Speed Control System Retrofit Of Steam Turbine

The original pure hydraulic adjustment system of the old units can be transformed into digital electro-hydraulic control system to realize the upgrade of the turbine control system and meet the requirements for flexible configuration of various control strategies.

electro-hydraulic control system renovation case:

A power plant installed a C25-8.83-0.981 turbo-generator set in the 1990s. Its control was based on a pure hydraulic adjustment system, with cam distribution mechanism. After more than ten years of operation, the turbine`s load control became insensitive, the operation sluggishness rate became large, the self-alignment of the thermoelectric load became lower, and the primary frequency modulation capability became weaker, and in case of partial load rejection, the adjustment performance cannot meet the operation requirements. In view of the above situation, the unit control system can be changed to the DEH system to improve the unit's control level and adaptability.

1.The original system adjustment principle introduction

The old system adopted a full-hydraulic pull-in adjustment system, which was mainly composed of speed-regulating oil pump, regulators (pressure converters, pilot oil valve and oil motives), pressure regulators, rotary diaphragm oil motives, synchronizers, etc.; and the electric and thermal loads of the turbine were respectively controlled by two loops-the speed control loop and the pressure control loop. The speed control loop receives the speed pulse signal (oil pump outlet pressure variation) of the speed regulating pump and the synchronizer given signal for controlling the speed and electric load; the pressure regulation loop receives the given signals of extraction pressure and the voltage regulator for controlling the extraction pressure and heat load.

1)Control process of speed control loop

The speed governing oil pump measures the turbine speed as a hydraulic speed sensor. The oil pressure difference between the inlet and outlet of the oil pump is proportional to the square of the unit rotation speed. When the rotation speed does not change much, the oil pressure change and the rotation speed change can be linearized, so the unit uses oil pressure as the pulse signal of the rotation speed change. One side of the pressure converter slide valve is controlled by the oil pressure of the revolution pulse signal, and the other side is controlled by the spring force of the operation signal through synchronizer. The pressure converter slide valve controls the overflow of the two pulse oil circuits. When the speed change of the unit causes a change in the pressure increase of the speed governing pump, or when the synchronizer is operated and the spring tension of the pressure converter is changed, the pressure converter valve slide up and down, and thereby change the opening of the pressure converter overflow window, hence causing two pulse oil circuit pressure changes, so that the pilot oil valve moves up and down to control the oil intake and discharge of the oil motive and hence cause the oil motive`s piston movement, and drives the adjusting steam valve and the rotating diaphragm to open or close at the same time to achieve the purpose of adjusting the speed of the turbine (single-unit operation) or the electric load (grid operation); When the oil motive piston moves, the overflow area of the feedback window is changed at the same time, so that the pulse oil is fed back and restore to the rated value, the pilot oil valve slides back to the middle position, the oil circuit of the oil motive is cut off, and the unit is in a new stable state.

2)Pressure regulating control loop

Bellows-type pressure regulator is used as a component for measuring the pressure change and the variation of the extraction pressure is used as pressure regulation pulse signal. The function of the bellows is to convert the change of steam pressure into displacement. This displacement makes the pressure regulator slide valve move, and thereby changes the opening of the transformer overflow window, causing the change of oil pressure of the two pulse oil paths. Since the change of opening of the two overflow windows of the regulator is opposite, the change of the oil pressure of the two pulse oil circuits is also opposite. That is, the high pressure oil motive and the low pressure oil motive moves in opposite direction. Revolving the upper handwheel of the pressure regulator can change the pretightening force of the pressure regulator spring so as to achieve the purpose of changing the steam extraction pressure of the steam turbine. When the thermal load is incorporated into the heat network, the static characteristic curve of the pressure regulator can be translated, thereby changing the heat load of the steam turbine.

2.Electro-hydraulic control renovation plan and its implementation

During the renovation of this unit, the steam turbine was still running, and according to the requirement of the owner, the shorter the renovation period, the better. In addition, after fully considering the construction period and the turbine`s control accuracy requirements, QNP decided to remove only the synchronizer, high-pressure oil motive, medium-pressure oil motive and pressure regulators and other adjustment assemblies in the original speed control system, retain the original security assemblies and oil supply system, thereby considerably reduce the workload of the renovation. At the same time, a direct-acting oil motive with an independent oil source was used as the hydraulic actuator of DEH, and Nanjing Keyuan DEH-NK series Electro-hydraulic control system was applied. After the renovation, the turbine was stable in operation and the operating parameters were excellent.

Description of the renovation :

1.Removed assemblies

1). High-pressure oil motive 2). Synchronizer  3). Medium-pressure oil motive 4). Pressure regulator, etc.

2.Retained assemblies

1). Electric oil pump set 2). Oil ejector 3). Oil overflow valve 4). Overspeed protective devices (including emergency governor, emergency governor pilot valve, electric overspeed protective devices, speed measuring devices, etc.) 5). Starting valve 6. Medium pressure oil motive bracket

3.Added assemblies

1). High pressure oil motive cover plate and bracket 2). Medium pressure oil motive cover plate 3). High and medium pressure oil motive adjusting screw rod 4). DEH electro-hydraulic control system and hydraulic Actuator 5). Regulating oil pipeline 6). Partial security oil road reconstruction 7). Speed sensor