miércoles, 27 de septiembre de 2017

What is synchronization and effects of poor synchronization in power plant

Synchronization of Turbo Alternator or an AC generator is the process of connecting the generator with grid power supply which is an interconnection of large pool of generators and power consumption loads. Simply the grid is parallel operation of some number generators with same frequency. So to connect the Generator in power plant in this pool of parallel running generators, The incoming generator parameters like frequency, phase angle and voltage should be matching with the existing grid frequency.

Before going detail description first let us understand what is the need of synchronization of generator. Generator is connected with the prime mover which provides the rotating magnetic field and hence this rotating magnetic field will induces the voltage in the stationary part. The frequency and phase angle of the voltage signal is controlled by the prime mover speed and magnitude of the voltage signal is controlled by the generator excitation. 

To understand the phenomenon let us correlate the entire operation with the person wants to catch a running travel bus. Consider the travel bus is grid power supply and the person is incoming generator. Now if the person wants to get in to the bus then he should equally or little faster than the bus same the generator tries connect to the grid should run equally or little faster than the grid. Here the speed is measured with the frequency because speed is proportional to the frequency( 50 Hz, 60 Hz). The person is now running with the same speed of the bus but the bus door is one end of the bus and he is at another end of the bus so he needs to match with the door to get in to the bus. Like the same if the generator is running at the same frequency of grid it cannot be synchronized until unless the phase of the two voltages matches.

What is the need of synchronizing two different power sources?
"Suppose you have a trolley that can only drawn by either pushing or pulling it ,two workers are there to drive it if one of them is pushing in one direction but the other one is in another direction ..What will happen?
they can't move it with different speed or different direction."
Similar is gonna happen with power source.
Phase angle indicates the direction, If there is a phase difference between two power sources it can't operate any load.

In order to synchronize a generator to the grid, four conditions must be met:
1. Phase Sequence
The phase sequence (or phase rotation) of the three phases of the generator must be the same as the phase sequence of the three phases of the electrical system (Grid).
The generator or transformer power leads could actually be interchanged during maintenance orthe potential transformer leads could be interchanged during maintenance..

2. Voltage Magnitude
The magnitude of the sinusoidal voltage produced by the generator must be equal to the magnitude of the sinusoidal voltage of the grid.
If all other conditions are met but the two voltages are not the same, that is there is a voltage differential, closing of the AC generator output breaker will cause a potentially large MVAR flow.

3. Frequency
The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.
The synchroscope would be rotating rapidly counter clockwise. If the generator breaker were to be accidentally closed, the generator would be out of step with the external electrical system. It would behave like motor and the grid would try to bring it up to speed.
In doing so, the rotor and stator would be slipping poles and damage (possibly destroy) the generator as described previously. The same problem would occur if the generator were faster than the grid.

4. Phase Angle
As previously mentioned, the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero.

The phase angle (0 to 360°) can be readily observed by comparing the simultaneous occurrence of the peaks or zero crossings of the sinusoidal waveforms..

Effects of poor synchronization:
Prime mover damages if the speed and rotor angle is not matches with grid voltage frequency and phase angle due to rapid acceleration or deceleration. Let us suppose generator has to connected to the grid frequency of 60 Hz. But the breaker has closed with poor synchronization at the generator frequency of 58Hz (i.e for two pole generator speed is 3480 out of 3600 rated), now once the breaker closes the generator is connected in the pool of parallel generators which forces the incoming generator to rotate at the same grid frequency. Due to this sudden acceleration of the rotor from 3480 to 3600 rpm and a sudden break at 3600 rpm damages the rotor mass. Same way in the reverse when the generator is running higher frequency than the grid frequency.
A large currents may suddenly flow through the Generator windings and Generator transformer windings due to poor synchronizations which damages the windings.
There will be power and voltage oscillations because of this sudden acceleration and deceleration of the rotor.
It may leads to activation of the generator protective relays which causes the major interruption so the process should be started once again after clearing the protection. 

viernes, 21 de julio de 2017

Mundo Siemens

sábado, 1 de julio de 2017

How to select Level Measuring technology?

Level Measurement Technology
Two Groups of Level Measurement -
1.Point Measurement (Discrete)
2. Continuous Measurement (Analog)
Common Types of Level Measurement Technologies
1.Plumb Bob
This is one of the oldest level measurement technology.
A worker drops a length of pre-measured rope until a float contact the material surface.
Advantages -
Disadvantages -
Human Non- reputability
Hazard exposure
Process damage from broken cable
Not instantaneous
Level Measurement - Plum bob
2.Ultrasonic (Non-Contacting)
Measure time of flight from transmission to received echo.This determines distance and calculates level.
Advantages of Ultrasonic Level Measurement-
Well Proven
Solids,Liquids,or Slurries
Control Capabilities (Pumping and alarm)
Potted/robust transducer for vibration and shock (No electronics in vessel)
Remote display
Disadvantages of Ultrasonic Level Measurement-
Sensitive to vapor changes in medium
Can be sensitive to foam
Limited pressure and temperature range
Level Measurement - Ultrasonic
3.Radar (Non-Contacting)
Measure time of flight from transmitted signal to return signal for distance measurement.
Advantages of Radar Level Measurement-
Insensitive to vapor and dust
Unaffected by temperature and pressure
Very long ranges (up to 100 m)
Disadvantages of Radar level measurement-
Can be sensitive to foam
Can be sensitive to very heavy condensate
Display and antenna are integral
Level Measurement - Radar Type
4.Guided Wave Radar (GWR) or Capacitance (Contacting)
Both technologies use a rod or cable which extends into the material being measured.
Advantages of Guided Wave Radar or Capacitance level Measurement
Able to measure the liquid-liquid interface of two imiscible liquids
Wide temperature / Pressure range up to 427 deg C or up to 431 bar
Disadvantages of Guided Wave Radar or Capacitance level Measurement
Can be sensitive to material build up
Wear in solids
Pull forces on roof-solids applications
Equipment damage from broken rods.
Level Measurement Capacitance type
5. Hydrostatic Level (Contacting)
Measure head pressure of material in vessel
Advantages of Hydrostatic Level Measurement
Easy to use
Complex internal geometry possible
Suitable for high temperature and pressure
Most common level measurement in all industries
Disadvantages of Hydrostatic Level Measurement
Contacting - chemical compatibility with seals
Susceptibility to specific gravity changes may require recalibration
Adds fitting/piping to system if not submersible
Hydrostatic Level Measurement

Source: https://automationforum.in/t/how-to-select-level-measuring-technology/882

viernes, 30 de junio de 2017

Temperature Sensor Installation for Best Response and Accuracy

The installation of the sensor can introduce errors, noise, and dynamics causing poor measurement and control loop performance. Here we look at best practices to get the most out of the inherent capability of the sensor. My next post will provide guidance on the communication of the sensor Thermovision image heating plant chimneysignal to the control room to provide the best total installation.
Thermowell Length
To minimize conduction error (error from heat loss along the sensor sheath or thermowell wall from tip to flange or coupling), the immersion length should be at least 10 times the diameter of the thermowell or sensor sheath for a bare element. Thus, for a thermowell with a 1 inch (2.54 cm) outside diameter, the immersion length should be 10 inches (25.4 cm). For a bare element with a ¼ inch (6.35 mm) outside diameter sensor sheath, the immersion length should be at least 2.5 inches (63.5 mm). This is just a rule of thumb. Computer programs can compute the error and do a fatigue analysis for various immersion lengths and process conditions. For high velocity stream and bare element installations, it is important to do a fatigue analysis because the potential for failure from vibration increases with immersion length.
The choice of thermowell length, location, and construction determines whether the temperature measurement is representative of the process, how much process noise is seen, how much delay and error is introduced, and the potential failure rate. This post provides some general guidance. This post provides some general guidance. For more details including the equations to predict eight sources of measurement error see Greg McMillan’s ISA book Advanced Temperature Measurement and Control, Second Edition
Thermowell Location
The process temperature will vary with process fluid location in a vessel or pipe due to imperfect mixing and wall effects. For highly viscous fluids such as polymers and melts flowing in pipes and extruders, the fluid temperature near the wall can be significantly different than at the centerline (e.g., 10 to 30°C; 50 to 86°F). Often the pipelines for specialty polymers are less than 4 inches (101.6 mm) in diameter, presenting a problem forgetting sufficient immersion length and a centerline temperature measurement. The best way to get a representative centerline measurement is by inserting the thermowell in an elbow facing into the flow (position 1 in the figure below). If the thermowell is facing away from the flow, swirling and separation from the elbow as can create a noisier and less representative measurement (position 2 in figure). An angled insertion (position 3 in figure) can increase the immersion length over a perpendicular insertion (position 4 in figure) but the insertion lengths shown for both are too short unless the tip extends past the centerline. A swaged or stepped thermowell can reduce the immersion length requirement by reducing the diameter near the tip.
The distance of the thermowell in a pipeline from a heat exchanger, static mixer, or desuperheater outlet should be optimized to reduce the transportation delay but minimize noise from poor mixing or two phase flow. Generally 25 pipe diameters are sufficient to ensure adequate mixing from turbulence if there is a single phase, turbulent flow, and no great differences in the viscosity of streams being combined. Two phases exist for desuperheaters, split ranged transitions from cooling water to steam in jackets, the use of lime ammonia as a reagent for pH control due to flashing and whenever slurries are involved.
The transportation delay will increase with distance adding more dead time to the loop. Consequently, there is a compromise between getting enough mixing to achieve a representative low noise measurement and creating too much additional dead time. In general, the transportation delay should be less than 10% of the PID reset time setting.
 Insight: Generally a distance of 25 pipe diameters between the equipment outlet and the temperature sensor is sufficient to provide a relatively uniform temperature profile of a single phase fluid. The presence of different phases (e.g. bubbles or solids in liquids and droplets in steam) and high viscosity fluids will require longer distances.
For desuperheaters, the distance from the outlet to the thermowell depends upon the performance of the desuperheater, process conditions, and the steam velocity. To give a feel for the situation there are some simple rules of thumb for the length of piping from the desuperheater to the first elbow known as straight piping length (SPL) and the total piping length from the desuperheater outlet to the sensor known as sensor total length (TSL). Actual SPL and TSL values depend on the quantity of water required with respect to the steam flow rate, the temperature differential between water and steam, the water temperature, pipe diameter, steam velocity, model, type, etc. and are computed by software programs.
SPL (feet) = Inlet steam velocity (ft/s) x 0.1 (seconds residence time)
SPL (m) = Inlet steam velocity (m/s) x 0.1 (seconds residence time)
TSL (feet) = Inlet steam velocity (ft/s) x 0.2 (seconds residence time)
TSL (m) = Inlet steam velocity (m/s) x 0.2 (seconds residence time)
Typical values for the inlet steam velocity, upstream of the desuperheater range from 25–350 ft/s (7.6 to 107 m/sec). Below 25 ft/s there is not enough motive force to keep the water suspended in the steam flow. Water tends to fall out and run down the pipe to a drain. When this happens the water no longer cools the steam and the system thinks it needs to add more water, which compounds the problem. Problems can also include pipe wall erosion and high thermal stress gradients in the pipe wall (i.e., a hot top and cold bottom, which can crack welds or warp the pipe to an egg-shaped cross-section). Current technology has an inlet velocity limitation of 350 ft/s (107 m/sec). Velocities higher than 350 ft/s cause the desuperheater to vibrate and damage the unit to the point where it breaks apart.
Thermowell Construction
The stem of a thermowell is the part that is inserted into the process stream. Stems can be tapered, straight, or stepped. The performance of a thermowell varies with its stem design. In general, a tapered or stepped stem provides a faster response, creates less pressure drop, and is less susceptible to conduction error and vibration failure.
If the thicknesses of the thermowell walls and the fit of the sensing element are identical, thermowells with straight stems have the slowest time response because they possess the most material at the tip (largest diameter). Thermowells with stepped stems have the fastest time response because they possess the least material at the tip (smallest diameter). A small diameter also results in the least amount of drag force. Thermowells with stepped stems also provide the maximum separation between the wake frequency (vortex shedding) and the natural frequency (oscillation rate determined by the properties of the thermowell itself). If the wake frequency is 80% or more of the thermowell natural frequency, resonance and probably damage can occur. Generally, thermowells with tapered stems are slightly more expensive as a result of a more complicated manufacturing process.
Insight: Swaged, stepped, and tapered thermowells offer a faster response, lower pressure drop, and less possibility of vibration damage from resonance with wake frequencies.
The tip of the sensor must touch the bottom of the thermowell. Spring loaded sensor designs help ensure this is the case despite different installation practices and orientation. The fit of the sensor should be as tight as possible to reduce the annular clearance since air acts as insulator. The sensor lag can increase by an order of magnitude for a sloppy fit. For liquid systems, the additional lag effectively becomes an additional equivalent dead time in the measurement.
Insight: The tip of the temperature sensor must touch the bottom of the thermowell and the fit must be tight to prevent introducing a large sensor lag due to the low thermal conductivity of air.
Take advantage of general guidelines on thermowell insertion length, location, construction, and fit to make sure the sensor is seeing the actual process temperature with a low probability of vibration failure and minimal noise, delay and lag.