Introduction
Most of the conventional protection in a power system derives information locally process locally and act locally . There are circumstances in which the relays and protection is required to operate based on the system wide information . Such a protection and control is broadly called as Wide area protection and control.
Some of the conditions which might require a relay and control to operate based on the system wide information is
1. Power swings and out of step protection
2. Islanding protection
3. Protection against voltage instability
4. protection against angular stability
5. frequency based protection and load shedding
All the above requirements and many more were accomplished traditionally based on the local information and control and protection action . But with the invention of Phasor measurement units , the above tasks can be accomplished in a more efficient way . Before we talk further on the subject let us discuss briefly about the phasor measurement units
Introduction to phasor measurement units
Phasor measurement units measures the magnitude of the voltage and current with phase angle of the system and sent to the control centre for further processing . This is achieved with time synchronisation using GPS - meaning the information about the voltage current , frequency , rate of change of frequency , MW and MVAr is sent to the control center with precise time stamp on it . The messages can be sent at different rates ( 10 , 25, 50 messages/sec ) The phasor measurement units are placed in different location of the power system stratagicaly and hence the information so received with same time refrence can be used in a better way to act against system disturbances
Phasor measurement network
Phasor measurement network consists of the following
1. Phasor measurement unit
As per the IEEE , this device could be a stand alone device or integrated as part of a relay or measurement unit
2. Phasor data concentrator
Phasor data concentrator is a function using a PC which can be stationed in local station . remote station . There can be local PDC , regional PDC and National PDC
The function of the PDC is process the information received from the PMUs and provide necessary analysis initiate control and protection process with the help of local protection and control devices
GPS clock
This clock gives the necessary time signal to the PMUs and PDCs so that they act in time synchronised manner.
Communication network
The communication network is required for the data transfer between PMUs and PDCs and vice versa . The existing communication network that is used to send the protection and communication network may be utilised for this purpose . However it is to be noted that as the messages are sent at a faster rate compared to the SCADA data , the communication requirement for the PMU network could be more than that is required for the SCADA for the same data to be transmitted ( to be continued )
The difference between the SCADA and PMU network
SCADA measurements are sent at a rate of 2 to 10 sec per second and it is not time synchronised with GPS . whereas the PMU measurements are time synchronised with GPS and it is sent at a faster rate up to 50 messages /sec . Hence by using the information received from the PMU measurement , it is possible to precisely estimate the state of the system including the disturbances and swings on real time .
Application of PMU based measurement
1. Power transfer and stability
We know that the power transfer between two nodes in a power system is given by the equation
P= E1 E2 Sin (delta)/X
where E1 is the voltage at the sending end and E2 is the Voltage at the receiving end and delta is the phase angle between E1 and E2 or is the rotor angle and X is the impedance of the connecting feeder. Hence the as the phasor measurement data directly gives the phase angle between the two ends , it has the possibility to measure the rotor angle between two ends of the feeder . The rotor angle shall not be more than 90 deg for the system to be in stable operation and whenever this angle goes beyond 90 deg it shows that , the system is heading for a out of synchronism condition
Assume that there are two parallel feeders used from a generating plant to transport power to the a grid and one of the feeder trips on a fault . As the power requirement is constant , the P in the above equation is constant but as one of the feeder is tripped the X value is doubled . hence the Sin delta increases to reach to the new equilibrium . If the increase in delta is crossing 90 deg permenantly , the system goes to unstable condition . This information can be used in the control center to take corrective action so as to bring the system to stability .
2.Intelligent Islanding
Using real-time data provided by the PMUs to more accurately determine whether a power system is heading to an unstable state and if a network separation is necessary to avoid a blackout.
Determine optimal islanding boundaries according to the prevailing system conditions. For example, establish which groups of generators will separate due to loss of synchronism and how to optimally balance load and generation in each island formed by coherent generator groups and loads.
3.Intelligent load shedding
There are two main reasons that , causes the stability of the system to be disturbed
Voltage instability
Angular instability
The basic reason for the voltage instability is due the gap between the VAR supply and Demand When the generators are unable to meet the Var requirement of the load , the voltage starts to collapse leading to wide area disturbance . If immediate remedial action s like under voltage load shedding is not carried out the whole system may collapse
. Whereas the reason for the angular instability is due the mismatch between the generation capacity and load requirement . As there is gap between the load and generation , the frequency starts decaying and the finally the generator go to unstable region and trip . Under frequency load shedding is the remidial action required to meet this situation
Also when the genration above the load requirement , such as a loaded transmission line trips , making the generation unable to supply the load due to the transfer capacity , results in to angular stability .Generation shedding is the remedy for this type of stability issues
The conventional voltage and frequency based load shedding protection scheme trips the load when the volatage or frequency at the bus goes below a specified limit . The local tripping relay has no information about the actual generation requirement . Hence , normally more unnecessry loads would be tripped due to overestimation
Hence in case of voltage instability , using PMU network the gap between the VAR generated and the Var requiremnent can be calculated and the loads that is suffiecient to meet the requirement can be shed . As this case of stability is an issue based on the VAR unavailability , the loads that requires more Var can be tripped selectively using the PMU based systems .
As the PMU based load shedding is based on the information exchange between the load and source , the unwanted tripping due to local disturbances can be avoided in the system
Load /Generator shedding ( frequency based )
Adaptive load shedding enable shedding a minimal amount of load that will allow frequency to recover. The angular instability can be because of a over load or it can be based on excess generation An intelligent scheme can be devoloped using PMU to identify the type of instability and opt for load shedding or generation shedding
This shedding can be made adaptive and only the required load or generation can be shed avoiding unwanted or excessive load /generation shedding
4. Distributed generation and antiislanding
Anti islanding is an important requirement for the Distribuetd generation (DG) . Anti-islanding is the ability of a scheme to detect when a generator is operating in an islanded system and to disconnect the generator from the system in a timely fashion. Failure to trip islanded generators can lead to a number of problems for the generator and the connected loads. The DG islanding shall be done prior to th e reclosing time of the CB
Traditional schems were using either the CB position transfererd from various remote locations , frequency , voltage dip , rate of change of frequency etc to detect the islanding condition and trip the generator in case the islanding is detected .
It has the disadvanges in complex schemes , difficulty to distingish between the system disturbance and Islanding etc
The antiislanding function using the synchro phasor can be implemented by placings the PMUs at point of common coupling (PCC) and DG . The phase angle difference between the PCC and DG can be directly used for arriving a decision on the Islanding and Isolation of the DG from the system.
5. State estimation to state measurement
( to be continued )
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