SEL SEL-387 Current Differential and Overcurrent Relay

SEL-387 Current Differential and Overcurrent Relay

Versatile Solution for

Power Apparatus Protection

Major Features and Benefits

The SEL-387 Relay offers restrained and unrestrained differential protection for two to four terminals.

Second-, fourth-, and fifth-harmonic elements, augmented by the dc element, provide security during trans

former energization and overexcitation conditions in a user-defined choice of either harmonic restraint or har

monic blocking. Overcurrent elements provide backup protection that contributes to relay versatility.

Oscillographic event reports, Sequential Events Recorder (SER), circuit breaker contact wear monitor, and

substation battery voltage monitor are all standard features. Four communications ports, local display panel,

and extensive automation features are also standard. Expanded I/O is available as an option. A restricted

earth-fault feature provides sensitive protection against earth faults for wye-connected transformers.

➤ Protection. Protect transformers, buses, generators, reactors, and other apparatus with a combination

of differential and overcurrent protection. Set the differential element with either a single- or dual

slope percentage differential restraint characteristic for increased security during through-fault condi

tions. Select the optional thermal modeling element for key information about transformer overload

capability.

➤ Metering. Interrogate the relay for instantaneous measurements of phase and demand current. The

recorded peak demand, including the date and time of occurrence, is provided.

➤ Monitoring. Schedule breaker maintenance when the breaker monitor indicates. Notify personnel

of substation battery voltage problems. Use the through-fault event monitor in the SEL-387-5 and

SEL-387-6 for information on system through faults and the resulting, cumulative I2t wear on trans

former banks. Monitor critical operating temperatures through use of the SEL-2600A RTD module.

➤ Automation. Take advantage of automation features that include 16 elements for each of

the following: local control and local indication with front-panel LCD and pushbuttons, remote con

trol, and latch control. Use the serial communications ports for efficient transmission of such key infor

mation as metering data, protection elements and contact I/O status, SER reports, breaker contact wear

monitor, relay summary event reports, and time synchronization. Select optional DNP3 Level 2 Slave

protocol with virtual terminal support for SCADA system interface capability.

➤ Relay and Logic Settings Software. ACSELERATOR QuickSet® SEL-5030 Software reduces engi

neering costs for relay settings and logic programming. The built-in HMI provides phasor diagrams

that help support commissioning and troubleshooting. The SEL-387-5 and SEL-387-6 are supported

by the QuickSet.

Protection Features

The SEL-387 contains a variety of protective elements

and control logic to protect two-, three-, or four- winding

power transformers, reactors, generators, and other appa

ratus. It includes current differential elements with per

centage restraint and harmonic blocking elements,

sensitive restricted earth-fault (REF) elements, and over

current elements. Use advanced SELOGIC® control equa

tions to tailor the relay to your particular application.

The relay has six independent setting groups. Use this

flexibility to configure the relay automatically for

virtually any operating condition including, for example,

loading and source changes.

Current Differential Elements

The SEL-387 has three differential elements. These ele

ments use operate and restraint quantities calculated

from the two-, three-, or four-winding input currents. The

differential elements are set with either single- or dual

slope percentage differential characteristics. Figure 2

illustrates a dual-slope setting. Slope 1 handles differen

tial currents resulting from CT errors and tap-changing.

Slope 2 prevents undesired relay operation resulting from

CT saturation for heavy external faults.

SEL-387-5 and SEL-387-6 relays guard against

harmonic conditions resulting from system and

transformer events that can cause relay misoperation.

Use the fifth-harmonic element to prevent relay

misoperation during allowable overexcitation conditions.

Even-harmonic elements (second and fourth harmonic)

guard against inrush currents during transformer

energization. The dc element, which measures dc offset,

adds to this security. The even-harmonic element offers

the choice between harmonic blocking and harmonic

restraint. In the blocking mode, you select either

blocking on an individual phase basis or on a common

basis, as per application and philosophy. Set second-,

fourth-, and fifth-harmonic thresholds independently,

and enable dc blocking and harmonic restraint features

independently.

All SEL-387 relays provide an additional fifth-harmonic

current alarm function that employs a separate threshold

and an adjustable timer to warn of overexcitation. This

may be useful for transformer applications in or near

generating stations.

In addition, a set of unrestrained differential current

elements simply compares the differential operating

current quantity to a setting value, typically about 10

times the TAP setting. This pickup setting is only

exceeded for internal faults.

Restricted Earth-Fault Protection

Apply the REF protection feature to provide sensitive

detection of internal ground faults on grounded wye-con

nected transformer windings and autotransformers. Only

one of the three ABC inputs of Winding 4 is used for

introduction of neutral CT polarizing current. Operating

current is derived from the residual current calculated for

the protected winding. A directional element determines

whether the fault is internal or external. Zero-sequence

current thresholds and selectable CT saturation logic

supervise tripping. One of the Winding 4 inputs is used

for the neutral CT, so a maximum of three winding

inputs may be used for normal differential and overcur

rent protection.

Overcurrent Protection

The SEL-387 has 11 overcurrent elements for each set of

3-phase current inputs, 44 elements total. Nine torque

controlled elements encompass one instantaneous, one

definite-time, and one inverse-time element for each

phase of the negative-sequence and residual currents.

The phase elements operate on the maximum of the

phase currents. The remaining two elements are phase

segregated to assist in phase identification for targeting

purposes or for level sensing type functions. These are

not torque controlled.

Two sets of combined overcurrent elements operate on

the vector sum of the Winding 1 and Winding 2 currents

and the vector sum of the Winding 3 and Winding 4

currents, respectively. The individual currents are scaled

by the appropriate ratio so that the combined current

accurately reflects the primary system current. Inverse

time phase and residual elements are available for each

of the combined currents. These combined elements

offer added flexibility when the relay is applied with

multiple breakers, such as in ring-bus or breaker-and-a

half applications. Different CT ratios are permitted on

the two windings that are summed to create the

combined current. For ideal CTs, these combined

overcurrent functions do not respond to circulating

current passing through both circuit breakers.

The time-overcurrent curves listed in Table 1 have two

reset characteristic choices for each time-overcurrent

element. One choice resets the elements if current drops

below pickup for at least one cycle. The other choice

emulates reset of an electromechanical induction disk

relay