FIELD OF INVENTION: -
This invention deals with providing a fast and generic load shedding scheme for industrial power system using PLC which balances the generation and load before any change in system parameter.
BACKGROUND OF INVENTION: -
For an industrial load power system, reliability and stability are very important. Loss of power leads to a high cost of production loss which affects the economy of the industry. A major disturbance leads to change in the configuration of the power system network because of opening of breaker(s) and this may result into generation-load gap. This causes the fluctuation in frequency and voltage. As the frequency decays, the efficiencies of the generators are affected and the ability to run the generator auxiliary systems is contributed to the problem. [2] The frequency drops faster in a power system with cogeneration (generally in industries) than in a simple transmission system. If the generation-load gap is not reduced faster, a cascading effect may come into existence which results into collapse of whole power system. To avoid complete system collapse, fast load shedding methods should be implemented in response to a contingency to preserve the generation-load gap to a minimum. A contingency is the loss of one or more in feeders (local generators or incomers from the grid). A fast load shedding scheme initiates load shedding much before the system frequency or voltage fluctuates unlike in the traditional under-voltage/under-frequency load shedding schemes. Thus, Fast load shedding scheme has the ability maintain the stability of the system intact. The loads in an industry can be divided into 3 parts: (i) critical (ii) non-critical and (iii) process-based or semi-critical. In fast load shedding method, the load shedding scheme removes the most non critical loads so as to improve the selective reliability for the most critical loads. Thus fast load shedding improves both stability and selective reliability of critical feeders in a contingency which is foremost requirement to an industrial power

system. For different reasons, a power system is run in different configurations. The fast load shedding scheme should such that it responds to every change in configuration and should provide its best result so that programme or logic should not be changed every time the configuration changes. Thus load shedding program should work for any dynamic change in the power system configuration as well as dynamic change in the power system parameters. Therefore, there is a need of GENERIC FAST LOAD SHEDDING SCHEME. The scheme will comply with the increasing complexity of power system network, commissioning of new unit generations etc.
Load shedding can be defined as the process of removing load from a power system in response to system disturbance which results in generator deficiency condition to keep the other portion of power system healthy. The possible disturbances include faults, loss of generations, lightning strikes, switching disturbance etc.
There are two main controlling loops to control the dynamic and transient changes in response to any kind of disturbances. One is excitation control loop which controls the reactive power and voltage of the system. Second one is prime-mover loop which controls the active power and frequency of the system.
Excitation loop of control:
Reactive power is essential for any rotating machine or transformer for the conversion of electrical energy into mechanical or vice versa. During a fault, fault currents demand heavy reactive power from generator(s)/ grid (which can be considered to be an infinite source of active and reactive power). After the fault is cleared the rotating machines has to balance the generation and load while rebuilding its magnetic energy. System may demand additional reactive power from motor loads under reacceleration. Voltage of the system depends on the reactive power provided by the generators for recovering the system stability the generators.

Prime Mover loop:
The system frequency directly depends upon the active power provided by the grid or prime mover of the generators. Mechanical energy available to the prime mover of the generator plays an important role in the dynamic and transient behaviour during a fault. As the stored energy is different in gas turbine, steam turbine and hydro-turbine, the prime movers and governors behave differently. When a disturbance such as lightning strike, switching disturbance, short circuit etc. occurs the speed regulator controls the closing/opening of fuel or gas inlet valve to regulate the generator-load balance. As mechanical system is involved, this loop is very slow to respond as per the dynamics of the system. A heavy fault may result into collapse of whole system. A proper load shedding method is thus required in order to maintain the generation load balance and consequently the system stability.
A number of conventional load shedding methods has been defined which have certain drawbacks.
Breaker Interlock Scheme
ln this scheme circuit breaker(s) of the generating source(s) is(are) interlocked to a set of circuit breakers of the loads via hardwired or remote signals. The set of breaker pre-selected to trip. When the configured source breaker(s) open(s), the pre-selected loads are tripped automatically. Since there is no processing required, system is fast.
Drawbacks
1. Difficult to change the priority of load shedding.
2. Excessive load shedding
3. For an industrial power system, this unplanned outage may result into high cost of production loss, equipment damage etc.

Under frequency based load shedding
In this method under frequency relays are used which detect either frequency deterioration or df/dt and initiate tripping of interlocked breakers of the loads. This operation is accomplished in different stages. After each stage a time delay is provided to avoid nuisance tripping and allows frequency to recover. Before first stage, time delay is provided to avoid nuisance tripping due to frequency spikes in transient conditions.
Drawbacks:
1. Slow response time of frequency relays. (30-40 cycles).
2. Operational time is more as operated in different stages with time delay within.

3. Incorrect load shedding as frequency relays are designed for most severe faults.
4. High cost of modification the tripping priority.
Conventional Programmable Logic Controller-Based Load Shedding
In conventional PLC based load shedding scheme, power system is divided into different subsystems and a PLC is used for each one to shed the sequence of loads in that subsystem until the parameter(s) of the substation come to operable range of parameter(s). Load shedding is initiated by deviation of frequency or other triggering conditions. The programming of load shedding based on no. of generators available, system loading etc.
Drawbacks:
1. Decision making time is more.
2. System wide operating condition is missing which results into
insufficient/excessive load shedding.
3. Modification of the logic requires changing of the ladder-logics that are
programmed in the PLCs.

OBJECTS OF THE INVENTION:
An object of the invention is to provide a fast and generic load shedding scheme for an industrial power system with cogeneration during contingency, which is fast enough to shed the load to maintain the available generation-load gap to a minimum before any significant change in system parameters like frequency so as to maintain the stability of the system intact at the same time it considers the system configuration, operating condition automatically so that no program has to be reconfigured for any change in the system configuration and/or operating condition.
SUMMARY OF THE INVENTION: -
This invention deals with providing a fast and generic load shedding scheme for industrial power system using PLC which balances the generation and load before any change in system parameter like frequency. The path function logic finds out whether a particular bus is connected to a particular source or not by considering the breaker and isolator status of sources, tie-feeders, trafo-feeders, bus-couplers and bus-sectionalizers in a power system. Different groups of BUSes are made considering the sources to which they are connected. Different islands types are considered for each BUS depending on the isolation of source from the group. The Power to be shed logic calculates the power to be shed for an island type in order to balance the load with generation. The tripping logic initiates a trip command to the feeders according to their process priority, criticality etc. The system is such that there is no need to reprogram for any change in the power system configuration and considers all possible islanding conditions.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS:-
Fig 1 shows sld of a double bus-bar system feeded from 2 inputs Fig 2 shows different group of bus based on source connected

Fig 3 shows power to be shed calculation
Fig 4 shows flow chart for individual tripping priority logic
Fig 5 shows architecture of a load management system
Fig 6 shows legend for fig. 7 to fig.10
Fig 7(a) shows configuration-l
Fig 7(b) shows configuration-2
Fig 8(a) shows configuration-1
Fie 8(b) shows configuration-2
Fig 9(a) shows configuration-1
Fig 9(b) shows gonfiguration-2
Fig 10 shows configuration
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION:-
The invention contains 4 Parts:
1. Path function logic
2. Island initiation and grouping
3. Power to be shed calculation &
4. Tripping logic
1. PATH FUNCTION LOGIC:
Path Function is a function which tells if a particular bus is connected with a particular source or not it has two arguments: Source Name and Bus Name and can have 2 states: 0 and:
Let us assume
S= Set of Sources

B=Set of Buses in the power system network.
S and b are any arbitrary source and bus such that: s e S and b e B
Mathematically Path Function P can be defined as follows:
P(s,b)= 0 ; path does not exist from source to load
P(s,b)= 1 ; path exists from source to load
IMPLEMENTATION IN PLC
This function can be built using simple LADDER logic. The status of circuit breaker and isolator can be used as a NO (Normal open) contact in the ladder logic to make COILS for building elements of path function.
For example:
A function bus coupler and bus sectionalizer can be made such using the breaker and isolator status.
ABC1= ABC1Q1*ABC1Q0*ABC1Q2
ABS1= ABSlQl*ABS1Q0*ABS1Q2
ABC2= ABC2Q1*ABC2Q0*ABC2Q2
ABS2= ABS2Q1*ABS2Q0*ABS2Q2
Bus A1A can be connected to source 1 in the following ways.
1. A1Q0 *A1Q1 or
2. A1Q0*A1Q2*ABC1 or
3. A1Q0*A1Q2*ABS2*ABC2*ABS1
A coil (S1, A1A) is configured which can have two values: 0 or 1. Now Bus A2A can be connected to source f. in the following ways:
1. A1Q0 *A1Q2 or
2. P (S1, A1A) * ABC1 or

3. P (S1, A1A)*ABS2*ABC2 *ABS1
A coil P (S1, A2A) is configured which can have two values: 0 or 1.
Now Bus A1B can be connected to source 1 in the following ways:
1. P (S1, A1A)*ABS1
2. P (S1, A2A)*ABS2*ABC2
A coil P (S1, A1B) is configured which can have two values: 0 or 1 Now Bus A2B can be connected to source 1 in the following ways:
1. P (S1, A1B)* ABC2
2. P (S1, A2A)*ABS2
A coil P (S1, A2B) is configured which can have two values: 0 or 1
ln the similar manner for source 2 i.e. coils P (S2, A1A), P (S2, A2A), P (S2, A1B) and P (S2, A1A) can be configured.
Now for the substation B, P(S1,A1A), P(S1,A2A), P (S1,A1B) and P(S1,A2B)can be used as sources configuring the coils P(S1,B1A), P(S1,B2A), P(S1,B1B) and P(S1,B2B) for source 1 connection and P (S2,A1A), P(S2,A2A), P(S2,A1B) and P(S2,A2B) can be used as sources configuring the coils P(S2,B1A), P(S2,B2A), P(S2,B1B) and P(S2,B2B).
LOGIC FOR ISLAND INITIATION AND GROUPING
An island is a portion of network which is isolated from the main network or from the main sources.
Let us assume power system which is connected to Grid (G) and has its own in-
house generation units (S1,S2,S3 Sn} which are connected to different buses
in the power system network.
No. of groups can be considered as follows:
Group 0 Group of buses connected to Grid G

Group 1 Group of buses connected to Unit S1
Group 2 Group of buses connected to Unit S2
Group n Group of buses connected to Unit Sn
ln the similar manner n type of islands can be created as follows:
island type 0:
A bus falls under the following condition considered in island type 0:
(a) Negative edge generated in the bus for coil P (G,B) where G=Grid and B= arbitrary bus
(b) At least bus B is still connected with any other unit i.e. S1,S2……...Sn. IL0(B)=1;
Island type 1:
A bus falls under the following condition considered in island type 1:
(a) Negative edge generated in the bus for coil P (S1,8) where s1=unit s1
and B= arbitrary bus,
(b) Bus B should not connected with Grid G i.e. P(G,B)=0, and
(c)At least bus B is still connected with any other unit i.e.S2 Sn.
IL1(B)=1;
A bus falls under the following condition considered in island type 2:
(a) Negative edge generated in the bus for coil P (S2,B) where S2=Unit S2 and B= arbitrary bus,
(b) Bus B should not connected with Grid G i.e., P(G,B)=0, and
(c) At least bus B is still connected with any other unit i.e., S1, S3, S4,
S5………..Sn.
IL2(B)=1;

A bus falls under the following condition considered in island type n:
(a) Negative edge generated in the bus for coil P (Sn, B) where Sn=Unit Sn and B= arbitrary bus,
(b) Bus B should not connected with Grid G i.e., P(G,B)=0, and
At least bus B is still connected with any other unit i.e., S1, S2, S3, S4, S5……….Sn-1.
ILn(B)=1.
ISLANDING BIT GENERATION FOR FEEDERS
For each feeder of each bus a Islanding bit is configured which is defined as IL (F1). This can have two values 0 or 1.
IL(F)=0, Feeder F can be considered for load shedding.
IL(F)=1; Feeder F need not to be considered for load shedding.
Let Feedes F1, F2, F3 Fm are connected to an arbitrary bus B.
when B falls under island type 0,1,2....n, Islanding bit of feeders connected to
the bus become set or IL(F1 )=IL (F2)= = IL(Fn)=1.
POWER TO BE SHED CALCULATION LOGIC
Power to be shed is calculated as sum of power flow from one group to another e.g. Group 1 to Group 2.
Let there are two links from Group I to Group J with two breakers in each link. Active power in link 1 is p1 from Group I to Group J and in active power in link 2 is p2 from Group J to Group I. The program is coded in such a way that it identifies if breakers are in parallel or in series accordingly power in it is used.
1. IF I≠0, (Source of Group I is not Grid)


2. IF 1=0, (Source of Group I is Grid)

IMPLEMENTATION IN PLC:
Islanding is created if a circuit breaker is opened. The CT arrangement is made in such a way that:
(1) In a TIE feeder when active power flows in or Imported it is considered positive and when exported it is considered negative.
(2) In a Bus Coupler when active power flows from upper bus to lower bus (Bus 1 to Bus 2) it is considered positive and when Active power flows from Lower bus to Upper bus (Bus 2 to Bus 1) it is considered negative'
(3) In a Bus Sectionalized when active power flows from right bus to left bus (Bus lA to Bus 1B) it is considered positive and when active power flows from left bus to right bus (Bus 18 to Bus 1A) it is considered negative.
Power shedding can be calculated as follows:
For island type k, AP(B)=Active power through the Breaker B.
Power(k)=0;
(i) For Tie feeder/Trafo feeders between bus B1(up) to B2(down).

(a) Breaker B1QO or B2Q0 is opened and
P (Sk, B1)=1 and
P(Sk, B2)=0
=> Power(k)= Power(k)+ AP(B1Q0).
(b) Breaker B1Q0 or B2Q0 is opened and
P (Sk,B1)= 0 and
P (Sk,B2)=1
=> Power(k)= Power(k)- AP(B1Q0). And same for all Tie feeders/Trafo feeders. (ii) For Bus Coupler between bus lA (up) to 2A(down).
(a) Breaker BCQ0 is opened
P(Sk,1A)=1 and
P(Sk,2A)=0
=> Power(k)= Power(k)+ AP(B1Q0).
(b) Breaker BCQ0 is opened
P(Sk,1A)=0 and
P(Sk, 2A)=1
=> Power(k)= Power(k)- AP(B1Q0).
And same for all Bus Couplers.
(iii) For Bus Sectionalizer between bus 1A(right) to 1B(left).
(a) Breaker BSQO is opened
P(Sk,1A)=1 and
P(Sk,1B)=0

=> Power(k)= Power(k)+ AP(B1Q0).
(b) Breaker BSQ0 is opened
P (Sk,14)=0 and
P(Sk,1B)=1
=>Power(k)= Power(k)- AP(B1Q0).
And same for all Bus Sectionalizers.
Now Power(k)=Power to be Shed in the islanding Type k.
In a power system all the loads can be divided into different based on its criticality and based on different processes/ functions which are independent.
Let us consider a power plant consists loads of different criticality such as C1,
C2, C3,…. Cn and independent processes as P1, P2, P3,...Pm such that tripping
priority C1>C2>….>Cn and P1 > P2 > P3 > Pn.

Here N (pi,cj) indicates No. of loads under Process Pi having criticality cj. There can be two types of tripping Logics:
1. Individual load wise tripping Logic and
2. Process Group Wise Feeder logic
In individual group wise tripping logic, the loads are categorized into different
processes rather only on criticality. Thus for a power system of groups: C1, C2,
C3……Cn. Let there are N1, N2…..Nn number loads under C1, C2 Cn

respectively. user need to set the priority of loads within C1, C2, C3….Cn. If 0(n^2) time and effort required to shuffle the n loads in a group, total time or effort required is proportional to [N1^2+N2^2+...+Nn^2] + [n^2] for changing the priority one time.
In process group tripping logic, the loads are categorized into different processes as well as into different criticality. Thus for a power system of n groups: C1, C2,
C3 ……Cn. Let there are N1, N2….Nn number loads under C1, C2 Cn
respectively. User need to set the priority of loads within C1, C2, C3...Cn for each process. The effort/time required will be proportional to
[N(P1,C1)^2+N(P2,C1)^2 +N(Pn,C1)^2 + N(P1,C2)^2+N(P2,C2)+ … …
+N(P1,Cn)^2+N(P2,Cn)^2+... …+N(Pm,Cn)]+m^2+n^2. Generally, tripping priority of loads in a group change rarely, thus effort or time required will only be m^2+n^2.
Clearly effort or time required for process group wise tripping is less than those in individual load wise tripping.
The flow chart for individual load wise tripping logic is:
Here Data Blocks are made one for Circuit Breaker Close status, Trip Signal for Breaker, islanding bit for feeders.
Ci = close status from field (If close then 1 and when open then 0)
Ni= priority based tripping (lesser the number higher the tripping priority is high)
Ti= Trip status from controller to field (if trip signal is provided, it is 1, else it is 0)
Ii= Islanding bit for each feeder (if bus to which feeder is connected is islanded, value=1, otherwise 0).
COUNT =No. of feeders in the priority table.
N= priority no.

For individual tripping logic, N starts from 1 to COUNT. Each N points to the serial no. of feeder having priority N.
For process based tripping logic, Np represents process priority and Np points to a 2*C no' of fields where C is the no. of groups based on criticality. First field points to starting address of the C1 group having process priority Np. Second field points to a no. Pc representing no. of feeders in C1 group having priority Np. of the C1 group having process priority Np. Similarly, 3rd and 4th represent for starting address and Pc for C2 group and so on.
Tripping proceeds first for C1 group having priority P1, P2,...Pn followed by C2 group having priority P1, P2…..Pn and so on. For Ci group of Pj , feeders are tripped just as in individual priority logic.
EXPERIMENTAL ANALYSIS:
The power system of generally has more than 700 breakers, more than 500 isolators and more than 55 buses from voltage level 220kv to 6.6kV. This power system has different loads distributed in different locations. At 33kv system and above, it has double bus bar system. The power system has divided into 6 PLC stations viz. MRSS PLC station, BF PLC station, HSM PLC station, CP PLC station, SP PLC station and SMS PLC station, redundant CPUs are used. Each PLC station has 2 redundant CPUs and redundant interface modules (ET2OOM) at different locations connected with the D1 and DO cards. The inputs are breaker close status, breaker open status, isolator status, active power. Breaker open and close status are collected from breaker auxiliary contacts (NO and NC) which are hardwired to the D1 cards. Isolator status and Active power data are collected from SAS (substation Automation system) which collect it from relay in field through a Communication Processor which is attached to the CPU. Outputs are Trip commands for each feeders which are sent from DO card of Remotes I/O station to field. Out of the six PLC station, BF PLC has considered to be the master PLC and all the I/O data of other substation communicate with BF CPU

through another Communication Processor.
ln general, power system is very critical due to continuous requirement of power and the production loss is very high. In first phase according to FDS (by TCE) it has a requirement of 244MW, but its in house generation is very less. Thus Load Management System (LMS) is very important.
TRIGGERING CONDITION: DUBURI INCOMER 1 BREAKER OFF
POWER TO BE SHED = AP of 200/132 trafo feeder 1+ AP of 200/132 trafo feeder 1
= 50 MW +50MW=100MW
ISLAND INITIATION:
As at the following buses, other than Grid different sources like CPP1, DG, CDQ and TRT are connected prior to Islanding triggering condition i.e. opening of breaker.
MSDS 132KV BUS 1A, 2A, TB, 2B
MSDS BF 132kV BUS 1A, 2A, 1B, 2B
MSDS BF 33kV BUS 1A, 2A, 1B, 2B
CP 33kV BUS 1A, 1B
BUS 1A & 1B of BF1A, BF1P, CP1P, CPUP, CPBP
Note:
1. No islanding initiation at CH1P1 BUS 1A, 1B as its both incomes were open prior to islanding condition.
2. No islanding initiation at MSDS BUS 1C, 2C, all buses in HSM and in other buses as prior to the fault there were only grid is connected or grid is not connected.
Fig 8(b) shows Configuration2 for case 2.

TRIGGERING CONDITION: MRSS to MSDS trafo2 CB OFF
POWER TO BE SHED = AP through MRSS to MSDS trafo2=5OMW
ISLAND INITIATION:
As at the following buses, other than Grid different sources like CPP1, DG, CDQ and TRT are connected prior to Islanding triggering condition i.e. opening of breaker.
MSDS 132kV BUS 1B, 2B
MSDS BF 132Kk BUS 1B, 2B
MSDS BF 33kV BUS, 1B, 2B
CP 33kV BUS 1A, 1B
BUS 1A, 1B of CPUP, CPBP, CP1P and CH1P.
Note:
1. No islanding initiation at other buses as prior to the fault there were only grid is connected or grid is not connected.
Fig 9(a) shows Configuration1 for case 3.
TRIGGERING CONDITION: DG FDR 2 BREAKER OFF
Fig 9(b) shows Configuration2 for case 3.
POWER TO BE SHED = AP through DG FDR 2 =10MW
ISLAND INITIATION:
As at the following buses, other than Grid different sources like CPP1, DG, CDQ and TRT are connected prior to Islanding triggering condition i.e. opening of breaker.
MSDS BF 132kV BUS 1B, 2B
MSDS BF 33kV BUS, 1B, 2B

CP 33kV BUS 1A, 1B
BUS 1A, 1B of CPUP, CPBP, CP1P and CH1P.
Note:
1. No islanding initiation at buses of Substation MRSS and MSDS as it is assumed that DG power to be available for BF 132, BF 33 and CP33 substation.
2. No islanding initiation at other buses as prior to the fault there were only DG is connected or DG is not connected.
Fig 10 shows Configuration2 for case 4.
TRIGGERING CONDITION: MSDS33 BUS COUPLER MADE OFF
Here 2 type of Islanding identified simultaneously.
1. TRT based Islanding
2. CDQ based Islandin
A. TRT BASED ISLANDING:
POWER TO BE SHED = AP through DG FDR 2 =+10MW (Right to Left)
ISLAND INITIATION:
As at the following buses, other than Grid different sources like CPP1, DG, CDQ and TRT are connected prior to Islanding triggering condition i.e. opening of breaker.
MSDS BF 33kV BUS, 1B, 2B
CP 33kV BUS 1A, 1B
BUS 1A, 1B of CPUP, CPBP, CP1P and CH1P.
Note:
1. No islanding initiation at buses of Substation BF and above as it is assumed that TRT power to be available for BF 33and CP33 substation.

2. No islanding initiation at other buses as prior to the fault there were only TRT is connected or TRT is not connected.
3. Loads at CH1P and CH1P1 are not tripped as tripping priority of these loads are lesser than the tripped ones and 10 MW has already been tripped.
CDQ BASED ISLANDING:
POWER TO BE SHED = AP through DG FDR 2 = -10MW (Left to right)
ISLAND INITIATION:
As at the following buses, other than Grid different sources like CPP1, TRT, DG are connected prior to Islanding triggering condition i.e. opening of breaker.
MSDS BF 33kV BUS, 1A, 2A
BUS 1A, 1B of BF1A & BF1P
Note:
1. No tripping since power to be shed <0MW.
2. No islanding initiation at buses of substation BF and above as it is assumed that CDQ power to be available for BF33 and CP33 substation.
3. No islanding initiation at other buses as prior to the fault there were only CDQ is connected or CDQ is not connected.
ln a power system all the loads can be divided into different based on its criticality and based on different processes/ functions which are independent.
Let us say a power plant consists loads of different criticality such as C1, C2, C3…..Cn and independent processes as P1, P2,P3,...Pm Such that tripping priority C1>C2>…>Cn and P1 > P2 > P3.... > Pn.


A power plant consists of different process plants like RMHS (Raw Material Handling system), CP (coke plant), CP(sinter Plant), BF(Blast Furnace), ASU(Air Separation Unit), CMS(Steel Melting shop), HSM(Hot Strip Mill), Utility. Depending on the production plan, planned or unplanned shut down plan, breakdown, maintenance requirement, and the criticality of the processes may change with time.
Accordingly, the tripping priority is decided by the operator. Again these processes has several power feeders which classified into 3 parts: 1. Non-Critical feeders, semi-critical Feeders and critical Feeders.
Very rarely it may so happen that in a particular case of process priority of feeders of plant has to be shuffled irrespective of their process groups be for which individual load or feeder wise tripping priority logic is required.
Thus there is a logical switch made to change the selection of priority logic.
In individual tripping logic, there is only a linear tripping priority levels in which all the feeders of plant are arranged from 1 to N where N is the total no. of available feeders in the plant. The tripping will start from feeders having tripping priority 1 to N, depending on the power to be shed becomes less than or equal to zero.
In process priority based tripping logic, there are two hierarchy of priority i.e., process priority and feeders in a process priority. The priority of feeders in a process does not change frequently; the operator has only to change the process priority which takes very less time to arrange. Again feeders in a process are divided into 3 parts: Non-Critical, Semi-Critical and Critical. The tripping of

feeders are as follows:
Non-critical feeders of Process with tripping priority 1-> Non-critical feeders of Process with tripping priority 2->... -> Semi-Critical feeders of process with tripping priority 1-> Semi-Critical feeders of process with tripping priority 2->.... Critical feeders of process with tripping priority 1-> Critical feeders of process
with tripping priority 2-> so on. The tripping initiation process will be stopped
when power of the tripped feeder >= power to be shed.
ADVANTAGES:
This work can be used for designing Load Shedding Scheme for ANY type of power system.
(i) The scan time of this program is within 5 ms. Additional time can added for communication to the main controller (PLC)which is in ms and breaker operation time which is 40-60ms.
(ii) This work can be used for designing GENERIC load shedding scheme for Any type of bus configuration (single bus, double bus, one and half bus etc.), for which no program required to be reconfigured for change in configuration of power system.
(iii) This scheme designed in such a manner that the scan time for processing the scheme does not differ much with small or large number breakers, isolators, buses etc. regardless of how complex the Power network is.
(iv) There is a provision of changing the priority processes, feeders with in a process or shuffling feeders according to the priority irrespective of processes or sub groups based on criticality of feeders.
(v) This scheme takes care of any kind of triggering conditions for Island formation i.e. opening of breaker(s) of tie-feeders, trafo-feeders, bus couplers and/or bus sectionalizes etc.

WE CLAIM:
1. A selectably operable load shedding system for industrial power system which
comply with any change in the system configuration and/or power system
parameter, the increasing complexity of power system network, commissioning
of new unit generations etc. to monitor the power system continuously and to
detect and protect the power system from system disturbance by maintaining
the generation load gap at minimum, the system comprising:
- determining a path by means of path function to analyze at least a bus coupled to a source;
- forming at least an island that includes a grid that comprises of plurality of generation unit connected to plurality of bus of a power system;
- computing the power flow from one assigned group to another group;
- tripping load after computation of the power flow to maintain power system stability;
wherein all the parameters of the power system are measured and a desired load shedding scheme is carried out depending on the obtained values.
2. The selectably operable load shedding system as claimed in claim 1, wherein the path function logic may be built by ladder logic.
3. The selectably operable load shedding system as claimed in claim 1, wherein the value of path function determines if path exists from source to load.
4. The selectably operable load shedding system as claimed in claim 1, wherein the island is a segment of network isolated from the main network.
5. The selectably operable load shedding system as claimed in claim 1, wherein the island is formed depending on circuit breaker condition.

6. The selectably operable load shedding system as claimed in claim 1, wherein the tripping logic may be of individual load wise tripping logic or process group wise feeder logic.
7. The selectably operable load shedding system as claimed in claim 1 and 7, wherein in the individual load wise tripping logic, loads are categorized on basis of criticality.
8. The selectably operable load shedding system as claimed in claim 1 and 6, wherein in the process group wise feeder logic load is categorized on basis of processes.
9. The selectably operable load shedding system as claimed in claim 1, wherein as illustrated in the accompanying drawings.