working of an energy management system
DESCRIPTION
Energy management system explainedTRANSCRIPT
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EMS-LECTURE 2: WORKING OF EMS
Introduction:
Energy Management systems consists of several applications programs which are used by the
operator in a control centre for effective decision making in the operation and control of a power
system. These applications programs are specific programs which monitor and calculate the
operating states of the power system. Examples of these programs are Load forecasting, State
estimation, Security Assessment, contingency analysis, optimal power flow etc.
All of these programs are networked together in such a way the output of one program is the
input to another program. This continues till all the programs from a closed loop. The output of
each program provides a result which can be used by the operator for decision making for
controlling the power system.
The working of an EMS is divided into two categories, namely real-time or On-Line or of-
line corresponding to the problem being addressed and the application it is handling.
a. Real time or online applications:
b. Extended real time of Off line applications
Real time or online applications:
These programs are usually those program which require real- time attention like control and
protection functions. For example the following applications can be classified under this
category
Control functions:
i) Load Frequency Control (LFC)
ii) Automatic Generation Control (AGC)
iii) Reactive Power Control (RPC)
iv) Voltage Control (VC)
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v) Reactive Power and Voltage Control (QVC)
Protection Functions:
i) Protective relaying
ii) Primary protection
iii) Secondary protection or backup protection
The protective functions are the first which are activated in a real time operation as a protective
measure. These are followed by essential control functions like LFC, AGC, RPC, VC, RPC,
QVC, etc..
Fig 2.1. On-Line and Off Line functions in an EMS
Figure 2.1 shows the here main important entities of power systems, EMS and SCADA. EMS
and SCADA are two important entities in the real time monitoring, operation control of power
systems. Figure 1(b) shows the information flow between the three modules. Power flows from
Power Systems through SCADA to EMS. SCAD forms the interface between Power Systems
and EMS.
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Fig 2.2 On-Line and Off Line functions in an EMS
Figure 2.2 describes the three main classifications about the operation, namely
i) On-line closed loop control
ii) On-line open loop control
iii) Off line study or simulation model
On-line closed loop controls are those which are performed without the presence of the operator
like LFC, AGC, etc.
On-line closed loop controls are those which are performed with the presence of the operator for
decision making like Operation and Dispatch. Examples are
i) Real power dispatch or active power dispatch (APD)
ii) Reactive Power Dispatch (RPD)
iii) Active and Reactive Power Dispatch
These may or may not involve economics of operation. The main concern is the matching of
power generation to the load requirement, which is termed as essential load balance criterion.
This ensures that the load requirement is met, which is also termed as the generation load
following.
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The following are the important functions which are carried out based on the real time functions
Measurements: Measurements of important parameters from the remote terminal unit (RTU)
like three phase voltage, current, power factor, power flows, etc.
Monitoring: Real time monitoring of the above parameters at frequent intervals of time. These
values are continuously monitored to check for any significant deviations from normal values
Control Actions: Important control functions or actions like preventive, corrective,
emergency , restorative, etc.
Network Topology Processing: Building a network model based on real time measurements.
State Estimator: Determining the best estimate from real time measurements.
Power Flow: Load flow analysis or Determining voltages and phase angles.
Security Assessment: Determining the secure or insecure state of a system.
Contingency Analysis: Impact of a set of contingencies to identify harmful ones.
Load flow analysis, Determining voltages and phase angles.
Optimal Power Flow: Optimization of a specified objective function with constraints.
Short Circuit Analysis: Determination of fault currents for various faults on the network
The above role and functions in an EMS are illustrated in figs 2.3 and 2.4. Fig 2.3 shows the
measurement of Voltage and Current, etc through Current Transformer (CT) and Power
Transformer (PT) or through remote terminal Units (RTU) or Phasor Measurement units (PMU).
These are inputs to the EMS of the control centre. Fig 2.4 illustrates the various EMS functions
in an EMS. State Estimation forms the first and the foremost function that is executed based on
SCADA measurements. The output of state Estimation is used by many other programs like On-
line Load Flow, Contingency Analysis, stability analysis and security assessment, etc.
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Fig 2.3 Measurement and monitoring Fig 2.4 Energy Management Systems functions
Fig 2.5 On-Line and Off Line functions in an EMS
The framework for computer aids for decision making in control centre for real time and
extended real time control is shown in Fig 2.5. It can be observed that both real time data and
predictive data are in an EMS.
The real time data functions are used for important EMS functions like topological analysis and
state estimation. Security analysis forms the important study which used real time information.
The output of state estimation is used along with the output network topology processor. The role
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of network topology processor is to update the status of circuit breakers and in the real time
model in the simulation study. The result of security analysis is used for dispatch and
optimization functions like generation dispatch and voltage and reactive power optimization. The
predictive data is used to perform other functions like load forecasting and generation
scheduling. These are used for interactive load flow and security analysis and short circuit
calculation.
Fig 2.6. Overall SCADA and EMS functions in an control centre.
Fig 2.6 shows the overall framework of SCADA and EMS functions in a control centre. The
SCADA received information for RTUs. It can be observed that the output of SCADA goes
simultaneously to various network analysis functions like i) state estimation, ii) on-line load
flow, iii) stability analysis and iv) short circuit analysis, etc. The outputs of these modules are
given to the operator for decision making.
Summary:
The working of EMS and the modules are described in section. On line and Off line functions in
an EMS are described in detail.