During the period of 2002-2006, Ukrainian Energy Machines (UEM) company (former TURBOATOM) carried out equipment supply on a turnkey basis for 4 power units of the ‘Kaiga-3,4’ and ‘Rajasthan-5,6’ Nuclear Power Plants under an order from the Nuclear Power Corporation of India Limited.
In accordance with the contract requirements, the following equipment was designed, manufactured, and supplied: K-240-4.0 steam turbine with a condenser and auxiliary systems, TGW-250-2PT3 generator with auxiliary systems, steam separators, moisture separator – repeater, and turbine automatic control systems. The contract included supervision of installation, commissioning, and operational testing, as well as personnel training.
In April 2007, the first start-up and synchronization of power unit No. 3 at the Kaiga NPP took place. In June 2008, based on the results of testing, the power unit Kaiga#3 was officially put into operation.
The project envisaged the retrofit of existing NPP where reactors manufactured by the Indian company BHEL with a capacity of 235 MW are in operation.
The K-240-4.0 turbines represent a further stage in the development of the K-220-44 turbines, which are successfully operated in Ukraine’s nuclear power plants and several other countries.
The contract stipulated the preservation of design solutions for the building part, foundations, condensate-feed system, cooling water, and other systems that were adopted for the K-235-4.0 BHEL turbines.
The turbine is a single-shaft two-cylinder unit consisting of a five-stage high-pressure cylinder (HPC) and a double-flow low-pressure cylinder (LPC). Each flow of the LPC has 5 stages. The length of the last stage working blade is 1030 mm. The longitudinal section of the turbine is shown in Figure 1.
Superheated steam is supplied through 2 detached stop valves and two governor valves located on the top of the HPC casing. The turbine steam distribution system operates on a throttling principle.
After the HPC steam passes through two steam superheaters, the superheated steam is directed to the LPC via 2 receivers. Each receiver is equipped with sequential stop and governor butterfly valves.
The turbine unit diagram includes 6 non-regulated steam extractions to regeneration system.
The stop valves are designed to be balanced from steam forces. Steam leakage sealing is achieved along the stop valve stems.
The governor valves are also designed to be balanced from steam forces. A cam-driven distribution mechanism is used as the main servomotor for both control valves.
The design of the steam part of the butterfly valves is identical.
The turbine is equipped with an electro-hydraulic control system. The general scheme of turbine governor is shown in Figure 2.
Turbine oil is used as the working fluid in the regulation system. The oil is supplied by an oil pump, which is located on the turbine shaft. During startup and shutdown, a separate starting pump is used.
The protection system consists of a mechanical safety regulator, slide valves of safety regulator, intermediate slide valves, two protective devices, switches and servomotors for stop valves, cut-off slide valves and stop butterfly valves.
The mechanical safety governor consists of two spring-loaded rings. The safety governor slide valves are two spring-loaded slide valves that act on the impulse line of the protection system. Between these slide valves, a rotary slide valve with bearings is installed. It is used during checks of the safety governor on the running turbine by filling oil and accelerating.
The protective devices are duplicated for reliability increasing. They consist of an electromagnet, a double-seated valve, and an actuating diaphragm. Safety devices are used for cock and knock the protection system.
The intermediate slide valves are a block comprising four spring-loaded tension springs for the actuating valves. Each pair of intermediate slide valves controls its side of the turbine. The use of intermediate slide valves allows the cutoff of oil supply from the main oil pump to the actuating servomotors of the stop and governor butterfly valves.
All servomotors of the stop governor butterfly valves are unidirectional and spring-loaded.
The feature of the regulation system is that it includes an Electronic Speed Controller (ESC) as a backup in the case of failure of the Electro-Hydraulic Control System (EHCS).
The EHCS consists of hydraulic and electronic components. The transmission of the electrical control signal is achieved using an electro-hydraulic converter. The Main Servomotor (MSM) is bidirectional. The control of the MSM is carried out by a four-slot Cutoff Slide Valve (CSV). The MSM drive has hydraulic feedback on the MSM and CSV positions. Additionally, electrical feedback on the MSM position is utilized, for which a position sensor is installed.
In parallel with the Electro-Hydraulic Converter (EHC), there is the Current Relief Mechanism (CRM) which performs unloading function of the EHC from the control current. Here a signal is sent to the CRM, causing the actuating traveling bush of the CRM to shift in such a way that the total oil flow from the EHC and CRM remains the same, while the amperage supplied to the control coil of the electro-mechanical converter of the EHC approaches zero. This ensures a smooth disconnection of the Electro-Hydraulic Control System (EHCS) in case of its failure. The maintenance of the MSM in the
desired position will be carried out by hydraulic feedback lines.
In the event of simultaneous failures of both the EHCS and the ESC, the turbine does not come to a stop but enters manual control mode. This mode allows the operator to gradually reduce the load for a planned turbine shutdown.
The main operating modes of the EHCS are as follows: ‘Turn mode’”, ‘Speed Control”, “Power Control”, “Pressure Control”, “Load shedding mode 1”, “Load shedding mode 2”, “ Tracking mode” and “Repair mode”.
In the ‘Turn mode’ the following actions are tested: the governor valves are tested on the nonoperational turbine; shaft speed increasing from revolutions when working with a turning gear to idle speed in both automatic and manual modes, safety governor testing during the shaft speed acceleration, testing of synchronization and connection to the power grid.
After the turbine is connected to the grid, the ‘Power mode’ is automatically activated. In this mode, the EHCS maintains the desired power output according to the standard ‘frequency-active power generator’ characteristic.
The ‘Pressure mode’ allows the EHCS to control the pressure at the turbine inlet. The turbine is controlled by a block pressure governor in this mode. The control signal in the form of current is sent to the electronic part of the EHCS, which then generates the appropriate signal to adjust the governor valves.
In the ‘Speed mode’, the EHCS operates according to the static characteristic of ‘change in rotational speed – change in the position of the MSM’.
When the generator switch is turned off, the ‘Load shedding mode 1’ is automatically activated. The purpose of this mode is to prevent the rotational speed from exceeding the safety governor trip setting and to stabilize the rotational speed at the design speed level.
In certain cases, during turbine operation, there might be a load shedding without disconnecting the generator from the grid. In such cases, the ‘Load Shedding mode 2’ is activated. Its function is similar to that of ‘Load Shedding mode 1’.
In case of a failure in the turbine equipment operation the EHCS switches to the ‘Repair’ mode. In this mode, the control signal to the EHC of the EHCS is physically disconnected. This allows repairs to be carried out on the electronic part without stopping the turbine.
After the cause of the malfunction is resolved, the EHCS switches to the ‘Tracking mode’. In this mode, the EHCS is physically reconnected to the EHC’s connection lines, but it does not participate in the regulation. EHCS monitors the turbine’s operating parameters and is ready to be smoothly re-engaged in operation.
The main operating modes for the Electronic Speed Governor (ESG) are ‘Speed mode’, ‘Load Shedding mode’, ‘Tracking’ and ‘Repair.’ The operation of the ESC in the last two modes is similar to that of the EHCS.
When operating in manual mode, turning off the Generator Switch leads to the automatic shutdown of the turbine.
To increase reliability, all main sensors and blocks are redundant with the use of 3 channels. The ‘two out of three’ principle is employed when forming the ‘true’ signals.
The operational experience has confirmed the correctness of the fundamental design decisions regarding the automatic control and protection system. The reliability of the turbine equipment is evident from the fact that Unit 5 has been continuously operational for 2 years from 2021 to 2023. The average power maintained by the unit was 235 MW, significantly higher than the power carried by turbines of other manufacturers for similar power units.
Turbine equipment repairs are carried out by Indian repair companies, and spare parts and engineering technical support are effectively provided by Trafalgar Epc Private Limited. The successful execution of repairs is facilitated by the good condition of the major steam distribution and control components. Based on experience, the tuning of the automatic control and protection system requires minimal intervention.
Conclusions
The operational experience of the K-240-4.0 turbines at Units 3 and 4 of the ‘Kaiga’ nuclear power plant and Units 5 and 6 of the ‘Rajasthan’ nuclear power plant has confirmed their high reliability and efficiency. These turbine units are capable of generating electrical power more than 10 MW higher than turbines from other manufacturers with similar specifications. The operational tests have demonstrated uninterrupted performance for a period of two years.
The technical solutions adopted during the project execution for these units allow for their easy adaptation to improve the existing nuclear power units in India with the same reactor installations. Specifically, replacing the flow sections of low-pressure cylinders would significantly increase the power output, and the implementation of electro-hydraulic control systems would enhance operational reliability, technical characteristics, reduce repair complexity, and enable the units to participate in grid frequency regulation or dynamic modes.
The successful implementation of the ‘Kaiga#3,4’ and ‘RAPPs#5,6’ projects highlights the significant advantages of using the developments of the ‘Ukrainian Energy Machines’ company, which has experience in supplying modern turbine equipment to nuclear and thermal power plants, as well as cost-effective modernizations of existing equipment, including turbines from other manufacturers.
BABAYEV Ivan Nikolayevich has done more the 45 projects, in-cluding 12 new turbine, more 33 project of modernization governor system of steam turbine in NPP and TPP of
Ukraine, Russia, Kazkhstan, Armenia, Bolgaria, Romania, Lithua, India. He carried out projects for most types of steam turbines from 2 MW to 1100 MW. He also took part in the
reactions and starts of turbine Kaiga#3,4 and RAPPs#5,6 in India.
