Unit ii

Unit iiDIGITAL PROTECTIONcontentsDigital signal processing Digital filtering in protection relays digital data transmission Development cycle of new numerical relay.Numerical RelayRelay hardware Relay softwareMultiple Protection characteristicsAdaptive Protection characteristics Data storage Instrumentation featureSelf check featureCommunication capability Additional functionsSize and cost effectiveness

Digital Signal ProcessingDSP is a technique of performing the mathematical operations on the signals in digital domain.

Need for DSPAnalog signal Processing has the following drawbacks: They are sensitive to environmental changes Uncertain performance in production units Variation in performance of units Cost of the system will be high ScalabilityIf Digital Signal Processing would have been used we can overcome the above shortcomings of ASP.Digital Signal Processing System

The Sampling ProcessADC process involves sampling the signal and then quantizing the same to a digital value. In order to avoid Aliasing effect, the signal has to be sampled at a rateatleast equal to the Nyquist rate. The condition for Nyquist Criterion is as given below, fs= 1/T > 2 fm where fs is the sampling frequency fm is the maximum frequency component in the message signal If the sampling of the signal is carried out with a rate less than the Nyquist rate, the higher frequency components of the signal cannot be reconstructed properly. The plots of the reconstructed outputs for various conditions are as shown in figureVerification of Sampling Theorem

Digital filtering in protection relays

Filters are used to remove the unwanted components in the sequence. They arecharacterized by the impulse response h (n). The general difference equation for an Nth order filter is given by,

Values of the filter coefficients vary with respect to the type of the filter. Designof a digital filter involves determining the filter coefficients. Based on the length of theimpulse response, digital filters are classified into two categories Finite ImpulseResponse (FIR) Filters and Infinite Impulse Response (IIR) FiltersFIR FiltersFIR filters have impulse responses of finite lengths. In FIR filters the present output depends only on the past and present values of the input sequence but not on the previous output sequences. Thus they are non recursive hence they are inherently stable.FIR filters possess linear phase response. Hence they are very much applicable for the applications requiring linear phase response.

IIR FILTERSUnlike FIR filters, IIR filters have infinite number of impulse response samples.They are recursive filters as the output depends not only on the past and present inputs but also on the past outputs. They generally do not have linear phase characteristics.

H (Z) = (b0+b1z-1+b2z-2+bLz-L) / (1-a1z-1-a2z-2-aNz-N) Stability of IIR filters depends on the number and the values of the filter coefficients.The major advantage of IIR filters over FIR is that, they require lesser coefficients compared to FIR filters for the same desired response, thus requiring less computation time.Factors to be considered in designing digital filter Travelling wave distortion : During a fault when there is a significant change of voltage at the fault point waves will be generated which travel from the fault point , waves will be generated which travel from the fault point into the adjoining power network.This effect is particularly prevalent in overhead lines.It causes corruption of relaying quantities.The dominant frequency of the travelling waves will be influenced by the position of the fault.Harmonics Rejection of harmonic effects are achieved by using protection algorithms which are immune to such effects.. Removal of low order harmonics will increase the operating time .Exponential offsets Induced in transducers , particularly (CVTs).It is better to remove actively all.That digital filters with dc rejection must be used.Digital data TransmissionIt includes many different functions .What information , when where to whom before saying how.Substation automation we can divide the communication into information and commands.They are send basically into four locations: Internally on the bay level, centrally on the substation level, remotely to and from the control level and remotely to and from the main office.ContdToday some utilities are implementing mobile workplaces for the operator and engineers.The utility owned are microwave carrier and power line carrier or the telephone system.Today available speed on the system is normally from 300 to 2400 bps.In some cases from 9600 to 19800 bps.Use of fiber optics and satellite communication the telephone companies are offering at speed greater than 1 Mbps.User can be divided into four personnel groups.

Information to be transmitted is coded as stream bits. The common types of codes are CCITT NO 5 ( 7 BITS ASCII), CCIITT S. 61 (Telex), EBCDIC( IBM Extended binary coded interchange code). A 7 bit code allows a total of 128 characters . 96 are alpha numerical .32 are control character.

Simplex ,half duplex and full duplex transmissionSimplex means the data transferred in only one direction . Example : measuring application.Half duplex: in both direction but one way at the time.Full duplex: in both direction at the same time.synchronous and Asynchronous transmissionA large number of character can be received without interruption of start and stop bits. only a small amount of data eg one character between start and stop bits . Between two master computers.

Numerical RelaysThe first protection devices based on microprocessors were employed in 1985.Developing the second generation numerical relays in 1990.Modern power system protection devices with multifunction's like protection, control , monitoring and measuring are available today in numeric power system protection devices. The communication capability of these devices facilities remote control monitoring and data transfer.

First generation of numerical relays were mainly designed to meet the static relay protection.Numerical protection devices offer several advantages in terms of protection, reliability and trouble shooting and fault information.Numerical protection devices are available for generation ,transmission and distribution systems.

Block diagram of numerical relay

Relay HardwareNumerical relays use a specialized DSP as the computational hardware along with associated software tools.The relaying voltages and currents are passed through an isolation transformers.The voltage inputs of the relay are scaled down from the nominal voltage to alow level determined by the ADC input range.ContdThe current inputs to the relay are scaled down from nominal5/1 A and are converted into equivalent voltages. These scaled signals are filtered by using a low pass filter to prevent aliasing of the high frequency component into the fundamental frequency component. The filtered signals are multiplexed using analogue multiplexer and amplified if needed by using a programmable gain amplifier.The multiplexed analogue signal is sampled and converted into digital data using ADC.Communication between the processors is provided by the dual ported memory.Flash memory is used for storing the programme and RAM is used for temporary storage variables ,target information ant oscillography.Contact inputs and outputs user interface and the the serial communication ports are interfaced to the host processor.The DSP executes a variety of signal processing algorithms to estimate several parameters of the digitized voltage and current signals , and transfers them to dual ported memory.The host processor receives these from the dual ported memory and performs relay logic and other timing functions to generate appropriate trip or alarm output signal.Host processor running under multi tasking can performs communication set point updates target updates and user interface.Providing back up for critical components.The analogue signal inputs are conditioned to withstand the harsh electrical environment.The design of relay input, output and power supply must filter to reduce EMI

Relay softwareThe software provided in a numerical relay is commonly organized in a series of tasks operating in real time.The main component is the RTOS.TO ensure that other tasks are executed as and when required on a priority basis.Types of SoftwareSystem services software: This controls the low level IO for the relay.HMI Interface software: high level software for communication with the user via front panel controls that are connected to a data link to another computer running suitable software storage and setting data. Application software : defines the protection function of the relay.Auxiliary function : other features offered in the relay.Key tests at various design phases1. Relay algorithm simulation testing.2. Static functional testing.3. Dynamic functional testing.4. Environmental and hardware related tests Site installation and testing

Multiple Protection Characteristics In simple Inverse definite minimum time lag (IDMTL) over current and earth fault relays multiple characteristics like NI, VI, EI, LTI and DTL characteristics are available in relay and the required characteristics.Adaptive Protection characteristicsIn motor protection relay : can be adapted for the starting condition and running condition .Over current protection, earth fault protection , unbalance protection . sensitive settings can be adapted for these protections during the running conditions of the motor.Transformer protection Greater stability can be achieved by monitoring the through fault current .Once the fault current is above the set value the relay bias automatically increases to facilitate greater stability for through fault condition. In distance relays for different system configurations like parallel feeder in and parallel out and grounded , different settings can be adapted for this condition.In IDMTL relays , depending upon the system condition like the number of incomers and outgoing feeders different settings can beb selected and better protection can be achieved.Data storageIt is done in three formatsFault record: information about the fault . It gives all information about the fault to the operating personnel.Event record : stores inside the relay all the changes taking place in the system like protection element like pick up , and drop off operation, output and input energisation and setting changes.Waveform record : nature of fault waveforms , relay operating time and CB operating time. It can also be fed to the harmonic analyzer software to find out the harmonic content in the fault quantities.

InstrumentationNumerical relays are provided with a metering function and separate panel mounted meters can be eliminated.A number of extra quantities can be derived from the measured quantities depend upon the input signal available. They may include a. Sequence quantities.b. Power , reactive power and power factor c. EnergyD. Maximum demand in a period, e. Harmonic quant ities f. Frequency g. Temperatures /RTD STATUSh. Motor start information i. Distance to fault.Self check feature

Self check FeatureData Acquisition System Testing: power supply voltages and ground are connected to the analogue i/p channels of multiplexer and checked against warning and failure thresholds.Memory testing: Flash ROM are checked by calculating the check sum and comparing it with the pre computed and stored. The RAM is tested by writing and reading test pattern. Set point testing : are stored in serial EEEPROM and copy in RAM for executing relay logicWatchdog timer: re set circuit to take the processor Through an orderly reset should the programme get lost due to hardware /software glitches.

Communication

Advantages of remote communicationChange of settings.Change of group settings.Control CB.Instrumentation viewingEvent recorder downloadingWaveform recorder and fault record downloading.

Additional Functionality CB fail.Loss of load , conductor brokenTrip circuit supervision . CB conditions monitoring including programmable digital output and inputs for various logic build generally for blocking. Auxiliary relays like Buchholz relay alarm /trip winding temperature alarm /trip contacts , which can be connected to a relay as logic input and details of relay operation can be seen in the event log sheet.PT/CT supervisionSize of Numerical Size

HV and Environment TestingNumerical Relays are tested as IEC 255 5 for dielectric , impulse voltage and insulation resistance. It should be suitable for 2 Kv dielectric voltage, Kv impulse voltage and insulation > than 1000 M ohms.Electrical EnvironmentRelay is tested for high frequency disturbance, fast transient electrostatic discharge, radio frequency and impulse as per IEC 100 4 AND ANSIC 37.94.Control system communication media and configurationCommunication media used for telephonic system include (i) pilot wire (ii) microwave (iii)fibre optic links and statellite.With existing media a communication speed of 300 to 2400 bps for remote communication Control station and substation.Alternative media Twisted pair : for telephonic communication upto 10 Mbps. Cheap, easily exposed to limited capacity/speed.Co axial cable: are more immune to disturbance upto 50Mbps.Optical fibre : high capacity and the disturbance immunity upto 5Gbps.Ity 51The simplest network configuration is called point to point .Direct connection between two station .Rapid development of desktop based computers lead to LAN.TOPOLGY OF LAN ARE STAR , RING AND BUS NETWORK.Twisted pair and co axial cable can be used.Fibre optic cable can be used to star and ring.By using an active star coupler LAN CONFIGURATION

Contd..Bus and star topologies have faster communication compared to ring.In the ring the data are transmitted from one point to next.Communication depends on all points being available star and busOne point can fail without affecting the rest of the network.Star coupler should have duplicated power supplies to increases the availability.Fibre cabling : within the substation environment.At station level in large station : LAN with a bus with coaxial cable or a active star with optical fibre cable.Optical ring can be used for information exchange at object level.