Featured post

Arduino FreeRTOS Mutex Examples

In this section, we will explain using an example to demonstrate mutexes in Arduino using freeRTOSThere are two freeRTOS Mutex examples in this tutorial, the first example demands some hardware (LCD) While the second does not need any hardware, you can try out both if you have the resources.

In the last tutorial, we considered in detail: semaphores and mutexes and we also established the difference between binary semaphores and mutexes.
Just for a review:Recall that a mutex is a locking mechanism that implements the take and gives functionality in itself, unlike a binary semaphore. See this tutorial if you have not before continuing.
Example 1: Protecting the LCD Resource Using Mutex in freeRTOS
Program Description

In this program, we Demonstrated the use a 16x2 LCD display to implement a mutex.

The LiquidCrystal library works with all LCD displays that are compatible with the Hitachi HD44780 driver. you can usually tell them by their 16-pin interface.

THE CIRCUIT

* LCD RS pin to digital…

Procedure for Electricity Generation in a power plant

The Procedure for Electricity generation in a thermal Power plant is a rather intricate and continuous process that utilizes many engineering applications, equipment, and control mechanisms so as to efficiently produce the required output. It encompasses the production and purification of feedwater, demineralization of the feedwater, steam generation process and the efficient use of the steam by the turbines for power generation.

Basing this post on the engineering training received from Nigerian's biggest Power Plant (Egbin), The Electrical Power generation process involves the following stages:

  1.  Deep-well Water Production Process.

  2.  Water Treatment Process.

  3.  Demineralization Process.

  4.  Steam Generation Process (Boiler).

  5. Steam Work Process (Turbine & Generator).

Water Production and Treatment Process


The Deep Well Water: The Deep-well can be said to be an industrial-sized borehole. The Deep-well is preferably used over the conventional stream or water body because it requires less modification, purification and process cost so as to meet process variable specifications. Each deep-well pumps water at a rate commonly 75m3/hr to the storage tanks/water reservoirs.

Water Treatment Process


The sequence of treatment operations carried out on the Deep Well water are:

    Bleaching in the Bleaching Tank: This is an open tank where water disinfection and pH modification take place. A pH probe is firstly used in measuring the acidity of the incoming water, in order to regulate the addition of the reagents:

    The Bleaching Tank is also equipped with a stirrer so as to aid the agitation (mixing) of the entire tank volume thereby to produce a homogenous mixture of CalciumHydroxide and CalciumHypochloride.

    Clarification Process: This is also an open tank in which flocculation, sedimentation, and coagulation occur so as to produce clear water. The Clarifier is able to remove suspended solids from water and particulates mixture coming in from the bleaching tank by allowing them to settle under the force of gravity (sedimentation) thereby producing mud (sludge) at the bottom. The sludge accumulated at the floor of the Clarifier is then pushed to the center by the scrapper and can be later recirculated or dumped.  In some cases, the addition of a flocculant is employed so as to increase the rate at which smaller particles coagulate into bigger particles thus, yielding faster sedimentation rate.



     Sand Filter Bed:  Here, the elementary idea of filtration using sand particles of various sizes is employed. The clarified water from the previous process is transported via open channels to the sand filter bed.  This water filters into the clear well. From here it is transported to the fresh water tank, apart from steam generation, it can also be used for:



      1. Fighting fire

      2. For domestic purposes (it contains minerals that the body needs but the plant does not)

      3. For steam generation after further processes.


      In addition, the Sand Filters are also equipped with Air Blowers and Wash Pumps so as to aid an operation which is termed Backwashing – an act of using compressed air and water to differentially clean the clogged pores of the filter beds which tends to reduce filtration efficiency.

       Clear Wells:  These are underground storage tanks that are used for temporarily storing processed water from the previous unit operations before being sent to the Fresh Water Tanks.

      Fresh Water Tanks: These are also closed tanks and water from the Clear Wells are sent here to be channeled to the Fire Fighting System for fire outbreaks and for Potable Water production.

      Water Demineralization Process




      The Demineralization Process is carried out so as to rid the feedwater of impurities such as acids, salts, bases and ions which are detrimental to the efficiency of the boiler system.  The Demineralization process is carried out in the Demineralization-Plant which consists of two separate trains A & B, equipped with the same process operation. Each train is specifically made to deliver a net service flow of 200m3/h.

      The sub-processes/operations in the Demineralization Plant are:

      1. a) Carbon Filter Vessel: Feedwater from the Fresh Water tanks are firstly channeled to this vessel which employs activated carbon as filter media. As a result of the physical adsorption mechanism of carbon, impurities such as free chlorine and organic substances are removed from the water.

      • Resin Exchanger Vessel: Contains resins for the removal of ions These resins are tiny substances that contain pores and are also called ion exchangers. Cation Resins removes positive ions while anion resins remove negative ions whereas residual traces of both cations and anions are removed by the mixed bed ion exchanger. The regeneration of this exchanger is carried out in another vessel called the Regeneration Vessel.

      • Regeneration System: This consists of 100% duty regeneration pumps, a backwash or rinse operation system, an external regeneration system for the mixed-bed exchangers, a sulphuric acid regeneration system for cation exchange resin, a caustic regeneration system for anion exchange resin and a mixing air unit for resin mixing and transfer.

      • Demineralization Storage Tanks: These are closed tanks which are used in storing demineralized water from the Demineralization Plant.

      Steam Generation Process (Boiler)


      Power generation in Egbin Power Plc employs steam turbines therefore in that perspective dry superheated steam is required to be generated for doing work. The Boiler System consists of tubes, pumps, heaters (heat exchangers), furnace, pressure vessels, and other unit operations which contribute to the generation and temperature efficiency management of the steam produced.

       The steam generated is at a high temperature of say 541 ֯c and pressure of say 12.5MPa.

      The process of steam generation from a cold ambient startup is as explained below:

      Economizers: These are a collection of tubes used in preheating the feedwater before reaching the Boiler Drum and does so by utilizing the heat from the flue gas. They also serve the purpose of preventing thermal shock in the boiler drum which may lead to rupture if the process water is fed in cold.

      Boiler Drum: This is a horizontal pressure vessel where saturated steam and high-pressure feed water gets separated.  It mainly functions as a phase separator for the steam-water mixture and also facilitates the recirculation of steam free water within the Boiler with the incoming feedwater for further steam generation. The Boiler Drum is equipped with components that help facilitate steam-water mixture separation and regulate pressure within the vessel.

      Superheaters: The Primary and Secondary Superheaters are used in converting saturated steam from the Boiler Drum into dry steam before being admitted into the High-Pressure Turbine. This conversion is carried out so as to increase overall cycle efficiency and to also avoid excessive condensation of the steam during expansion in the turbine, thus protecting the turbine blades from corrosion.

      Reheater: The Reheater basically performs the same function as the Superheaters. Expanded steam from the High-Pressure Turbine is admitted into the cold reheat line and is reheated back to the required process temperature before being sent into the Intermediate Pressure Turbine through the hot reheat line. This is done to also increase cycle efficiency and reduce the relative condensation of steam during expansion in the Intermediate Pressure Turbine.

      The steam from the Intermediate Pressure Turbine after expanding, is admitted into the Low-Pressure Turbine through the crossover pipe, expands and falls into the Condenser which is located below the Low-Pressure Turbine.

      The Condenser facilitates the differential condensing of the steam. The Lagoon Water is made to flow through the tubes while the steam flows through the shell and as a result of the temperature gradient, instantaneous heat loss and heat gain are experienced by the steam and Lagoon Water respectively. The condensate falls into the hot well where the Condensate Extraction Pump (CEP) is used in moving it into the Gas Air Ejectors.

      The condensate is also made to flow through the CPP -  Condensate Polishing Plant (the condensate Polishing Plant is required only when the conductivity of water is high) which is a mini demineralization plant and in some cases, Acid and Caustic are injected into the line so as to improve the water properties.

      After the polishing process, the Condensate Booster Pump (CBP) takes on the responsibility of transferring the polished condensate to the Drain Coolers as well as the Low-Pressure Heaters. The preheated water is passed onto the Deaerator where oxygen is relatively isolated from the flow.

        Boiler Feed Pumps (BFP): These are pumps used in transferring feedwater from the Deaerator into the High-Pressure Heaters.  After the feedwater might have been also preheated in the High-Pressure Heaters, it is again sent into the Economizer then to the boiler drum where the whole steam generation process takes off and continues as a cycle.

        Other equipment and unit operations which significantly contribute to the steam generation process include:

        Furnace: This is a combustion chamber where the combustion of the fuel-air mixture takes place. It is integrated into the Boiler System and is also significantly lagged so as to increase thermal efficiency and reduce relative heat loss. It is made up of Burners, Soot Blowers, Site Glasses, Furnace Walls (water tube) and Flame Detectors.


             Forced Draft Fan (FDF): This is a motor-driven fan which serves the purpose of supplying air into the Furnace for combustion. It is also used in purging the combustion chamber before start up. The supply of air into the combustion chamber has to be sufficient enough to produce a clean combustion and this is known as the air-fuel ratio. Too much air supply and the heating effect of the combustion will be reduced while an insufficient supply of air will result to fuel wastage, reduction in efficiency and incomplete combustion (production of unburnt carbon particles). There are twelve FDF’s in total (two per Unit).

            Gas Air Heater (GAH): This is used in preheating the air supplied by the FDF before being sent to the combustion chamber.

            The flue gas is employed in heating the air and in the process of heat exchange, the air is preheated to a reasonable process temperature for clean combustion while the flue gas is cooled down to a temperature which is safe enough for the environment.

             Steam Work Process (Steam Turbine)


            The Steam Work Process is one which is similar to the conventional thermodynamic heat engine process. In the power plant, a multi-stage steam turbine is utilized in converting thermal energy from steam (working fluid) into shaft work which in turn drives the generator to produce electricity (explained further in the next section), after which the steam is then taken down to a lower state temperature in the condenser.

            The unit operations involved in the Steam Work Process in a typical power plant:

            1. Work in the High-Pressure Turbine: This is the first stage in the multi-stage setup where steam expansion is used in driving the shaft. The steam from the Superheaters is admitted into this stage for expansion after which the expanded steam is sent into the Reheater so as to improve cycle efficiency and maintain the steam process temperature as much as possible. The High-Pressure Turbine was designed to share boundaries with the Intermediate-Pressure Turbine and it consists of 8-blade stages through which the steam expands.

            2. Steam work in Intermediate-Pressure Turbine: This is the second stage in the multi-stage setup where steam expansion through the turbine blades also occurs. The steam from the Reheater is admitted into this stage and the 6-blade stage configuration of the Intermediate-Pressure Turbine helps in converting the thermal energy from the steam into shaft work. After the expansion of steam here, it is directly sent into the Low-Pressure Turbine through the crossover pipe.

            3. Low-Pressure Turbine: This is the last stage in the multi-stage setup where steam expansion further drives the shaft. Steam is admitted directly into the Low-Pressure Turbine from the Intermediate-Pressure Turbine through the crossover pipe. The Low-Pressure Turbine consists of a dual 5-blade stage through which steam expands before falling into the condenser for cooling.

            4. Condenser: This is a heat exchanger that facilitates heat transfer between the steam and the lagoon water (coolant). The Condenser is a shell and tube type where the steam is made to flow through the shell while the lagoon water flows through the tubes. The temperature gradient in the Condenser is the primary driving force of the heat exchange which leaves the steam converted into water (condensate) while the lagoon water relatively increases in temperature.

            5. Hot Well: The condensate is temporarily stored in the hot-well before being transferred by the Condensate Extraction Pump (CEP) to begin the recycling process.

            Feed Water Heaters


            Feed Water heaters are placed on the lines of the feed water to gradually increase the temperature of the feed water before letting it into the boiler drum so as to prevent thermal shock and increase efficiency. There are six feed water heaters which are divided into classes:

            1. Low Pressure Heaters

            2. High Pressure Heaters

            3. Deaerator

            The heating element is extraction steam tapped off from different points in the turbine sections and it does not mix with the feed water because it is in a shell type arrangement.

            Ideally, heaters are added to the system in order to increase the efficiency of the boiler thus increasing the efficiency of the system. To learn more about feed water heaters see this post.

            Condensate Polishing Plant


            The condensate polishing plant (CPP) is a mini demineralization plant which is used to further purify the feed water when the need arises (Usually when the conductivity of water is higher than expected). Notice, during steam work as a result of contact with pipes, turbine blades etc, it is very likely that the working fluid must have picked up dirt and impurities which will negatively impact the system and can lead to boiler tube explosion, blade corrosion etc. In order to rid the process of these impurities without passing it through the main demineralization plant discussed above, a mini-demineralization plant – CPP is introduced to the system. The Condensate Polishing Plant implements a sequential control system with a programmable timer that completes one activity then begins the next sequence.

            The Generator


            Generators are basically devices that transform mechanical energy to electrical energy by electromagnetic induction. The Generator  is a large power generator eg. 16KV/50HZ 220MW which is always a Synchronous generator. If there exists a relative motion between the magnetic flux and conductors, then an emf is induced in the conductors. The rotor of the generator is connected to the single shaft which is mechanically coupled to the turbine.

            During my training, the generator I worked with has the following features:

            • Hydrogen cooled (210MPa)

            • synchronous generator

            • two magnetic poles

            • static armature(stator)

            • rotating magnetic field (rotor).

            • 16KV Output voltage

            • Rotor speed 3000rpm

            • Frequency of 50Hz

            Why Static Armature? 

            It can be easily deduced why the armature was made static as you cannot rotate such heavy bulk of coils as such there would be serious sparks if not fire outbreak if you try to use carbon brushes to output such high current. In addition to the above,

            • the stationary armature coils can be insulated easily,

            • Higher peripheral speed can be achieved in the rotor,

            • Cooling of the winding is more efficient.,

            • Only two slip rings are required to give DC supply to the field system.

            • Output current can be easily supplied to the load circuit.

            • Also you may not be able to completely specify the output phase sequence with a rotating armature.

            Note: The field windings are on the rotor.

            The Generator Excitation


            The generator consists of a stator in a rotating magnetic field. The magnetic flux that is essential to the production of the electric power is produced by the field coils wound on the rotor. The rotor is a rotating electromagnet that requires a DC (Direct Current) electric power source to excite the magnetic field. The totality of the processes involved in making the rotor possess magnetic properties is called excitation.

            The stronger the magnetic field created, the stronger the electrical power produced. The strength of the magnetic field is adjusted by controlling the current to the rotor, this is a major function of the AVR (Automatic Voltage Regulator) which contains the processors and I/O devices that monitor generator terminal voltage and current, field voltage and current, rotating exciter field voltage and current, control switches, breaker status etc. Outputs include Annunciation, alarms, meters, and a full range of data for the distributed control system (DCS).

            Field Flashing: Due to insufficient residual magnetism in the rotor, an external source of DC Power is required to generate the initial electromagnetic field until the generator creates enough power to self-excite and sustain itself. The process of energizing the rotor coils for a short period (4-8 seconds) using DC energy from the battery room is known as field flashing. Each unit has its own battery bank.

            The SCR Transformer: After field flashing occurs and voltage is established on the armature, the generator generates electricity which is tapped by the SCR Transformer, the SCR transformer is a dry type step down transformer (16KV/660V). The output of the SCR Transformer is sent to the thyristor stack for rectification. The maximum output voltage from the thyristor stack is 440VDC this is due to voltage loses due to rectification. This DC voltage is sent back to the rotor windings through the slip rings and the process continues indefinitely – thus the generator sustains itself. Unless there is a system disturbance i.e a system collapse.

            The AVR (Automatic Voltage Regulator): The Automatic Voltage Regulator (AVR) monitors the output of the generator using instrument transformers (Current Transformers and Potential transformers, it also works in conjunction with the Boiler Management Control System) in order to ascertain the firing angle of the thyristors i.e to ensure that the excitation voltage is not greater / less than the needed value. Thus, it prevents the dangers associated with over-excitation and under excitation.

            The Thyristors: The Thyristor or Silicon Controlled Rectifier is a four-layer, three terminal, solid state device with the ability to block the flow of current, even when forward biased, until the gate signal is applied. The thyristors perform a function of supplying rectified DC voltage back to the rotor of the generator. Note that there are some filter networks which perform filtration on this rectified output. The maximum output voltage is 440VDC, the output from the thyristor is dependent on the firing angle from the AVR.

            Power Transmission


            The Generator Transformer: The generator transformer is an oil cooled large power transformer rated say 270MVA and steps up voltage output from the generator to the high voltage of the grid through transformer action. Like other transformers, it works on the principle of electromagnetic induction

            Synchronization to the Grid: Nigeria operates a centralized grid system where different generating power stations connect to a single grid network after meeting the Grid network requirements. Synchronization is a common term among the operators in a power plant which basically means a system of interconnecting various independent systems to behave like one. There are three requirements before synchronization can take place:

            1. The voltage of the grid and output voltage of the system must be the same

            2. Phase sequence must be the same as that of the grid

            3. Frequency must be the same as the grid

            In the past, this is done manually but now, a synchro-check relay (Synchronism check relay) compares these values and synchronizes when conditions are met. There are two synchro-check relays used for double checking.

            Once synchronization is established, the system becomes susceptible to the disturbance in the grid which could result to damage, system collapse due to load and frequency changes.

            NOTE: The steam turbine is actually sensitive to low frequencies and can knock itself off from the grid when frequency goes too low, this is a protective measure that is triggered by the under-frequency relay which changes state when the frequency of the grid/generator goes lower than the designed minimum value (46Hz). The alarm stage is 47Hz while the trip stage is 46Hz.

            Comments

            Popular posts from this blog

            Arduino FreeRTOS Tutorial 05: Binary Semaphores and Mutexes

            Arduino FreeRTOS Beginner Tutorial: Task Creation, Scheduling, Deleting

            Understanding the operation of I2C Bus