Tripping means the interruption in electricity supply. An electric line is tripped if it starts carrying the fault current or it get broken or due several other reasons. It is a protective measure which essentially isolates the faulty lines from the rest of the healthy sections. So basically cascade tripping is a tripping of protective devices to isolate the part or parts of the system to avoid the damage to the load and the system equipment’s. Sometimes tripping of power grid takes place due to unbalanced condition is also called as cascade tripping.
Modern power systems experience many disturbances and majority of that are eliminated by relay protection and emergency control system. According to historical data , relay miss-operation is one of the contributing factor of 70% of major disturbances in the power system. With rapid growth of loads , transmission distances , HVDC and FACTs devices , the dynamic behaviors of power system are getting more and more complicated. A single fault is unlikely to destroy a modern power system , but information deficiency , hidden failures of relays , faults of other secondary system or human errors may be the cause of cascading events and the system, however strong may evolve in to power calamity. Cascading outages may happen in AC lines, DC lines, AC-DC line, sending area and receiving area as well.


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When the load demand is larger than the power generated the frequency of the machine goes down. When the value of his frequency lowers than the nominal value of the frequency the grid trips and the loss of power occurs. These power failures are not1 normal, they sometimes lead to blackout conditions if there is no backup protection available.

In power grid when one of the component fails and then its load is shifted to nearby system. Those nearby systems may push to work beyond their capacity so they get overload and shift their load to another system. Cascade failure mainly seen in high voltage systems where a single paint failure on a completely loaded or somewhat overloaded system results into surges of spikes across the system.

Once power is generated it has to be supplied to somewhere else. If it is than the load demand it will cause voltage surge or if it is less it will cause voltage dips. For proper operation of system, synchronization must be kept with the load. Power grid can withstand a single event that is single generator failure or single transmission line failure. This is called as ‘N-e’ contingency planning. The system can collapse if several failures takes place in rapid succession.


Most of power distribution or utility .companies relay on manual labor to perform the distribution tasks like interrupting the power to loads ; all parameter hourly checking .SCADA implementation in distribution reduce the manual labor operation ; cost. The PLC ; SCADA allows detecting the exact location of fault ; without waiting SCADA gives an alarm system to the operators for identifying ; prevent it.

Much attention has been given to the use of PLCs (Programmable Logic Controllers) in substation and distribution automation applications in recent years. Innovative engineers and technicians have been actively seeking new applications for PLCs in substations and SCADA (Supervisory Control and Data Acquisition) systems.

PLCs have an important place in substation automation and their use in substation applications will grow. As the use of PLCs in substation automation applications increases and the demand for substation and distribution automation increases. Utility engineers are required to field more projects with fewer available resources.

Moreover the blackout condition can be prevented maintaining the balance of power supply and load demand. Proper amount of reactive power should be connected to the grid to maintain proper voltage level. By providing high supporting backup systems which can withstand the load of one blacked out area.

There have been so many researches done in its theoretical ; application field which are briefly discussed in this chapter.
1 V.K. Mehta ,Rohit Mehta, PRINCIPLES OF POWER SYSTEM, presented in his book regarding requirements of distribution system and its objectives.
2 Nicholas Honeth, Substation Automation Systems, Royal Institute of Technology. Presented SCADA refers to a system that enables on electricity utility to remotely monitor, co-ordinate, control and operate transmission and distribution components, equipment and real-time mode from a remote location with acquisition at date for analysis and planning from one control location.



There are 5 generators as shown in the figure, which are interconnected with each other through tie line.
As shown in figure Generator 1, 2 and 3 are supplying the power to the substation 1 through grid and another generator 4 and 5 are supplying to the substation 2.
Load demand is balanced by using PLC through which cascade tripping and backup condition is possible.
If generator 1 load demand will increase then we have to share the load. So for that we are doing load sharing in which some amount of load will be shared on another generator 2.
If load demand is increasing out of capacity in that condition we have to do use load shading in which all over load will be disconnect from generator and generator will be safe.
During blackout, PLC gives a feedback signal to the generator 5 which as result acts as a backup system.
Power continuity in blackout condition is prevented by using Programmable Logic Controller.


PLC compares the reference value of main alternator and connect to second alternator through relay according to reference voltage Designing an efficient and cost effective solution for replacing or changing the alternator that is Problem related to alternator when it was fail then it can be handle and control by PLC.
Flowchart Description:

1. Supply is provided to a single transformer under normal condition and remaining transformers are connected to each other in a parallel manner.

2. A current transformer measures the load current continuously and feeds it to the relay by converting it to a corresponding D.C value in order to compare with the reference value set by the user.

3. Whenever the load current increases above its rated value given to the PLC then PLC send high signal to relay The relay coil thus passes a tripping signal to the load which is
Connected of the slave transformer.

4. That effect load on both transformers is identical. The current transformer still measures the load current and compares it with the reference value.

5. When the load current decrees bellow the reference value given to PLC then one transformer get shut down and avoid overloading.

6. If the load value increases further beyond the capacity of two transformers, load will be cut-off from the main supply based on the priority level set by the user. This is done to Provide UN-interrupted power supply to higher priority loads.


With a Programmable Logic Controller (PLC) scheme, load shedding is initiated based on the total load versus the number of generators online and/or detection of under-frequency conditions. Each substation PLC is programmed to initiate a trip signal to the appropriate feeder breakers to shed a preset sequence of loads.
Adaptive load shedding techniques employ a power swing equation to shed the required amount of load. The power imbalance within the system can be obtained by using this equation.
Where ?P is the power imbalance, H is inertia constant of generator, f is nominal
frequency (Hz), and df /dt is the rate of change of frequency (Hz/s).
This equation can be applied to an isolated power system having only a single generator as well as to an interconnected power system
Where, S – load to be shed.
dv – change in voltage w.r.t time.
dq – change in power w.r.t time.


In this work, a fault detection and diagnostic module is described based on internal PLC program signal data which is acquired through OPC Server. The observed or real-time PLC signal data is compared with normal PLC signal data to find out possible faults or deviations. The data acquisition procedure and the techniques used have been explained in this paper.


According to the International Electro technical Vocabulary, backup protection is intended to operate when a power system fault is not cleared or an abnormal condition is not detected in the required time because of failure or inability of other protection to operate or failure of the appropriate circuit-breaker(s) to trip. The backup protection is, by definition, slower than the main protection. Back-up protection may be obtained automatically as an inherent feature of the main protection scheme, or separately by means of additional equipment.
Time graded schemes such as over current or distance protection schemes are examples of those providing inherent back-up protection; the faulty section is normally isolated discriminatively by the time grading, but if the appropriate relay fails or the circuit breaker fails to trip, the next relay in the grading sequence will complete its operation and trip the associated circuit breaker, thereby interrupting the fault circuit one section further back. In this way complete back-up cover is obtained; one more section is isolated than is desirable but this is inevitable in the event of the failure of a circuit breaker. Where the system interconnection is more complex, the above operation will be repeated so that all parallel in feeds are tripped.
If the power system is protected mainly by unit schemes, automatic back-up protection is not obtained, and it is then normal to supplement the main protection with time graded over current protection, which will provide local back-up cover if the main protective relays have failed, and will trip further back in the event of circuit breaker failure.

Such back-up protection is inherently slower than the main protection and, depending on the power system configuration, may be less discriminative. For the most important circuits the performance may not be good enough, even as a back-up protection, or, in some cases, not even possible, owing to the effect of multiple in feeds. In these cases duplicate high speed protective systems may be installed. These provide excellent mutual back-up cover against failure of the protective equipment, but either no remote back-up protection against circuit breaker failure or, at best, time delayed.




Relay Module
Energy Meter
Current Transformer (C.T)
Toggle Switch
Lamp Load
Programmable Logic Controller (PLC)


5.2.1 Relay Module

This is an easy to use 4 channel relay board that works on 12V. Use it to control four 240V from PLC.
The board uses high quality relays which can handle a maximum of 7A/240V A.C. or 7A/24V D.C.


Energy Meter
This is used in the electrical control panels for measurement of power ; energy.
It has enabled with RS 485 communication port and has accuracy level class 1.0.


They can be design for multiple arrangements of digital and analog I/O extended temperature ranges, resistance and vibrations and impact etc.
This function is discrete inputs are given a unique address and PLC instruction can test if the inputs state is on or off.



A Programmable Controller is a specialized computer. Since it is a computer, it has all the basic component parts that any other computer has; a Central Processing Unit, Memory, Input Interfacing and Output Interfacing. A typical programmable controller block diagram is shown above. Selecting a PLC:
After the planning phase of the design, the equipment can be ordered. This decision is usually based upon the required inputs, outputs and functions of the controller. The first decision is the type of controller, mini, micro, or software based. This decision will depend upon the basic criteria listed below.
Number of logical inputs and outputs.
Memory – Often 1K and up. Need is dictated by size of ladder logic program. A ladder element will take only a few bytes, and will be specified in manufacturer’s documentation.
Number of special I/O modules – When doing some exotic applications, a large number of special add-on cards may be required.
Scan Time – Big programs or faster processes will require shorter can times. And, the shorter the scan time, the higher the cost. Typical values for this are 1 microsecond per simple ladder instruction
Communications – Serial and networked connections allow the PLC to be programmed and talk to other PLCs. The needs are determined by the application.
Software – Availability of programming software and other tools determines the programming and debugging ease.

The process of selecting a PLC can be broken into the steps listed below.
1. Understand the process to be controlled (Note: This is done using the design sheets in the previous chapter).
List the number and types of inputs and outputs.
Determine how the process is to be controlled.
Determine special needs such as distance between parts of the process.
2. If not already specified, a single vendor should be selected. Factors that might be considered are, (Note: Vendor research may be needed here.)
Manuals and documentation
Support while developing programs
The range of products available
Support while troubleshooting
Shipping times for emergency replacements
The track record for the company
Business practices (billing, upgrades/obsolete products, etc.)
3. Plan the ladder logic for the controls. (Note: Use the standard design sheets.)
4. Count the program instructions and enter the values into the sheets Pin configuration of ABB PLC:

This figure shows the various pins that are used by the ABB PLC. Each pin has its own importance.
1 – Location of DIN rail.
2 – Plate Fixture with Unit earthling.
3 – Lock for DIN rail mounting.
4 – Location for external dual connector.
5 – Location of the cable connectors of the CS31 Bus of the 24V DC power output for the inputs.
6 – Visualization set for the status of 8 inputs/6 outputs.
7 –location of the connector for the connection of input/output extensions.
8 – Location of
The serial port for programming or communication ASCII/MODBUS.
Connector for the central unit power supply cabling.
Connector for the output cabling.
9 – Location of the Potentiometers and the ON/OFF switch.
10 – Location of the connector for connection to a coupler.
11 – PLC status visualization area.
POWER: power ON
RUN: Program running.
ERR: Error(s) present.

Modbus Communication

It is a serial communication protocol with PLC. It is developed with industrial application as well as in power system too.
The main reason to use it is moves raw bits or words without placing many restrictions on vendors.
Modbus enables communicates many devices connected to the same network.


Indicator consisting of a light to indicate whether power is ON or the lamp load is in operation.
Indicator is device for showing the operating condition of some system.

Lamp Load

The load capacity at a constant load is usually characterized in the technical data by the specification of a maximum output current whe the signal is at 1.
The specification for the lamp load allows fot the fact that the switch ON current on a filament lamp is n times greater then the rated current.


There are many applications for PLCs in substation automation, distribution automation and SCADA systems. As utility engineers become more familiar with the capability of PLCs and PLC manufactures develop new substation specific products, the number and type of potential

Applications continue to increase.

• Analog and Discrete I/O
• Metering and station information management
• Protection and control
• Circuit breaker lockout
• Protective relay interface/interaction
• Dynamic protective relay setting for dynamic station topology
• Automatic switching