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)

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.

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.

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.

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

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.