Date Code 20120504 SEL Application Guide 2012-03

Application Guide Volume I AG2012-03

Applying SEL-311L Relays on 120- to 240-Kilometer Lines

Alex Rangel and David Costello

INTRODUCTION The SEL-311L Line Current Differential System is a digital line current differential relay with an integrated communications interface. A typical two-terminal line application consists of two SEL-311L Relays. Direct communication between the relays is performed through a fiber-optic cable at a distance of up to 120 kilometers. When this distance is longer than 120 kilometers, a third SEL-311L may be used as a pseudo repeater. This can extend the application of direct fiber-optic cable to lines up to 240 kilometers long.

OVERVIEW Consider the application shown in Figure 1. At each line terminal, there is an SEL-311L that measures and communicates local current. A third (master) SEL-311L is used to extend the total communications distance between the two line-end relays. The master relay does not measure any local current. It communicates with the two line-end relays, receives the line-end current measurements, makes a differential trip decision, and communicates that decision back to the line-end relays. This is useful when the distance between the two terminals is longer than 120 kilometers, the maximum distance of a single point-to-point application. This application is similar to a three-terminal line with only two communications channels. However, one of the terminals is essentially open all of the time (i.e., it measures no local current).

Figure 1 240-Kilometer Two-Terminal Line Application Using Fiber-Optic Cable

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SEL Application Guide 2012-03 Date Code 20120504

THREE-TERMINAL DIFFERENTIAL ELEMENT In Figure 1, Relay 3 is configured as the master relay by setting E87L = 3. It is the only relay that receives all current information, performs the differential element operation, and sends transfer trip signals to the remote line-end terminals for internal faults. Relay 3 must be ordered with two fiber-optic interfaces. Relay 1 and Relay 2 are configured as remote relays by setting E87L = 3R. These relays transmit current information and receive direct transfer trip signals for internal faults. Relay 1 and Relay 2 may each be ordered with a single fiber-optic interface.

In a normal three-terminal application (where each terminal measures current), two of the terminals are combined (vectorly added) to produce the equivalent remote current. The remaining (uncombined) current becomes the local current in the calculation of operate (difference) current and the Alpha Plane ratio of remote to local currents. In other words, the SEL-311L converts the three-terminal line into an electrically equivalent two-terminal line and then applies two-terminal line algorithms. Additionally, the SEL-311L considers all three possible combinations of local and remote currents (shown in Table 1) and selects the maximum terminal current as the local current. Remember from Figure 1 that Relay 3 (master) never measures current, so the third combination in Table 1 should never be used by the master relay in this application.

Table 1 All Possible Combinations Used for Local and Remote Currents in a Three-Terminal Application

Possible Combination Local Current Remote Current

1 I1 I2 + I3

2 I2 I1 + I3

3 I3 I1 + I2

A simplified representation of the three-terminal differential element processing in the master SEL-311L is shown in Figure 2.

Z InL Z InR+

Z InR

Z InL

Z InR 2

Z InR 3

Figure 2 Phase, Negative-Sequence, and Zero-Sequence Differential Element Processing for the First Combination in Table 1

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Date Code 20120504 SEL Application Guide 2012-03

METERING AND THE ALPHA PLANE The terminal command MET B displays the physically local relay currents and voltages. For Relay 3, these values are always zero because there are no current transformer (CT) and potential transformer (PT) connections. For Relay 1 and Relay 2, MET B displays the actual currents and voltages measured at Terminal 1 and Terminal 2, respectively.

The terminal command MET shows currents at all terminals. At Relay 3, local currents are always zero, Channel X currents match Relay 1 currents in primary amperes, and Channel Y currents match Relay 2 currents in primary amperes. At Relay 1 and Relay 2, local currents are shown and Channel X currents are always zero.

The terminal command MET also shows differential quantities. Because Relay 1 and Relay 2 are configured as remote relays by setting E87L = 3R, differential or Alpha Plane quantities are not displayed. Relay 3 displays the differential quantities (vector sum) and zeros for the Alpha Plane. This is because the MET command always uses the physically local relay current (which is zero in this application) for local differential current. Note that the actual differential algorithm determines local current to be the maximum terminal current and does not use the physically local relay current, as described in the previous section.

To visualize the operation of the differential element and the Alpha Plane in the master Relay 3, use ACSELERATOR QuickSet SEL-5030 Software to view an event report triggered by Relay 3. Select VIEW ALPHA PLANE, select 3T for the primary channel, and select the terminal (Channel X or Channel Y) with the largest current as the 3-Terminal Channel Reference.

SEL-311L SETTINGS The channel settings TIMRX and TIMRY must be set as shown in Table 2 for proper synchronization.

The relay setting E87L determines the number of terminals in the 87L protection zone. Because there are only two communications channels, Relay 1 and Relay 2 are set as remote relays and Relay 3 is set as the master, as shown in Table 2. Note that for this application, all of the relays must have the same nominal current rating.

Table 2 Relay Settings

Setting TIMRX TIMRY E87L 52A CT Ratio

(CTR) CTR_X CTR_Y

Relay 1 E 3R IN101 Actual

Relay 1 CTR 1

Relay 2 E 3R IN101 Actual

Relay 2 CTR 1

Relay 3 I I 3 1 1 Actual

Relay 1 CTR Actual

Relay 2 CTR

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SEL Application Guide 2012-03 Date Code 20120504

The logic setting 52A is typically set to follow a breaker status input wired to IN101. In Relay 1 and Relay 2, this should be the case. Relay 3 does not measure local current. Therefore, 52A is the only local setting that determines if the local 3PO (three-pole open) Relay Word bit asserts. Differential operate setting 87LPP (phase) doubles and 87LGP (ground) and 87L2P (negative-sequence) triple when the local 3PO Relay Word bit is asserted and for three cycles after it deasserts. This allows for inrush current after a breaker close operation. Set 52A to 1 to maintain sensitivity at set pickup values. Note that remote terminal 3PO operation also acts to adjust settings for security during breaker close operations.

The differential pickup settings 87LPP, 87L2P, and 87LGP are in units of secondary amperes, referenced to the maximum CT ratio. No CT is connected to Relay 3. Therefore, set the CTR setting associated with Relay 3 to 1 in all relays.

In order to obtain successful communication between the relays, the proper transmit and receive addresses must be specified in Channel X settings TA_X and RA_X (TA_Y and RA_Y for Channel Y). Figure 3 shows one possible channel address configuration.

Relay 3

Channel X Channel Y

TA_X = 2

RA_X = 1

TA_Y = 4

RA_Y = 3

Channel X

TA_X = 1

RA_X = 2

Relay 1

Channel X

TA_X = 3

RA_X = 4

Relay 2

Figure 3 Channel Address Configuration

SEL-311L TEST PROCEDURE Test this application the same as any SEL-311L two-terminal application, with one exception. Relay 3 is the only relay in this system that calculates differential element operation and issues trip signals to Relay 1 and Relay 2. For assistance with testing procedures, refer to the SEL-311L Instruction Manual and the Microsoft Excel spreadsheet available for download with this application guide at http://www.selinc.com.

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Date Code 20120504 SEL Application Guide 2012-03

CONCLUSION When the distance between two SEL-311L Relays is longer than 120 kilometers, a third SEL-311L may be used as a pseudo repeater. This can extend the application of direct fiber-optic cable to lines up to 240 kilometers long.

In this case, the line current differential function of the SEL-311L is essentially a two-terminal application. The third relay measures no physically local current but receives current measurements from the two line-end terminals and acts as the master, deciding when to trip the line. The master relay needs to have X and Y communications channels; however, the line-end terminal relays only need to have one communications channel.

In this application, it is especially important to set 52A = 1 and CTR = 1 in the master relay. In the line-end terminal relays, it is important to set CTR_X = 1. This ensures that the relay operates as expected based on the 87LPP, 87L2P, and 87LGP settings.

The SEL-311L has many other functions besides line current differential. However, in this application guide, we only cover the most important line current differential settings for two-terminal applications that exceed 120 kilometers.

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 info@selinc.com

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SEL Application Guide 2012-03 Date Code 20120504

2012 by Schweitzer Engineering Laboratories, Inc. All rights reserved.

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

*AG2012-03*