intelligent load shedding scheme

1. ABSTRACTConventional methods of system load shedding are too slow and do not effectively calculate the correct

amount of load to be shed. This results in either excessive or insufficient load reduction. In recent years,

load shedding systems have been repackaged using conventional under-frequency relay and/or breaker

interlocks schemes integrated with programmable logic controllers to give a new look to an antiquated

load preservation methodology. A truly modern and intelligent load shedding system with a computerized

power management system should provide fast and optimal load management by utilizing system

topology and actual operating conditions tempered with knowledge of past system disturbances. This

paper demonstrates the need for a modern load shedding scheme and introduces the new technology of

intelligent load shedding. Comparisons of intelligent load shedding with conventional load shedding

methods are made from perspectives of system design, system engineering, project implementation, and

system operation.

Intelligent Load Shedding System

2. INTRODUCTIONLoad shedding can be defined as the amount of load that must almost instantly be removed from a power

system to keep the remaining portion of the system operational. This load reduction is in response to a

system disturbance (and consequent possible additional disturbances) that results in a generation

deficiency condition. Common disturbances that can cause this condition to occur include faults, loss of

generation, switching errors, lightning strikes, generation-load mismatch etc. When a power system is

exposed to a disturbance, its dynamics and transient responses are mainly controlled through two major

dynamic loops. One is the excitation (including AVR) loop that will control the generator reactive power

and system voltage. Another is the prime-mover loop, which will control the generator active power and

system

The frequency conditions of the overall system will directly depend on the amount of active power that

the generator prime movers could deliver to the system. For gradual increases in load, or sudden but mild

overloads, unit governors will sense speed change and increase power input to the generator. Extra load is

handled by the unused capacity of all available generators operating and synchronized to the system. If all

generators are operating at maximum capacity, the spinning reserve is zero, and the governors may be

powerless to relieve overloads.

Sudden and large changes in generation capacity through the loss of a generator or main inter-tie

impacts the dynamic response of the prime mover and can produce severe generation and load imbalance,

resulting in rapid frequency decline. For some switching disturbances (that results in a loss of generation

or system islanding condition), the cascading effects may be of the primary concern if the load shedding

action is not set and timed correctly.

Gas turbines are very sensitive to critical speeds affecting their low pressure blades. These critical

speeds may be close to the rated operating speed leaving a small margin on the allowed frequency range

before reaching a protective trip. Typically a protective instantaneous low-speed trip on gas turbines may

be set at 96% of the nominal system frequency.

Furthermore, system generation and stability are at risk as the frequency drops. This is specially the

case for a thermal generation plant where power output mostly depends on motor-driven auxiliary loads,

such as boiler feed water pumps, coal pulverizing, and draft fans. The drop in system frequency instigates

a rapid fall of power output to the auxiliary loads, causing further reduction of the energy input to the

turbine generator. This sequence of events further deteriorates the system frequency endangering the

entire plant stability.


To halt the drop in frequency, it is necessary to intentionally, and automatically disconnect a portion of

the load equal to or greater than the generation deficiency in order to achieve balanced power economics

while maintaining system stability.

3. CONVENTIONAL LOAD SHEDDING Breaker interlock load shedding

Under frequency load shedding

PLC based load shedding

3.1 Breaker interlock load shedding

This is the simplest method of carrying out load shedding. For this scheme, the circuit breaker

interdependencies are arranged to operate based on hardwired trip signals from an intertie circuit breaker

or a generator trip. This method is often used when the speed of the load shedding is critical. Even

though, the execution of this scheme is fast, breaker interlock load shedding possesses a number of

inherent drawbacks:

Load shedding based on worst-case scenario

Only one stage of load shedding

Almost always, more load is shed than required

Modifications to the system are costly


3.2 Under frequency load shedding In this a fixed amount of load is removed at a fixed system frequency level. Upon reaching the

frequency set point and expiration of pre-specified time delay, the frequency relay trips one or more load

breakers. This cycle is repeated until the system frequency is recovered, e.g., 10% load reduction for

every 0.5% frequency reduction. Since this method of load shedding can be totally independent of the

system dynamics, total loss of the system is an assumed possibility. Additional drawbacks of this scheme

are described below.

Slow Response Time

Incorrect / Excessive Load Shedding

Analysis Knowledge Is Always Lost


3.3 PLC based load shedding 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. This static sequence is continued until the frequency returns to a normal, stable level.

A PLC-based load shedding scheme offers many advantages such as the use of a distributed network

via the power management system, as well as an automated means of load relief.

Drawbacks of this system are

• Monitoring of power system is limited to portion of network


• system wide operating condition is missing from decision making process result in incorrect load

shedding

• Priority load list doesn’t take into account the dynamic behavior of the system

4. INTELLIGENT LOAD SHEDDINGIntelligent Load Shedding (ILS) provides optimal, fast load shedding for electrical disturbances and loss

of generation. ILS calculates the minimum required MW to be shed for each subsystem according to the

type and location of the disturbance, actual operating generation, spin reserve, loading, configuration,

load distribution, and priority. ILS then selects the best combination of loads (CBs) that will satisfy this

requirement. All of this is executed in less than 100 ms after a disturbance occurs in the system.

4.1 Need of ILS system


Due to the inherent drawbacks of existing load shedding methods, an intelligent load shedding system is

necessary to improve the response time, accurately predict the system frequency decay, and make a fast,

optimum, and reliable load shedding decision. This system must have the following capabilities:

Able to map a very complex and nonlinear power system with a limited number of data collection

points to a finite space.

Automatically remember the system configuration, operation conditions as load is added or

removed, and the system response to disturbances with all of the system configurations.

Recognize different system patterns in order to predict system response for different disturbances.

Utilize a built-in knowledge base trainable by user defined cases.

Adaptive self-learning and automatic training of system knowledge base due to system changes.

Make fast, correct, and reliable decisions on load shedding priority based on the actual loading

status of each breaker.

Shed the minimum amount of load to maintain system stability and nominal frequency.

Shed the optimal combinations of load breakers with complete knowledge of system

dependencies.

4.2 Requirements of ILS system Pre-disturbance operating condition Total system load demand

Total system power exchange to the grid

Generation of each on-site unit

Spinning reserve for each on-site unit

Control settings for each running unit

Settings and loading conditions for all major rotating machines

System configurations

Post-disturbance operating condition New system load demand

Remaining generation from on-site generation

Spinning reserve for each remaining unit

Time duration to bring up the spinning reserve

New system configurations

Status of each sheddable load

Nature and duration of disturbance


Electrical and/or Mechanical faults

Complete or partial loss of power grid connection

Complete or partial loss of on-site generation

Load addition (impact)

Location of disturbance

Duration of disturbance and its termination (selfclearance,fault isolation, protection device

tripping, etc.)

Subsequent system disturbances

System transient response due to disturbance System frequency response (decay, rate-of change, final frequency)

System voltage response

Rotor angle stability of each remaining unit

Operation of protective devices

4.3 Block diagram of ILS system


Knowledge Base- utilizes carefully selected input and output data under different cases, based on

off-line system studies and simulations. System dynamic responses including frequency variation are

amongst the outputs of the knowledgebase.

Advanced Monitoring- constantly surveys the system operating condition changes, and calculates

tie-line flows, on-site generations, transformers and feeder loading as well as evaluates status of the

sheddable loads.

Network Models- contain system topology, connection information, and electrical properties of

system components.

Trigger List- is compiled based on pre-specified system disturbance types.

Load Shed Optimizer- computes optimal load shedding tables corresponding to system changes.

Distributed Controls utilize- PLCs to rapidly execute the load shedding actions based on

detection of disturbance triggers from the system.

4.4 Implementation configuration of ILS system


• ILS knowledge base and computation engine reside in an ILS server computer

• The server interfaces with an advanced real-time power system monitoring and acquires real time

system data

• It calculates and updates the load shedding table and sends it to PLC’s

• PLC’s then initiate the load shedding and circuit breakers are tripped

4.5 Benefits of ILS system

• Time-variant load shedding tables, which reflect true status, and loading conditions for the

sheddable loads.

• Optimal combination of sheddable loads to maximize load preservation.

• Fast response to disturbance triggers (less than100 ms in most cases).

• Environment to accelerate operator training with the ability to simulate and validate load shed

decisions.

• Reduction of spinning reserve requirement

5. COMPARISON


5.1 Comparison of ILS with frequency relay load shedding

Frequency relay load shedding will be delayed until the system frequency drops below the relay set point

(Stage1). Additional load shedding will be needed if the system frequency does not recover to normal

(Stage 2). Thus the total response time for the frequency relay based load shedding is much longer than

ILS system.

ILS requires only one load shedding stage and has a much faster response time (less than 100 ms in

most cases).

5.2 Comparison of ILS with PLC based load shedding


A comparison of ILS system response time with that of PLC-based load shedding is illustrated in Fig. 7.

The PLC based load shedding will take longer time to respond to the fault due to lack overall system

topology, calculation time, and time delays associated with frequency relays.

6. CONCLUSIONIntelligent Load Shedding System

Load shedding serves as the ultimate guard that protects the power system from a disturbance-induced

collapse. Normally, this critical load preservation is done with the use of under-frequency relaying and

PLC-based schemes. Common drawbacks of these schemes include the lack of detailed system operating

information such as pre- and post-disturbance data, system topology and configuration, generation and

load distribution, type of disturbances, duration of the disturbances, and other pertinent information. This

paper has introduced an intelligent, optimal, and fast load shedding technology referred to as ILS. ILS

combines online data, equipment ratings, user-defined control logics, and a knowledgebase obtained from

power system simulation studies, to continually update dynamic load shed tables. This system can

perform optimal load shedding in less than 100 milliseconds from the initial occurrence of a disturbance.

ILS technology has been successfully installed and operational at several industrial facilities.


7. REFERENCES Farrokh Shokooh1, Fellow IEEE, J J Dai1, M. IEEE, Shervin Shokooh1, M. IEEE, Jacques

Tastet2, M. IEEE, Hugo Castro1, M. IEEE, Tanuj Khandelwal1, M. IEEE, Gary Donner1, Fellow

IEEE “An Intelligent Load Shedding (ILS) System Application in a Large Industrial

Facility” page no.417-425

Shervin Shokooh ,Tanuj Khandelwal ,Dr. Farrokh Shokooh, Jacques Tastet , Dr. JJ Dai

“Intelligent Load Shedding Need for a Fast and Optimal Solution” IEEE PCIC Europe 2005

D Andersson, P. Elmersson, A. Juntti, Z. Gajic, D. Karlsson and L. Fab “Intelligent Load

Shedding to Counteract Power System Instability” page no.570-574


intelligent load shedding scheme

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