Electrical Power Engineering Practical Knowledge Questions 2

126, For a DC system there occurs positive ground or negative ground What are the hazards of running them?

 A: DC system has a positive ground fault protection caused by the possibility of malfunction. Because the electromagnetic drive mechanism of the trip coil is usually connected to the negative power supply, if these circuits occur over the poor insulation it will lead to protection of mis operation. DC system negative ground, if there is another point in the circuit ground, it may make tripping or closing circuit to a short circuit, resulting in protection or circuit breaker refusal to move, or burn the relay, or fuse. 

127, Transformer gas protection light-up, What is the reason?

 A: TTransformer gas protection light-up reasons are: 

(1) due to oil, fuel or cooling system is not tight so that air gets into the transformer. 

(2) due to temperature drop or oil leakage caused by oil surface gas relay light gas below the float. 

(3) transformer failure to produce a small amount of gas. 

(4) the short circuit. 

(5) gas relay or secondary circuit failure. 

128, For AC fuse loop , DC control loop  and signal circuit fuse how to choose? 

A: The choice of fuse should be: 

(1) AC circuit fuse protection device rated current of 1.2 times the selection.

 (2) DC control, signal loop fuse generally used 5 ~ 10A. 

129, If the transformer appears false oil level, which may be caused by?

 A: The transformer appears false oil level may be caused by the following reasons: 

(1) oil standard pipe block. 

(2) respirator plug.

(3) airway vent plug. 

(4) film-protected oil pillow in the refueling air is not exhausted.

 130, transformer winding insulation damage that is caused by what?

 A: Transformer winding insulation damage reasons are: 

(1) line short-circuit fault. 

(2) long-term overload operation, a serious aging insulation.

 (3) winding insulation damp.

 (4) Winding joint or tap-changer connection is bad. 

(5) lightning wave intrusion, so that winding over-voltage.

Electromagnetic Electrical Engineering Interview Questions 1

2. What is the Hall effect? What is the Rogowski coil?

The Hall effect is a phenomenon in which a lateral potential difference occurs across a current-carrying conductor in a magnetic field.

Rogowski coil is a kind of electromagnetic induction principle and the use of the full current law, measuring large inrush current (tens of kA to several hundred KA) or the rate of change of impact of the current.

3, What is Poynting vector?

The Poynting vector is the cross product of the electric field strength and the magnetic field strength, and the direction represents the direction of energy flow, and the size represents the unit time

The energy per unit vertical area. The unit is watts per square meter.

Poynting's theorem is the energy conservation theorem in the electromagnetic field, which means that the energy transfer is equal to the reduction of energy stored and the algebraic sum of the power supplies.

4, What is the the difference between free charge and bound charge, dielectric polarization and electrostatic induction and types?

Free charge is a free electron in the conductor (metal), ion (gas or liquid) and so on when the electric field force let the free movement of the charge. Bounded charge is defined as a charged particle in a dielectric that is bound by an atomic force, an intra molecular force, or an inter molecular bond,

They can move a little under the force of an electric field, but can not leave the range of molecules.

Polarization of the dielectric refers to the electric field in the dielectric molecules or atoms to form electric dipoles, the surface of the dielectric appears positive

And the phenomenon of negative binding charge. After polarization, the electric field created by the bound charge in the dielectric can generally attenuate the applied electric field.

Electrostatic induction of a conductor refers to the free-charge movement of the conductor in an applied electric field, accumulating on the surface of the conductor and establishing an electric field until The surface charge of the electric field and the external electric field in the conductor everywhere is the phenomenon. Due to electrostatic induction, electrostatic field In the conductor is internally charged, the charge can only be distributed in the conductor surface; charge inside the conductor is zero; its just near the conductor surface field strength and the surface perpendicular to the conductor is an equipotential body, the conductor surface is equipotential surface.

5, What is the difference between electrostatic shielding electromagnetic shielding?

Electrostatic shielding is the use of electrostatic equilibrium of electric field within the field strength of the conductor is made zero for shielding device.

Electromagnetic shielding is the use of good conductors in the vortex that can prevent high-frequency electromagnetic waves into this conductor that can be made while shielding the electric field and Magnetic field shielding device. It can suppress radiated interference and high-frequency conducted interference.

Magnetostatic shielding is the use of magnetic field in the high magnetic flux law of ferromagnetic materials that can make most of the magnetic field concentrated in the ferromagnetic circuit. A shielded device is provided which can shield the magnetic field.

The effect of static shielding is similar to that of electrostatic shielding, but the effect is not good for static shielding, since the conductivity of the metal conductor is of several dozen orders of magnitude than the air conductivity , and ferromagnetic material and air permeability difference of only a few orders of magnitude.

6, given the second type of boundary conditions, why the potential value of uncertainty?

Only partial derivatives do not have a base value

7, E at the interface between the two media is changing?

Tangent direction E unchanged; normal direction D unchanged.

8, In a circular coil vertical through a current into the wire, there is no induced coil emf

This depends on whether or not there is a change in the incoming current, since the principle of the induced electromotive force is that the induced magnetic field produces an induced electric field,

If there is a change in the current coil is induced electromotive force, if the current does not change then it is not.

9, In a balloon, the surface covered with a uniform charge, with the balloon constantly blowing large balloons inside and outside the electric field are how to change?

External constant, the internal field strength is zero

10, For the stability of the electromagnetic field conditions, physical significance When the frequency of the alternating current is low?

it is much smaller in the vicinity of the current than the wavelength of the electromagnetic wave at that frequency in the vacuum, Of the region, you can ignore the delayed effect of electromagnetic fields, such areas of the electromagnetic field is a quasi-stable or quasi-stable field).

What is capacitor, equations, units , working prinicple and different types with applications

What is Capacitor? Capacitor in parallel, Capacitor in Series, Working principle of capacitor, Ceramic capacitor applications, polarized capacitors, film capacitors, unit of capacitor, capacitor charge equations.

Known concepts of basic electrical engineering will help understand what it is and how one of the basic elements of electrical engineering, electronics and especially - capacitor. The arrangement of two conductors (pads) separated by a thin dielectric layer (i.e., non-conductive material) is called capacitor.

What is Electric Field, Electric Potential and Formulas and Definitions

The electric potential connection with electric field, their formulas, working principle and equations. Definition of Voltage and Electric Potential?

Defining electric field was necessary to explain the next interconnection with electrical potential. As for the potential of the electric field. Its value at a given point field strength E is determined by the work, they do force moving the unit positive charge from a given point of the infinitely distant point (outside the box). 

What is Definition of Electric Field, Permittivity and their Formulas.

Definition of Electric Field, Permittivity and their Formulas.

Before we define, electric filed we need explanation of several other important concepts. We will then electric charge. As already mentioned, the smallest carrier of electric charge is an electron. Since it is assumed that a negative charge is simple and intuitive definition of the loads of the bodies (particles) that is reduced to overflow or underflow of body electrons.

What is the purpose of Power System Protection

The power system protection can not prevent system faults, but it can limit the damage caused by short circuits while protecting people and equipment from damage. Goals are to selectively clearing faults in milliseconds and protecting equipments from overload conditions. The damages are due to short circuits in a system, abnormal conditions in the system and majorly human mistakes.

The basic purpose of installation of protective relays are:

1. To protect very expensive equipment like transformers, generators, motors, cables and other installed components.

2. To ensure the interruption of supply to minimum number of consumers in case of system fault.

3. To minimize the production losses of the factories.

4. To avoid major shutdowns.

Since the protection system limits the damages/losses and keeps the system running therefore it is considered a vital importance for the system.

Power Electronics Engineering Interview Questions set 3

What is a BJT (Bi Polar Junction Transistor)?

A bipolar junction transistor (BJT), unlike a thyristor-like device, is a two-junc­tion, self-controlled device where the collector current is under the control of the base drive cur­rent.

Why Darlington pair is used and what are its advantages and disadvantages?

The dc current gain of a power transistor is low and varies widely with collector current and temperature. The gain is increased to a high value in the Darlington connection,How­ever, the disadvantages are higher leakage current. higher conduction drop. and reduced switch­ing frequency.

How to control collector leakage current and enable base bias voltage?

The shunt resistances and diode in the base-emitter circuit help to reduce collector leakage current and establish base bias voltages.

What is avalanche breakdown (1st breakdown) and 2nd (second) breakdown in a BJT and how they destroy BJT?

Power transistors have an important property known as the second breakdown effect. This is in contrast to the avalanche breakdown effect of a junction, which is also known as first break­down effect. When the collector current is switched on by the base drive, it tends to crowd on the base-emitter junction periphery, thus constricting the collector current in a narrow area of the reverse-biased collector junction. This tends to create a hot spot and the junction fails by thermal runaway, which is known as second breakdown. The rise in junction temperature at the hot spot accentuates the current concentration owing to the negative temperature coefficient of the drop, and this regeneration effect causes collapse of the collector voltage, thus destroying the device.

What happens when inductive load is turned off in BJT circuitry?

when an inductive load is turned off the base-emitter junction becomes reverse-biased, the collector current tends to concentrate in a narrow area of the collec­tor junction. which causes the 2nd breakdown and BJT is destroyed after junction hotspot leading to thermal runaway of device.

What is a MOSFET, abbreviation and definition?

Unlike a BJT, a power MOSFET (metal-oxide semiconductor field-effect transistor) is a unipolar, majority carrier, "zero junction." voltage-controlled device.

Can you explain the working of a MOSFET?

If the gate voltage is positive and beyond a threshold value. an N-type conducting chan­nel will be induced that will permit current flow by majority carrier (electrons) between the drain and the source.

What is the major advantage/disadvantage of MOSFET over BJT?

An important property of a MOSFET is that it does not have the second breakdown problem of a BJT. If localized and potentially destructive heating occurs within the device. the positive temperature coefficient effect of resistance forces local current concentration to be uniformly distributed across the total area. The conduction loss of a MOSFET is large for higher voltage devices. its turn-on and turn-off switching times are extremely small, causing low switching loss. The device does not have the minority carrier storage delay problem associated with a bipolar device.

Please name a few applications of MOSFET?

MOSFETs are extremely popular in low-voltage, low-power. and high-frequency (hundreds of kHz) switching applications. Application examples include switching mode power supplies (SMPS), brushless dc motors (BLDMs). stepper motor drives. and solid-state dc relays.

Power Electronics Engineering Interview Questions set 2

What is GTO (Gate Turn Off Thyristor)?

A gate turn-off thyristor (GTO), as the name indicates, is basically a thyristor-type device that can be turned on by a small positive gate current pulse, but in addition, has the capability of being turned off by a negative gate current pulse.

How is the enhanced capability of a GTO obtained?

The turn-off capability of a GTO is due to the diversion of P-N-P collector current by the gate, thus breaking the P-N-P / N-P-N regenerative feedback effect.

What ratio of negative gate current is required to turn off a GTO?

The turn-off current gain of a GTO, defined as the ratio of anode current prior to turn-off to the negative gate current required for turn-off, is very low, typically 4 or 5. This means that a 6000 A GTO requires as high as –1500 A gate current pulse. However, the duration of the pulsed gate current and the cor­responding energy associated with it is small and can easily be supplied by low-voltage power MOSFETs.

What are Common applications of a GTO?

GTOs are used in motor drives, static VAR compensators (SVCs). and ac/dc power supplies with high power ratings. After large-power GTOs became available. they ousted the force-commutated, voltage-fed thyristor inverters.

What are series and shunt snubber and what is their application in power electronics?

The snubber consists of a turn-on component (R, L) called a series snubber and a turn-off component (R, C, and D), called a shunt snubber. This type of con­verter is typically used for a subway dc motor propulsion drive.

What do you mean by snubber loss?

The snubber loss may be substantial in a high-power and/or high-frequency converter. This, in turn, may reduce the converter's efficiency and put a burden on the cooling system. To combat this problem, various regenerative or energy recovery schemes have been proposed.

What are methods to cope with snubber loss

Passive energy recovery and the other is regenerative snubber.

What is Passive energy recovery method to overcome snubber loss?

When the GTO is turned off, the snubber capacitor„ charges to the full supply voltage, as usual. At the subsequent turn-on of the GTO, the stored energy is transferred to capacitor resonantly through the inductance and diode. When the GTO turns off again, the energy in capacitor is absorbed in the load and capacitor, charges again to voltage V.

What is regenerative snubber scheme to overcome snubber loss?

This scheme uses an auxiliary chopper. At GTO turn-off, the snubber capacitor, charges to supply voltage. At subsequent turn-on of the device, the energy is resonantly transferred to capacitor, as before. The energy in capacitor is then pumped to the source through a dc-to-dc boost converter.

What are common applications of regenerative snubbers and RC snubbers?

The idea of a regenerative snubber appears very attractive, but its application should be carefully weighed against the extra cost, loss, complexity, and equipment reliability. High-power GTO converters normally use regenerative snubbers. Otherwise, RC snubbers are commonly used.

Communication Systems Basics Interview Questions Set 1

What is an Address in Communication Systems?

It should be called as designator for the location of data in a storage device; allows the retrieval of data by reading the contents of a specific location. Also, the identity of a peripheral device. These (normally unique) allow individual devices on a single communications line to recognize and respond to messages directed at them.

What do you mean by ARP?

APR stands for address resolution protocol. A transmission control protocol/internet protocol (TCP/IP) process that maps an ID address to an Ethernet address; required by TCP/IP for use with
Ethernet.

What is the role of ARQ in data transmission?

Automatic request for transmission. A request by  the receiver for the transmitter to retransmit a
block or a frame because of errors detected in the originally received message.

What is ASCII standard ?

American Standard Code for Information Interchange. A universal standard for encoding
alphanumeric characters into 7 or 8 binary bits. Drawn up by ANSI to ensure compatibility
between different computer systems.

What do you mean by asynchronous Communications?

Communications in which characters can be  transmitted at an arbitrary, unsynchronized time,
and where the time intervals between transmitted characters may be of varying lengths.
Communication is controlled by start and stop bits at the beginning and end of each character.

What is bandwidth in communication?

The range of frequencies available, expressed as the difference between the highest and lowest
frequencies, in hertz (cycles per second, abbreviated Hz).

What does band pass filter do?

A filter that allows only a fixed range of frequencies to pass through. All other frequencies
outside this range (or band) are sharply reduced in magnitude.

Unit Baud describes what?

  Unit of signaling speed derived from the number of events per second (normally bits per second).
However, if each event has more than one bit associated with it, the baud rate and bits per second are not equal.

What are system BIOS?

The basic input/output system for the computer, usually firmware-based. This program handles
the interface with the PC hardware and isolates the operating software (OS) from the low-level
activities of the hardware. As a result, application software becomes more independent of the particular specifications of the hardware on which it runs, and hence more portable.

Differentiate between bits & bytes?

One bit is one binary digit,either a binary 0 or 1. One byte is the amount of memory needed to store
each character of information (text or numbers). There are eight bits to one byte (or character),
and there are 1024 bytes to one kilobyte (KB). There are 1024 kilobytes to one megabyte (MB).
Data acquisition boards typically take 2-byte samples; a board acquiring data at a 20 kHz
sample rate is actually gathering 40,000 bytes of data per second.

What is the purpose of buffer in electrical transmissions?

An intermediate temporary storage device used to compensate for a difference in data rate and data
flow between two devices (also called a spooler for interfacing a computer and a printer).

What is the significance of the terminology bus in communications?

A data path shared by many devices, with one or more conductors for transmitting signals, data or
power. Also, the expansion connector built into a computer. Boards are inserted into this connector,
and all communication between the computer and the board occurs through the computer’s bus.

What is cache memory?

A fast buffer memory that fits between the CPU and the slower main memory to speed up CPU
requests for data.

What is clock pulse?

A rising edge, then a falling edge (in that order), such as applied to the clock input of an 8254
timer/counter.

What is CMRR?

Common mode rejection ratio – A board’s ability to measure only the voltage difference between the leads of a transducer, rejecting what the leads have in common. The higher the CMRR, the
better the accuracy.

What does counter data register do?

The 8-bit register of a (8254 chip) timer/counter that corresponds to one of the two bytes in the
counter’s output latch for read operations and count register for write operations.

What is CRC error detection method in communications?

Cyclic redundancy check. A basic error checking mechanism using a polynomial algorithm based
on the content of the frame and then matched with the result that is performed by the transmitter
and included in a field appended to the frame. Also referred to as CRC-16 or CRC-CCITT.

What does the term CSMA/CD defines?

Carrier sense multiple access/collision detection. When two stations transmit at the same time on a
local area network, they both cease transmission and signal that a collision has occurred. Each then
retries again after waiting for a random time period.

Industrial Automation and Electrical Control Systems Interview Questions Part 4 (Distribute Control System)

 

What are Three types of expert system development tools?

Skeleton Type Expert System:

　　It is the basis for the successful expert system application, removing the specific content of knowledge retention and the shell reasoning of knowledge representation and a special tool to enhance knowledge acquisition subsystem functionality.

Universal Type Expert System:

　　That is a knowledge representation via language, such tools provide a more generic configuration and this type of language corresponds to the reasoning facts. The user can enter production in the form of knowledge and facts, such as OPS-5 production system-oriented language which is equipped with a forward reasoning for process which rely on internal reasoning mechanism for the solution to the problem.

Toolbox type Expert System:

　　That type is in middle of general and special development tools. It provides the framework for the several expert systems components (such as reasoning frame, blackboard framework, etc.), and each frame is composed of several modules. Description added according to the needs of designers and system automatically generate the expert system for process with certain areas targeted. Example of such development tools are AGE, ZDEST-1

What is Artificial Intelligence?

Artificial Intelligence (AI) language is adapted for the field of artificial intelligence and knowledge engineering, symbolic processing and logical reasoning ability combined in  computer program. That computer program can be used to solve numerical/non-numerical calculation, intelligent knowledge processing, reasoning, planning, decision-making, such as a variety of complex issues. The typical AI language are LISP, Prolog, Smalltalk, C + +, etc.

What are the basic components of Artificial Intelligence?

The basic components are:

• Knowledge representation technology
• Knowledge, reasoning techniques
• Fuzzy logic
• Neural network technology
• Genetic algorithms
• Expert systems
• Machine learning
• Swarm intelligence

What is model reference control?

The basic idea of ​​adaptive control lies in control system design, a constantly measurements of the states of the controlled object, performance and parameters, thus finds out the master of the current health of the system which can have positive effects on system state. Analyze the current state of system performance indicators and expectation indicators so as to make decisions based on the comparison results and to change the structure and parameters of the controller and to change state variables according to the laws of the adaptive control action to ensure that the system is running in optimal state.

Generally speaking, the adaptive control system is coupled with feedback control systems with basic control loop and adaptive mechanism is also constituted. It has three functions:

(1) Real time identification.
(2) The decision-making control.
(3) Real Time correction

What is PLC?

 

PLC (Programmable Logic Controller ) is a control device which uses memory that can be programmed for its permanently stored program, that is used for the implementation of logical, sequential control, timing, counting and arithmetic operations such as user-oriented instruction and digital or analog input / output control of various types of machinery or production processes.

What is the working principle of PLC?

When programmable logic controller is put into operation, the course of their work are generally divided into three :

1. Input sample stage
2. User program execution stage
3. Output refresh stage

What is the input stage execution of PLC?

In the input sampling stage, the programmable logic controller sequentially scan/reads the status of all inputs and data and they are stored in the corresponding cell in the I/O memory area.

What is user program execution stage?

Programmable logic controllers are always followed by a top-down order to scan the user program (ladder/FBD). Scanning each ladder by the contact form on the left of the control line and process the first left and right line and execution continued the first after the other and according to the result of logic operation it refresh the status of the corresponding logic bit of the coil in the system RAM storage area.

What is the output refresh stage of PLC?

After the end of the scan and execution of user program, programmable logic controllers starts the output refresh stage. During this period, the CPU status and data in accordance with the I/O from memory area refresh all of the output latch circuit.