Setting the Pickup Point_CT_PT

7/24/2019 Setting the Pickup Point_CT_PT

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Setting the pickup point

The standard over current relay is designed to operate from a ratio-type CT with a standard 5A

secondary output. The output of the standard CT is 5A at the rated nameplate primary current,

and the output is proportional to the primary current over a wide range. For example, a 1!5ratio CT would have a 5A output when the primary current "the current #eing sensed and

measured$ is 1A. This primary-to-secondary ratio of %-to-1 is constant so that for a primary

current of 1A, the secondary current would .5A& for %A primary, 1.A secondary& for 5Aprimary, %.5A secondary& etc. For 1A primary, the secondary current is 5A, and similarly for

all values of current up to the maximum that the CT will handle #efore it saturates and #ecomes

nonlinear.

The first step in setting the relay is selecting the CT so that the pic'up can #e set for the desiredprimary current value. The primary current rating should #e such that a primary current of 11 to

1%5( of the expected maximum load will produce the rated 5A secondary current. The

maximum availa#le primary fault current should not produce more than 1A secondary currentto avoid saturation and excess heating. )t may not #e possi#le to fulfill these re*uirements

exactly, #ut they are useful guidelines. As a result, some compromise may #e necessary.

+n the 5!51 overcurrent relay, the time-overcurrent-element "device 51$ setting is made #y

means of a plug or screw inserted into the proper hole in a receptacle with a num#er of holesmar'ed in CT secondary amperes, #y an adusta#le cali#rated lever or #y some similar method.

This selects one secondary current tap "the total num#er of taps depends on the relay$ on the

pic'up coil. The primary current range of the settings is determined #y the ratio of the CT

selected.

For example, assume that the CT has a ratio of 5!5A. Typical taps will #e , 5, , /, 0, 1, 1%,

and 1A. The pic'up settings would range from a primary current of A "the A tap$ to 1A

"the 1A tap$. )f a A pic'up is desired, the A tap is selected. )f a pic'up of more than 1A orless than A is re*uired, it would #e necessary to select a CT with a different ratio or, in some

cases, a different relay with higher or lower tap settings.

arious types of relays are availa#le with pic'up coils rated as low as 1.5A and as high as A.

Common coil ranges are .5 to %A, for low-current pic'up such as ground-fault sensing& 1.5 toA medium range& or to 1A, the range usually chosen for overcurrent protection. CTs are

availa#le having a wide range of primary ratings, with standard 5A secondaries or with other

secondary ratings, tapped secondaries, or multiple secondaries.

A usa#le com#ination of CT ratio and pic'up coil can #e found for almost any desired primarypic'up current and relay setting.

The instantaneous trip "device 5$ setting is also adusta#le. The setting is in pic'up amperes,

completely independent of the pic'up setting of the inverse-time element or, on some solid-state

relays, in multiples of the inverse-time pic'up point. For example, one electromechanical relay isadusta#le from % to 0A pic'up& a solid-state relay is adusta#le from % to 1% times the setting of

the inverse-time pic'up tap. +n most electromechanical relays, the adusting means is a tap plug


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similar to that for the inverse-time element. 2ith the tap plug, it is possi#le to select a gross

current range. An uncali#rated screw adustment provides final pic'up setting. This re*uires

using a test set to inect cali#ration current into the coil if the setting is to #e precise. +n solid-state relays, the adustment may #e a cali#rated switch that can #e set with a screwdriver.

Setting the time dial

For any given tap or pic'up setting, the relay has a whole family of time-current curves. The

desired curve is selected #y rotating a dial or moving a lever. The time dial or lever is cali#rated

in ar#itrary num#ers, #etween minimum and maximum values, as shown on curves pu#lished #y

the relay manufacturer. At a time-dial setting of 3ero, the relay contacts are closed. As the time

dial setting is increased, the contact opening #ecomes greater, increasing relay operating time.

4ettings may #e made #etween cali#ration points, if desired, and the applica#le curve can #e

interpolated #etween the printed curves.

The pic'up points and time-dial settings are selected so that the relay can perform its desired

protective function. For an overcurrent relay, the goal is that when a fault occurs on the system,

the relay nearest the fault should operate. The time settings on upstream relays should delay their

operation until the proper overcurrent device has cleared the fault. A selectivity study, plotting

the time-current characteristics of every device in that part of the system #eing examined, is

re*uired. 2ith the wide selection of relays availa#le and the flexi#ility of settings for each relay,

selective coordination is possi#le for most systems.

4electing and setting other than overcurrent relays are done in similar fashion. etails will vary,

depending on the type of relay, its function in the system, and the relay manufacturer.

Relay operation

An electromechanical relay will pic' up and start to close its contacts when the current reaches

the pic'up value. At the inverse-time pic'up current, the operating forces are very low and

timing accuracy is poor. The relay timing is accurate at a#out 1.5 times pic'up or more, and this

is where the time-current curves start. This fact must #e considered when selecting and setting

the relay.

2hen the relay contacts close, they can #ounce, opening slightly and creating an arc that will

#urn and erode the contact surfaces. To prevent this, overcurrent relays have an integral auxiliary

relay with a seal-in contact in parallel with the timing relay contacts that closes immediately

when the relay contacts touch. This prevents arcing if the relay contacts #ounce. This auxiliary

relay also activates the mechanical flag that indicates that the relay has operated.

2hen the circuit #rea'er #eing controlled #y the relay opens, the relay coil is deenergi3ed #y an

auxiliary contact on the #rea'er. This protects the relay contacts, which are rated to ma'e

currents up to 6A #ut should not #rea' the inductive current of the #rea'er tripping circuit, to


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prevent arcing wear. The dis' is then returned to its initial position #y the spring. The relay is

reset. 7eset time is the time re*uired to return the contacts fully to their original position.

Contacts part a#out .1 sec "six cycles$ after the coil is deenergi3ed. The total reset time varies

with the relay type and the time-dial setting. For a maximum time-dial setting "contacts fully

open$, typical reset times might #e sec for an inverse-time relay and up to sec for a very

inverse or extremely inverse relay. At lower time-dial settings, contact opening distance is less,

therefore reset time is lower.

A solid-state relay is not dependent on mechanical forces or moving contacts for its operation #ut

performs its functions electronically. Therefore, the timing can #e very accurate even for currents

as low as the pic'up value. There is no mechanical contact #ounce or arcing, and reset times can

#e extremely short.

CT and PT selection

8 current transformer

)n selecting instrument transformers for relaying and metering, a num#er of factors must #e

considered& transformer ratio, #urden, accuracy class, and a#ility to withstand availa#le faultcurrents.

CT ratio. CT guidelines mentioned earlier are to have rated secondary output at 11 to 1%5( of

expected load and no more than 1A secondary current at maximum primary fault current.2here more than one CT ratio may #e re*uired, CTs with tapped secondary windings or multi-

winding secondaries are availa#le.

CT #urden. CT #urden is the maximum secondary load permitted, expressed in voltamperes "A$

or ohms impedance, to ensure accuracy. A94) standards list #urdens of %.5 to 5A at :(power factor ";F$ for metering CTs, and %5 to %A at 5( ;F for relaying CTs.


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CT accuracy class. A94) accuracy class standards are .6, ., or 1.%(. 7atio errors

occur #ecause of 7 heating losses. ;hase-angle errors occur #ecause of magneti3ing core

losses.

CTs are mar'ed with a dot or other polarity identification on primary and secondary windings so

that at the instant current is entering the mar'ed primary terminal it is leaving the mar'edsecondary terminal. ;olarity is not re*uired for overcurrent sensing #ut is important for

differential relaying and many other relaying functions.

;T ratio. ;T ratio selection is relatively simple. The ;T should have a ratio so that, at the rated

primary voltage, the secondary output is 1%. At voltages more than 1( a#ove the rated

primary voltage, the ;T will #e su#ect to core saturation, producing voltage errors and excessheating.

;T #urden. ;Ts are availa#le for #urdens from 1%.5A at 1( ;F to as high as A at 05(

;F.

;T accuracy. Accuracy classes are A94) standard .6, ., or 1.%(. ;T primary circuits,

and where feasi#le ;T secondary circuits as well, should #e fused.

CTs and ;Ts should have ade*uate capacity for the #urden to #e served and sufficient accuracy

for the functions they are to perform. ?owever, more #urden or accuracy than necessary will

merely increase the cost of the metering transformers. 4olid-state relays usually impose lower

#urdens than electromechanical relays.

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Setting the Pickup Point_CT_PT