Protection Application Handbook (англ. яз.)

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REQUIREMENTS ON LINE PROTECTION

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- limit the ionization at the fault which will increase the chances of a suc-

cessful Auto reclosing and thus shorten the dead interval.

- increase the power transmission capability of the network without de-

creasing the safety margin for the network stability.

REQUIREMENT ON SPEED

The maximum fault clearance time is

important i. e. including the back-up protection function and the

possibility of a breaker failure. The network must be stable under

maximal conditions. Times from 250 ms up to several seconds

can occur depending on line type, location in the network, sourc-

es etc.

Sensitivity

Sensitivity means the capability to detect all types of fault.

It is important to detect all faults even if the fault current is smaller

than the load current. Equipment damages due to induction in

low voltage equipment, or person injuries due to rise in earth po-

tential, can occur also for low magnitude faults. High resistive

earth faults are quite likely to occur at long transmission lines and

the relay system must be able to detect such faults before the

faults developes further or people will get seriously injured. Sen-

sitivity is therefore the second important aspect in the perfor-

mance of line protection relays.

REQUIREMENT ON SENSITIVITY The requirement on the line pro-

tection systems sensitivity at earth faults is often discussed and

varies between utilities. The mid-span fault often requires higher

sensitivity than what can be achieved by the primary protection

relay used. A maximum sensitivity of approximately “R

<50 Ω”

can be achieved by the primary protection.

An acceptable sensitivity can only be achieved utilizing zero se-

quence components in overcurrent, directional overcurrent or di-

rectional comparison schemes. These relays can only detect

earth faults.

Selectivity

Selectivity i. e. the capability to determine the fault location and

only disconnect the faulty object.

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The consequences of a fault must be limited and the power sup-

ply to the consumers secured. The protection system must there-

for be capable of distinguish between an external and an internal

faults also for low magnitude faults on a heavy loaded object, or

for parallel objects where close to similar parameters exist for

both healthy and faulty object.

Requirement on selectivity

In order to fulﬁl these requirements the protection relays has to

be able to distinguish between the normal operating condition of

the protected object and an electrical fault i.e. give a reliable fault

detection unaffected by normal operating conditions such as

load, inrush currents etc. In some cases it is also required that

the protection relay must be able to detect also other abnormal

operating conditions such as overexcitation, overload, broken

conductor etc. These aren’t electrical faults but may still damage

the protected object or other apparatus in the network. Since

power apparatus in many applications have to operate near their

rated limits it is important that this part of the protection system

exactly can distinguish between permissible and none permissi-

ble operating conditions.

Electrical faults are normally required to be cleared instanta-

neously. Other abnormal operating conditions, can be accepted

to result in time delayed action.

Dependability and security are contradictory to each other but

have to be evaluated together due to the linking of the two quali-

ties. In redundant protection schemes the whole scheme has to

be evaluated not only the individual relays.

To achieve maximum dependability combined with maximum se-

curity the communication demands shall be at minimum. Wide

band transmission is not only expensive it will also be more ex-

posed to interference. The latter is apparent when power line car-

rier is used. The communication demand is therefore linked with

dependability and security.

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Operate time, sensitivity and dependability are contradictory to

security, therefore none of the ﬁrst qualities have to be exagger-

ated.

3. MEASURING PRINCIPLES

The basic type of measuring principles can be used for line pro-

tection relays. One, or combinations, of the measuring principles

below can be used to create the total line protection scheme.

Overcurrent protection relays

are undirectional, or directional,

current measuring relays with a back-up feature due to the cur-

rent measuring principle.

Pilot wire, optical line differential and phase comparison

measuring principles

gives exclusive unit protection without

any back-up feature. New types of Optical line differential relays

with Distance back-up functions do now exist on the market

which solves the back-up function problem.

Distance and directional impedance measuring

principles

basically gives time/distance selective protection with a back-up

function. By using communication an unit protection function can

be achieved together with the time/distance function with

back-up.

Travelling vawe protection

is a unit protection where the two

line ends are communicating through a fast channel in a permis-

sive or blocking scheme.

3.1 OVERCURRENT RELAYS

Overcurrent relays are normally used in networks with system

voltage below 70 kV where fault infeed is from one direction only

and where relatively long operating time is acceptable. At higher

voltage levels in the transmission lines the directional, or undirec-

tional, overcurrent relays are used as back-up protection to the

instantaneous primary protection relays. The overcurrent protec-

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tion will then operate as a back-up to the primary protection and

for special types of faults e. g. high resistance earth faults which

cannot be detected by the primary protection.

3.2 PILOT WIRE, OPTICAL LINE DIFFERENTIAL AND

PHASE COMPARISON RELAYS

Introduction

Pilot wire and phase comparison relays are unit protection relays,

which only detects faults in the zone between the relays. No

back-up function is included, so a separate back-up relay is nor-

mally required.

Optical line differential relays are also unit protection normally

without back-up function. However there exists now such relays

with a built-in back-up Distance protection function.

Communication between line ends is required and can be ac-

complished with any of the following:

- Pilot wires

- Power line carrier

- Radio link

- Optical ﬁbre

Pilot wires

can be placed in soil or at towers. The resistance will

limit the possibility to use pilot wires. The use is mostly restricted

to distances less than 10km.

Power line carrier (PLC)

equipment is based on a capacitive

connection of signals, with a frequency of 50-500 kHz, in the

power line. A frequency keying is normally used to transmit a trip,

block or phase angle signals to the remote end.

PLC equipment can also be used in very long lines and for re-

mote control.

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Figure 2. Communication through a high frequency signal, interposed at the

high voltage line (PLC).

Radio links

are reliable, but expensive, communication links and

are therefor rarely used.

Optical ﬁbres

have two big advantages, the insensitivity to noise

and its possibility to transmit huge amount of information. An in-

crease in the use of optical links have been noticed during the

last number of years. Cost has decreased and new Optical line

differential relays utilizing the transmitting capability has been de-

veloped giving improved protection capability. See separate sec-

tion below.

Principle design of pilot wire differential relay

Pilot wire differential schemes gives an absolute selectivity and a

short operating time. It can use wires of metal or, with recent de-

veloped relays, optical ﬁbres. The use of optical ﬁbres in trans-

mission tower top lines will increase during the forthcoming

years, and several new products using optical ﬁbres will be avail-

able. Probably also combined products where a single phase dif-

ferential relay is combined with back-up functions, the lack of

which up to now have been a draw-back for the pilot wire differ-

ential relays.

A short description of the pilot wire differential relay princi-

ple:

The measuring principle is based on a comparison of amplitude

and phase angle at the two line ends according to the differential

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current principle (Kirchhoffs ﬁrst law). A relay is of practical rea-

sons, provided at each line end to detect the faults and to trip the

circuit breakers.

A summation transformer is often used to transform three phase

systems into single phase form enabling the use of one pair of pi-

lot wires only. With the voltage balance principle, the current un-

balance is changed into a voltage balance. The principle of a

voltage balance differential relay is shown in ﬁg 3.

A relay with a matching current ratio is connected to the current

transformer at the line ends (1 A can be used at one end and 5 A

at the other but the same primary current is required). The pilot

wires resistance “R

” and the settable resistor “R

/2” are con-

nected to the secondary of the summation transformer. A relay is

connected between the pilot wire terminal and the mid-point of

the summation transformers secondary winding. These points

will with the used dimensioning of the pilot resistance and the

built in resistor have the same potential at normal service condi-

tions (see ﬁg 3a).

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Figure 3. A voltage balance current differential relay

At high through fault currents the voltage across the relay will be

zero although the voltages “2E

” respectively “2E

” will increase

to the maximum value “E

Amax

” respectively “2E

Bmax

”, dependent

of the two regulating diodes. The relay operating voltage is select-

ed to prevent maloperation due to small differences e. g. caused

by saturation of current transformers cores (see ﬁg 3b).

If a fault, fed from both directions, occurs within the protected

area the summation transformers outputs will be in phase oppo-

sition. No currents can therefor circulate in the pilot wires. The

secondary voltages will be limited by the regulating diodes to

“2E

Amax

” resp. “2E

Bmax

”. Operating voltages will occur across

/2 R

/2R

a) Normal operating conditions

Amax

Bmax

Amax

Bmax

b) External fault ideal transformers

c) Internal fault, double end infeed

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the relays and the differential relays at both end will trip (see ﬁg

3c).

Also at an internal fault fed from only one line end, voltages will

occur across the relays and both relays will operate (see ﬁg 3d).

Principle design of Optical line differential functions

The optical line differential functions have been introduced on the

market during the last number of years together with the start of

provision of optical ﬁbres between the two stations, giving a

broad band connection enable direct differential measurement

on a phase per phase basis.

The differential current principle is shown in ﬁgure 4. One relay is

provided at each line end. The communication can be through

separate dedicated ﬁbre, through a MUX’ed 54/64 kbit/s channel

complying to CCIT and EIA standards or even through separate

pilot wires in special cases.

Figure 4. The principle for an optical line differential relay with one protection

relay at each line end communicating through a communication channel.

The differential measurement is stabilized with the current scalar

form as per below ﬁgure 5. The degree of stabilization for through

faults and the minimum operating current is settable. Modern re-

lays also normally have a CT supervision detector to lower the re-

quirement on current transformer cores.

An advantage with these modern optical relays based on numer-

ical principles is the possibility to set the CT ratio adaption. Dif-

ferent CT ratios can then be used at the two line ends.

Communication

channel

Interface to MUX

dedicated fibre etc.

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An intertrip channel for direct intertrip from e. g. Breaker failure

protection is normally also available in the relays.

In some types of relays the differential functions can be comple-

mented with an back-up protection in form of a Distance protec-

tion or a simple short circuit and earth fault protection. This

function can then operate in parallel with the differential function

or only as a back up when the communication channel is missing.

Figure 5. The stabilization characteristic.

Principle design of phase comparison relay

Phase comparison relays is used in some countries. However the

relays haven’t been able to take big market shares mostly be-

cause of the fact that they provide only unit protection not includ-

ing any back-up functions, but also due to the high requirement

on a fast communication between ends which mostly means a

separate PLC equipment for this protection relay only.

The “phase comparison principle” indicates that the phase angle

of the current at the two line ends are compared each cycle as

shown in ﬁgure 6.

diff

bias

MinOp

MinSat

LVL1/2Cross

DiffLVL2

Stabilization at

normal faults

Stabilization at

CT saturation

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Figure 6. Measuring principle of a phase comparison relay. The angle differ-

ence, between the currents at the two line ends, is compared.

The measured time for the zero crossing is transmitted to the oth-

er end with a short, and well deﬁned, channel time.

At normal load, or through faults, the phase angle at the two ends

is rather alike. A small difference will though occur due to the ca-

pacitive generation of the line. An amplitude requirement is often

added to the phase angle requirement.

To limit the transmission of signals to fault conditions a start cri-

teria is normally added. Overcurrent, Overcurrent/Undervoltage,

I1 I2

Ø> Ø>

Load

e1-e2

Operating

area