Optimal Load Scheduling

Submitted by: Submitted to: Mayank Sharma(10EJCEE032) Mr. S.N. Jhanwar VIIIth Sem. H.O.D.(EE Dept.) J.E.C.R.C.,Jaipur

Introduction:The electrical load schedule is an estimate of the

instantaneous electrical loads operating in a facility, in terms of active, reactive and apparent power (measured in kW, kVAR and kVA respectively).

The load schedule is usually categorised by switchboard or occasionally by sub-facility / area.

A key feature of this divisible load distribution scheduling theory (known as DLT) is that it uses a linear mathematical model.

Need Of load Scheduling:Essential for some of the key electrical design activities

(such as equipment sizing and power system studies)It provides the preliminary details of process / building /

Organisation Load.

The electrical load schedule can typically be started with a preliminary key single line diagram (or at least an idea of the main voltage levels in the system)

Procedure to calculate the load Scheduling: Step 1: Collect a list of the expected electrical loads in the

facility Step 2: For each load, collect the electrical parameters, e.g.

nominal / absorbed ratings, power factor, efficiency, etc Step 3: Classify each of the loads in terms of switchboard

location, load duty and load criticality Step 4: For each load, calculate the expected consumed load Step 5: For each switchboard and the overall system,

calculate operating, peak and design load

Step 1: Collect list of loadsProcess loads - are the loads that are directly relevant to

the facility.

Example-Motors, Heaters, Compressors, Conveyors.• Non-process loads - are the auxiliary loads that are

necessary to run the facility.• Example-lighting, utility systems (power and water),

DCS/PLC control systems, fire safety systems

Step 2: Collect electrical load parametersRated power is the full load or nameplate rating of the load and

represents the maximum continuous power output of the load. For motor loads, the rated power corresponds to the standard motor size (e.g. 11kW, 37kW, 75kW, etc).

Absorbed power is the expected power that will be drawn by the load.

Power factor of the load is necessary to determine the reactive components of the load schedule. Typically 0.85 for motor loads >7.5kW, 1.0 for heater loads and 0.8 for all other loads).

Efficiency accounts for the losses incurred when converting electrical energy to mechanical energy. Typically 0.85 or 0.9 is used when efficiencies are unknown.

Step 3: Classify the loadsVoltage Level :What voltage level and which switchboard

should the load be located?

Loads <150kW-LV System (400V - 690V)

150KW<Load<10 MW- MV System (3.3kV - 6.6kV)

Loads >10MW-HV Distribution System (11kV - 33kV)Load duty-

Continuous loads -are those that normally operate continuously over a 24 hour period.eg. process loads, control systems, lighting and small power distribution boards, UPS systems.

Step 3(2): Intermittent loads -only operate a fraction of a 24 hour

period, e.g. intermittent pumps and process loads, automatic doors and gates.

Standby loads -are those that are on standby or rarely operate under normal conditions, e.g. standby loads, emergency systems.

Load criticality- Normal loads-run under normal operating conditions. Essential loads are those necessary under emergency

conditions, when the main power supply is disconnected and the system is being supported by an emergency generator, e.g. emergency lighting, key process loads that operate during emergency conditions, fire and safety systems

Critical Loads-are those critical for the operation of safety systems and normally supplied through a U.P.S. Battery.eg. Escape lightning .

Step 4: Calculate consumed loadThe consumed load is the quantity of electrical

power that the load is expected to consume. For each load, calculate the consumed active and reactive loading, derived as follows:

;

Step 5: Calculate operating, peak and design loadsOperating load -The operating load is the

expected load during normal operation.

Peak load -The peak load is the expected maximum load during normal operation.

Step 5(2):Design load -The design load is the load to be

used for the design for equipment sizing, electrical studies.

or

Parts of Load Scheduling:

Coordination(Yearly,Monthly Or Weekly)

Unit Commitment (Weekly Or

Daily)

Economic Load Dispatch (Hourly)

Hydrothermal Coordination problem:It is the first stage in the solution of the

hydrothermal generation scheduling problem. The HCP consists of determining the optimal amounts of hydro and thermal generation to be used during a scheduling period .The HCP is also decomposed in three Parts. depending on the reservoirs storage capacity.

1.Long Term 2.Mid Term 3.Short Term

Unit Commitment-The electrical unit commitment problem is

the problem of deciding which electricity generating units should be running in each period so as to satisfy predictibly varying demand of electricity.

Load of power system varies through out of the demand reaches a different peak value from one day to another. so which generator to start up and the sequence in which units should be operate and for how long.The computational procedure for making such decision is called unit commitment

Economic Load DispatchIn power generation our main aim is to

generate the required amount of power with minimum cost.

Economic load dispatch means that the generator’s real and reactive power are allowed to vary within certain limits so as to meet a particular load demand with minimum fuel cost

This allocation of loads are based on some constraints.

DIFFERENT CONSTRAINTS IN ECONOMIC LOAD DISPATCH

INEQUALITY CONSTRAINTS Voltage constraints Vmin ≤ V ≤ Vmax , δmin ≤ δ ≤ δmax Generator constraints KVA loading of generator should not exceed

prescribed value Pmin ≤ P ≤ Pmax Qmin ≤ Q ≤ Qmax

Running spare capacity constraints This constraints are needed to meet forced

outage of one or more alternators in the system and also unexpected load on the system

Transmission line constraints flow of power through transmission line

should less than its thermal capacity Transformer tap set

for autotransformer tap t should between 0 & 1 For two winding transformer – between 0& k

Equality constraints Real power Pp= Vp Σ Ypq Vq cos(θpq-(δp+δq)) Reactive power Qp= Vp Σ Ypq Vq sin(θpq-(δp+δq))

OPERATING COST OF THERMAL PLANT

The factors influencing power generation at minimum cost are operating efficiencies of generators, fuel cost, and transmission losses.

The most efficient generator in the system does not guarantee minimum cost as it may be located in an area where fuel cost is high.

If the plant is located far from the load center, transmission losses may be considerably higher and hence the plant may be overly uneconomical.

The input to the thermal plant is generally measured in Btu/h, and the output is measured in MW

In all practical cases, the fuel cost of generator can be represented as a quadratic function of real power generation

a) Heat rate curve b) Fuel cost curve

• By plotting the derivative of the fuel-cost curve versus the real power we get the incremental fuel-cost curve

Incremental fuel-cost curve

The incremental fuel-cost curve is a measure of how costly it will be to produce the next increment of power.

ECONOMIC DISPATCH NEGLECTING LOSSES

It is the simplest economic dispatch problem Assume that the system is only one bus with

all generation and loads connected to itA cost function Ci is assumed to be known for

each plant

The problem is to find the real power generation for each plant such that the objective function (i.e., total production cost) as defined by the equation

Is minimum ,subjected to the constraints

when losses are neglected with no generator limits, for most economic operation. all plants must operate at equal incremental production cost

Production from each plant can be found by

This equation is known as the coordination equationFor analytic solution we can find λ by

REFERENCESPower System Analysis - Hadi SaadatPower system Analysis - Nagrath and KothariOpenelectrical.org/load scheduling

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