Sample Load Shedding System Requirement of load Shedding System The load shedding system is the one which has to ensure the availability of electrical power to all essential and most critical loads in the plant. This is achieved by switching off non-essential non-es sential loads in case of a lack of power in the plant electrical electri cal network, or parts of the refinery electrical network. A lack of electrical power can be caused caused by loss loss of generat generation ion capaci capacity ty or discon disconnec nectio tion n from from the Public Public Power Power Company (PPC supply. The load shedding system has to be able to cope with the above mentioned conditions. The re!uirements of a lower priority that have to be fulfilled by the load shedding system are" #oad shedding should disturb the production process as little as possible, shed only the amount of power that is really necessary. The load shedding system should not operate if it is not re!uired by the situation in the electrical network. The system should be reliable and the number of spurious operations should be limited during the lifetime of the system. The load shedding system can use the under-fre!uency shedding merely as a backup system to the primary load shedding system. The primary load shedding system continuously calculates the energy balance in the network by subtracting the electrical loads in the network from the available electrical power. As soon as a lack is predicted, the load shedding starts operating and sheds the surplus in the demand for power. The operating time from opening a circuit and read in by a digital input in any of the Programmable Controllers to giving the shed command at a particular output in any of the Programmable Controllers is ma$imum %&& msec.
Allocation of load shed functions 'ain input to the load shedding system is the register received from another part of software, called Configuration etermination that determines the configuration of the electrical network. The configuration is determined from the checked and approved circuit breaker positions (open and closed positions are checked against each together with the power flow, if available. )econdly load shedding needs data from the load flow in the network. ata regarding the loads on the buses and the power generated by each generator is sent to the central #oad shedding by analog inputs and software has already checked the validity of the data. #oad shedding uses data representing the load flow in the network which are at least % seconds *old*. This is done to prevent load shedding from using data which which was obtained while the network was already in a faulty and thus unstable condition.
The third group of data needed by load shedding is the operator input. The operator has to define for every load shed group the priority and whether a group is inhibited or not. This data is used by the load shedding system to calculate the dynamic load shed tables.
Primary load shedding The following main functions are included in the primary load shedding system" Continuously check changes in the configurations Continuously check the energy balance in every configuration Calculate the dynamic shed tables +enerate shed command when needed Change loads andor priorities nform and guide the operators
Contingency based logic for generator tripping The load shedding system continuously checks whether changes in the configuration have occurred. As soon as a change occurs, for e$ample, tripping of a generator, load shedding starts checking all the individual configurations. As long as a configuration is connected to the PPC supply, there is always enough power available in that particular configuration, so that this reserve power can be used by the system for a particular period of time. f the time period e$ceeds the set value, based on network configuration, loads are identified in the island connected to the grid and are in /01 status and the #oad )hedding is activated. 2or every configuration that is not connected to PPC the energy balance has to be calculated. f the load in this configuration e$ceeds the available generated power then it is necessary to shed the surplus of loads. As mentioned before, data used for this calculation is at least % seconds old. 2or every configuration, which is momentarily in a steady state condition, but which is not connected to PPC, the load shedding system has to monitor continuously the power available from the generators in that particular configuration, since losing a generator or losing part of the capacity of a generator might cause the need for load shedding. After the calculation of real time shortage in power , an additional 3&4 is added to the calculated power shortage ,and the load shedding is based on this final value.f suppose there is a power shortage of 35.6 'w ,because of a generator tripping , loads e!uivalent to 37.86 'w ( 35.6 'w $ 3.3 must be shed. Then , the reserve power available in other running generators are summed up and subtracted from the calculated power shortage. The resultant is the actual amount of load that has to be shed . f the total reserve power available in other generators is 33.95 'w, then the re!uired amount of loads that must be shed is 5.&8 'w. As soon as load shedding is started due to a change in the configuration of the electrical network, not meaning that a real shed command is generated, the system starts calculating the dynamic shed tables. nput for this calculation is the data in the configuration and the load tables for the several buses. 2or every bus on which there are shedable loads such a load table is assembled.
The load table consists of the name andor number of the load shed group, the load in that particular group (dynamic, the priority and the status of that particular load shed group. The load table for every bus is assembled in order of priority. Calculation of the load table is done in the background. 2rom this load table an accumulated load table per bus is obtained. As soon as load shedding is started, an accumulated load table for the whole configuration is calculated. f the result of the energy balance calculation leads to the conclusion that load shedding is needed, the amount of power to be shed is also obtained from this energy balance calculation. This amount is now compared with the contents of the accumulated load table for the configuration. The shed command is generated and sent to the unit where the loads are actually connected. :ecovery of the network after load shedding has to be done manually by the operator . There are no features offered for automatic restoration of the network included in the load shedding system.
Frequency / back-up load shedding This function is more or less the backup for the primary fast load shedding. ;hen fast load shedding fails due to wrong inputs this function will shed the loads. 2re!uency shedding not only takes the absolute fre!uency limits into account but also calculates the dfdt< this gives a more accurate load shedding. The fre!uency limits are read in from fre!uency relays by digital inputs in the Programmable Controller.
Loadshedding under arious conditions The )cope of =igh-speed #oadshedding is limited to selective load shedding scheme for reliable tripping of non-critical loads of all the 39 )ubstations in the >ser configurable Priority Table. The Priority Table will be site alterable by any of the >ser of the )CAA )ystem. Condition !" Contingency #ased load shedding $ith %&'# Po$er
#oss of nternal generation ?ut the )ystem is connected to the grid 7.7 @. n this case the #oad )hedding )ystem will calculate the 'a$imum capacity of the available generators based on the predefined ata furnished for various generators and Transformers. The )CAA system will be having user assignable figures for Contract emand, 'a$imum Capacity of Bach +enerator and Transformers. The )ystem will also calculate the rawl of power in the with the >ser )ettable time nterval of 3 7& 'inutes in the ncrements of 3 'inute interval. f there is no spinning reserve available in the +eneration of all the +enerators as well as if the contract demand with T0B? e$ceeds for the >ser defined Time nterval then this type of #oad shedding will come into action. The )ystem will raise an Alarm condition while reaching the D54 of the Contract demand. The )ystem will calculate the Total eficit after taking into the Consideration the 'a$. Contract demand with T0B? as well as the )pinning :eserve Capacity of all the +enerators and will shed only e$actly whatever will be the total deficit of the load by using the Priority table
There will be one table provided in the )CAA =' which will allow user customer operator to configure define all the +enerator and Transformer1s capacity in terms of ';. /nce the user has entered all the alues and is ready to download the same into P#C, =e has to do it manually with the =elp of ='. The )ystem will ask for following details in the table C/0T+B0CE ?A)B #/+C 2/: +B0B:AT/: T:PP0+"Tripping of any +enerator ?reaker
Current 'a$ emand G Contract emand with
Time delay more than set value, Calculate e$cess '. B$cess ' F ' Contract emand
Based on network configuration identify the loads in the island connected to the grid and are in ‘ON’ status
Trip the identified loads as per priority, to the e$tent of calculated e$cess '. >se the Priority Table for the same.
+rid Power Available
Calculate the load fed by the +enerator 5 seconds prior to tripping +enerator ?reaker
Add 3&4 and calculate the total shortage (A
Calculate the reserve capacity available in other running generators (?
Calculate the load to be shed (A-?
Based on network configuration identify the loads in the island connected to the tripped generator and are in ‘ON’ status
Trip the identified loads as per priority, to the e$tent of load calculated to be shed
C/0T+B0CE ?A)B #/+C 2/: TB T:A0)2/:'B: 2BBB: T:PP0+
Tripping of any Tie 2eeder Transformer ?reaker in network
Calculate the load fed by the tripped breaker 5 seconds prior to tripping
Add 3&4 and calculate the total shortage (A
Based on Based on network network configuration configuration identify the loads in the island identify the island formed due to Calculate Trip the identified the Calculate reserve loads the capacity as per formed due to tripping that are tripping and identify the otherintie priority, available to inload the above e$tent to be identified of load ‘ON’ status feeders/ generators /shed transformers feeders (? calculated (A-? be shed feeding theto island
Condition 2: Contingency Based load shedding without TNEB Power #oss of nternal generation and the )ystem isolated from the grid 7.7 @.
n this case, the system will check any status change in any one of the Critical ?reakers ()uch as all the +enerator transformers ?reakers, ?Cs ?reakers, TB ?reakers, CT ?reakers. )ystem will check the eficit of the total system as well as on all the slands #oad buses that are separated from the total grid due to the Tripping of any of the TB ?reakers or CT ?reakers tripping and as there is no e$ternal source of supply such as T0B?, )ystem will take into Consideration the #oad ?alance at each of the island or for the total system and will go on shedding the load for the Particular subgroup island of by taking into consideration respective island priority table feeders or will shed the load for a total system when there will not be any island formed till the time #oad balance is achieved in the system )ystem shall always monitor the current plant loading (+eneration or +rid )upply at the instant of the trip. :eadings of Total plant +eneration and load consumption shall be continuously monitored by the system. Condition 3: Slow Load Shedding (Transformer Overload Load shedding)
The )low load shedding will be initiated in case of overload of a 33& @ 7.7 @ grid transformers T: A, ?, C, , B, 2 and +, nterconnecting Transformers (CT 3, %, 6 and H reactors in CPP system e$ceeds their rated capacity. The rated capacity of each transformer is mentioned in table !()( The Capacity can be changed as when re!uired by )CAA /perator. This type of #oad shedding will be initiated when actual loading of above e!uipments e$ceeds the rated capacity in either direction. n this case the #oad fed by those transformers reactors in the direction of power flow need to be shed as per the Priority of particular subgroup. #/A )=B0+ >B T/ T:A0)2/:'B: /B:#/A0+ ()#/; #/A )=B0+
Transformer is overloaded
Calculate e$cess loading
s Contingency based load shedding triggered
0o Identify the loads in the island to which transformer is connected that are in ‘ON’ status
Condition )" *anual Load Shedding +*anual control of
Trip the identified loads as per priority, till the load reduces below set value. breakers,
/perator will be allowed to trip any of the outgoing feeders at his own choice with )elect before B$ecute1 feature. Also the groups of 2eeders can be defined to shed the load in larger blocks. Condition " .nder frequency Load shedding +#ack-up protection to PLC based
2or all the 33 load buses, bus PT signal will be provided to the H-stage under fre!uency relay. n the event of the 2ailure of the P#C ?ased #oad shedding system, under- fre!uency load shedding will be initiated. The >nder fre!uency relay will continuously monitor the fre!uency of all 33 load buses. n the event when due to sudden loss of generation the relay will generate the output contact based on the rate of change of fre!uency and the base fre!uency data. The relay will be provided with H )tages of Configuration of >nder fre!uency stages interlocked with dfdt values. ?y using this H contacts at various stages of under fre!uency we can trip load connected to the system in H different groups connected to that bus where there will be an actual power deficit. FR'.'&C0 #AS'1 #AC2 .P L3A1 S4'115&6
>nder fre!uency relay acted in any of the load buses
fdt G set value I time delay for grid islanding
s Contingency based load shedding triggered
0o
dentify the loads in the bus having under fre!uency that are in /01 status
Trip the identified loads as per priority as per the )tages that are being operated in the system.
