Achieving a primary frequency response with CCGT steam turbines

Genelba power station is located in Marcos Paz, 50 km from Buenos Aires, Argentina. It is a Siemens 2+1, 660 MW combined cycle plant based on V94.3A gas turbines. It is owned by PECOM Energía and went into commercial operation in May 1999.

PFR as a business opportunity

In large state owned utilities, PFR services were distributed among those generators best technically suited for the purpose. But in the deregulated market, where IPPs and MPPs are significant players, economic considerations come into play: To offer PFR with a generation unit, it is necessary to lower its energy production. Consequently in deregulated markets PFR is usually very well paid, enough to compensate for the reduction in income from base supply, and at the same time give additional motivation to operate the plant in a mode that responds to load swings.

In some regions, a genco has the opportunity to sign a contract with the independent system operator - ISO - to provide a specific amount of PFR. In Argentina electricity market rules oblige all generation units to provide a minimum amount of PFR. If a generation unit is not able to supply that minimum, it has to 'buy' it.

Alternatively, when a generator is able to supply PFR in a proportion higher than the minimum required, that unit has the opportunity to sell its additional PFR capacity in a way that compensates for the deficiency of other generators. In the Argentine electricity market, the PFR price is at least equal to the energy sale price. That means a generator providing PFR receives a payment equivalent to the one it would get if the unit was operated in base load. But a generator's spinning reserve does not consume any fuel Furthermore, when there is a PFR deficit in the grid, the PFR price rises above the energy price, representing additional income for frequency responsive generators.

Accepted truths

CCGT steam turbines were originally expected by their makers to operate only in the so-called sliding pressure mode. CCGT steam turbines have habitually been considered incapable of providing PFR in that mode, and no significant advances in that direction were made by manufacturers despite encouragement from MPPs.

The supposed inability and unsuitability of operating combined cycle steam turbines other than in sliding pressure mode persuaded both parties - plant owners and OEMs - to quickly give up the idea of providing PFR with the steam turbine.

An initial approach to solving the problem was to compensate for it by arranging for gas turbines to provide a bigger proportion of the PFR. Some generating companies had in fact implemented this approach. The problem it creates is that when the gas turbine's output is lowered, in order to increase spin reserve, the steam turbine output power decreases in almost the same proportion. And the lost production in the steam turbine is paid for neither by the PFR fee nor the sale of energy.

The Genelba approach has shown that the received wisdom was not based on fact. Maybe this is one of the most outstanding aspects of this innovative approach - changing the attitude to a set of paradigms previously accepted by the whole industry.

The Genelba approach

The system operates by accumulating energy, in the form of steam p-v energy, by throttling the control valves of the steam turbine up to a specific position under certain conditions. This accumulated energy provides the steam turbine with a spin reserve that is almost immediately available at its output and thus applicable to the PFR function.

Figure 2 shows the Genelba approach as a block diagram. Of course, the approach makes use of the usual 'droop action' calculation. That is, an operator enters the frequency reference, together with the actual grid frequency, the pre-set dead band, the pre-set droop, and the proportion of PFR required (referred to in Figure 2 as % PFR), and a PFR control signal is calculated.

Two specialised control blocks are required for the correct functioning of PFR in a CCGT steam turbine.

The first is the steam turbine load set point tracking logic block, which is dedicated to setting a proper steam turbine load set point on a continuous basis and prevents the operator from altering it. In a CCGT the steam turbine should generate the electrical energy appropriate to the current gas turbine condition. The basic concept of the combined cycle renders the steam turbine a "follower turbine". So, at the appropriate times, this special function block corrects and enters the ST load set-point.

  There is another way, such as incorporating a mechanical governor, to regulate frequency without having a load controller. In this approach the need to have an ST load set-point is suppressed because the load controller is absent. However, that is certainly a poorer method because it is difficult to predict whether or not the power contribution to frequency control would be sufficient. That situation usually causes problems for the grid operators, who have responsibility for maintaining system quality.

The second special block is the one labelled energy store calculation and control in Figure 2. Its main function is to maintain permanently a pre-determined level of stored energy. It is necessary to ensure that the steam turbine will be able to supply the maximum load that it could be called on to provide. On the other hand excessive throttling of the steam turbine control valves could produce either a loss of power generation or a boiler side overpressure condition.

The energy store calculation and control block sets what is called a 'control valve reference value' based on the required PFR proportion - % PFR in Figure 2 - and varied according to steam pressure. The load controller moves the steam turbine control valves around this value.

Even when it is possible to use this approach to provide PFR with the steam turbine alone, best results are obtained when the associated gas turbines also provide PFR.

Testing

Figure 3 shows a screen shot of a test carried out at Genelba in which a grid frequency step drop was simulated at the same instant in all three turbines providing PFR, resulting in a 5 per cent increase in each of the output power values. Steam turbine output (light blue curve) goes up from about 200 MW to a final value around 210 MW in about 20 seconds, this final value being maintained. The gas turbine output (red and yellow curves) goes through a similar change, helping to maintain the ST power increase over time. Figure 3 shows Genelba's steam turbine behaving rather like a third gas turbine. This is one of the many tests that CAMMESA, the Argentine ISO, required of Genelba before it would approve its steam turbine PFR service for commercial operation.

Operating experience

The project implementation schedule was:

• January 2000: initial tests and conceptual design. During this stage several tests were carried out on the energy storage and its function of responding quickly when additional power is called for.

• Control loop design and implementation. New controls were incorporated to ensure that the stability of the plant process was not affected by its capacity for PFR effectiveness. Specifically, steam overpressure and excessive boiler depressurisation events had to be avoided. Steam turbine by-pass valves as well the steam turbine minimum pressure controller had always to be ready for operation.

• Tests of response to frequency variations.

• Additional test required by local ISO in order to gain approval to provide PFR commercially.

• April 2000: start of PFR commercial operation.

Since then, the experience gained providing uninterrupted steam turbine PFR to the electricity market has proved that the system is very effective both in normal daily frequency oscillations and during large grid disturbances. The water/steam process in all its pressure stages has shown very stable behaviour.

The plant carried out several empirical performance tests as well as theoretical analyses along the way to determine exactly how much plant performance is affected. The effect turned out to be so small that it was very difficult to measure. It can be said that plant performance is affected by less than 0.2 per cent.

Figure 4 shows a trend graphic taken from the plant distributed control system. The graphic corresponds to a normal operational day in the Genelba power plant, with its three turbines operating in the PFR mode. The three lines for the three turbine outputs move together in response to grid frequency variations.

Lines for the HP drum level (brown line) and HP drum pressure (green line) of one of the boilers are included. Figure 4 shows that no instabilities are introduced into the steam process.

Grid disturbance

The system has successfully negotiated several large grid disturbance events. Figure 5 shows the response of Genelba's turbines to one of those events. The grid frequency (green curve in Figure 5) fell to a minimum value of about 49.25Hz (50Hz is the nominal value in Argentina) and then stabilised at around 49.7Hz. Genelba's turbines were at that point supplying 5 per cent PFR with load set-points of about 200 MW. As a result the turbines increased their output power incrementally by about 10 MW. It can be seen in Figure 5 that the steam turbine output power (light blue curve) provided a very effective reponse.

Recognition

As previously mentioned, the approach described here was developed entirely by Genelba plant staff. The project has achieved industry recognition - it was awarded the 'Innovation' award at PowerGen Europe 2001, and the plant is the subject of a 'patent pending'.

Plant personnel believe that the CCGT ST PFR project has not arisen in Genelba by chance: it came from accepting that it is the operator's responsibility to detect, explore and develop every 'up-side' opportunity. There is plenty of room for improvement even in power plants based on recent technology.