Digital Relays

7/27/2019 Digital Relays

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ElectricalEngineeringDepartment

POWERSYSTEMRELAYINGEE466-TermProject/Part-1

Doneby:

AbdullahYahya

FahadAl-khaldi TurkiAl-harbi

Preparedfor:DR.MOHAMMADALIABIDO



TABLE OF CONTENTS

I. INTRODUCTION TO DIGITAL RELAYS………………………………….3

II. NUMERICAL RELAYS……………………………………………………….4

a. Numerical distance relay features

b. Advantages of numerical protection relays over static

III. HARDWARE………………………………………………………………......6

IV. RELAY SOFTWARE………………………………………………………….8

V. ADDITIONAL FEATURES OF NUMERICAL RELAYS………………….9

a. Measured values display

b. VT/CT supervision

c. CB Control/State Indication /Condition Monitoring

d. Disturbance Recorder

e. Time Synchronization

f. Programmable Logic

g. Provision of Setting Groups

VI. NUMERICAL RELAY ISSUES………………………………………………11

a. Software Version Control

b. Relay Data Management

I. CONCLUSION…………………………………………………………….12



I. INTRODUCTION TO DIGITAL RELAYS

Relays had a jump-change in technology introduced by digital protection relays.

Analogue circuits in static relays are replaced by microcontrollers and

microprocessors. Around 1980s, early examples are invented into service. Most relays

applications are still using this technology with significant enhancements in

processing capability. Such technology, digital relays, is called nowadays as

numerical relays. Conversion of analogue values to digital ones, by A/D converter,

and algorithm implementation using microprocessors, with some counting technique,

are main distinctions of digital relays compared to static. Compared to static,

functionality of digital relays is greater in accuracy and has variety range of settings.

So, the main difference between digital and numerical relays is the processing

capacity. Waveform of samples per cycle is reduced due to limited power of

microprocessors, which limits operation speed in some applications. Therefore, a

digital relay for a particular protection function may have a longer operation time than

the static relay equivalent. This may lead by intuition to have larger tripping time.

However, this is not significant in overall effects and in terms of stability. Examples

of digital relays are shown in Figure 1.1.

Figure 1.1: Digital relay from Siemens



II. NUMERICAL RELAYS

As a result of advances in technology, numerical relays can be viewed as natural

developments of digital relay. For the computational hardware, they use a digital

signal processor (DSP). After converting the signal to digital, it will be processed with

desired algorithm. High power microprocessor is required for digital signal processing

Additionally, ‘one-box solution’ is an approach for latest relays hardware, by

reducing the cost of digital devices and providing different items in single item. The

use of multiple microprocessors would enhance the performance within this single

item. Figure 1.2 shows typical numerical relays, and a circuit board is shown in

Figure 1.3.

Figure 1.2: Typical Numerical Relays

Figure 1.3: Circuit board of numerical relays



a. Numerical distance relay features

• Distance Protection.

• Overcurrent Protection (directional/non-directional).

• Switch-on-to-Fault Protection

• Power Swing Blocking

• Voltage Transformer Supervision

• Negative Sequence Current Protection

• Under voltage Protection

• Overvoltage Protection

• CB Fail Protection

• Fault Location

• CT Supervision

• VT Supervision

• Check Synchronization

• Auto reclose

• CB Condition Monitoring

• CB State Monitoring

• User-Definable Logic

• Broken Conductor Detection

• Measurement of Power System Quantities (Current, Voltage, etc.)

• Fault/Event/Disturbance recorder

b. Advantages of numerical protection relays over static

• Several setting groups

• Wider range of parameter adjustment

• Remote communications built in

• Internal Fault diagnosis

• Power system measurements available

• Distance to fault locator

• Disturbance recorder



• Auxiliary protection functions (broken conductor, negative sequence, etc.)

• CB monitoring (state, condition)

• User-definable logic

• Backup protection functions in-built

• Consistency of operation times - reduced grading margin

The functionality that requires several discrete relays is now emerged into single

item, where different relay elements are represented as software routines. Reliability

and availability are main concerns against putting many features. Any failure in the

relay would cause lost of more functions where one function is lost if hardware is

separated.

III. HARDWARE

Figure 1.4 shows the architecture of numerical relay. It consists of DSP

microprocessors, (I/O) digital and analogue input/output, memory and power supply.

Usually, only one DSP is used for protection algorithm while the others implement

other logics and deal with human machine interface (HMI). By using printed circuit

board, I/O can be easily added up to hardware limits. Hardware is linked through

internal communication bus, which is an important component in design process. Low

voltage levels, high speed and immunity to noise are essentials in designing. Isolating

digital input, by using transformers, does prevent transients to transmit into internal

circuit. The amplitude of input must be limited to a void any distortion as shown in

Figure 1.5.



Figure 1.4: Relay module

Figure 1.5: Example of distorted signal

The conversion of analogue signals is done by A/D converter, which is followed

by a multiplexer. Signals may be initiated to number of sample-and-hold circuits

before multiplexing. Alternatively, each input has its own A/D converter and a

predefined logic to make sure all measurements are performed simultaneously.



Recall the Nyquist criterion, the sampling frequency will as follows:

fs ≥ 2 x fh (equation 1)

Where:

fs = sampling frequency

fh = highest frequency of interest

Aliasing appears in the input signal if low sampling frequency was chosen.

This result will appearing high frequencies in the range of interest of our signal,

which will end up with incorrect results. This is can be solved by adding an anti-

aliasing filter. A modern numerical relay may sample input into different samples per

cycle, between 16 and 24 samples per cycle. Any other subsequent processing is

carried by software. Digital outputs use relays for isolation purposes or via external

bus.

IV. RELAY SOFTWARE

All tasks are organized by the software, which is operating in real time. Real Time

Operating System (RTOS) is an essential component, which main function is the

supervision of tasks. It will ensure every task is executed when needed with priority

basis. Some of these tasks are summarized as follows:

1. System services software (i.e. drivers for the relay hardware, boot-up

sequence, etc.)

2. HMI interface software –high-level software for communicating with a

user, via the front panel controls or through a data link to another

computer running suitable software.

3. Application software – this is the software that defines the protection

function of the relay.



V. ADDITIONAL FEATURES OF NUMERICAL RELAYS

In numerical relay, DSP is sufficient to perform processing while maintaining the

basic protection function. Although the protection function will not occupy much part

processing, excess to perform other functions must be considered for over capacity

processing. This over capacity processing may compromise protection function.

Typical functions that may be found in a numerical relay besides protection functions

are described in this section.

a. Measured values display

This is the simplest function to implement, as it needs least processing time. It

measures the values that the relay will operate for the protection function It is

therefore displayed in the front panel. The number of extra quantities may be

able to be derived from the measured quantities, depending on the input

signals available. These might include:

o Sequence quantities (positive, negative, zero)

o Power, reactive power and power factor

o Energy

o Max demand in a period (kW, kvar; average and peak values)

o Harmonic quantities

o Frequency

o Temperatures

o Motor start information (start time, total no. of starts, total running

time)

o Distance to fault

The accuracy of measurements is dependent on accuracy of transducers.

Nevertheless, it would be easy for an operator to assess system conditions.



b. VT/CT supervision

The general principle of supervising transducers is the calculation of a level of

negative sequence current that is inconsistent with voltage value of negative

sequence.

c. CB Control/State Indication /Condition Monitoring

Operators will have to know the condition of all circuit breakers at all time. The

communication bus is linked to remote control center to send inductions of CB status.

Periodic maintenance is required for circuit breakers to ensure that the fault capacity

in not affected. The numerical relay can measure all parameters affecting the tripping

function, hence, alarm is sent when maintenance is due.

d. Disturbance Recorder

Every relay has a memory, which has a minimum number of cycles to be stored

for correct signal processing. Also, it can act as a disturbance detector for a monitored

circuit. This implies a record of disturbance when detecting a fault by freezing the

memory when faults occur. In transmission networks, it should have its own single

recorder for monitoring disturbances, as it’s not the case for small distribution

networks.

e. Time Synchronization

Time synchronization from an external clock is a feature of numerical relays. The

standard normally used is an IRIG-B signal, which may be derived from a number of

sources, the latest being from a GPS satellite system.



f. Programmable Logic

Microprocessors are widely used to implement logic functions. Such logics as:

inter-tripping and auto-reclose. The cost of manufacture would be reduced as there

will be no need for separating relays version, as it is easier to customize a relay for a

specific application and discard unnecessary devices

g. Provision of Setting Groups

Power system change their configuration due to operational reasons. For example,

changing supply from normal to emergency generation. This could cause a problem

with static relay, since there is only one group of settings. A number of setting groups

are an effective feature in numerical relays. Switching arrangements between these

groups can be performed by a remote command or by programmable logic system.

VI. NUMERICAL RELAY ISSUES

There is nothing works just perfect and this implies to numerical relays. Some of

limitations are listed as follows:

a. Software version control

b. Relay data management

c. Testing and commissioning

a. Software Version Control

Functions are performed in numerical relays by means of software, which

include the challenge of developing error-free code. Manufacturers should consider

that errors might occur. In this context, a new version of the software is necessary.

Advising users to have the new version by reporting errors with pervious version.



b. Relay Data Management

A record of all relay data must be kept for backup purposes. Data entered to

static relay is about 10 times less than numerical relays. Thus, the percentage of error

in entering data mistakenly to numerical relays is much higher.

Conclusion

Digital relays are the last generation relays. Relays applications grow rapidly

due to evolution in communication in general and in micro-processing specifically.

Numerical relays would eliminate the need for other measurement devices in

substations. Relays are no longer consistent with protection function, although its

main function, as it is becoming an integral part of automated subs-stations schemes.

Documents

Digital Relays