State estimation of Power System

State Estimation:

It is a process where the state of the power system is estimated.All the telemetry values of power flow, voltage levels are considered.These values are used to calculate the phase angle of the voltage which helps in determination of the power flow between two nodes in the power system as the power flow is directly proportional to sine of that angle.

The power flow monitoring is important for the power system as any kind of overloading on the lines can lead to cascade tripping and can also lead to black out.

 For the accurate state estimation the data received from the field in SCADA system should be accurate so that computer can build the topology of the network and can give accurate snapshot of power system.

Data received in the SCADA has various flags

1. GOOD (G) : - Data received is accurate
2. SUSPECT (S) : - Data received is not correct due to communication problem
3. MANUAL (M) : - Status or Data is set manually by operator

Data received by SCADA is of two types:

1. Analog Data

• Analog data contains the active and reactive power flow on the lines, voltages of the buses
• This data is received from the meters installed at the field
• This analog data is mapped to a particular physical address at substation.For avoiding the mismatch the mapping should be done properly.
• It declares analog data GOOD(G flag) or SUSPECT(S flag) according to the connectivity and reception of data from the field
• According to the flag status, the weightage of each data is decided and then it is inserted in the system for calculation
• E.g. Low weightage is given for the data which is having S flag
• The Direction of the power flow is also important.

2. Digital Data

• It contains the status of circuit breakers and isolators.
• There are 4 possibilities defined for status of any equipment:
  CLOSE - The breaker or Isolator is close
  OPEN - The breaker or Isolator is open
  FAULTY - The breaker or Isolator is faulty
  TRANSIT - The transition in breaker status is taking place. E.g. CLOSE to OPEN or OPEN to CLOSE
• This 4 status are received by the combination of 2 bits (00, 01, 10, and 11) which is configurable.
• This data is received from the relays installed at the field
• This data is mapped to a particular input point at substation.For avoiding the mismatch the mapping should be done properly.
• It declares the status GOOD (G flag) or SUSPECT (S flag) according to the connectivity and reception of data from the field. There is also provision of MANUAL (M flag) status in which operator can set the faulty breaker status to CLOSE or OPEN as in field by contacting the substation for avoiding the false calculation 
• According to the flag status, the topology of the network is built and is used for further calculations. The topology development is very important task in the state estimation procedure. As there is no weightage system as like in analog data, a single wrong status can severely affect the accuracy of the state estimation. 

 State variables are the voltage magnitudes and relative phase angles at the system nodes.

Above fig shows the inputs and out puts of the state estimator.

• Analog measurements and the topology of the power system are taken as inputs 
• The topology can only built perfectly when we know all the circuit breakers of the system which are acquired from the digital data.
• the network observability check is also don by comparing the telemetry values with the calculated value.
• the bad data processor processes the data and try to minimize the errors with a feed back to the analog values. 

There are many solution methodologies for state estimation but here we will see mainly about weighted least square method WLS.
Weighted Least Square (WLS)method:

• Minimizes the weighted sum of squares of the difference between
  measured and calculated values .
• In weighted least square method, the objective function 'f ' to be
  minimized is given by 

Each error of the measurement is given its own weightage. For example a meter is having error of

+/- x% it is equated to the 3*(sigma) thus in this way the most inaccurate measurement is given less weightage as the value of sigma would be more.

Thus

State estimation forms the backbone for the real time monitoring and control functions.
•In this environment,a real-time model is extracted at intervals from snapshots of real-time measurements.
•Estimate the nodal voltage magnitudes and phase angles together with the parameters of the lines.
•State estimation results can be improved by using accurate measurements like phasor measurement units.