Showing posts with label Electrical & Instrumentation. Show all posts
Showing posts with label Electrical & Instrumentation. Show all posts

Generator and Turbine intertripping


 On tripping of class A trip of Generator

1-Generator breaker opens

2-Excitation switches off

3-Turbine trips on 86 T relay and followed by closing of ESV and all extractions

4-Turbine speed increases a bit by around 2-3% and gradually speed recedes


 On tripping of class-B trip of Generator

 1-Only generator breaker opens and excitation switches off

2-Turbine rotates on rated speed

3-After normalization of problem, generator synchronized with auxiliary power supply unit and later with grid

4-In case of small turbines, STG clubbed with 86GA to trip Turbine also


 On tripping due to Turbine fault

 1-Relay 86 T gets activated to close turbine ESV all extractions

2-Low forward power or Reverse Power Relay activates first which has time delay, during this time entrapped energy gets consumed by generator

3-Subsequently 86 G relay gets activated to open Generator breaker and followed by excitation gets off

4-Turbine speed does not increase in this case

 Trend of rotor speed on tripping of Turbine and Generator

Read>>>>Powerplant O&M reference books

 On tripping of Generator, instantly generator breaker gets opened and Turbine emergency valve closes.So steam entrapped in Turbine casing causes increase in speed initially and later slows down.

On the other hand,when turbine trips first, generator breaker opens after some time delay through Low forward power or Reverse Power Relay.This time delay helps turbine to consume entrapped steam in casing.As a result turbine generates power and does not allow speed to increase more.

 Relays and significance

Sl No.

Type of relay

Used for



Generator lock out relay



Turbine lock out relay



Generator over voltage @110% alarm and 120% trip



Generator under voltage @90% alarm and 80% trip



Generator Under frequency



Generator over frequency



Generator Reverse Active Power  5 % of Active Power



Generator Reverse Reactive Power



Low forward Power



Negative Phase Sequence 



Generator Over Load



Generator Instantaneous O/C    



Restricted E/F



Rotor E/F2nd Stage

10-Difference between fixed nozzle and Variable nozzle de-super heating


 De-superheating is the process of reducing the temperature of superheated steam. This is typically achieved by injecting a cooling medium, such as water, into the steam flow. The nozzles used in the desuperheating process can be classified as either variable nozzle or fixed nozzle de-superheaters. Here's

 The differences between these two types are:

Sl No.

Variable nozzle de-super heater

Fixed nozzle de-super heater


Variable nozzle desuperheating systems have adjustable nozzles that allow for controlling the flow rate of the cooling medium injected into the steam flow.

Fixed nozzle desuperheating systems have non-adjustable nozzles, meaning the cooling water flow rate and the degree of desuperheating are fixed


The nozzle opening can be adjusted to vary the amount of cooling water injected, thereby controlling the degree of desuperheating and achieving the desired steam temperature.

Separate control valve is required to adjust the water flow


More flexibility in adjusting the cooling water flow rate and achieving precise temperature control.

Not much accuracy in temperature control


Variable nozzle desuperheating systems are often used in applications that require tight temperature control,

USed where there is much tolerance in temperature control,

Ex: In process industries

They are commonly used in applications where a constant degree of desuperheating is sufficient, such as in industrial processes with steady steam loads or in small-scale power plants.


Complex design

Simple design


More costlier than fixed nozzle de-super heaters

Less costlier


Little bit complicated operation

Simple operation


Can be used for variable inlet flow & temperature

Used only for fixed flow & temperature


Size of nozzle is variable

Size of nozzle is fixed


Maintenance is difficult & costlier

Maintenance is simple & cheaper

 The choice between a variable nozzle and fixed nozzle desuperheater depends on factors such as the required temperature control accuracy, steam flow variability, plant operating conditions, and budget considerations. Variable nozzle desuperheaters are often preferred in applications where precise temperature control and flexibility are crucial, while fixed nozzle desuperheaters can be suitable for applications with relatively stable operating conditions and lower cost requirements.

11-differences between star connection and delta connections


11-differences between start connection and delta connections

Sl No.

Star connection

Delta connection


Two types of star connections are possible

A-4-wire, 3-phase system

B-3 wire. 3-phase system

Only 3 wire. 3-phase system is possible


Out of 4 wires, 3 wires are the phases and one is neutral

All 3 wires are phases in Delta connection


In Star  connection, one end of all the three wires are connected to a common point in the shape of Y to form neutral

In Delta connection every wire is connected to two adjacent wires in the form of triangle.And all the three common points of the connection form the three phases


Line current and phase currents are same

Line current and phase current are different.Line current =Phase current


Line voltage and phase voltages are different, Line voltage =Phase voltage

Line voltage and phase voltages are same


Since line voltage is more than phase voltage, insulation required for each phase is less

In Delta connection line and phase voltages are same hence more insulation is required


Star connections are used for both transmission and distribution applications/networks

This connection is generally used for distribution networks


Since insulation required is less, these connections are used for longer distances

Since insulation required is more, these connections are used for shorter distances


Star  connections are used where less starting current and starting torque is required

Delta connections are used where starting current and Torque is more


Power calculation in Start connections.


P = 3 X Vp X Ip X Cosθ




P = (√3 X VL) X IL X Cosθ

Power calculation in Delta connections.


P = 3 X Vp X Ip X Cosθ




P = (√3 X IL) X VL X Cosθ




In star connections different voltage levels are used as line voltage & phase voltages are different

In Delta connections, only single voltage is used

For more articles on power plant and calculations read >>>>power plant and calculations

15-Emergencies in power plant operation

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