Beckwith Electric M-3401 Load Shedding Relay

• Voltage Collapse Mitigation providing 4-step undervoltage and 4-step

underfrequency load shedding functions

• Undervoltage blocking and negative sequence overvoltage blocking for

system faults provides load shedding security

• Local and remote serial communications (MODBUS Protocol) for

monitoring and control functions

• 5 programmable outputs and 2 programmable inputs Protective Functions

• 4-Step Phase undervoltage (27) protection, single-phase and positive sequence

• 4-Step Phase undervoltage, selectable as single phase or positive sequence

responding, with Negative sequence overvoltage and three phase undervoltage

supervision

• Phase overvoltage (59) protection

• Four-step over/under frequency (81) protection

• Rate of Change of Frequency (81R) protection

• IPSlogic™ takes the contact INPUT status and function status and generates

OUTPUTS by employing (OR, AND, and NOT) boolean Logic and a timer.

Standard Features

• 5 programmable outputs, 2 programmable inputs, and 1 self-test output

• Oscillographic recording (COMTRADE file format)

• Time-stamped sequence of events (SOE) recording for 32 events

• Metering of Voltage and Frequency

• One RS-232 port (COM1) on front and one RS-232 or 485 port (COM2) on

rear

• M-3812 IPScom® For Windows® Communications Software

• MODBUS protocol

• Supports both 50 and 60 Hz applications

• Relay voltage inputs can be directly connected (no VT required) for

voltages ≤ 480 Vac

• Continuous Self-Diagnostics

Optional Features

• M-3801D IPSplot® PLUS Oscillograph Analysis Software

• Horizontal and Vertical panel mount versions available

Description

The M-3401 Load Shedding Relay provides voltage load shedding,

frequency load shedding or supervised voltage or frequency load

shedding to assist in voltage collapse mitigation. 

The M-3401 Load Shedding Relay can be integrated into load shedding

schemes that are part of an Energy Management System (EMS), or into

automatic load shedding schemes as defined by the IEEE Power System

Relaying Committee, Substation Protection Subcommittee, Working

Group K-12.

In load shedding as part of EMS application, a control signal is sent to

multiple remote switches at participating blocks of customers to interrupt

loads for predetermined intervals. This method of load shedding can be

manually performed by dispatchers at the EMS control station, or they

may be automatic from the EMS using the logic, measurement and

intelligence of the M-3401.

In the Automatic Load Shedding scheme where voltage instability is

caused by sudden loss of critical transmission equipment or VAr

generating equipment (i.e. very short collapse time), the M-3401 provides

the means to quickly arrest fast voltage drop by disconnecting selected

large blocks of customer load allowing voltage to recover.

The security of an undervoltage load setting scheme is increased with

multiple phase detection rather than single phase, proper time coordination

between fault clearing and the time delay for load shedding and also by

using various supervision techniques that prevent nuisance tripping for

voltage conditions that do not lead to collapse.

This M-3401 offers three-phase line to line or line to ground configuration,

optimal pickup/ seal-in time delays from 2 to 8160 cycles, and negative

sequence voltage (47S) as well as undervoltage (27S) supervisory functions

that ensure security and reliable operation.

To ensure that all data is collected during a load-shedding event the M-3401

has oscillographic recording, storing 180 cycles of all measured parameters.

Additionally, the M-3401 has a Sequence of Events (SOE) recorder built-in to

capture and store a total of 32 events with 1mSec resolution time stamp.

Undervoltage Load Shedding

Area undervoltage can occur when there is a lack of reactive support for the

load. This lack of reactive support manifests itself as an undervoltage condition,

with the undervoltage most severe at the load area requiring the reactive support.

As load in the transmission systems are approximately symmetrical on all three

phases, the undervoltage condition is seen on all three phases. If the power

system voltage profile over the system is viewed as a plane, the undervoltage

event from lack of reactive support can be conceptualized as a depression from

the 1.0 pu level nominal) to some lower level, with the lowest point the area of

highest reactive support requirement.

A method of gaining selectivity for load shedding is to employ multiple time

undervoltage elements. In that manner, the localized areas with the lowest voltage

(highest reactive power support requirements) are shed first.

When implementing an undervoltage load shedding scheme, it must be secure

from asymmetrical voltage depressions occurring from unbalanced faults; single

phase-to-ground, phase-to-phase, and phase-phase

to phase-ground faults as well as three-phase faults and complete system

de-energizations.

• Asymmetrical voltage depressions occurring from single to phase-to-ground, phase

-to-phase, and phase-phase to ground faults are detected by using a negative

sequence overvoltage element (47S) as a supervision. Typically set to 0.05 to 0.1 pu

of nominal voltage, if the adjustable threshold is exceeded, the undervoltage load

shedding is blocked.

• Three-phase faults that decrease the voltage symmetrically across all three phases,

as well as complete de-energizations of the parts of the power system due to fault

clearing or other sectionalizing, are detected by using undervoltage supervision

(27S) on all phases. The undervoltage supervision is typically set lower than the lowest

survivable undervoltage that could occur from lack of reactive support before the entire

system voltage collapses, typically from 0.9 to 0.7 pu. If any phase voltage is lower

than the adjustable setting, it is assumed that the condition is from a fault (any type,

including three-phase) or complete de-energization of that part of the system.