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FULL ELECTRICAL LNG PLANTS: HIGHEST AVAILABILITY AND ENERGY EFFICIENCY THROUGH OVERALL SYSTEM DESIGN

Dr. Edwin Lerch Siemens AG

Infrastructure and Cities Sector, IC SG SE PTI, Germany Phone: 49-9131-7-34052 Fax: +49-9131-7-35159

edwin.lerch@siemens.com

KEYWORDS: liquefied natural gas, combined cycle plants availability, generator outage reliability, co-ordinated process control with co-ordinated countermeasures, load control to support generation reserve.

1. ABSTRACT

In the last decades the importance of LNG (liquefied natural gas) has been increasing continuously. Worldwide a large number of plants have been erected using classical compressor technology driven by direct-coupled gas turbines. This approach limits the availability due to maintenance restrictions and the missing spinning redundancy. Modern concepts are based on large full electrical compressors which allow controlling the process with reduced energy consumption. The corresponding maintenance concept improves the LNG plant availability significantly.

The generation efficiency can be increased using large gas turbines in open cycle or in combined cycle mode. Beside the electrical demand the LNG process requires large amounts of process steam. LNG plants very often have no grid connection, i.e. the LNG plant must be supplied under island conditions.

In order to guarantee a high availability of the electrical and steam generation, it is necessary to overcome the planned and unplanned outage of a generation unit, considering both the steady state and the dynamic behaviour.

The operational concept has to take into account gas turbine maximum output limitations by the ambient air temperature variations. Furthermore, in case of a combined cycle power plant the electrical output of the steam turbine is cross linked with the variation of the steam demand, the boiler capacity and the dynamics of the supplementary firing (SF).

Additional reserve on the consumption side might also be available through variation of the operation of the large converter drives and through temporary load shedding of large consumers or process parts.

In order to simulate and coordinate all necessary actions in case of severe outage cases, a model of the system has been built, including the relevant equipment with all necessary and relevant controllers. The model comprises both the electrical and the power plant process systems. This allows to optimize the basic electrical and steam control concept and to design the load or generation shedding sequences with the corresponding load steps and process-coordinated timings, depending on the severity of the outage case.

The paper shows the advantages of this comprehensive simulation approach which comprises all relevant characteristics of the system, providing the basis for development of a process management system which is able to handle even the most severe operational situations.

2. INTRODUCTION

In the oil and gas industries (O&G) energy is used differently along the O&G value chain. In the upstream 75 % is used in rotating equipment like compression and pumping, in the midstream 99 % for compression

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and pumping transportation and in the down stream more than 90 % for heat transfer. O&G is one of the most energy intensive industries with a high CO2 emission (O&G consumers about 20 % of its products in its own process and emitted 12 % of the global energy related CO2 per year).

The O&G industry will play a major role in the climate debate in the next years.

In addition fuel costs and CO2 emission can take different pathways which allow to use efficiency technology to reduce the costs and the CO2 emission.

In the last years the importance of LNG has been increasing and this development is expected to continue. Depending on the size of one liquefaction train, the power demand is 300 to 450 MW, mainly used for coolant compression. World wide a large number of LNG-plants have been erected using classical compression technology with compressors driven by gas turbines directly coupled on a single shaft. This approach is limited in availability and flexibility through high maintenance restrictions and missing spinning redundancy. In addition the energy efficiency is relatively low and therefore CO2 emission and fuel consumption is relatively high.

A modern concept is based on large electrical driven compressor units, supplied by a set of generators, based on combined cycle plants (E-LNG). Fig. 1 shows the different plant concepts.

Figure 1: Different LNG concepts

The modern concepts combine a high electrical efficiency with a proportionally lower CO2 emission, lower fuel consumption with a highly improved productivity and plant availability. The steam, electricity and process demands have to be co-ordinated to guarantee high availability in case of large disturbances in the system like generation outage, compressor outage, process chain outage or sudden loss of a production train. To find a suitable solution the LNG configuration can be modelled in a power system simulation tool [2] including all models for generation, process equipment (variable speed drives (VSD), pump motors plant, house load) and the control of all relevant equipment (governors, voltage controllers and control of variable speed drives).

These models allow simulating, tuning and optimizing the system reaction in case of severe system disturbances and allow defining a suitable system behaviour using adapted countermeasures in a whole system approach.

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direct drives, efficiency approx. 34%

open cycle gas turbines: higher availability

combined cycle gas turbines: higher availability and efficiency

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3. INTEGRATED MODEL OF STEAM AND ELECTRICITY

Simulation of electrical networks today is a standard technology even for highly complex systems. Beside steady state load flow calculations, the main focus of such simulations is the short term stability of the system in case of disturbances. Within those simulation systems, generator models are well established as far as related to the characteristics of their electrical parameters. Modulation of their active power output generally is limited to load setpoint variations, to frequency response capabilities and the corresponding load change rates, which are directly related to grid frequency stability.

Any process parameters like pressures or temperatures within the steam system of thermal power plants or any output limitations of the generators due to external parameters as e.g. ambient air or cooling water temperatures are normally not represented.

Within such plants the time constants of the thermal systems are much larger than the typical electrical time constants and hence allow a separated investigation (maybe with different models and tools) of electrical on one and process dynamics on the other side.

However, in case of the combined supply of electrical energy and process steam to a LNG plant by a combined cycle power plant (CCPP) under island conditions, the interaction between steam and electrical dynamics is of crucial importance, in particular in case of large disturbances. Sudden losses of gas turbines or steam turbines e.g. not only affect the electrical balance, but at the same time have large dynamic influence to the steam system, requiring control actions and limitations, and lead again to secondary effects on the active power side.

The number of variables within the process and control system together with the twofold target of controlling the electrical and the steam output necessitates therefore an integrated model, representing the dynamic behaviour of all relevant process and electrical parameters for all interesting disturbance cases.

The principle of a CCPP for this purpose is shown in Fig 2. Several gas turbines are the main generators for electrical energy. Their hot exhaust gas generates high pressure steam (HP) in heat recovery steam generators (HRSG), which is used both a steam driven turboset and for process steam supply on one ore several pressure levels.

The supplementary firing (SF) allows an increase of the steam generation and a better control flexibility due to separation of the power and steam control loops.

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/LP steamHP

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Figure 2: CCPP with process steam extraction and supplemental firing

4. MAIN CONTROL INTERACTIONS

Fig. 3 shows a typical configuration of a large E-LNG plant with VSD drives up to 80 MW which will be planned in 2 phases (train one and train two). The generation concept in this example is a combined cycle plant with 4 GT (gas turbines) and 2 ST (steam turbines) installed in phase I and additional generation of 2 GT and 2 ST in phase II. The first phase configuration is more critical in case of large outages of process components or generators (largest drive 80 MW, size gas turbine 176 MVA, size steam turbine 154 MVA).

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Figure 3: Example of a large E-LNG plant

SGT = 176 MVAr, SST = 154 MVAr The grid frequency is the integral process value for the balance of generation and demand of active power. Whereas in a large interconnected system the influence of one generator on the overall frequency might be neglectible, under island conditions the control of the frequency is one of the main tasks of the power plant. In case of insufficient control and sustained imbalance, the island frequency would lead rapidly to a trip of generators due to high or low frequency. Fast balancing is realized as a droop control using the measured frequency deviation with a proportional influence to the output. A slower integral control loop working like a local secondary frequency control brings the frequency back to nominal value. The droop is realized on the gas and steam turbines. Whereas a change of the gas turbine output leads to a relatively slow change of steam generation, a change of the steam turbine output has direct influence on the process steam supply to the LNG plant

Like grid frequency for the active power balance, the process steam pressure is the integral control parameter for the balance between steam generation and demand. Since the process steam is extracted from the steam turbines, a change in steam demand has a direct consequence on the electrical steam turbine output and hence on the frequency.

A large disturbance like a gas turbine trip requires on one hand a rapid increase of electrical output in order not to reach frequency trip thresholds; on the other hand it implies a rapid loss of steam generation. In such case not only the active output has to be increased, but also the loss of steam generation and its effect on the steam turbine output needs fast compensation. Since an increase of steam generation is limited by relatively slow thermal processes (increase of supplemental firing), the use of thermal storage has to be taken into account in the control concept and hence in the model.

This is shown in Fig. 4, where the electrical loss of one GT is compensated rapidly by the other turbines, but where the steam pressure loss (visible as the steam turbine output drop) takes some time to recover. The process steam supply itself is not reduced after a transient oscillation.

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Figure 4: GT/ST reaction due to reduced steam generation

Simulations of severe fault and outage scenarios show rapidly that the design of the power plant and its reserves of turbine active power and steam generation must not be based only on several steady state cases, but has to take into account a thorough knowledge of the transient behaviour.

Depending on the severity of the outage scenario, it will be necessary not only to adjust the turbine outputs in order to stabilize the electrical balance (i.e. keep the rotational frequency within allowed limits), but also to switch off either complete generator sets or to reduce or switch off selected consumers.

These steps are sorted in “priorities”, i.e. depending on the severity, the most suitable priority is selected.

Fig. 5 depicts the simulation results regarding the case of GT outage for the more critical phase I. The outage of a GT has been investigated in case of all generators connected (N op., red curves), as well as in case of maintenance of one generator (N-1 op., blue curves). In this last condition, some load reduction steps and load shedding are necessary. The figure shows the results in case of high ambient temperature T2 (40°C) and no load shedding (priority 0), VSD speed reduction (total 48 MW, priority 4), VSD string trip and load trip (154 MW, priority 8). Under low ambient temperature T1 (15°C) a load shedding with VSD speed reduction and load trip (90 MW, priority 7, black curve) is sufficient.

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Figure 5: Outage of a gas turbine in phase I

Fig. 6 summarises the system response regarding the case of process outage. In case of process outage all generators are assumed as connected (N op.). In case of partial process outage, i.e. approx. 150 MW (blue curves), the turbine governors can suitably react without exceeding the dynamic band (in 5-6 sec. the speed deviation is again in the static band, ± 1%). In case of total process outage, i.e. a 320 MW train trips (red curves by temperature T1 and black curves by temperature T2), the turbine governors are not fast enough to avoid overfrequency generator trip. Therefore, the trip of one or more generators is required in a suitable time after the outage.

The large 80 MW VSD compressors can be used to reduce load in emergency situations up to 10 % without critical influence on the LNG-process (emergency load variation).

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Figure 6: Outage of process components (VSD compressors) in phase I

5. INTEGRATION OF THE STEAM SYSTEM

In an open cycle gas turbine process the time constants of the thermal system much larger than the typical electrical (electromechanical) time constants. This allows investigating the individual process reaction separately. In an combined cycle power plant concept the interaction of steam dynamic and electromechanical dynamic is of high importance after severe changes in the electrical configuration (outage of gas turbine, outage of steam turbine, outage of compressor drives, outage of a train). To investigate the electrical and steam balance after a disturbance and to show the influence of the steam system on the required control actions (and limitations) a system model was developed which allow the investigation of the combined electrical and steam process of the generation and distribution system. Figure 7 depicts the combined simulation model which is a combination of the classical electrical model interconnected with a steam process model of the combined cycle steam/gas process.

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Figure 7: Integration of the steam process into the electrical simulation model

The model includes the gas turbine, the supplementary firing, the steam generation, the steam pipes, the steam generation and condenser, the feed water pump, train and the necessary control (fig. 8).

/ LP steam HP

HV bus

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gas turbine ( s )

HRSG

steam to LNG

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Figure 8: The closed loop steam process

This combined model allows a suitable design of the steam and gas generation in combination with an optimized supplementary firing system.

The system control and management of the power system can be tuned to adapt the load variation and load shedding in case of severe system outages. This increases the availability and flexibility of the plant.

The system is based on an object oriented result data base which allows selecting result quantities after the simulation for the studying process. Selected results can be compared and optimized.

As an example some process quantities after outage of a gas turbine are shown (steam control behaviour, steam change in electrical power and change in pressure). Figure 9 shows the system characteristics with unfavourable load shedding (LS) concept (overshoot of LS).

Figure 9: Overshoot of LS after outage of a GT

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Fig. 10 depicts the system stress reaction with a suitable LS concept.

Figure 10: Adapted LS concept, system reaction after outage of GT

6. PERSPECTIVE

The integrated model of the electrical system with all generators and consumers and the steam system will allow the development of the overall control concept. Beside the design of the main control loops the concept also comprises detection of certain critical situations which require additional action either on the generation or on the demand side in order to improve the stability and hence the availability of the total system. Such determined actions can be triggered by a logical combination of availability of machines and of reserves, in combination with measurement and interpretation of current load and frequency data.

In principle there is also the possibility of integrating the compression process model into the model, as far as relevant for the behaviour of the LNG process. This would allow a further detailing of the design of the steam, the electrical and the process system. Specific events with known consequences for the supply can be counter-acted specifically in the power plant, the control of steam and compression and vice versa.

7. CONCLUSION

To guarantee a high level of availability under varying conditions for the combined cycle generation in the LNG plant a simulation approach was established. These simulations of the generation with its control together with the relevant process characteristics have been used to build a process management system that co-ordinates all necessary countermeasures to overcome generator outages, individual process equipment faults, depending process chain outages and even outages of a whole process train. The generation control has been chosen, parameterized and co-ordinated with the process load control to use it as a surplus for generator outages reserve. The process management includes the time critical trip of generator units in case of process train outage together with a suitable generation reduction and the co-ordination of load shedding and process load reduction in case of generation outage. The simulation approach allows to model the generation, electrical loads and the steam system to find suitable system overall control and management of steam, generation and process demand. A combined model of electrical and steam behaviour allows to develop a suitable design concept and to optimize the overall

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concept together with the optimization of the power plant equipment. These simulation produces additional informations of the closed loop electrical and steam process to tune, stabilize and optimize components, control and power management. Critical situations can be detected in more details and counter measures can be realistically coordinated. The approach results in a higher availability and reliability of the plant with modern VSD and CCPP technology to reduce CO2 emission and fuel consumption. The concept allows analysing the compression process, too, to investigate and optimize the behaviour in case of electrical disturbances. To investigate the overall plant behaviour the whole LNG-process as a closed loop simulation can be integrated in the future, too.

8. LITERATURE

[1] Kleiner, F.; Kauffman, Steve: All Electric Driven Refrigerations Compressors in LNG Plants offer Advantages, Publication by Siemens AG Erlangen, 2005

[2] Ruhle, O.: Power System Dynamic Simulation Software Packages. Released in Styczynski, Z.: Haubrock, J. (Editors): Renewable and Dispersed Power Generation in Power Systems; International Workshop in Athens, September 2007. Res Electricae Magdeburgenses – Magdeburger Forum für Elektrotechnik. Vol. 19; ISBN: 978-3-929757-44-6

9. AUTHOR

Edwin N. Lerch (1953) received his B.Sc. in engineering from the University of Wuppertal/Germany in 1979 where he also completed his PhD in electrical engineering in 1984. Since 1985, he has been a member of the systems planning department at Siemens in the system dynamic group. He is currently working in the Siemens PTI department as a principal expert in the areas of power system stability, dynamics of multi-machine systems, control, optimization and identification of electrical power systems