Design Features of Gas Turbine Power Conversion

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Design Features of Gas Turbine Power Conversion System for HTR-10GT

Wang Jie, Huang Zhiyong, Zhu Shutang, Yu Suyuan Institute of Nuclear and New Energy Technology, Tsinghua University

Beijing 100084, China

Abstract The 10MW high temperature gas-cooled reactor (HTR-10) is the first reactor

worldwide which has inherent safety feature. The HTR-10, a pebble bed type reactor, reached its first criticality in 2000 and began to operate on full power in 2003. The power conversion system of the HTR-10 is a steam turbine generator system. Based on the success, a new project, the gas turbine power conversion system, was launched recently, which is denoted as HTR-10GT.

For the HTR-10GT, the gas turbine direct cycle is utilized instead of steam generator and steam turbine. The HTGR gas turbine cycle is expected to have higher efficiency and better performance theoretically. The arrangement of helium turbine and electric generator is selected as single shaft configuration with identical speed which is designed as 15000r/min. The rotors are supported by active magnetic bearings to avoid the contamination from any lubricant. The reactor core outlet temperature is designed as 750 and inlet 330 , and the latter is restricted not to exceed the value by use of a temperature adjustor. The output power is controlled by total mass inside the primary system gradually and by bypass valves for fast changing. This paper presents the main design features and parameters for the HTR-10GT. Meanwhile the main performances and control method are also described.

1. Introduction The 10 MW high temperature gas-cooled reactor (HTR-10) is a pebble bed type

gas-cooled reactor incorporating a graphite moderated, helium cooled core with ceramic-coated fuel particles, of which the configuration is a side-by-side one and the energy conversing system is a steam turbine generator (Wu, Lin, and Zhong, 2002). The HTR-10, a modular reactor with inherent safety, reached its first criticality in 2000 and began to operate on full power in 2003. It is the first modular high temperature gas-cooled reactor (HTGR) worldwide, which is currently operating.

The HTR-10 project has developed and demonstrated the modular HTGR technology. After that the energy conversion system with high efficiency becomes the next focus for HTGR researchers. Instead of the steam turbine generator of the HTR-10, the gas turbine generator is expected to be installed inside the primary system to simplify the reactor system and elevate the plant efficiency. Therefore the Chinese government has launched a new project of HTR-10GT to couple the modular HTGR of the HTR-10 with gas turbine cycle (Zhang, and Yu, 2002). It is scheduled to supply electric power to the external grid by the HTR-10GT in 2008.

Corresponding author. Tel: +86-10-6279-4678; Fax: +86-10-6277-1150. E-mail address: [email protected]

2nd International Topical Meeting on HIGH TEMPERATURE REACTOR TECHNOLOGY Beijing, CHINA, September 22-24, 2004 #Paper D05

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The initial basic design was a joint one by the Institute of Nuclear and new Energy Technology (INET), Tsinghua University, and State Unitary Enterprise I.I. Afrikantov, Experimental Design Bureau of Mechanical Engineering (OKBM), Russia. The engineering design is currently carried out by INET. This paper provides the main design features and parameters for the HTR-10GT and discusses its performance and control method.

2. Power conversion system 2.1 Layout of the power conversion system

Fig. 1 shows the layout of the HTR-10 primary system. The left side pressure vessel contains the reactor core and the right side one contains the steam generator and helium circulator, which are connected by a horizontal hot gas duct pressure vessel. The nuclear energy generated in the core is transported to the steam generator by the helium circulation within the primary system forced by the helium circulator, which forms the nuclear steam supply system.

Fig.1. Layout of the HTR-10 primary system

For the new project of the HTR-10GT, the helium circulator installed above the

steam generator will be removed and the gas turbine generator assembly, called the power conversion unit (PCU), inserted into and installed on the original steam generator pressure vessel. The power conversion system of the HTR-10GT is illustrated in Fig. 2.

DESIGN FEATURES OF GAS TURBINE POWER CONVERSION SYSTEM FOR HTR-10GT #D05

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PCU is designed for converting thermal energy generated in the reactor core into the electric one in direct gas-turbine cycle, as well as for removing heat from the reactor core due to coolant circulation through the core in the reactor shutdown cooling mode. PCU configuration includes: PCU vessel, turbine, high-pressure compressor, low-pressure compressor, generator, connecting coupling, recuperator, precooler, intercooler, generator gas cooler, and in-vessel metalworks.

Fig.2. Layout of the power conversion system of the HTR-10GT

Helium flow chart is depicted in Fig. 3. The helium from the core with high

temperature and pressure expands in the turbine and drives turbine with the compressors and generator together. The exhaust helium enters the recuperator to heat helium in the other side from the compressor outlet. Then the helium is further cooled in the precooler and pressurized to high pressure through the two-stage compressor with intercooler. The high pressure helium is preheated through the other side of the recuperator as mentioned above and then enters the temperature adjustor, and then the reactor core to be heated to complete the Brayton cycle.

HTR 2004 Beijing, CHINA, 2004.9

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Fig.3. Schematic layout of HTR-10GT cycle 2.2 Turbomachine

The turbomachine (TM) uses a single shaft configuration in vertical arrangement (Fig.4). The components of the turbomachine include turbine, low pressure compressor, and high pressure compressor. The rotation speed of the turbomachine is 15000r/min.The turbomachine is connected with the electric generator by flexible coupling. All the rotation components are supported by active magnetic bearings.

The turbomachine is provided with a repair seal below the upper radial electromagnetic bearing, which separates the generator cavity from the main circulation circuit, allowing its depressurisation and access of personnel to the generator upper part. Repair seal is actuated by helium supply with the turbomachine shut down.

The generator is of synchronous type, and the generator configuration includes main generator, brushless exciter with rotary diode rectifier, and exciter lead-outs. The generator is located in the cavity separated from the main coolant circulation circuit in the PCU.

Core Generator

Turbine

Intercooler

Precooler

Recuperator

Control Valve

High-pressurecompressor

Low-pressure compressor

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2a

2b 2

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Temperature adjustor


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Exciter

Radial catcher bearing

Radial electromagnetic bearing

Generator

Radial-thrust catcher bearing

Axial electromagnetic bearing

Flexible coupling

Turbine High-pressure compressor

Low-pressure compressor

Repair seal

Buffer seal

Stator seal


Radial catcher bearing


Axial electromagnetic bearing

Radial-thrust catcher bearing


Fig.4. Turbomachine structure diagram of HTR-10GT


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2.3 Active magnetic bearings

Helium gas of high purification quality is used as the reactor coolant. In order to avoid possible contamination, such as the leakage of mechanical bearing lubricating oil into primary circuit, the active magnetic bearings (AMB) are adopted to support the TM rotor and electric generator rotor in the PCU vessel. There are totally two axial and four radial electromagnetic bearing to support the rotors. Besides, there also include catcher bearings to support the TM rotor in the cases of shutdown, failure and exceeding dynamic load. Fig.5 and Fig.6 show the structure of radial and axial electromagnetic bearings respectively.

1-upper magnetic circuit, 2-lower magnetic circuit, 3-power winding, 4-thrust disk, 5-rotor shaft, 6-sleeve, 7-unit of axial displacement sensors, 8-radial-axial catcher bearing.

Figure.5. Axial magnetic bearing assembly

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8

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5

6


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1-stator casing, 2-stator magnetic circuit, 3-power winding, 4-electromagnet, 5-rotor magnetic

circuit, 6-rotor shaft, 7-unit of radial displacement sensors, 8-radial catcher bearing.

Figure.6. Radial magnetic bearing assembly 2.4 Recuperator

The recuperator is designed to ensure high-effective of the gas-turbine cycle by heating up primary coolant supplied the core inlet by the heat of the primary coolant flowing out of the turbine. The recuperator is arranged inside the PCU vessel. Helium

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of the low-pressure cavity goes from the turbine to the recuperator low-pressure path, transfers heat to high-pressure path helium, leaves the recuperator and enters the precooler. Helium of the high-pressure cavity goes from the HPC to the recuperator high-pressure path, and then leaves the recuperator to the temperature adjustor, and then enters the core.

The type of the recuperator is vertical with plate-fin heat exchange surface. The recuperator is based on 38 heat exchange modular elements, and they are located around the circular ring, being adjacent to each other by lateral sides. The helium circulates in the recuperator on the basis of counter-flow scheme. The heat exchange elements are surrounded by distribution and collection headers in the low-pressure cavity on the internal and external sides of the circular ring in the places where low and upper openings are located. There are nozzles for helium supply and removal in each header (Fig.7).

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2

5

67

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1-LP helium outlet, 2-Outlet LP header, 3-Heat exchange element, 4-LP helium inlet,

5-Inlet LP header, 6-Outlet HP header, 7-HP helium outlet

Figure.7. Structure of the recuperator


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3. Main design parameters Main design parameters of HTR-10GT in the mode of operation at full power are

presented in Table 1.

Table 1 Main design parameters of HTR-10GT Parameter Value

General parameters Structure layout Vertical single shaft Thermal circulation efficiency, % 25.11 Generated electricity efficiency, % 21.44 Total relative pressure loss, % 11.62 Rotor speed, r/min 15000 Total relative helium leaks, % 3.617 PCU mass, t 51.9 PCU height, mm 7940

Core Reactor power, MW 10.00 Temperature at core inlet/outlet, C 330.0/751.8 Pressure at the inlet/outlet, MPa 1.5899/1.5747 Helium flowrate, kg/s 4.56

Turbine Power, MW 5.861 Adiabatic efficiency, % 88.9 Helium flowrate through turbine, kg/s 4.64 Helium temperature at inlet/outlet, C 750.0/497.8 Helium pressure at inlet/outlet, MPa 1.5616/0.6931 Turbine expansion ratio 2.2531

Low-pressure compressor Power, MW 1.788 Adiabatic efficiency, % 84.6 Helium flow rate, kg/s 4.70 Helium temperature at inlet/outlet, C 35.2/108.3 Helium pressure at inlet/outlet, MPa 0.6690/1.0570 Blade system compression ratio 1.580

High-pressure compressor Power, MW 1.797 Adiabatic efficiency, % 84.6 Helium flowrate, kg/s 4.72 Helium temperature at inlet/outlet, C 35.5/108.7 Helium pressure at inlet/outlet, MPa 1.0253/1.6200 Blade system compression ratio 1.580

Generator Nominal power, MW 2.144 Efficiency, % 94.2 Voltage, V 2500 Phase current, A 571 Current frequency, Hz 250


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4. Performance and control PCU and its components ensure the fulfillment of functions in the following

typical modes: (1) startup; (2) operation at any steady-state level of power within the automatic control range from 30 to 100% of full electric power release to the external grid through the frequency converter; (3) scheduled shutdown; (4) operation in the mode with the loss external load.

4.1 Startup mode

The reactor is in subcritical state. All the plant systems and equipment are checked and prepared for startup. The circuit is filled with helium with mass making 14 % of the nominal value at 100 % of load. During the reactor startup this helium mass is necessary to ensure primary circuit filling pressure similar to atmospheric. Besides this pressure determines the power consumed by the generator operating in the motoring mode and limited by the maximum output power of frequency converter (FC) by generator side - 0.4 MW.

TM rotor is suspended by the electromagnetic bearings, and the generator is connected to the electric power grid through FC, and TM shaft is accelerated to the nominal rotation speed of 15000r/min by means of FC control. The reactor is brought out of the subcritical state to the minimum controllable thermal power level - ~ 3 % of the nominal value at 100 % of load. At the reactor power increase up to 7 % of nominal, helium mass is kept at the level of 14 %.

With the increase of power and temperature of helium at the reactor outlet (at the turbine inlet) and maintenance of TM rotor speed at the nominal level power consumed by the generator and FC from the grid will be decreasing. When power consumed from the grid decreases to zero, the "generator - frequency converter" system switches over to the mode of generated electric power release into the grid with frequency 50 Hz and of TM rotor speed maintenance at the nominal level. By further increasing reactor power up to 36 % of nominal and temperature of helium at the reactor outlet (at the turbine inlet) and by adding helium mass into the circuit, the power is increased to the minimum level of the automatic electric power control range 30 %. The rate of helium temperature change at the reactor outlet (at the turbine inlet) within this power range may be from 30 to 100 C/h, and at certain helium temperature levels pauses shall be made in order to stabilize the metalworks temperature and let helium get purified in the circuit. 4.2 Mode of operation within the range from 30 to 100 % of electric load

Control within the mentioned range from 30 to 100 % of electric load at electric power change rate up to 5 %/min is supposed to be effected by changing reactor power and helium mass in the circuit. With such electric power control the highest efficiency at partial loads and almost constant helium temperatures at the reactor inlet and outlet, equal to the nominal ones at 100 % of power, are ensured.

At electric power change rates exceeding 5 %/min control is effected by means of bypass control valves, bypassing helium from the compressor suction side to the


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turbine outlet. Corresponding opening of valves makes it possible to control electric load at the generator. This electric power control is performed at almost permanent reactor power, whose balance is ensured by changing removed thermal power in the precooler and the intercooler.

In case of rapid electric power reduction with rates above 5 %/min down to the specified level for a long-term period, the combined control method is used, performed by opening DN 50 bypass control valves and reducing helium mass in the circuit and reactor power. PCU capacity changes lead to corresponding changes of feed water flowrate in the temperature adjustor are performed in order to restrict helium temperature at the reactor inlet with the value of 330 C and exclude increased rates of helium temperature change at the reactor inlet (exceeding 100 C/h). 4.3 Shutdown and cooling mode

Scheduled shutdown and cooling mode is performed similar to the startup mode but in the reverse order, excluding the time pauses for metalworks temperature stabilization and helium purification in the circuit. Emergency mode of shutdown and cooling by PCU due to failures in the reactor section differs from the scheduled one, which is important for the development of requirements for equipment. Generator with FC, when power of electric energy generation is reducing, slow down TM rotation speed, and when power of electric energy generation is reduced to zero, they change over to the motoring mode to ensure cooling at that maintaining TM at reduced rotation speed, thus excluding the increase of helium temperature change rate at the reactor outlet (exceeding 100 C/h). 4.4 Electric load loss mode

In case of the loss of electric load the frequency converter with the generator disconnect from the grid and TM rotor starts accelerating above the nominal speed of 15000r/min. When the emergency TM rotor speed setpoint is reached (110 % of nominal value) emergency protection signal is generated, reactor is shutdown, the three DN 80 shutoff valves of the TM control and protection system fully open within 0.25s. The shutoff valves, effecting reactor and turbine bypassing by damping part of the coolant flowrate from the HPC suction side to the turbine outlet, quickly reduce turbine power and stop TM rotor. TM rotor acceleration above the allowable speed (120 % of nominal value) is eliminated by fast opening of valves with necessary flow area. TM rotor should stop with no violation of allowable rotor speed in case of failure of one of the three shutoff valves. In this mode helium temperature at the reactor outlet remains constant for certain time due to reactor sluggishness, therefore helium temperature at the outlet from the decelerating turbomachine increases to the value at its inlet, equal to the temperature value at nominal 100 % power.

5. Conclusion For the HTR-10GT program in China, the direct gas turbine cycle is adopted for

higher efficiency. The arrangement of helium turbine and electric generator is selected


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as a single shaft configuration at the high speed of 15000r/min, and the rotors are supported by advanced active magnetic bearings. The main design features and parameters for the HTR-10GT are presented; and it is expected to supply electric power to the external grid in 2008. The successful demonstration of HTR-10GT would make great contributions to the HTGR development for global nuclear power supply.

Acknowledgements The authors wish to express their thanks to the OKBM specialists for their

significant contributions to the HTR-10GT project during the cooperation.

References Wu, Z.X., Lin, D.C., Zhong, D. X., 2002. The design features of the HTR-10. Nuclear

Engineering and Design 218(1-3), 25-32. Zhang, Z.Y., Yu, S.Y., 2002. Future HTGR Developments in China after the

Criticality of the HTR-10. Nuclear Engineering and Design, October 2002, 218(1-3), 249-257.


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