FILED OF INVENTION
The present invention generally relates to protection systems for steam turbines. More particularly, the invention relates to an electronic overspeed protection system for the steam turbines.
BACKGROUND OF INVENTION
When large capacity Steam Turbines operate, huge amount of centrifugal forces are exerted on the rotating components i.e. rotors, blades, windings etc. Mechanical design of the machine permits only brief excursions above the rated operating speed. Thus, over-speeding of the turbines has the potential to cause catastrophic failures which can endanger human lives and cause long outages.
Protection against the over speed is the most critical control aspect of the steam turbine vis-a-vis the protection system. The governing system represents the first line of defence against such over speeding, and the protection system represents the - second and final line of defence against over speeding. For proper functioning of the Governing system, a reliable and fast response speed measurement is a must.
Steam turbine power plants typically employ three detectors for speed measurement and the electronic speed control loop of the governing system. Overspeed protection is provided through two channel overspeed protection system for example, two hydro-mechanical emergency governors mounted on

the main turbine shaft. Some machines are provided with 3 channel overspeed protection system configured using one sensor for each channel, resulting in a lower reliability.
Typical Steam Turbines are equipped with an electronic speed control system to control the speed during start-up, shut-down and during load rejections or generator breaker opening. Steam admission valves of the turbine are controlled in relation to the energy demand. The speed control needs actual speed measurement as an input for realizing the control. The dynamic response of the speed control is dependent on fast response measurement of the speed.
In case of Generator breaker opening, the speed of the turbo set may go above the overspeed set limit and cause turbine tripping and is also harmful for the rotating components.
In addition to normal speed control, steam turbines are generally equipped with additional overspeed protection devices. When the speed control system malfunctions or the conditions are beyond its capacity to arrest speed rise, only turbine overspeed protection system can prevent damage to the turbine. For this, generally two shaft mounted hydro-mechanical emergency governors are provided.
Conventionally, these emergency governors consist of a spring loaded eccentric bolt installed on the turbine shafts or an additionally coupled shaft. Under overspeed conditions, the bolts come out forced by the centrifugal force and strike a lever which results in depressurization of the hydraulic circuit. This

phenomenon ultimately causes the steam admission valves to close arresting the overspeed.
Non-operation of these devices for years together, reduces their reliability for example, the mechanical parts may get stuck-up or their response may become slower or the spring properties may change over time. In view of this, it is mandatory for these devices to be tested periodically.
The testing is normally envisaged in two-ways - one is online testing by injecting test oil in the over speed bolt assembly. The normal trip circuit is by passed in this test to avoid turbine trip. Such online test is normally recommended once in a fortnight or once a month. However, an overspeed trip being the most important element for the safety of the machine and the operating personnel, it is mandatory as per international practice to test the complete hydraulic overspeed system by carrying out actual over-speeding of the machine. This is normally recommended to be done once a year.
During such a test, if the trip device fails to operate, it is difficult to manually react and prevent damage to the machine due to overspeed. Such testing causes fatigue in the machine and also requires the electricity generation process to be interrupted causing loss of revenue for the utility.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an electronic overspeed protection system for the steam turbines, which eliminates the disadvantages of prior art.
Another object of the invention is to propose an electronic overspeed protection system for the steam turbines, which incorporates a device for an accurate speed measurement of the turbine.
A still another object of the invention is to propose an electronic overspeed protection system for the steam turbines, which incorporates a device for speed measurement which device is capable of providing faster response to the protection system.
Yet another object of the invention is to propose an electronic overspeed protection system for the steam turbines, which is capable of being periodically tested online to ensure reliability of the system.
A further object of the invention is to propose an electronic overspeed protection system for the steam turbines, which constitutes a two channel, fail-safe protection system with three independent probes feeding each channel.

SUMMARY OF INVENTION
Accordingly, there is provided an electronic overspeed protection system for protecting steam turbines, comprising; at least six ferro-static type speed probes for measurement of the speed of a rotor shaft of the turbine, the shaft having a plurality of slots configured on a rotor collar such that the slots pass below the probes when the shaft rotates; at least six hardwired speed evaluation modules each connected to each one of the six speed probes, the overspeed protection being realised under two-channel electronic system, three probes with three speed evaluation modules assigned to each channel, the speed evaluation modules acquiring speed signals in analog form respecting the shaft rotation speed; at least one processing circuitory provided to each speed evaluation modules (SEM) which process the acquired data to ensure that the overspeed response time is minimized, the modules having diagnostic features for signal interruption, short circuit, deviation monitoring, and supply failure including a display device; at least one test generator provided to each of the channels to generate a test frequency representing overspeed conditions; and tripping devices comprising a turbine stop valve and a control valve, each valve being provided with a trip solenoid valve, overspeeding when occurs causes all the trip solenoids to be de-energized through trip relays incorporated in a trip relay module.
The developed system offers advantages like faster response, better reliability, no drift of setting over time due to change in spring properties, precise setting of overspeed limit, very easy initial setting. A major advantage of the electronic

overspeed protection system is that it can be tested using simulated overspeed signal. Testing of Emergency Governors by carrying out actual over speeding of the machine, once a year, causes undue overstressing of the machine and is especially harmful for older machines.
The inventive protection system is electronic type and protects the turbo set against overspeed. Mechanical emergency governors are no longer required. The system can be automatically tested online with an adjustable periodicity, thus increases its reliability.
The system is enabled to generate faster response of speed measurement and protection, which limits the transient speed rise to below unacceptable limits. Any fault in power supply system causes the values to close resulting in enhanced safety of the machine as the system gets tested online by simulating overspeed, actual over speeding to test the system is not necessary, thus avoiding undue overstressing of the rotating components of the turbo set.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. - 1 shows a typical Steam Turbine shaft with milled slots.
Fig. - 2 block diagram of the Speed Evaluation Modules of the overspeed protection system.
Fig. - 3 configuration of Overspeed Protection system of the invention.

DETAILED DESCRIPTION OF THE INVENTION
As illustrated in figs. 1 & 2, Six Ferro static type probes (pi to p6) are used for measurement of speed. As the turbine shaft (1) rotates, slots (2) milled onto the shaft pass below the probes (pi to p6) and the air gap between a first probe (pi) and the shaft (1) changes. Voltage pulses are generated in the probes (pi -p6) corresponding to the rotational frequency of the turbine. Fixing of separate pole wheel to the turbine shaft (1) is avoided as it can get loosened over time or due to faulty fitting, and machining. Instead the slots (2) are milled directly in a collar (3) on the turbine shaft (1).
The slots (2) and the probe locations are selected in such a way that they are not substantially affected by the thermal expansion of the rotor. Figure 1 shows the details of the slots (2) in the rotor collar (3). The target slotted wheel for speed measurement probes is designed to ensure reliable and fast response to meet the dynamic and safety requirements of the Turboset. At least sixty slots (2) are made on the rotor collar (3) to meet the accuracy and resolution requirements of the speed signal.
Speed signals are acquired using hardwired Speed Evaluation Modules (SEM1, SEM2, SEM3), one each being connected to each of the six speed probes (P1-P2). The sensors and processing circuitory (PC1, PC2, PC3) is selected to ensure that overspeed protection response time is kept within permitted limits, which is of the order of 200 to 250ms. Three analog speed signals are passed on to the electronic turbine controller for governing the speed control.

The speed evaluation modules (SEMs) have integrated fault diagnostic features for signal interruption, short circuit, deviation monitoring with other module, supply failure and plausibility checking.
The overspeed threshold is adjustable in the module (SEMs) and is normally kept at 110% of rated speed for large steam turbines. It is also displayed in digital form on the front side of the module. When the threshold is exceeded, a relay changes over, which is used further in the trip circuit for denergizing the trip relays (4) in the trip relay modules (5).
Refer Fig. 2 for block diagram of speed evaluation modules (SEMs).
The overspeed protection is realized as a two channel electronic system with online cyclic testing. Three probes (p1-p3) are used for each protection channel (CH1, CH2) for extreme reliability. Both the channels (CH-1, CH-2) are hardwired to achieve fast response and for utmost reliability. Fast response ensures that the transient speed rise is kept within defined limits.
The system is designed on "failsafe de-energized to trip" technology. Tripping devices on the machine side are decentralized i.e. each turbine stop and control valve is provided with two separate trip solenoid valves (6). Actuation of either of the valves is sufficient to shut down that particular valve. Exceeding of speed beyond the preset limit causes all the trip solenoids (6) to be de-energized through the trip relay modules (5). The trip relay modules (5) are configured in two-out-of-three logic for each of the two channels (CH-1, CH-2).

When the machine is in operation, the trip solenoid valves (6) are energized with redundant 24V DC supply. Any interruption in the power supply causes the machine to be tripped in fail-safe mode.
Refer Fig. 3 for configuration of the overspeed protection system.
In each of the channel (CH-1, CH-2), a test generator (7) is provided which generates a test frequency representing overspeed condition. This simulated overspeed signal is used for testing of each protection section of each channel (CH-1, CH-2). Thus all the six sections get tested alongwith the trip relays (4).
However final trip does not take place due to action of only one relay at a time against the requirement of minimum two in any channel. This testing is carried out automatically in a periodic manner. The periodicity of testing is adjustable.

WE CLAIM
1. An electronic overspeed protection system for protecting steam turbines, comprising:
at least six ferro-static type speed probes (pi to p6) for measurement of the speed of a rotor shaft (1) of the turbine, the shaft (1) having a plurality of slots (2) configured on a rotor collar (3) such that the slots (2) pass below the probes (pi to p6) when the shaft (1) rotates;
at least six hardwired speed evaluation modules (SEM) each connected to each one of the six speed probes (p1 to p6), the overspeed protection being realised under two-channel (CH1, CH2) electronic system, three probes with three speed evaluation modules assigned to each channel, the speed evaluation modules acquiring speed signals in analog form respecting the shaft rotation speed;
at least one processing circuitory (PC-1) provided in each speed evaluation modules (SEM) which processes the acquired data to ensure that the overspeed response time is minimized, the modules (SEMs) having diagnostic features for signal interruption, short circuit, deviation monitoring, and supply failure including a display device;

at least one test generator (7) provided to each of the channels (CH1, CH2) to generate a test frequency representing overspeed conditions; and
tripping devices comprising a turbine stop valve and a control valve, each valve being provided with a trip solenoid valve (6), overspeeding when occurs causes all the trip solenoids (6) to be de-energized through trip relay incorporated in a trip relay module (5).
2. The system as claimed in claim 1, wherein actuation of either of the Trip Solenoid valve (TSV) causes a Closure of Stop and Control Valves, thereby shutdown of the Turbine.
3. The system as claimed in claim 1 or 2, wherein the trip solenoid valves (6) are energized with redundant 24-V DC supply.
4. The system as claimed in claim 1, wherein at least sixty slots (2) are configured on the rotor collar (3) to ensure the accuracy and resolution requirements of the speed signals.
5. The system as claimed in claim 1, wherein the overspeed response time is maintained between 200 to 250 ms.

6. The system as claimed in claim 1, comprising trip relays for deenergizing the trip solenoid valves in case the threshold value exceeds.
7. An electronic overspeed protection system for protecting steam turbines, as herein described and illustrated with reference to the accompanying drawings.

This invention relates to a turbine speed control and overspeed protection, which are very critical for safe and reliable operation of the turboset. Hydro-mechanical overspeed emergency governors have been replaced with electronic overspeed protection system. This is realized using six probes connected in 2 out of 3 channels. The system is of failsafe de-energised to trip design. For achieving faster response, hardwired speed monitroring modules are used based on relay based protection circuits. The system can be tested online simulating an overspeed signal using a test frequency generator. This obviates the need for actual overspeed test on the machine. Speed signals are also used for turbine governing system.