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Wednesday 13 September 2017

Designing issues in PLC

When we were going to design our PLC, we found choices for hardware design and software architecture. The main issue was, on what criteria should we choose to develop among a variety of possible designs.
Following are the few points that we decided to keep in consideration during the designing of our PLC.
    Our PLC should fulfill the current industrial requirements.
    Our PLC should be flexible and cheap.
    Our PLC should be compact and repairable.
    It should follow the most common standards of PLC.
    Programming method should be easy and graphical one.
    It should provide the enhanced features of PLC.
    It should have expandability feature for the future needs.
 
Modular or Non-Modular  
There are two types of PLC’s Modular and Non-Modular. Modular PLC gives more flexibility in adding and removing modules and also provides enhanced features of programming. Therefore we decided to fabricate a modular PLC with five types of prefab modules.
i.    PC Module
is.    ADC Module
iii.    Input Module
iv.    Output Module
v.    DAC (Optional)    
The number of input modules and output modules attached to the PC module (CPU) depend up on the requirements. Our PLC has a module expandability feature up to 32 modules of each input, output and ADC modules.
Due to this feature of multiple modules system, our PLC has become more flexible and powerful.




PC Based PLC
The main module or the Central Processing Unit of a PLC can be designed by using micro controllers and microprocessors based systems.
We decided to use a complete PC and Central Processing Unit of our PLC and named it, as “PC Module” following are the reasons for this choice:
I.    PC Processor can support multiple module processing and attachment without sacrifice of processing speed.
II.    PC Module provides Processing, Debugging, Programming and Visual Display feature with in a single module. This feature makes our PLC a versatile one in the era of PLC’s.
III.    Just changed the processor can increase the number of attachable modules and the processing speed of our PLC.
Cost of PLC
When we go through the prices of available PLC’s we find that the cost of straightforward and cheap PLC range is from at least few hundred US dollars to thousands of US dollars depending upon the types and features provided. We choose to design a PC based PLC to reduce the cost, skill in designing and labor in the fabrication of our PLC. This is why our PLC is cheaper than any other PLC’s available in the market with the same set of features as our PLC.  
Method of Programming
There are three standard methods of PLC programming which have been discussed earlier in the previous chapter.
But the programming method used by us is new, unique and a versatile one by providing a user-interface, a large instruction set, and an enhanced capability of programming.
This programming method is named as “PLC Visual Script.” We designed software for VB Script editing, compiling, debugging and printing. This software is powerful and easy to handle. It fulfills all the requirements of PLC programming.
Hardware of our PLC
Our PLC Hardware consist of six types of modules which are given below:
1)    PC Module                             (Main processing unit)  
2)    Input Module
3)    ADC Module                  

4)    Output Module
5)    DAC Module (Optional)


6)    MAX-232 Serial Interface
The main processing unit module (PC Module) for our PLC is only one but the number of input and output may vary as per requirement up to 32 Module of each one. Physical timer and counter Modules are replaced by the built-in software timers and counters.
Software of our PLC
The software of our PLC consists of many separate programs each program is written for the specific purpose and specific place (Module) depending upon logic.
We can categorize our software into two parts: -
1.    CPU (PC Module) related program
2.    I/O Module similar program
CPU (PC Module) related programs are given as: -
I.    PLC Hardware and Abstraction Layer Program (PLC server)
II.    PLC Debugger Program
III.    PLC VB Script compiler
I/O Module related programs
I.    Input Module Micro controller Programs
II.    ADC Module micro controller Programs
III.    Output Module Micro controller Programs
     In the CPU related program, all the source codes are written in a Visual Basic language. Hardware abstraction layer program enables the CPU to communicate with I/O Module while Debugger program helps and provides the useful information about the changing states of input and output Module according to the terms and condition are given in the Visual Script program. The other part of a program is a low-level programming used for Microcontrollers operations being used as controller units of different input and output Modules.

programming methods of plc part 2

Unlike circuit diagram, the ladder diagram is constructed schematically and does not allow how the various components are arranged. This type of programming was developed from the writing (circuit) diagram. If a circuit diagram exists for a control problem, the simplest method of programming would, therefore, be to convert this into a ladder diagram.




2.    Function Chart:
The Function table may be used both for just scanning programs and for the representation of sequence programs. In its schematic version (with commentary), it can be used as a flow chart; if on the other hand, a flow chart is presented along with a problem this can be easily converted into a Function Chart. It is also called “Control System Flow Chart (CSF).”
Logic elements shown by a basic rectangular symbol a function designation and negated inputs are indicated by a circle in front of the symbol.
If a flow chart of several steps is available, the function table should be divided into levels areas (boxes). Such an area contains the level member (starting at the top with 0) and a freely selectable commentary. The step area connects all inputs belonging to it (combination of data) to the corresponding executive elements. When a step has been fulfilled, it passes on automatically to the next Function Chart. This programming has been developed from the logic diagrams of electronics. However, these do not include a direct representation of sequence steps. Thus for PLC programming of course programs, it was necessary to introduce a chronological sequence with levels.
Statement List
Unlike LAD and CSF, the report list (STL) does not present the program graphically but describes it textually.
The announcement list is made up of individual instruction lines. It is possible to write a comment (in everyday language) to the right of each instruction line, giving a more precise description of the switching elements. The direction lines in the statement list are numbered consecutively. The set of instructions (statement) comprise various conditional and executive instructions.
The instruction (command) are listed in abbreviated form. L (“Load”) signifies the start of instruction; the logic elements AND, OR and NOT are shortened to A, O, and N respectively. The assignment “set, otherwise reset” is expressed by ‘=.' “Set” otherwise reset” signifies: this output should be set at one if the logical signal is present and at 0 if the 0 logic signal is present.


The DIN standard for the statement list does not recognize any steps but must, in the case of sequence programs work with level flags (with ladder diagrams, too, such projects must be programmed using step colors). However today there is also a form of STL programming which lists the individual stages of the program and their related instructions verbally in chronological order. With this STL even complex control problems can be presented. Boolean equations for control task and displacement step diagrams can be written with the case as STL.
Flags (Memory Relays)
Flags are internal 1-bit memories. These functions units are of extreme importance in the PLC. They store results, which are later required by the program sequence. Flags are treated as if they were simultaneously inputs and outputs. Thus they can be placed at any point instead of inputs or outputs and interrogated. Outputs can be interviewed in the same way. Flags are outputs without cards, i.e., without connection between the internal electronics of the PLC and the actuators.  
PLC VISUAL SCRIPT
A newly developed technique for programming a PLC in which each module is allotted a specific Alphabetical ID. For example Input module = X Output = Y and Analog module is = ADC. Condition codes and instructions, for performing different tasks, can be written in any of Windows based language. This idea provides the facility of language platform independence for a PLC system. Example.
if
        X. 2.1 = 1  &  X 2.3 = 1 then
        Else
    Y 2.4 = 1


PLC Basic  operations :
These operations are used in SIEMENS S5 series PLCs.

Programming Methods Of PLC

Methods of Programming
While using SIEMENS PLCs, Programs are written by using PGs (Programmers) and then transferred to the CPU module of PLC.
PGs are used to write the control programs for programmable controllers, and can also be used to test these programs when the system is put into operation.
Extensive user-friendly testing and debugging functions support troubleshooting and error recovery and a wide variety of documentation functions are available for adding program commentary, and documenting and printing out user programs.  

Programming Languages:
The Programming languages serve as a mean of communication between the user and automation and are the language in which user formulates his control task.

There are four possible programming languages:

1.                 The Ladder diagram (LAD)
2.                 The control system flowchart (CSF) or Function Chart
3.                 The statement list (STL)
4.                 PLC Visual Script

1.                 Ladder Diagram:
Like a ladder, the diagram is made up of two vertical lines. The left one represents the connection to the voltage source, the right one the connection to the earth. The various current paths (rungs) run horizontally from left to right between the two Inputs are shown by these symbols:
-] [-    :         contact; normally open contact
-] / [-  :         negated contact; normally closed contact
AND-elements from inputs are actuated by the switching of contacts in series, OR-elements by parallel switching. In a ladder diagram, a negated input is designated by a normally closed contact.
Output has the symbols  –( )–  at the right-hand end of the current path in question. When programming, each symbol is accorded a real PLC address or a short form (symbolic address).
The ladder diagram (LAD) is suitable for some control tasks, particularly if a circuit diagram is available.  



Components Of PLC

The Components of a PLC

Component
Solutions to control problems realize technically with PLC applications and may vary in their complexity. However, the following building blocks are always necessary:
Hardware:  
By hardware, we understand the electronic modules, through which all functions of the installation or machine to be controlled are addressed and actuated in the logical sequence.
Software:  
By software we mean the programs in which logic operations and the related triggering of components within the installation or machine are precisely specified. The software is filed in particular hardware memory and can be modified if required. The control sequence changes with the new program. It is not necessary to modify the hardware.
Sensors (Sensors):  
By attaching these components directly to the machine or installation to be controlled, information regarding the current status of these devices can be transmitted to the PLC. Sensors are, e.g., limit or proximately switches.
Actors (Actuators):  
These are the components fitted directly to the machine or installation to be controlled, which can be used to change status via the PLC. In other words, sequences can be influenced, or changes in status can be announced. Examples of Actuators are buzzers and solenoid valves.
Programmer:  
The developer is used to create the software and to transfer it to the PLC Memory. In most cases, it can also be used for testing software Sensorics and Actorics.

Hardware  
The most important components of the PLC device is the central control unit (CCU) which corresponds to a computer in structure. The processes taking place in the central control unit are designed processors (processing).  
The data processed and stored in the CCU are in the form of binary signals, each as a rule composed of a single bit (with status zero or one).
The input and output modules make the connection between the central control unit and the actuators/sensors. Each of these modules contains a fixed number of inputs and outputs which can assume the binary values, Zero and one. A critical component of the central control unit is the flag memory. Flag is one-bit memories, through which the PLC can “Note” the status of binary signals.
Software  
PLC programs have a concrete structure, determined in the Central Control Unit. They are created by the programmer from source programs, in principle, the programmer can create these programs by four different methods:
     As a statement list program (STL)
     As a ladder diagram (LAD)
     Function Chart or Control System Flow Chart (CSF)
     PLC Visual Scripts

Sensors
Sensors are signal generators, which can be used by the PLC to integrate the status of the installation or machine to be controlled. As the PLC works “Electrically,” the sensors must convert non-electric signals into electric ones so that they can be understood by the input module. Examples of sensors are:
 I) Limit Switches
Normally closed contacts, normally open contacts or changeover contacts.
II) Initiators
Contacting and contactless switches, which emit one or zero signal when approached by an object, inductive generators react to metal parts, capacitive generators also to other materials.
III) Light Barriers
Contacting and contactless switches, which emit one or zero signal when an optical connection is interrupted.
IV) Temperature Sensors
Contacting and contactless switches emitting one or zero signal on attainment of a set temperature value.

Actors
Actors are executive elements used to amplify binary signals from the output modules to create switching signals or to convert them into signals of another energy type.
The distinction is made between electric, electro hydraulic and electro pneumatic actuators. They can be used to create switching processes and linear or rotary movements. Examples of actuators are:
     Alarms
     Lamps, Buzzers, Bell

     Pneumatic Cylinders (with value systems)
 Single and double acting cylinders, cylinders with through rod, tandem cylinders, multi positions cylinders.
           Electro Motors
DC motors, stepping motors, synchronous, AC motors, rotating vane motors.
           Hydraulic actuators (with value system)  
Single or double acting cylinders, flow control valves, hydraulic motors.
            Electro Hydraulic Motors  
           Stepper motors, servo drives etc.

Programmer
The developer is used to write and correct programs to translate them into the PLC machine code to transmit them to the PLC and to test them. It can be just like a simple or unique personal computer.
Personal Computer
In the past, this was mainly carried out using specialized tools, tailor made for individual manufacturers control system. Today conventional personal computers are increasingly being used as programming tools loaded for the task with programming software specific to the control system involved.
Programming System
Programming Systems based on the personal computer mostly offer several methods of programming, i.e., the programmer creates a source program graphically or in plain text, which the programming system translates into machine code, understood by the PLC being used. If the personal computer processes a data interface suitable for the PLC, programs can be transmitted directly into the program memory of the PLC.
To gain a better overall view and for the purpose of programming methods documentation, it is possible to print out the programs created.

PLC Definition and Introduction

A Programmable Logic Controller is a user-friendly electronic computer that carries out control functions of many types and levels of complexity. It can be calculated, monitored and operated by a person unskilled in operating computers. The programmable logic controllers mostly draw the lines and devices of ladder diagrams. The resulting drawing in the computer replaces much of the external wiring required for control of a process. The programmable logic controllers can operate any system that has output devices that go ON and OFF. It can also run any system with analog outputs. The Programmable Logic Controllers can be performed on the Input side by ON / OFF devices or by analog input devices.
In 1978, the National Electrical Manufacturers Association (NEMA) released a standard for PLC’s. This specification, NEMA Standard ICS3-1978, was the result of four years of work by a committee made up of representatives from PC manufacturers, Part ICS3-304 of the standard defines a PLC as:

“A digitally operating electronic apparatus, which uses a programmable memory for the internal storage of instructions for implementing specific

functions such as logic, sequences, timing, counting and arithmetic to control, through digital or analog Input / Output modules, various types of machines of a process.”
Evaluation to the present PLC  
The first PLC system evolved from the conventional computers in the late 1960s and early 1970s. The first PLC’s were mostly installed in automotive plants. Traditionally, the auto plants had to be shut down for up to a month at a change over time. The first PLC’s we used along with other new automation techniques to shorten the changeover time.
Through late 1970, the improvements were made in PLC programs to make them somewhat more user-friendly. In 1978, the introduction of the microprocessor chips increased computer power for all kind of automation and lowered the computing cost. Robotics, automation devices, and equipment of all types, including the PLC, consequently underwent many improvements. PLC programs became more understandable to people. PLC’s became more affordable, as well.    
In the 1980s with more computer power available per Dollar, the use of PLC rises exponentially. Some electronics and computer companies and some different corporate electronic divisions found that the PLC became their greatest valued product. The market for PLC’s grew from a volume of $80 million in 1978 to 1 billion dollars per year by 1990 and is still growing.
Advantages of PLC  
The following are some of the major benefits of using a Programmable Logic Controller.
Flexibility
In the past, each another electronically controlled production machine required its controller; fifteen devices might require fifteen drivers. Now it is possible to use just model of a PLC to run anyone of the fifteen machines. Furthermore, you would probably need fewer than fifteen controllers, because one PLC can easily run many machines. Each of the fifteen devices under PLC control would have its distinct program.
Implementing changes and correcting errors
With a wired relay type panel, any program alterations require time for rewiring of panels and devices. While a PLC program can be modified from a Keyboard sequence in a matter of minutes. No extra rewiring is required for a PLC controlled system. Also, if a programming error has occurred in a PLC control ladder diagram, change can be typed in quickly.
Large quantities of contacts
The PLC has a lot of contacts for each coil available in its programming. Suppose that a panel wire relay has four connections and all are in use when a design change, requiring three more contacts, is made. It would mean that time must be taken to procure and install a new relay or relay contact block. Using a PLC, however, would only require that three more contacts be typed in. These three connections would be automatically available in the PLC. Indeed, hundreds of contacts can be used from one relay if sufficient computer memory is available.
Lower Cost
Increased technology makes it possible to compact more functions into smaller and less expensive packages. In the 1990s, you can purchase a PLC
with numerous relays, timers, counters, a sequencer and other features for a few hundred dollars.
Pilot Running
A PLC programme circuit can be pre-run and evaluated in the office or lab. The record can be typed in, tested, observed and modified if needed, saving valuable factory time. In contrast, conventional relay system has been best tested on the plant floor, which can be very time-to consume.

Visual Observation
A PLC circuit’s operation can be seen during operation directly on a CRT screen. The operation or miss-operation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done quicker by visual observation.    
Speed of Operation
The relay can take an un-expectable amount of time to actuate. The operational speed for the PLC programming is breakneck. The rate of PLC logic operation is determined by scan time, which is a matter of milli seconds.
Ladder or Boolean Programming Method
The PLC programming can be accomplished in the Ladder mode by an electrician or technician, alternately, a PLC programmer who works in digital or Boolean Control systems can also easily perform PLC programming.        

Reliability
Solid-state devices a more reliable, in general, than mechanical or electrical relays and timers, the PLC is built up of solid state electronic components with very high-reliability rates.


Simplicity of Ordering Control System Components
A PLC is a device with one delivery date when the PLC arrives; all the counters relay another component also arrive. In designing a relay panel, on the other hand, you may have twenty different relays and timers from twelve different suppliers. Obtaining the parts on time involve various delivery dates and availability. With a PLC you have one product and one lead-time for delivery. In a relay system, forgetting to buy one component would mean delaying the startup of the control system until that part arrives with the PLC one more relay is always available providing you an ordered PLC with enough extra computing power.  
Documentation
An immediate printout of the real PLC circuit is available in minutes if required. There is no want to look for the blue print of the course in remote files. The PLC print out actual circuit in operation at a given moment often the file prints for relay panels are properly kept up to date. PLC print out is the course at present; no wire tracing is needed for verification.
Security
A PLC program change can’t be made unless PLC is properly unlocked and programmed. Relay panels tend to undergo undocumented changes. People on the late shift don’t always record panel alternations made when the office area is locked up for the night.
Ease of Changes by Reprogramming
Since the PLC can be reprogrammed quickly, so a mixed production processing can be accomplished. For example, if a part B comes down the assembly line while part A is still being processed, a schedule for part B’s processing can be reprogrammed into the production machinery in a matter of seconds.







Disadvantages of the PLC
Following are some of the disadvantages of or perhaps precautions for using PLC’s.
Newer Technology
It 's hard to change some personnel’s thinking from ladders and relays to the PLC computer concepts.
Fixed Program Application
Some applications are single function application. It can’t play to use a PLC that includes multiple programming capabilities if they are not needed.
Environmental Consideration
Individual process environments such as high heat and vibrations interfere with electronic devices in PLC’s, which limits their use.
Fail Safe Operations
In relay systems, the stop button electrically cuts, the circuit; if the power fails the operation ends. Furthermore, the relay system doesn’t automatically restart when power is restored. This, of course, can be programmed into the PLC however in some PLC plans you may have to implement an input voltage to produce a device to stop. These systems are not “Fail Safe.” These disadvantages can be overcome by adding safety relays to a PLC system.
Fixed Circuit Operations
If the circuit in operation never altered, a fixed control system might be less costly than a PLC. The PLC is most useful when periodic changes in operations are made.






Tuesday 7 February 2017

Interview Questions and Answers of Specialty Power System Electrical Engineering Set-4

Electrical Equipment

1. What is a power distribution unit? What devices does it include?

A: The distribution device is the switch electrical appliances. Current carrying conductor and protection and measurement of electrical equipment, according to certain requirements formed from the electrical construction, known as the distribution device. A device for receiving and distributing electrical energy. Power distribution devices, including switchgear (circuit breakers. Isolating switch. Load switch, etc.). Protection of electrical appliances (fuses. Relays. Surge arresters, etc.) Measurement of electrical (current transformer, voltage transformer, ammeter, voltmeter, etc.) and bus and Current carrying conductor. Power distribution devices according to their settings can be divided into indoor distribution equipment and outdoor power distribution devices, according to their voltage level is divided into high-voltage power distribution devices and low-voltage power distribution devices, according to its structure is divided into complete sets of power distribution equipment (switchgear) and assembly.

2. High-voltage circuit breaker. Load switch. Fuse role?What are their main structures and main types?

A: 1) high-voltage circuit breakers: high-voltage circuit breakers, also known as high-voltage switchgear in power plants and substations it is an important power distribution equipment. Its role has two aspects: 
① under normal circumstances, the separation of high-voltage circuit in the no-load current and load current. (In accordance with the requirements of power grid operation, starting for some of the power lines to stop. Transmission control role); 
② When the power system failure, it works with relay protection devices. Cooperated to quickly cut off the fault (short circuit) To reduce the power outage, to prevent the accident to expand, to ensure the safe operation of power systems. (I.e., protection)
The main structure of high-voltage switch can be divided into: 
① diversion part; 
② part of the arc device;
 ③ part of the insulation;
 ④ operating mechanism. 
The main types of high-voltage circuit breakers, according to arc quenching medium: oil switch (oil and less oil.) Compressed air breaker. Sulfur hexafluoride breaker. Vacuum breaker. At present the original high-pressure (110kV.220kV) and ultra-high voltage (330kV.500kV) power system plays a major role in the low oil breaker.Compressed air breaker are almost all replaced by SF6 switch; 10kV ~ 35kV power switch in the basic vacuum switch .
SF6 circuit breaker due to the development of SF6 gas it has excellent insulation properties and arc performance, making the switch off capacity greatly improved (a single fracture of the rated breaking current of 50kA ~ 63kA, 80kA), switch size and weight To reduce the maintenance and maintenance of the switch has become a great convenience and simple.
The development of vacuum switch and contact materials are closely related to the use of CuCr alloy contacts, making 12kV vacuum circuit breaker breaking current for a reach of 73kA or even 100kA. Vacuum switch interrupter is not overhauled, once the leak can only be replaced.
2) high-voltage load switch: high-voltage load switch is a small-capacity high-voltage circuit breakers. It has an arc extinguishing mechanism, but the interrupter capacity is small, it is mainly used to cut off or connected to the normal load current, can not be used to cut short-circuit current. In most cases, the load switch with the use of high-voltage fuses, load switch from the no-load or load current, by the fuse for short-circuit protection.Load switch for a large number of load switchgear. Ring power supply units and prefabricated substation. Load switch is mainly produced by gas-type load switch. Pressure gas load switch. Vacuum load switch. SF6 load switch. Vacuum and SF6 load switch in urban and rural areas in a large number of use.
3) High-voltage fuses:
High-voltage fuse is divided into current limiting and drop-type.
In the factory power supply system, the capacity is relatively small and relatively less important load, widely used fuses for power distribution lines and power transformers (including 35KV and below level voltage transformer) overload and short circuit protection. It is economical and can meet a certain degree of reliability, its structure is simple, easy maintenance and repair.
Fuse is dynamic. Static contact. Insulation support and indicators and other components. The action of the fuse has an inverse time characteristic, that is, the greater the short-circuit current through the melt, the shorter the time of the melt-blown current.
fuse usually made of Lead, tin, lead antimony (ti), aluminum alloy or zinc, etc. made of different specifications of round wire or flake,its specification is rated current. 

Interview Questions and Answers of Specialty Power System Electrical Engineering Set-2

8. Synchronous generators and power grid side by side What are the methods?

A: synchronous generator and grid parallel methods are self-synchronous method and quasi-synchronous method.Usually in order to reduce the impact of parallel current are generally used quasi-synchronous method, parallel to adjust the generator excitation current and speed, in the generator and the grid phase sequence of the same frequency equal to the voltage equal to the same phase and the same phase.

9. What are the main data on the nameplate of asynchronous motor?

A: Rated power. Rated voltage. Rated current. Rated speed. Phase number. Model. Insulation class. Work. Allowable temperature rise. Power factor. Quality. Date of manufacture.

10. Asynchronous motor by structure how to classify?

A: divided into squirrel cage and winding type.
Power Systems

1. What is the power quality indicator?

A: Frequency. Voltage. Waveform. Symmetry

2. What are the indicators to characterize the economics of power system operation?

A: coal consumption (g/kw×h). Net loss rate

3. What are the voltage levels of our standard?

A: 0.22KV, 0.38KV, 0.66KV, 3kV, 6kV, 10kV, 35kV, 60kV, 110kV, 220kV, 330kV, 500kV, 750KV, 1000KV.

4. Which two types of power lines are divided according to the structure? The basic components of what?

A: The overhead line: wire. Lightning wire. Tower. Insulator
Cable line: conductor. Insulating layer

5. 220kv above the line often used split wire What is the purpose?

A: Improve the parameters of the line (to reduce line reactance) and to prevent the occurrence of corona phenomenon

6. What are the contents of the power flow calculation?

A: power loss calculation; current and power distribution calculation; voltage loss and node voltage calculation

7. What is the trend to calculate?

A: system design and design to select the knot line. Select the electrical equipment and wire cross-section; determine the system operation mode and develop maintenance plan; for relay protection. Automatic device design and setting calculation; Short circuit and stable calculation.

8. What are the losses of the transformer?

A: The changing losses (ie winding loss, with the load size and change); fixed loss (ie no-load loss or excitation loss)

9. What is the active power of the system? What is the use of spare capacity?

A: The active power supply is the generator; the standby capacity is used as: standby load. Accident backup. Overhaul spare. National economic reserve; Standby forms: cold standby (standby).
use)

10. What are the reactive power sources of the system?

Power supply - Static electricity compensator - Charging power for high - voltage lines

11. What are the system's voltage regulation measures?

A: The use of the generator voltage regulator; adjust the transformer variable voltage regulator; series capacitor voltage regulator; parallel compensation reactive power equipment regulator

12. What are the main uses of a capacitor in a system?

Improve the economy (increase power factor, reduce power loss); improve the stability of the system (series compensation, reduce the system reactance); improve the economy (increase power factor,

13. What are the short-circuit faults occurring in the power system? What is the highest probability of occurrence? Which equipment has the highest probability of a short-circuit fault?

Symmetrical short circuit: three-phase short circuit; the highest probability of occurrence of the short circuit: one phase short circuit; the highest probability of occurrence of short-circuit fault device : Power lines

Interview Questions and Answers of Specialty Power System Electrical Engineering Set-1

Interview Questions and Answers of Specialty Power System  Electrical Engineering Set-1

1. What is the role of the transformer in the power system?

A: The transformer is an important electrical equipment in the power system, play a role in the transfer of electricity, from the power plant to the user's entire hair. Transmission process, the transformer plays a role in raising and lowering the voltage.

2. When the secondary side of the transformer short-circuit what is the danger?

A: The transformer secondary side of a sudden short circuit, the winding will have a huge short-circuit current, the value of up to 20 to 30 times the rated current, such a large current damage to the transformer are:
(1) under the action of a huge short-circuit current, the winding will produce a large electromagnetic force, its value up to 400 ~ 900 times the rated electromagnetic force, so that the mechanical strength of the winding damage;
(2) a huge short-circuit current will make the winding heat, resulting in high temperature, may cause the winding burned.

3. What are the conditions of parallel operation of the transformer?

A: Two or more transformers running in parallel conditions: (1) high and low voltage side of the rated voltage are equal, the actual operating ratio should be equal; (2) to connect the same group; (3) short-circuit voltage per unit value (Percentages).

4. Why should the provisions of the allowable temperature of the transformer?

A: Because the higher the operating temperature of the transformer, the faster the insulation aging, not only affects the service life, but also because of insulation brittle and fragmentation, the loss of insulation winding protection. In addition, the higher the temperature the lower the dielectric strength of insulating materials, it is easy to be high voltage breakdown caused by failure. Therefore, the transformer can not exceed the permissible value during operation.

5. What are the basic structure of the synchronous generator?

A: The stator includes: stator core, stator winding, frame, end cover, cooler and so on. The rotor includes a rotor core, a rotor winding, an inter-ring, a slip ring, a fan, and the like.

6. Synchronous generator "synchronization" What does this mean?

A: After the three-phase symmetrical load is applied to the generator, the three-phase symmetrical stator current produces a combined magnetic field which is in the same direction as the rotor and rotates at the same speed.

7. Synchronous generators according to the rotor magnetic pole shape how to classify? What are uses in the power plants?

A: The synchronous generator according to the rotor pole shape is divided into salient and hidden pole machine. Salient pole machine is usually more pole pairs, the rotor speed is low, generally used in hydropower plants.The hidden pole machine is usually a small number of pole, the rotor speed is high, usually in thermal power plants. Thermal power plants and nuclear power plants to use.

Sunday 5 February 2017

Electrical Power Systems Interview Knowledge Questions 6


Electrical Power Systems Interview Knowledge Questions 

power cable inspection which inspection items?

A: Power cable inspection inspection items are:
(1) Check the cable and terminal box for leakage of oil, insulating plastic is softened overflow.
(2) Insulator is clean and complete, whether there are cracks and flashover traces, wire connectors are intact and not heat.
(3) exposed cable sheath is complete, whether the solid support.
(4) the skin is good ground.

 main control room, relay protection indoor and 10 (6) kV power distribution indoor equipment normal patrol what projects? 

A: Indoor equipment, the normal inspection items are:
(1) no sound and Jiao smell.
(2) All instruments, signals, indicator windows should be consistent with the operating conditions, the instructions are correct.
(3) to protect the connection piece position is correct (should be consistent with the actual).
(4) system three-phase voltage balance (approximate), and in the specified range.
(5) power line, the main switch three-phase ammeter approximate balance.

 transformer oil shortages on the operation of what harm?

A: The transformer oil level is too low will make light gas action; serious lack of oil, the core and winding exposed to the air easily damp, and may cause insulation breakdown.

 forced oil circulation transformer stopped the oil pump why not allowed to continue to run?

A: The reason is that the transformer housing is flat, the cooling area is very small, and even can not transform the transformer no-load loss of heat generated by scattered out. Therefore, forcing the oil circulation transformer to completely stop the cooling system operation is dangerous.

take the operation of the gas in the transformer one should pay attention to what safety matters?

A: The security issues that should be noted are:
(1) to take gas gas must be carried out by two people, one person operation, one guardianship.
(2) climbing transformer should take a safe distance from the gas to prevent high-throw.
(3) to prevent accidental collision probe.

 What causes the transformer to issue unusual sound?

A: The following causes the transformer to make an unusual sound:
(1) Overload. (2) internal contact is bad, discharge spark.
(3) individual parts loose. (4) The system is grounded or shorted. (5) large motor starting to make a big change in load. 

 voltage transformer in operation, why not allow a short circuit twice?

A: The voltage transformer in normal operation, the secondary load impedance is very large, voltage transformer is a constant voltage source, the internal impedance is very small, small capacity, a winding wire is very small, when the transformer secondary short circuit, Current is great, if the second choice of improper fuse, the fuse can not be fused, the voltage transformer can easily be burned.

What happens when the capacitor should immediately exit the operation?

A. The capacitor should be removed from operation immediately upon any of the following conditions:
(1) casing flashover or serious discharge.
 (2) joint overheating or melting. 
(3) expansion of shell deformation.
(4) the internal discharge sound and discharge equipment abnormal sound.

 arc suppression coil switching tap the operation of what provision?

A: Operational requirements are:
(1) The tap-changer tap should be switched according to the position of the tap assigned by the dispatcher.

single busbar 10kV system single-phase grounding, the line-by-line power failure to find, grounding phenomenon is still not disappear What is the reason?

A: There are two reasons:
(1) Both lines are earthed at the same time.
(2) the bus station equipment grounding.

 fixed-time over-current protection is the setting principle?

A: The definite time overcurrent protection action current setting principle is: the action current must be greater than the load current, the maximum load current protection device does not act, when the next level line external short circuit, if the current relay has been in action, Subsequent protection After the fault current has been removed, the protection of this stage should be reliably returned.

 distribution lines why only installed over-current does not install quick-break protection?

A: 10kV distribution line power supply distance is short, the line short-circuit current and the end of the first little difference between the value, quick-break protection by bypassing the line short-circuit current setting, protection is too small; In both cases, it is not necessary to install a current quick-break protection.

 export intermediate relay contacts Why should series current coil?

A: The purpose of contact series current coil is to prevent the protection action after the contact jitter, vibration or a very short time can not switch on and off tripping. So to protect the middle of the outlet is usually used in contact with a series of current coil self-sustaining wiring. In the event of a fault, the contact is closed and the current coil in series is energized to produce a self-holding to ensure a reliable trip of the switch.

 transformer differential protection is based on what the principle of installation?

A: The transformer differential protection is installed according to the principle of circulating current. Two current transformers with the same model are installed on both sides of the transformer. In the normal and external fault, there is no current in the differential relay, and in the transformer internal phase short circuit occurs, the differential relay will have a great current flow.

transformer zero sequence protection under what circumstances put into operation?

A: Transformer zero sequence protection should be installed in the transformer neutral point directly to the ground side, to protect the side of the winding internal and lead-wire short-circuit, can also be used as the corresponding bus and line ground short-circuit protection, so when the Transformer neutral grounding knife into the joint, the zero sequence protection can be put into operation.

 What line of differential protection device installed cross-linked? What is the fault of the transverse differential protection response?

A: In the same impedance of the two parallel lines can be installed cross differential protection. Transverse differential protection protects against internal faults in parallel lines and does not respond to external faults in parallel lines.

synchronous reclosing under what circumstances do not move?


A: No action in the following cases:
(1) If the line has a permanent failure, with no reclosing the circuit breaker reclosing immediately after the break, the same reclosing will not move.
(2) no pressure reclosing refuses to move, the same period reclosing will not move.
(3) synchronous reclosing refuses to move.

 Under what circumstances will the reclosing of the circuit breaker exit operation?

A: The reclosing operation is exited in the following cases:
(1) the circuit breaker interrupter capacity is less than the bus short-circuit capacity, the reclosing out of operation.
(2) the number of circuit breaker failure trip exceeds the requirements, or though not more than the provisions, but the circuit breaker serious fuel injection, smoke, etc., should be agreed after the operation of the reclosing out.
(3) line with live operation, when the duty dispatcher command reclosing will be out of operation.

Electrical Power System Interview Knowledge Question 5

 power grid operation and management of the basic principles of what?

A: The basic principle of grid operation is:
(1) power grid operation to implement unified scheduling, hierarchical management.
(2) No unit or individual shall illegally interfere with the dispatch of the power grid.

What is the main content of the electric power law?

A: The power law of ten chapters and seventy, the main content: General provisions to determine the legislative purpose, scope, the development of basic principles of the power industry and the power management system. The power generation and power grid management, electricity supply and use, electricity and electricity, rural electric power construction and rural electricity, power facilities protection and supervision and inspection, legal responsibility, comprehensive construction and development of the electric power industry Made a specification.
The Supplement provides for the implementation of the Electricity Act, 1 April 1996.

 electric power production safety management system which includes the main content?

A: According to the provisions of Article 19 of the Electric Power Law, electric power enterprises shall strengthen the management of work safety and establish and improve the responsibility system for production safety, including the following specific systems:
(1) safety production responsibility system. (2) production duty system. (3) operation of the ticket system.
(4) work permit system. (5) operation and monitoring system. (6) work interruption, transfer, end system.
(7) safety education system. (8) power facilities on a regular basis maintenance and maintenance system.

 anti-accident exercises should focus on what to consider?

A: Anti-accident exercises should focus on consideration of the content is:
(1) the enterprise and other business accidents and lessons learned anomalies. (2) the main defects and weaknesses in the equipment. (3) accidents that may occur before and after the new equipment is put into operation, and seasonal accidents that affect the safe operation of the equipment. (4) special mode of operation and operation of the weak links. (5) major complex system equipment operation.

 team trainers specifically responsible for what training?

A: The team responsible for the training of trainers are:
(1) the preparation of team training program, under the leadership of the squad leader to seriously implement and strive to complete the breakout (workshop), the work area (county) layout of the training tasks. (2) organization of team members to learn the technical specifications of the equipment under the jurisdiction of the equipment, familiar with the rules and regulations and systems. (3) to rely on team personnel, with team characteristics to carry out team training activities. (4) to help the squad leader to do the work of the daily team training, such as on-site examination, security knowledge Q & A, technical explanation, technical questions and answers, accident expectations. (5) to assist the squad leader of the new arrangements for job training, supervision, inspection of mentoring the implementation of the contract, do a good job of new staff examination and examination work. (6) timely do a good job of the registration management training and regularly summarized to the sub-field (workshop), the work area (county) training engineers (staff) to report on the implementation.

new hire production staff training is the basic content?

A: According to the relevant provisions of the recruitment of new production personnel into the factory (bureau), must be into the factory (bureau) education, as follows:
(1) political ideology and fine traditions, factory (bureau) history, factory (bureau) discipline education. (2) electrical professional ethics education. (3) law-abiding and civilized courtesy education. (4) the relevant laws, regulations and safety knowledge of production education.

new duty on staff before the training step is what?

A: The new staff on duty in the independent duty work, must be the scene of the basic system of learning, attendant study and pilot study of three training stages, each stage to develop training plans, according to plan training.

anti-accident drill purpose?

A: The purpose of the anti-accident exercise is:
(1) Regular inspection of operational personnel to deal with accidents.
(2) the production staff to master the rapid handling of accidents and anomalies of the correct way.
(3) to implement anti-accident measures to help production personnel to further master the site procedures, familiar with the equipment operating characteristics.

 technical staff running what learning content?

A: The operating personnel technical learning content:
(1) electrical safety work rules, operating procedures, accident handling procedures, technical grade standards, job specifications and the relevant rules and regulations system. 
(2) the factory (bureau) occurred in the accident, obstacles, over the years accumulated equipment anomaly data collection and anti-accident technical measures. 
(3) anti-accident exercises exposed weaknesses and anti-accident measures, countermeasures. 
(4) the existing equipment and new equipment structure, principles, parameters, performance, system layout and operating methods of operation. 
(5) safe and economical operation mode and advanced working methods. 
(6) equipment maintenance or transaction after the operation mode and the use of new technologies. 
(7) the impact of seasonal changes on the operation of equipment and preventive measures. 
(8) to run professional theory or operational skills demonstration.
(9) the relevant part of the scheduling procedures.

 What should be observed when inspecting equipment?

A: The following rules should be observed when inspecting the equipment:
(1) No other work shall be carried out and shall not be removed or crossed.
(2) thunder and rainy days need to inspect outdoor equipment, should wear insulated boots, shall not be close to lightning rod and arrester.
(3) high-voltage equipment grounding occurs, the indoor shall not be close to the point of failure within 4m, the outdoor shall not be within 8m near the point of failure, into the above-mentioned range of personnel must wear insulated boots, contact equipment shell or frame should wear insulated gloves.
(4) after the inspection of the high pressure chamber must be readily locked.
(5) special weather increased special patrol.

 transformer gas relay inspection items which?

A: Transformer gas relay inspection items are:
(1) The valve on the gas relay connection should be in the open position.
(2) Transformer respirator should be in normal working condition.
(3) gas protection connecting piece into the right.
(4) check the oil level of the oil pillow in the appropriate location, the relay should be filled with oil.
(5) gas relay waterproof cover should be firmly.

voltage transformer normal patrol projects which?

A: voltage transformer normal tour of the project are:
(1) porcelain pieces with or without crack damage or abnormal discharge phenomenon.
(2) oil standard, oil level is normal, whether oil spills.
(3) Whether the terminal is loose.
(4) whether the overheating of the connector discoloration.
(5) suction Whether the discoloration.
(6) No abnormal voltage indication.

 arrester inspection which inspection items?

A: The arrester inspection inspection items are:
(1) check the porcelain part of whether there is damage, cracks and discharge phenomenon. 
(2) Check whether the discharge logger is operating. 
(3) Check whether the lead wire connector is secure. 
(4) Check whether the arrester internal abnormal sound.