AG2011-13_20110714

Date Code 20110714 SEL Application Guide 2011-13

Application Guide Volume VIII AG2011-13

Setting the SEL-710 Motor Protection Relay to Replace Existing Electromechanical Motor

Protection Relays on Induction Motors Jim Buff

INTRODUCTION Modern microprocessor-based motor protection relays, such as the SEL-710 Motor Protection Relay, use a thermal element instead of an inverse-time overcurrent element to protect the motor. To set the thermal element correctly, nameplate, data sheet, and system data are needed. Some motors may be lacking the data sheet. Either these data were never obtained, or over time, after many years of company changes, these data are missing from the files.

When all of the information is known, setting the SEL-710 is simply a matter of setting the relay with the values the motor manufacturer has given and then entering the specific system parameters for the motor position. Some enhancement to the protection can be gained if the motor overload curves are also available.

When all of the motor data are not available, the next best solution is to use the existing relay information to help set the motor protection relay thermal element. By using the existing electromechanical phase time-overcurrent relay settings, the hot locked rotor time can be assumed. This assumption is often conservative and can only be determined if the existing relay is correctly set to protect the motor from locked rotor conditions. If the relay is not already correctly set and all of the motor data are not known, then the relay setting must be adjusted after the motor is started by using the relay motor start report as a reference.

This application guide uses an example application to illustrate how to set the SEL-710 to replace existing electromechanical relays.

MOTOR PROTECTION REQUIREMENTS Properly protecting a motor requires that it be protected against the following:

• Three-phase and phase-to-phase faults in the motor or cable.

• Ground faults in the motor or cable.

• Locked rotor motor conditions.

• Overload motor conditions.

Optionally, the following may also be provided by the motor protection relay:

• Current unbalance or single-phase protection.

• Load jam protection.

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SEL Application Guide 2011-13 Date Code 20110714

GATHERING THE NECESSARY DATA

Motor Data

To properly replace a motor protection relay, the following information must be known about the motor:

• Full-load amperes (FLA) – should be on the motor nameplate.

• Service factor – is also on the motor nameplate.

• Locked rotor current – may be a value or a code letter.

• Safe stall time – is on the motor data sheet.

If the locked rotor current is in the form of a code letter, the following additional information must be known:

• Motor horsepower – should be on the motor nameplate.

• Rated voltage – should also be on the motor nameplate.

System Data

The following system data must be gathered:

• System phase rotation – should be known or on the drawings.

• Phase current transformer (CT) ratio – should be on the drawings.

• Ground CT ratio – should be on the drawings.

• Maximum ground fault current – is available from the fault study.

Electromechanical Relay Data

If the safe stall time and/or phase and ground fault currents are not known and it is assumed that the electromechanical relay was correctly set to protect the motor from overload, locked rotor, and phase and ground fault conditions, then the following electromechanical relay information is needed:

• Phase and ground relay type – is on the relay nameplate or test report.

• Phase and ground relay tap – is on the relay tap block or test report.

• Phase and ground relay time dial – is on the relay dial or test report.

EXAMPLE DATA The motor data for the example in this application guide are the following:

• Motor horsepower = 600 hp

• Rated voltage = 2300 V

• FLA = 133 A

• Service factor = 1.15

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• Locked rotor current = Code D

• Safe stall time = unknown

The following are the system data for this example:

• System phase rotation = ABC

• Phase CT ratio = 200/5

• Ground CT ratio = 50/5

• Maximum ground fault current = 400 A

The example electromechanical relay data include the following:

• Phase relay type = GE IAC66K

• Phase relay tap = 5.6 A

• Phase relay time dial = 5.0

• Ground relay type = GE PJC11

• Ground relay tap = 2.0 A

EXAMPLE MOTOR AC CONNECTIONS TO THE SEL-710 Figure 1 shows the CT connections to the SEL-710 for the example motor.

2.30 kVBus

IAINZ08 Z07 Z01Z02

IB Z03Z04

IC Z05Z06

123

50/5 200/5

SEL-710

Circuit Breaker

Air Compressor

Motor600 hp,2.30 kV

Figure 1 Example Motor AC Connections to the SEL-710

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EXAMPLE MOTOR DC CONNECTIONS TO THE SEL-710 Figure 2 shows the dc connections to the SEL-710 for the example motor.

Figure 2 Example Motor DC Connections to the SEL-710

CALCULATING LOCKED ROTOR CURRENT FROM A CODE LETTER When the locked rotor current is given as a code letter, the approximate locked rotor current can be calculated with the following formula:

A • B • 577

LRAC • D

⎛ ⎞= ⎜ ⎟⎝ ⎠

(1)

where:

LRA = approximate per-unit (pu) locked rotor current.

A = kVA/hp multiplier (see the appendix).

B = motor horsepower.

C = motor rated voltage.

D = motor FLA.

The following is an example calculation:

( )( )

4.25 • 600 • 577LRA 4.8 FLA or 638 A primary

2300 •133= = i (2)

CALCULATING SAFE STALL TIME FROM RELAY INFORMATION When the safe stall time (hot locked rotor time) is not known, the approximate locked rotor time can be calculated, assuming that the electromechanical motor protection relay was properly protecting the motor for locked rotor conditions.

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Step 1

Calculate the approximate locked rotor amperes as a multiple of the electromechanical pickup with the following formula:

( )( )LRA • D

MOPCTR • E

= (3)

where:

MOP = multiple of electromechanical pickup.

LRA = approximate pu locked rotor current.

CTR = phase CT ratio.

D = motor FLA.

E = phase relay tap.

Step 2

Using the electromechanical relay curve, find the intersection of the multiple of pickup and the phase relay time dial. Locate the time to trip at MOP. This will be the approximate hot locked rotor time.

The following is an example calculation:

( )( )4.8 •133

MOP 2.8540 • 5.6

= = (4)

Using GE Instruction Manual GEK-86722, Figure 1, and a time dial of 5.0, the trip time is 30 seconds. The approximate hot locked rotor time is 30 seconds.

SEL-710 DEFAULT SETTINGS Some of the default settings for the SEL-710 do not need to be changed for the relay replacement. These settings include the following:

• E2SPEED:= N (enable two-speed motor protection). Unless the relay is replacing two-speed motor protection, this should remain set to N. If it is set to Y, two separate values of motor parameters (one for each speed) must be entered as thermal protection settings.

• E49MOTOR:= Y (enable thermal motor protection). The relay uses a thermal element based on motor current and motor parameters to protect the motor for locked rotor and overload conditions. This setting enables this protection.

• SETMETH:= RATING (motor parameters used in the protection element). The rating thermal method configures the thermal protection based on the motor FLA, service factor, running state time constant, locked rotor amperes, hot locked rotor time, and locked rotor trip time dial settings. This application guide addresses the case when all these parameters may not be available.

• FLS:= OFF (use full load slip and locked rotor torque to calculate rotor resistance). This setting is used for induction motors that have slip and locked rotor torque data. When these data are not available, this setting should be turned to OFF.

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• TD1:= 1.00 (locked rotor trip time dial setting). For the relay replacement, this is set to 1.00 so the locked rotor curve will be used to its full potential.

• RTC1:= AUTO (running state time constant). For the relay replacement, it is assumed that the thermal overload curves are not available for the motor. If they are available, then they may be used. This application guide does not address a case when these curves are available to the user.

• TCSTART:= OFF (thermal capacity used to start). Without knowing the nature of the acceleration of the load, it is difficult to set this feature. This feature keeps the trip active if there is not enough thermal reserve available for a safe start to be attempted. This setting may be set after the motor is started.

• 49RSTP:= 75 (thermal overload reset). This specifies that if a thermal overload does trip the motor, it will not allow a start until the thermal capacity used (TCU) has decreased from 100 to 75 percent.

• TCAPU:= 85 (overload warning level). This specifies that if TCU is above 85 percent, then the Relay Word bit 49A (overload alarm) asserts. This Relay Word bit can be routed to an output to indicate that TCU is getting near the overload trip point.

• 50P1D:= 0.00 seconds (phase overcurrent time delay). This high-set phase overcurrent protection should be instantaneous in operation and should be set without any intentional time delay.

• E47T:= Y (enable phase reversal logic). This element protects for the incorrect current phase sequence. If the current has the wrong phase sequence, then it is desirable to trip.

• 52A:= 0. This is recommended when there is no breaker status wired.

SEL-710 MAIN IDENTIFIER SETTINGS These settings help identify the relay and associate it to a particular motor. This information is extremely helpful during analysis of motor start reports and event reports. These settings include the following:

• RID:= 600 HP MOTOR (relay or motor identifier). This identifies the motor that is being protected by this relay. The identifier may be up to 16 characters in length.

• TID:= ACME PLANT (terminal or plant identifier). This may identify the terminal or plant where the motor resides. The identifier may be up to 16 characters in length.

SEL-710 MAIN CONFIGURATION SETTINGS The following settings are used to configure the relay to accurately scale measured values:

• CTR1:= 40 (phase input CT ratio). The CTs are 200/5 or 40/1.

• CTRN:= 10 (neutral input CT ratio). This CT is a 50/5 zero-sequence CT or 10/1.

• FLA1:= 133 (motor full-load primary amperes). This is from the motor nameplate information.

• PHROT:= ABC (system phase rotation). This setting is actually a global setting but is used for sequence calculations in the relay. The correct system rotation must be entered.

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SEL-710 THERMAL OVERLOAD AND LOCKED ROTOR SETTINGS The following settings are used to configure the thermal overload and thermal locked rotor model in the relay:

• SF:= 1.15 (service factor 1.01 is the minimum). This is from the motor nameplate.

• LRA1:= 4.8 pu (locked rotor amperes). This is from (2).

• LRTHOT1:= 30 seconds (hot locked rotor time). This is from the GE IAC66K relay curves.

• COOLTIME:= 157 minutes (stopped motor cool time). This setting is dependent on the preceding settings. First, the running time constant (RTC) that the relay is using (RTC:= AUTO) must be calculated. Once RTC is known, it can be multiplied by three to determine the minimum motor stopped cool time.

( )

( )( )

2 2

2 2

TD1 0.2 • LRTHOT1RTC

LRA1 0.81•SF60 • Ln

LRA1 SF

+=⎛ ⎞⎛ ⎞−⎜ ⎟⎜ ⎟

⎜ ⎟⎜ ⎟−⎝ ⎠⎝ ⎠

(5)

( )COOLTIME 3• RTC 1≥ + (6)

where:

Ln = natural logarithm function.

The following are example calculations:

( )

( )2 2

2 2

1.2 • 30RTC 52 minutes

4.8 0.81•1.1560 • Ln

4.8 1.15

= =⎛ ⎞⎛ ⎞−⎜ ⎟⎜ ⎟

⎜ ⎟⎜ ⎟−⎝ ⎠⎝ ⎠

(7)

( )COOLTIME 3• 52 1 157 minutes= + = (8)

SEL-710 OVERCURRENT SETTINGS The following settings are used to configure the phase and ground overcurrent elements in the relay to detect phase and ground faults:

• 50P1P:= 9.6 pu of FLA1 (phase overcurrent pickup) = 2 • LRA1. A setting of twice the locked rotor current setting gives the instantaneous phase overcurrent plenty of sensitivity for phase faults without tripping for motor starting conditions. (Do not use this function when the interrupting device is a contactor and not a circuit breaker.)

• 50N1P:= 10 A (neutral ground overcurrent pickup) = CTRN. As long as the maximum ground fault current is at least twice this value, there should be no problem with this setting. This setting should offer enough sensitivity for ground faults without tripping for transient conditions.

• 50N1D:= 0.1 second (neutral ground overcurrent delay). This is a definite-time delay of 0.1 second or 6 cycles. This is the recommended delay setting to avoid tripping for transient conditions.

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OTHER OPTIONAL RECOMMENDED SETTINGS The following are the recommended optional settings for current unbalance and load jam protection:

• 46UBT:= 15 percent (current unbalance trip threshold).

• 46UBTD:= 5.00 seconds (unbalance definite-time delay).

• LJTPU:= 2.00 pu (load jam trip threshold).

• LJTDLY:= 2.00 seconds (load jam definite-time delay).

TRIP LOGIC SETTINGS The trip logic is based on the elements that were set previously. The logic statement contains many Boolean OR functions that make the trip equation act as a set of parallel tripping elements. The following are the trip logic settings:

• TR:= 49T OR 50P1T OR 50N1T OR 47T OR 46UBT OR JAMTRIP, where:

− 49T = locked rotor and overload thermal element.

− 50P1T = phase instantaneous overcurrent element.

− 50N1T = neutral ground definite-time overcurrent element.

− 47T = current phase sequence element.

− 46UBT = current unbalance definite-time delay element.

− JAMTRIP = load jam definite-time element.

• OUT103FS:= N. OUT103 will not be in a fail-safe mode.

• OUT103:= TRIP. This output will be the trip output.

PLOTTING THERMAL CURVES The SEL-710 uses the 49T thermal element for locked rotor and overload conditions. Plotting these locked rotor and overload curves requires the use of (9), (11), and (12).

The locked rotor element is used for currents ranging from 2.5 • FLA to 12 • FLA. The following equation, which can also be used for testing, plots the locked rotor protection curve:

( )2

2

LRTHOT1• LRA1Tp

I= (9)

where:

I = current in multiples of FLA.

LRTHOT1 = hot locked rotor time setting.

LRA1 = locked rotor ampere setting.

The following is an example calculation at 4 pu:

( )2

2

30 • 4.8Tp 43.2 seconds

4= = (10)

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The following equations plot the stator overload protection curve. The stator overload element is used for currents ranging from service factor to 2.5 • FLA. Equation (11) is the cold curve equation, which can be used for testing, and (12) is the hot curve equation.

( )

2

2 2

ITp 60 • RTC • Ln

I SF

⎛ ⎞⎜ ⎟=⎜ ⎟−⎝ ⎠

(11)

( )( )

( )

22

2 2

I 0.9 •SFTp 60 • RTC • Ln

I SF

⎛ ⎞−⎜ ⎟= ⎜ ⎟−⎜ ⎟⎝ ⎠

(12)

where:

I = current in multiples of FLA.

RTC = running time constant calculated for cool time.

SF = service factor setting.

The following are example calculations at 1.5 pu, where (13) is the cold curve calculation and (14) is the hot curve calculation:

2

2 2

1.5Tp 60 • 52 • Ln 2765 seconds

1.5 1.15

⎛ ⎞= =⎜ ⎟−⎝ ⎠

(13)

( )22

2 2

1.5 0.9 •1.15Tp 60 • 52 • Ln 748 seconds

1.5 1.15

⎛ ⎞−⎜ ⎟= =⎜ ⎟−⎝ ⎠

(14)

Figure 3 shows a plot of the new SEL-710 hot overload curve compared to the old GE IAC66K overcurrent curve that was protecting the motor.

Tim

e (S

econ

ds)

Figure 3 SEL-710 Hot Thermal Curve Versus GE IAC66K Curve

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MOTOR START REPORT Figure 4 is an example of the motor start report text using the MSR terminal command to retrieve the report. The data may also be retrieved using ACSELERATOR QuickSet® SEL-5030 Software. Some of the important information that is shown in this report is as follows:

• Motor and plant identifier.

• Relay firmware version and setting revision.

• Date and time of motor start.

• Motor accelerating time.

• Starting TCU.

• Maximum starting current.

• Sampled phase and neutral currents and TCU every 5 cycles.

Figure 4 Abbreviated Example of a Motor Start Report

Figure 5 shows a plot of the motor start report using Microsoft® Excel® to plot the data.

500

450

400

350

300

250

200

150

100

50

00.0 2.0 4.0 6.0 8.0 10.0

Mot

or C

urre

nt (

% F

LA)

% T

herm

al C

apac

ity

Time (Seconds)

40

35

30

25

20

15

10

5

0

% I

% TC

Figure 5 Plot of Motor Start Report

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If it is known that the electromechanical protection was not set correctly, the locked rotor ampere setting (LRA1) and the hot locked rotor time setting (LRTHOT1) can be adjusted using the motor start report as a reference. The settings can be adjusted as follows:

MSI

LRA1FLA1

= (15)

where:

MSI = motor start report maximum start current.

FLA1 = FLA setting.

LRTHOT1 ST 3 seconds= + (16)

where:

ST = motor start report start time.

The following are example calculations:

612

LRA1 4.6 pu133

= = (17)

LRTHOT1 8.3 3 11.3 seconds= + = (18)

These settings replace the LRA1 and LRTHOT1 settings that were initially used to protect the motor. Using an average of data from multiple motor start reports can improve (15) and (16).

CONCLUSION The SEL-710 has the following advantages over the old electromechanical relay:

• It requires less panel space, offers more reporting capabilities, and needs no ongoing calibration.

• It is much better at protecting the motor than the old electromechanical phase time-overcurrent relays due to the thermal model.

The SEL-710 settings can be made with the following guidelines:

• Locked rotor current can be calculated if it is not known, as long as the code letter is given. Otherwise, the locked rotor current must be measured.

• Hot locked rotor time can be approximated if the existing phase time-overcurrent relay settings are safely protecting the motor and the time is not known. Otherwise, the hot locked rotor time can be set to the motor acceleration time with 3 seconds added.

• Motor start reports can be used to verify the thermal model settings or to improve those settings using measured values.

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APPENDIX

Table 1 Locked Rotor Code and kVA/hp Multiplier

Locked Rotor Code kVA/hp Multiplier

A 1.575

B 3.350

C 3.775

D 4.250

E 4.750

F 5.300

G 5.950

H 6.700

J 7.550

K 8.500

L 9.500

M 10.60

N 11.85

P 13.25

R 15.00

S 17.00

T 19.00

U 21.20

V 22.40

FACTORY ASSISTANCE We appreciate your interest in SEL products and services. If you have questions or comments, please contact us at:

Schweitzer Engineering Laboratories, Inc. 2350 NE Hopkins Court Pullman, WA 99163-5603 USA Telephone: +1.509.332.1890 Fax: +1.509.332.7990 www.selinc.com • [email protected]

© 2011 by Schweitzer Engineering Laboratories, Inc. All rights reserved.

All brand or product names appearing in this document are the trademark or registered trademark of their respective holders. No SEL trademarks may be used without written permission.

SEL products appearing in this document may be covered by U.S. and Foreign patents.

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