled, we set ours for fast and were allowed to use them for spinning reserve -- this cost us extra maintenance due to the starts, but it was worth the money otherwise... (an emergency start took as little as 2 to 3 minutes, but cost an immediate overhaul, as I recall) -- check with the OEM on starting ramps and any recommendations / maintenance penalties vs. the Pool requirements for spiining reserve -- be sure upper management is clued in and they acknowledge and support (ha, ha -- wait till budget time) the extra maintenance inspections and costs....

 

[Bize]

The goal of the "keepers" of electric grids is to keep the grid frequency fixed (usually at either 60Hz or 50Hz.) If the power generated is less than the power used, the grid frequency will drop (and vise versa.) Below are three ways that the grid system uses to maintain the frequency (They are in the order of how often they are used.) The ramp rate of most generators is limited in different ways in each of these methods:

1) Inertia of the Generators -- Power dips or spikes can come and go so quickly that, for all practical purposes, the power output of the prime mover is not affected. Thus there is no need for rate limiting.

2) Dispatchers in the central office send signals (either verbally or automatically) to the power stations to balance the grid (i.e. make power generated in their sector equal power used.) Some may also fine tune the system to keep electric clocks accurate. When given verbal commands, the machine operators are usually supposed to limit the ramp rate of the prime mover to that recommended by the manufacturer. Automatic changes may be programmed to include a rate limiter, either in the dispatcher's system or at the plant. In recent years Turbine Stress Evaluators (TSE) have optimized this rate limit in the turbine controllers, allowing the rate to be as fast as possible.

3) The speed change that accompanies the frequency change causes an automatic response from the speed governor on the prime mover. Typically a 1% drop in speed (or frequency) outside of some deadband might attempt to increase the load by 20% (and vise versa.) Since the grid in the US is so stable (because of the above two responses,) governor is seldom needed. There are many potential automatic limiting methods on governor response. Some prime movers will make small changes instantly, but for large load changes, limit the rate using a TSE. Others might use firing temperature to limit the response. Others might not limit the response at all. Others run at full capacity continously (so they can't respond.) There must be hundreds of variations.

 

[ScottyUK]

I work at a large CCGT plant comprising 8x 169MW gas sets and 2x 305MW steam sets.

Our gas turbines have an operating mode known as 'system reserve' which basically allows over-firing of the engine to increase power output. This is achieved by raising the setpoint of the temperature limiter to a value above the normal value. This allows the unit to accept additional load up to the electrical and thermal limit of the generator.

The trade-off is that over-firing really eats into the life of the hot parts, and can significantly reduce the period between maintenance outages.

The above only applies to turbines which are already at their thermal limit; units below this limit can accept fairly steep load rates (>0.5MW/second), although this again comes at the expense of component life.

Our steam sets can accept a high loading rate, limited by the mechanical stresses in the turbine. The rate limiter is a complex calculation within the turbine logic using temperature and pressure values measured at the machine. The steam required is available through use of supplementary firing in the exhaust duct of the gas turbine, somewhat similar to an afterburner, to provide additional input to the HRSG.