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DIGITAL PROTECTION OF POWER SYSTEMS
(Hardware Organization in an integrated system)
MODULE - 3
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Hardware organization: Computers for relaying, substationenvironment, Industry environmental standards, countermeasures against EMI, Redundancy and Back up.
System relaying and control: Measurement offrequency and phase, sampling clock synchronization,Application of phase measurements to static anddynamic state estimation, system monitoring
Text Book: COMPUTER RELAYING FOR POWER SYSTEMSArun G. Phadke and James S. Thorp
OVERVIEW:
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Severalhardwarerelatedquestionsmayarise such as the computer hierarchy in the substation,
subsystems of a computer relay,
and the analog to digital converters.
The requirements stems from the functional needs of relaying, and from relaying application considerations.
IntroductionHardware Organization:
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Level I computers
are the protection computers,
usually are placed inside the substation control house.
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Level II computers
are central to the substation, and are therefore placed in the control house also.
Which are extremely important.
Different issue is that of hardware reliability through redundancy.
traditional practice of achieving dependability is through duplication of hardware.
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Discussing considerations that affect and set limits for acceptable performance for computers to be used for relaying.
What about the speed of distance relay?
Limited by transient phenomena that accompany a fault.
Considering the entire fault clearing process,
it is not significantly beneficial to achieve relay speeds faster than a quarter of the period of the fundamental power frequency.
Computersforrelaying
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The fastest desirable speed of a relay response may be taken to be between 4 and 5 milliseconds (depending upon at 60 Hz or 50 Hz).
To make a secure decision for relaying over this period, more than two data samples are needed.
Conclusion that the analog input sampling frequency should be at least 12 times the fundamental frequency of the analog input signals with a sampling interval of the order of 1.4 milliseconds.
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Since the relaying algorithms must execute between samples,
it is not advantageous to increase the sampling rate to double or triple (i.e. to 1440 or 2160 Hz), as the algorithm execution time would then be reduced to 0.7 or 0.5 milliseconds respectively.
As most relaying algorithms seem to be accommodated by a couple of thousand machine language instructions, a sampling period of 0.5 millisecond would call for an average instruction time of the order of 250 nanoseconds.
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In fact such an algorithm would not leave any margin of safety.
Hence conclusion is that, with computers having an average instruction time of 100 to 300 nanoseconds, satisfactory relaying algorithms could be built around sampling frequencies of between 36 times and 12 times the power system frequency.
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In an integrated computer system, the sampled data may also be used for oscillography.
In such cases, it may be desirable to sample at much higher rates to reproduce the higher frequency transients.
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If higher sampling rates are used in the data acquisition units, the sampled data must be converted to lower frequency samples before relaying algorithms are invoked.
This data reduction must also simulate correct anti-aliasing filters that would go with the lower sampling rate.
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The main features of computer relaying's are: Computer instruction execution time,
word length,
and A/D converter resolution, which influence the quality of measurement performed by a relay.
Other features which make for good overall performance are immunity to interference,
low power consumption,
adequate peripheral equipment,
and assured supply of spare parts.
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Physical conditions within an electric utility transmission substation are among the most severe.
In Outdoor station yard, the temperatures can become very high: 49C in very hot regions of the world,
and very cold: 51C in cold climates during the winter.
Addition to extreme temperature and humidity, the substation equipment must withstand, there is also a hostile electromagnetic environment.
Thesubstationenvironment:
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During switching operations, high levels of electromagnetic fields are set up in the yard and control house.
Faults within the substation or near it also cause ground currents and ground potential rise which may influence all protection and control equipment within the station.
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In addition, various high voltage apparatus may have corona discharges of varying severity depending upon the weather.
Furthermore, relay operation within the control house may generate transient fields which may affect other relays and control equipment.
And finally, the relays and other protection and control equipment may be affected by fields produced by hand-held walkie-talkie type radio communication equipment.
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The control house where most protection equipment is located
generally provides considerable shielding from radiated interference originating in the substation switchyard.
Careful wiring practices and equipment shielding techniques must be employed in designing relaying equipment in order to ensure that the equipment will perform
satisfactorily under all reasonable service conditions
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Industry standard provides operating temperature and humidity specifications for
supervisory control,
data acquisition and
substation automatic control equipment.
Although later revisions of this standard have indicated that these specifications may not apply to protective relays.
we could use the standard as a typical environmental specification document.
Industryenvironmentalstandards
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For buildings without air-conditioning, temperature range is 0C to 55C with an allowable rate of
temperature change.
The humidity range is specified to be 1095% without condensation.
For outdoor equipment, the applicable temperature range is 25C to 60C.
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The 1974 ANSI/IEEE standard C37.90a for relaying equipment specifies the permissible ambient air temperature close to the relays to be 20C to 55C.
A computer relaying system built for service in the field has used a temperature specification of 0C to 55C.
Standards for seismic shock withstand are defined by another standard.
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The control house within a substation provides substantial shielding from radiated EMI.
The wiring from the switchyard to the control house penetrates this shield, and consequently the EMI induced in this wiring and conducted to the protection equipment remains a major source of concern.
The ANSI/IEEE standard C37.90a of 1974 and its revision provide the standard for SWC to be built into protective equipment.
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EMI can reach the relay- Input ( analog & Digital)
Digital output,
Power supply,
A specification is provided for EMI level at the terminals where these wires enter the relay system.
The EMI may be transverse (differential mode), or longitudinal (common mode).
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These two terms refer to interference voltages between any pair of wires (transverse mode) and all wires and ground (common mode).
Hence specifications are given for a group of wires at a time while the relay is operating normally.
while the relay inputs and outputs are connected normally, one group of wires is subjected to the SWC test.
Where the surges are coupled through capacitors, and they are restricted to the relay under test by blocking inductors in series with each wire.
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The design of relays computer based or
solid-state must incorporate SWC filtering on all wires.
SWC must reduce the transient surges to acceptable levels within the relay.
Hence each SWC filter is an integral part of the relay system and is designed to accommodate the specifications of the computer and its peripheral hardware.
Counter-measures against EMI
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The substation design and wiring must also follow a procedure which will limit the EMI signals.
ThestrongestsourceofEMIinthesubstationisadisconnectswitch.
As the current is interrupted, the arcing contacts reignite several times - each time a high frequency high voltage oscillation is initiated.
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produce induced voltages in the control and signal wiring that connects the CT and
CVT windings and
circuit breaker control wiring, to the relay located in the control house.
The shield should be grounded to the station ground mat at both ends and preferably at as many points along the cable run as practical.
The signal circuit should be grounded at one point only, generally inside the control house.
Shielding should be extended to all wiring that penetrates the control house.
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Redundant protection system provides backup in the event of misoperation in the relaying.
Redundant protection uses a different principle of protection for example, a step distance relay may be the redundant
system for a phase comparison system.
The redundant system would guard against the failure to trip, making the overall protection more dependable.
Redundancy and backup
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The presence of microprocessor based devices in the substation, which are constantly processing data received from the system.
A relay can be regarded as a measuring system. Measuring function is a logical complement to the
protection function. The two functions can coexist if proper priorities are
maintained.
System relaying and control
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During normal system conditions the measuring function would be active.
During fault conditions measuring would be suspended and the measurements used for relaying decisions.
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useful result can be obtained by examining the SCDFT calculations of positive sequence voltage.
Considering nominal power system frequency and write the phase voltages as
Measurementoffrequencyandphase
(1)
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compute the positive sequence voltage with a full cycle window, (corresponding to K samples per full cycle of the nominal frequency, ending at sample L.)
Using equa (1)
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Last term is zero,
Consider
can be evaluated as
Thebracketedterm,representsanerrorincomputingthephasor.
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The part of above equation that depends on the recursion number L is the angle L t.
let the phase angle of the phasor computed at time L be denoted by L then,
since the time between samples is t seconds, the angular velocity of is given by
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The technique suggested has much to recommend it for the measurement of frequency and rate of change of frequency.
It uses all three phase voltages therefore less sensitive to error terms than techniques based
on a single phase.
It uses much more information than methods based on zero crossing times and is more immune to noise and harmonics.
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in an integrated substation protection system it is assumed that all sampling of voltages and currents would be synchronized.
advantage is that, allowing data sharing between modules in a backup mode.
The voltage signals used in transformer protection might be obtained from line protection modules in such a system.
Sampling clock synchronization
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The sampling clock synchronization would have advantages and all the phasors computed in the substation would be on a common reference.
if all the recursive calculations are begun at the same instant and updated synchronously, the phasors would all be with respect to the same reference angle.
The required accuracy of synchronization can be determined by observing that at a 60Hz power system frequency a timing error of 1 sec. corresponds to an angular error of 0.0216 degrees.
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Other important factors of a synchronizing system are the cost and uninterrupted operation of the system over a long time span.
Real-time operation of the bulk power system has been greatly enhanced by state estimation algorithms
Which use measurements of available quantities: real and reactive power flows,
real and reactive power injections,
Applicationofphasor measurementstoStaticstateestimation:
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bus voltage magnitudes,
breaker status to estimate the state of the system.
The transducers that used in state estimation are called remote terminal units (RTUs).
The RTUs communicate with the supervisory control and data acquisition system (SCADA) over dedicated telephone lines and/or
microwave channels.
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Commonly accepted state estimation algorithms are a form of the weighted least squares (WLS) technique.
If the state of the system is taken as the vector V of bus voltage magnitudes, and the vector of bus voltage angles, then the measurement vector, z , can be
These measurement is nonlinear, since the line flows and injections are nonlinear functions
of the states.
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The estimator is static because the entire set of measurements are assumed to be
taken from the system in a fixed (static) state.
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Give as Assignement
Phasor measurements in dynamic state estimation
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