Ground or Earth fault protection Relay - RCB, RCD

Connection types of Ground / Earth fault protection

In star point of an over current relay -

                           Fig: Ground fault relay connection in star point of over current relay

RCB-Residual circuit breaker to detect earth fault - how it works

The RCB or residual circuit breaker is a protective device to sense the earthing fault in circuit. Basically RCB is used in circuit to protect human life from electrocution or electric shock.

If a person is somehow electrified, the RCB will sense it and disconnect the circuit. If somehow an electrical equipment like motor or utility like panel is electrified, hence is grounded or earthed – RCB will detect it and disconnect the circuit.

Why we need to use RCB - Residual circuit breaker

As we know that only 40 mili amp current is enough to kill a strong man, so saving human life from lethal electric shock is an obvious thing to consider.

Generally circuit breaker are of minimum 5 amp rating, so as usual circuit breaker is assigned to protect the circuit from overload or short circuit other than human from electric shock.

Arc Fault Circuit Interrupter (AFCI) - Circuit breaker that prevents electrical fire causing arcing

In an electrical system, a circuit breaker protects the system from overload and short circuit fault. But without this type of fault happening there might be severe arcing in the circuit that cause electrical fire.

The AFCI- Arc fault circuit interrupter circuit breaker is special type of circuit breaker is to protect the circuit from arc happening. If there is a possibility of lethal arc , the breaker will break the circuit like other normal circuit breaker do with a fault.

So general circuit breaker detects and prevents circuit from overload, short circuit condition, over voltage etc. But these does not cover another potential risk “fire by arcing”. In case of a lethal fire due to arcing, circuit breaker might not be tripped. As there might be no fault happened at all but arcing happened already.

How to select Fuse or Circuit Breaker for group of motor in same branch circuit as per NEC

To select the proper fuse or circuit breaker for Group Motor installations, you must apply the specific rules of the National Electrical Code (NEC) for Group Motor installations. This applies to a single circuit breaker or fuse for a group of motors in the branch circuit that is single upstream short circuit protection in a Group Motor installation.

                                                    Fig: Motor in the same branch circuit

Refer to NEC (National Electric Code) section 430-53C and section 430-53D for proper conductor ampacity selection. This applies to a single circuit breaker or fuse for a group of motors in the branch circuit that is single upstream short circuit protection in a Group Motor installation.

Frequency characteristics and switching, operating characteristics of Circuit breaker

Rated frequency of circuit breaker

Two frequencies are currently used throughout the world: 50 Hz in Europe and 60 Hz in America, a few countries use both frequencies. The rated frequency is either 50 Hz or 60 Hz.

The performance of a circuit breaker is same for both the frequencies 50HZ and 60HZ ac network. Same thermal effect and other performance for up to 60HZ of a circuit breaker. Special consideration is required for frequency above 60HZ.

Function and rated characteristics of circuit breaker

The circuit breaker is a device that ensures the control and protection on a network. It is capable of making, withstanding and interrupting operating currents as well as short-circuit currents.

The circuit breaker have to sustain and withstand the following currents- Normal current, Over load current or thermal current & Short circuit current.

So a circuit breaker must carry current in normal condition and should be capable to break current, make current under both normal and fault condition. Plus it must have withstand capability of sustaining short circuit current for minimum 1-3 sec. Short circuit current might vary 1KA (1000amp) to higher value as per design.

Why and how: Reactive Power symbol is small letter and all other electrical symbol is small letter.

All of the symbols in electrical engineering are in capital letter such as “V” for voltage, “A” for ampere etc. You might heard or read of some confusions regarding to the symbol of reactive power. What it should be “VAR” or “var”.

The IEC (International Electro technical Commission) is sole responsible for defining standard for electrical system as well as providing symbols for different parameters.

Correct symbol for reactive power

“var” is the correct symbol for Reactive power. Thus reactive power symbol should be lower case.

As per IEC, the symbol of reactive power should be of lower case when all others are in upper case. Therefore, “var” volt-ampere reactive is the correct unit measurement of reactive power.

According to IEC Publ. 27-1, the unit abbreviation is to be in lowercase letters, while other power unit abbreviations are to be in upper case letters. Also, as per EU directive 80/181/EEC (the “metric directive”), the correct symbol is lower-case “var”.

The term “var” first proposed by Romanian electrical engineer Constantin Budeanu in 1930.

Additionally, the unit “var” is allowed by International System of Units (SI). As indicated in IEC website for SI zone, Active power symbol is “P”, Apparent power symbol is “S” and reactive power symbol is “Q” and also, “var”.

Is there any needs of reactive power in electrical system?

The answer is yes! There is needs of reactive power in electrical system. In fact, without reactive power the present electrical system will not work at all.

It is said that active power in the electrical system is do things and reactive power is a waste and need to be controlled/ rectified. Nevertheless, the reactive power is needed for a crucial purpose-

Reactive power is needed to generate magnetizing force! Transformers, induction motors etc. runs through magnetic flux or field!

Active power converts to mechanical energy, light energy, thermal energy. On the other hand, reactive power converts to develop magnetic fields.

And we know that, the necessary parts of electrical system is power transformer, distribution transformer, induction motor and many more electrical equipment that works on the basis of usage of magnetic force.

The reactive power is used to generate the flow necessary for the conversion of powers through the electric or magnetic field and it is index of the transfer of energy between supply and load. Without this, there could be no net transfer of power.

What is power factor correction in electrical system?

Power factor correction means managing the reactive power component in electrical system. Usually by any correction, it is meant of removal or suppression.  However, by power factor correction it does not mean anything related to removal or suppression of reactive component of electrical power.

We need reactive power for power transformers and other equipment that works by the principle of using magnetic force. 

Power factor correction means - generation of reactive power at the load point. Thus relief the source (usually generator) to supply reactive power to the load side.

Basic design of SCDA – Supervisory control and data acquisition

SCDA means – supervisory control and data acquisition. It is a computer-based system, which performs measurement, data acquisition, data transmission, operating and control function. In modern power generation, transmission and factory with automation system has a computer-based interface where operator can watch the data in real time updated, interact with it, operate-control system by clicking or putting settings data.

SCDA design of a simple steam generation unit

Let’s say a simple steam generation unit is to be watched and control. As we know that, a steam boiler unit has two basic safety- the steam pressure, level of water inside boiler (high, low). Now we can design a basic SCDA to control this small steam unit as follows.

    Fig: SCDA animation- courtesy Wikipedia

Basic of Power Plant automation system

Power plant Automation system is the complete solution of safe and efficient operation. In a power plant, there are lots of complex system required to work and more complication is that this system are inter related. Not only interrelated, they are required to follow exact time sequence in operation. Say in a heavy oil power plant- the engine run by HFO, at the same time there are at least dozens of direct process need to smoothly operate.

The recognized facts & parameters are – pressure of fuel supply, temperature of fuel, engine cylinder temperature, engine lube oil temperature & pressure, cooling water temperature in different location, engine speed etc.

For the safe and optimal operations, these facts are controlled by automation. These are categorize as per their importance and risk level. Say in Diesel engine based power plant the most severe risk factors are -over speed, engine jacket water high temperature & low lube oil pressure to engine.

International standards to Sensor naming code - to identify their type and function

According to the international standards like IEEE, IFP sensors must have universal code to identify them, there function etc. These sensor naming code comprise of letter and numeric decimal. Say PTZ 201 means “lube oil inlet pressure transmitter sensor” plus it is used for emergency purpose like it is attached to direct shut down or direct tripping of the s machine. P=pressure, T=transmit, Z=emergency, 2=lube oil, 01= inlet. This sensor numbering technique used in Wartsila engines and auxiliaries according to ISO 3511 part 1 and 2.

Therefore, we have this that every sensor has maximum three letter and three number like PT201, TE501, TEZ501. We shall learn each letters and decimal numbers significance here. At first, some example will be helpful.

Pt-100 temperature sensor – working principle and test procedure

Pt-100 or pt100 is one of the most used temperature sensor. It is one of the most used temperature sensor. Cheap but robust for use and capable to directly interface with PLC. The use of pt-100 is limited to below 150°c. Moreover, it is not suitable for internal use, with its hard plastic body as it has own limitation.

Pt-100 temperature naming and working principle

Pt-100 is named as such because platinum is used as the sensing material and at with perfect calibration it has 100-ohm resistance at 0°c. For increment or decrement of temperature, the resistance value of platinum changes linearly. Thus it can be used to identify the temperature by comparing the resistance value and temperature-resistance variation log value (provide by manufacturer).

How to make connection between thermocouple and PLC

Connection between thermocouple and PLC is an important thing to consider. The cost & efficiency both factors are needed to take in to consideration. Also the environment (temperature) of the sensor location, distance to the PLC is a factor.

Thermocouple connection in a system comprises with connection channel cable and cold end temperature generator usually pt-100 temperature sensor. As pt-100 temperature sensor has this option to directly making interface with PLC plus it is cheap, it is more popular to use thus.

Basic working principle of Thermocouple - temperature sensor

As for Basic working principle of Thermocouple- Thermocouple is temperature sensor that measure temperature in terms of voltage according to the temperature difference between two ends of the metal pair.

Thermocouple working principle

Thermocouple made by two different metal wires joined at one end, this joint ends is placed in a temperature zone where temperature should be measured called “hot zone” and the other end of thermocouple where two metal wire are open (not connected or joined) placed in a low room temperature called “cold zone or reference temperature.”

Details on P/I converter- pressure to current converter

P/I converter or Pneumatic pressure-to-Current is a transducer. It converts pressure input to a current in mili-ampere range. P/I converter is a common instrument in automation system. Like a temperature senor or level sensor, it transmit data to other source like PLC, instrument to control systems only here the input is pressure.

let’s say we have to control a valve that is to open it, shut it, or control the openings by percentage that is we may choose it to be open for 70% to control a fuel flow or steam flow. The valve is in a hazardous place so we cannot reach it; we can only observe this in SCDA (supervisory control and data acquisition).

How to control level of a tank with float switch

To design an automatic control of maintaining a level of a tank, First thing to consider from two option – Keep the tank empty – for dirty water, dirty wastage liquid. Keep the tank filled– for fresh water, fresh liquid.

Floating switch

Components: Level switch (a simple floating switch will do), Relay & A simple pump motor.

Two types of float switch is constructed –

Filling – switched on when level is low and switched off when level is high.

Emptying – switch on when level is high (pump out); switch off when level is low.

Float switch – how it works

Float switch is the basic form of a simple automation system. Its function is to operate or actuate a system by the level of liquid or fluid. 

Float switch can transmit only two types of signal - level zero & level high. 

A circuit can be design to start stop some process by receiving the signals from float switch. Common processes are sending low or high level alarm, starting or stopping pump motor.

Basic insight on power transistor, JFET and MOSFET

A simple npn or pnp transistor has not much use in circuits. The hybridization of npn/pnp transistor has offered us with BJT- Bipolar junction field effect transistor, Field Effect Transistors (FET), Junction Field Effect Transistors (JFET), Metal oxide semiconductor field effect transistor (MOSFET). Thus, we can have the application of transistor with much greater power handling capacity with reduced loss.

                                       Fig: Simple transistor circuit, Water-in-pipe analogy

Solenoid types, parameters and practical application area

Like relays, Solenoid takes more voltage to pull in the solenoid than it takes to hold it in the retracted position, Thus, if a solenoid is left on, it is drawing more current than it really needs, and it tends to heat up. So there types, parameters are need to be known to apply them in practical application.

Solenoid types

According to operation, an intermittent-duty solenoid is designed to operate for a short time and then take time to cool. Moreover, a continuous-duty solenoid is designed to deal with the heat, so it can operate all the time.

Solenoid basic working principle and a practical Solenoid valve

A solenoid is a simple electromagnetic device that converts electrical energy directly into linear mechanical motion, but it has a very short stroke (length of movement), which limits its applications.

Basic working principle of solenoid

The solenoid consists of a coil of wire with an iron plunger that is allowed to move through the center of the coil. Above figure shows the solenoid in the un-energized state.

                                               Fig: Basic working principle of solenoid

  

Notice that the plunger is being held about halfway out of the coil by a spring. When the coil is energized, the resulting magnetic field pulls the plunger to the middle of the coil. The magnetic force is unidirectional — a spring is required to return the plunger to its un energized position.

Functions, characteristics of Varistor – a transient voltage protection component

What is Varistor

Varistor are voltage dependent, nonlinear device which have an electrical behavior similar to back-to-back zener diodes.

Fig: A Varistor

How varistor suppress transient voltage

When a transient occurs, the varistor resistance changes from a very high stand-by value to a very low conducting value. The transient is thus absorbed and clamped to a safe level, protecting sensitive circuit components.

Rectifier, Inverter, Capacitor, Inductor - Basic what need to know

Rectifier

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC). The process is known as rectification.

Usual rectifiers:

Vacuum tube diodes, mercury-arc valves, solid-state diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches.

Rectifier Circuit

Single phase Half Wave Rectifier:

                                                      Fig: Half wave rectifier circuit with a diode

Half and full wave rectification from AC to DC

Comparison between half and full wave AC to DC rectification -

Optocoupler or Opto-isolator and its function in circuit

Optocoupler or Optoisolator is a safety component that transfers electrical signals between two isolated circuits by using light signal. A common type of opto-isolator consists of an LED and a phototransistor in the same package. Optocoupler are mainly used in delicate system like between sensor and PLC.

Why optocoupler is used

Optocoupler functions as a galvanic isolation component. That is it maintains the connection between two devices or component without any direct conduction. Components or devices like PLC, SCDA, Sensor etc. Say a sensor is to connected to a PLC, if it is directly connected – any fault in the circuit would damage the PLC.

Frequency converter function and basic

Frequency converter converts the frequency of ac current, that is it converts a 50 or 60 hz ac current to ac current of any desired frequency. The device may also change the voltage if it is required.

                                                       Fig: Frequency converter Basic

Why electrical equipment rated in KVA not in KW

The ratings of all electrical equipment are all rated in KVA or Kilo Volt Amp. However, the practical use is in KW. We can say it also that electrical equipment are rated in apparent power (KVA or VA) other than active power (KW or W).

Electrical equipment rated in KVA - the main reason

Electrical equipment rated in KVA – is due to the variety of power factor rating in consumer side. KVA is independent of power factor. The KW depends on Power factor of the consumer utility. 

Low power factor effect in transmission and consumer electrical system

Low power factor effect has some adverse impact on both the transmission line and also in the consumer electrical system. To cope up the adverse effect better understanding of low power factor is needed.

Power factor varies from 0 to 1.

Low power factor means the reactive power is higher than the active power, as power factor is the ratio of active power to reactive power. So with a low power factor we got to deal with high amount of reactive power. Thus low power factor disadvantage is actually the disadvantage of excessive reactive power. 

Relationship between phase voltage and line voltage in 3 phase wye connection Or Root 3 factor between line and phase voltage, current in delta, Wye connection

There is √3 function in between wye and delta voltage and current relationship in case of phase and line, this might have easy for you but forgot in time. As this a simple tricky question asked in the interview board so better to understand this basic electrical facts.

In case of Wye connection iL = ip, but vL = √3 x vp. And in case of delta connection iL = √3 x ip, but vL = vp. The core reason behind this is the vector sum of two parameters. Say for simple arithmetic sum p + p = 2p, but with vector sum p+p by angle 120 = √3p. 

What is power line carrier channel

High frequency signals are transmitted through the transmission line conductor for the purpose of communication, protection, signaling and monitoring.

Carrier current equipment are set at receiving and sending end of transmission line channels. This signal have several uses like communicating data between electrical equipment like relay for trip signal.

What is electrical power?

We know of active and reactive power from text books and other sources. But the basic concept to electric power and its constitutes are most of the time misleading and vague as these are some virtual terms. After studying and searching through internet I have gathered some concept on electrical power- active, reactive power, apparent power.

Electrical power

In simple word we can define electrical power as the multiplication of voltage and current at any instant. We know P=V*I, Power= voltage*current. But this needs some extended information. 

Busbar Trunking System or BBT

BBT or Bus Bar Trunking system is a replacement to traditional cable and panel distribution. MCB, MCCB in BBT is plugin component as bus is extended to consumer area.

Fig: Busbar trunking

Electrical cables and Wiring- as per electromagnetic pick up and radiation protection

Types of Electrical cable according to the protection from Electromagnetic pick up and radiation are Twisted Pair cable, Co-axial cable, Sheathed  cable. And according to materials used Copper cable, aluminum cable.

Details on Electrical cable types

Twisted Pair cable

Two conductors of a single circuit are twisted together for the purposes of canceling out Electromagnetic interference (EMI) from external sources.

Details of over speed and Pneumatic over speed trip device of Diesel engine

The over speed of diesel and gas engine is very severe unlike that of turbine. In case of turbine if over speed occurs, alternator can be disconnect safely by an operator while turbine can run in over speed for a while. With over speed the voltage fluctuates but that is the part of alternator that has extensive protection to handle these types of faults.

In case of diesel engine with its several number of cylinders which has moving piston and rotating crankshaft the over speed has severe impact. If over speed sustain for a long time, the engine will be mechanically damaged or break down.

Turning gear or Electrically driven turning device in engines

The turning gear is an electrically driven system to turn the engine flywheel manually in diesel engine. It consist of a motor, gear arrangement. When engaged the gear arrangement mounts on the engine shaft gear and rotate the flywheel by about .60 RPM (it varies). It is needed for maintenance and inspection.

Basic speed control mechanism with speed governor in a diesel engine system

Speed Governor is an electromechanical device in diesel engine. Its function is to maintain & control the speed of the engine by controlling the amount of fuel to engine.

The core concept in power generation is that, the speed of engine or turbine must be constant or +-x of it. A fluctuation in speed would cause fluctuation in voltage and frequency. 

                                                                      Fig:  Typical diesel engine

Governor maintain the speed of running engine by controlling the amount of fuel supply to engine. If the speed is low or high other than nominal value, then it increase or decrease the amount of fuel to engine. The signal governor receives from a certain control system.

Basic Operation of Diesel / HFO engine

We know that diesel engine is effective for peak load power plant. A diesel engine can go to full load within 5 minutes but a steam plant unit might need over 12 hours to go to loading. With this advantage diesel engine has its own limitation that it is very uncertain and not reliable. There is always prone to some problems or difficulties. You can stop a good running engine and then might fail to restart it – some problem is likely to occur.

So care should be taken to start, stop, run, loading, unloading & specially preserving preserve a diesel engine.

Lubricating oil system of Wartsila W20V32 engine

The lubricating oil is stored in the oil sump under the engine. Return oil from the engine system is led back to the oil sump. The lubricating oil separator (if used) is connected directly to the engine oil sump. 

Fig:  Lubricating oil system of Wartsila W20 V32 engine

Lubricating oil level monitoring in running engine

Lubricating oil is the vital fact for running engine. If for some instance lube oil is not sufficient then engine will be damage permanently that is a mechanical seize will occur due to friction and cooling failure in moving parts. Lubrication oil do two things cooling and smooth the friction in moving parts.

On power plant and marine installations with several engines, a system with real-time monitoring of the lubrication oil level is essential for avoiding costly damage. There might be two thing happening – firstly the lube oil level might be low and secondly there might be leakage water in the lube well storage sump below engine. We have to set a lube oil monitoring system to cope up both fact.

Pneumatic system – why & how it works

Pneumatic system

A centrally located and electrically powered compressor that powers cylinders and other pneumatic devices through solenoid valves can often provide motive power in a cheaper, safer, more flexible, and more reliable way than a large number of electric motors and actuators.

Basic concept of hydraulic system

Hydraulic system theory

The concept in hydraulic system is to utilize liquid power or hydraulic power. By hydraulic power of liquid, a small force can be converted to a larger force, only by changing the area of liquid surface.