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MV SWITCHGEAR-DCS INTERFACE SIGNALS DESCRIPTION
A. Introduction
This document must be read in conjunction with the document describing the philosophy on
the interfacing of DCS with switchgear (i.e. Tutuka LV/MV Station Electrical Reticulation
and Process Load Feeders Interface Document) as well as the document titled ‘C&I
Interface Signals for MV Switchgear'.
The trips, alarms, indication, control and measurement signals that are exchanged via the
interface between the Medium Voltage (MV) Switchgear panels and the Distributed Control
System (DCS) are as described below.
B. Descriptions of Signals (in alphabetical order)
1.
BPS Interlock
This is a specific signal that is sent to the Boiler Protection System (BPS) to be used in their
safety interlocks.
The signal is activated every time the motor (drive) that is affected in the BPS safety
interlocks is running.
2.
Breaker Racked In
This is a status indication used to inform the operator at the DCS workstation that the circuit
breaker is in a connected position where it could be closed to switch the feeder on.
This status indication should also be reflected on the IED mimic.
3.
Breaker Racked Out
This indication is initiated once the circuit breaker is moved to a disconnected position. Only
electrical and DCS functional tests could be done on the switchgear while the circuit breaker
is in this position.
The signal will serve as information to the operator that the circuit breaker cannot be
controlled from the remote location unless it is for test purposes.
This status indication should also be reflected on the IED mimic.
4.
Busbar Earth-switch Applied
This indication is initiated once the busbar earth switch has been applied. A signal should be
sent to the DCS to indicate the status of the busbar earth-switch.
5.
Busbar Voltage
Just like in the case of the cable voltage measurement above, the busbar voltage is
measured using a busbar connected VT and then sent to the DCS via a transducer in the
form of an analogue (4-20mA) signal.
This will then serve as an indication that there is voltage applied to the busbars of a particular
board. The signals will also be recorded to assist with the data acquisition and analysis in
case of need.
The measured values should also be shown to the IED mimic.
6.
Cable Earth-switch Applied
This indication is initiated once the cable earth switch has been applied. A signal should be
sent to the DCS to indicate the status of the cable earth-switch.
This status indication should also be reflected on the IED mimic.
7.
Cable Voltage
This is a cable voltage measurement, normally obtained from the Red to White phases but
any combination of the three phases can be used. The analogue signal is send to the DCS
through a transducer which gets the input from a single-phase cable connected Voltage
Transformer (VT).
This will serve as an indication that there is voltage applied on the cable where the
measurement is taken. The signals will also be recorded to assist with the data acquisition
and analysis in case of need.
The measured values should also be shown to the IED mimic.
This status indication should also be reflected on the IED mimic.
8.
Chop-over Failure
Following an initiation of a chop-over either automatically on switchgear or manually by the
operator, the transfer might not be successful depending on the conditions of the system. If
the Chop-over Failure is realised, a signal must be sent to the EOD to notify the operator of
such an event.
Chop-over Failure function is applicable to specific interconnected and incomers where
provision for chop-over is made.
9.
‘Circuit’ Closed
The term ‘Circuit’ will be replaced by the relevant circuit type applicable for the following:
 Incomers (including Diesel Generator Incomer)
 Interconnectors
 Transformer Feeders
 Bus sections
 Maintenance Isolators
The ‘Circuit’ Closed signal will be sent to the DCS to indicate that the circuit breaker of the
feeder/ incomer concerned is closed.
Unlike in the case of motor feeders, the types of circuits described in Section 18 could remain
closed even if power to the feeder/ incomer concerned is cut.
This status indication should also be reflected on the IED mimic.
10.
‘Circuit’ Open
The signal ‘Circuit’ Open will be sent to the DCS when the circuit breaker of the feeder/
incomer concerned is in an open state.
This status indication should also be reflected on the IED mimic.
11. Command Chop-over
Provision of this command signal makes it possible for the transfer between two independent
power supply sources can be initiated by the operator from the DCS workstation. The signal
will close the circuit breaker of the incomer or interconnector concerned.
Command Chop-over function is applicable to specific interconnected and incomers.
12. Command Close
Similar to the Command Run signal above, the operator need to be able to close the feeder
and incomer circuit breakers from the remote DCS workstation.
It is applicable to all circuit breakers other than those used on the motor feeders where the
terminology Command Run is used.
13. Command Open
It applies the same concept as the Command Stop command signal above, except it is for the
other circuits such as feeders (cables and transformers), bus sections and incomers.
14. Command Run
Provision of this command signal makes it possible for the drive to be started by the operator
from the DCS workstation. The signal closes the circuit breaker of the MV motor circuit
concerned.
Command Run function is applicable to all MV drives.
15. Command Stop
Provision of this command signal makes it possible for the drive to be stopped by the operator
from the DCS workstation. The signal opens the circuit breaker of the MV motor circuit
concerned.
Command Stop function is applicable to all MV drives.
16. Current
This is a current measurement that is normally presented by an analogue (4-20mA) signal
from the white phase Current Transformer (CT) via a transducer.
When shown at the HMI on the control desks it will serve as an indication that the circuit
concerned has some load drawn from it. The signals will also be recorded to assist with the
data acquisition and analysis in case of need.
The measured values should also be shown to the IED mimic.
17. DC Control Power Supply Healthy
The Direct Current (DC) power supply to the protection system including the IED’s and
protection circuit is critical and need to be monitored.
If the supply fails, the controls and protection functions on the board will be disabled. Hence,
the Control Power Supply Healthy status should be monitored through the DCS by the
operators. If the conditions for Control Power Supply Healthy are not met, the signal will fall
away and this will require the operator’s attention.
18. Emergency Trip
Emergency Trip 1 is allocated to the Boiler Trip signal which is normally send from the DCS
to the MV switchgear motor feeder panel to isolate a particular drive in reaction to an
emergency situation on the process side.
The Emergency Trip 1 signal is send to the IED for recording purposes and also directly to the
second trip coil of the feeder circuit breaker to limit the time delay. The critical drives
considered for this application are:
 Boiler Feed Pump (BFP)
 Forced Draft (FD) Fans
 Induced Draft (ID) Fans
 Primary Air (PA) Fans
 Mill Motors
(NB: Depending on the requirement from process control, the signal might be disabled for
some of the drives)
Emergency Trip 2 function will be allocated to the emergency trip signals initiated from the
field such as conveyor safety trip. The mode of operation of this function is the same as that
of Emergency Trip 1.
In case a DCS has two channels for the Boiler Trip facility, both Emergency Trip 1 and 2
functions will be used. The two emergency trip signals will be sent separately to operate the
two trip coils (i.e. main and second trip coils) directly through two separate interposing relays
and also send a signal to the IED for recording purposes.
Emergency Trip Initiated signal will be sent to the DCS in response to any external trip
described above.
19. Emergency Trip Initiated
The Emergency Trip Initiated indication is used to inform the operator that the trip was
initiated by external sources including the DCS due to emergency situation/s.
20. Frequent Start Inhibitor
Due to limitations for the thermal stresses that a motor can withstand, the number of starts
over a specific period of time is limited. Algorithm is developed and programmed in the IED to
help with the monitoring of this situation and inhibit the closing of the breaker once the set
parameter is reached.
When the number of allowed operation is exceeded, the inhibit indication should be sent to
the DCS to inform the operator that the drive cannot be started.
21. Interlock Active
The Interlock Active indication signal is initiated any time the electrical interlocks are activated
to prevent a particular feeder from being operated by the DCS. The electrical interlocks will be
implemented on switchgear.
The signal will always be kept on as long as it is not safe to close the breaker.
22. Motor Stopped
The Motor Stopped signal will be sent to the DCS to indicate that the circuit breaker of the
motor feeder concerned is opened signifying loss of power supply to the motor.
NB. Motor feeders will be tripped for undervoltage conditions that would result in a loss of
power.
This status indication should also be reflected on the IED mimic.
23. Motor Running
The Motor Running signal will be sent to the DCS to indicate that the circuit breaker of the
motor feeder concerned is closed signifying power being transferred to the motor.
Exception will be in a Test position where there will be no power transferred to the motor to
drive it.
This status indication should also be reflected on the IED mimic.
24. Overvoltage Trip
Overvoltage conditions will prevail if the supply voltage goes above a set value (e.g. 10%
above the nominal value). The overvoltage conditions are monitored by IEDs using voltage
inputs from the cable VTs.
Diesel generator incomers are tripped when such conditions occur to prevent equipment
damage. If tripped, the diesel generator could be restarted once it is confirmed that it is the
external abnormal factors that caused an overvoltage condition.
25. Protection/ Control Healthy
The Protection Healthy alarm signal will be lost when any condition that might lead to the
malfunction of the protection system is triggered. Such conditions are:
 Faulty IED that will initiate the Internal Relay Failure (IRF) alarm
 Loss of power supply to the IED
 Fuse fail protection pick-up
This is monitored on all IEDs used for both protection and control purposes. This includes the
IEDs that will be used for control purposes only like those on bus section or maintenance
isolator protection schemes whereby the term Control Healthy is used.
26. Protection Trip (Generic)
This is a common or generic trip signal which is triggered by an initiation of trip by a protection
scheme of a particular electrical plant. The Protection Trip signal can only be reset on the IED
of the affected panel to release the latched trip signal.
The different protection functions shall be lumped such that if any fault occurs on the system,
the Protection Trip signal will be passed to the DCS. The details on the protection operation
including the sequence of events will be stored in the Intelligent Electronic Device (IED) and
also transferred to the Engineering Station for diagnosis and future reference purposes.
Examples of the protection trip functions that might be lumped as part of the Protection Trip
(Generic) are as follows:
 Differential
 Overcurrent
 Earth fault
 Negative phase sequence
 Buchholz
 Transformer winding temperature
 Transformer oil temperature
 Internal arc
 Buszone
27. Reverse Power Trip
Reverse power conditions will prevail if the diesel generator incomer goes into motoring mode
drawing current instead of supplying power to the board. The reverse power conditions are
monitored by IEDs using voltage inputs from the CTs and cable VTs.
Diesel generator incomers are tripped when such conditions occur to prevent equipment
damage. If tripped, the diesel generator could be restarted once it is confirmed that it is the
external abnormal factors (e.g. cable fault) that caused an underfrequency condition.
Otherwise, the diesel generator should be isolated and inspected to see if there is an internal
fault.
28. Switchgear Healthy
This is a ‘fail-safe alarm’ which is kept on as long as the switchgear and controlgear are safe
to operate. If any of the components in the switchgear or controlgear is faulty, it will lead to
the alarm going off.
One of those conditions will be the operation of the Trip Circuit Supervision (TCS) that is used
to monitor the condition of the control circuit including the circuit breaker trip coils and
auxiliary contacts. The other condition to be monitored would be the charging of the spring
rewind mechanism.
29. Switchgear on Local Control
This is an indication to inform the operator at the DCS workstation that the Circuit Breaker is
on local control whereby the process interlocks are disabled.
This allows the Test Run (i.e. motor directional tests and phasing verifications) operations to
be executed from the switchgear.
30. Switchgear on Remote Control
This is an indication to inform the operator at the DCS workstation that the Circuit Breaker is
on remote control whereby the process interlocks are enabled. The process interlocks can be
disabled for test purposes.
There is no operation that could be done locally on switchgear while in this status.
31. Thermal Overload Alarm and Trip
Thermal Overload Alarm and Trip signals are initiated by the overheating conditions on the
motor.
The IED will be used to model the temperature of the motor using the current input and initiate
a Thermal Overload Alarm if the temperature exceeds preset limits. Higher limits will be set
for a Thermal Overload Trip so that the motor feeder could be tripped if the overheating
conditions persist.
The plant operator is expected to check for overloading on the process once the thermal
overload alarm is initiated. The operator should be able to reset the alarm after the plant
returns to normal operation (i.e. without excessive overloading).
If tripped, the motor could be restarted once it is confirmed that it is the process related
overloading conditions that led to a protection trip.
NB: The same philosophy would apply for the diesel generators.
32. Underfrequency Trip
Underfrequency conditions will prevail if the frequency on the diesel generator incomer is
below a set value (e.g. 2% below the nominal value). The underfrequency conditions are
monitored by IEDs using voltage inputs from the cable VTs.
The underfrequency condition is normally liked to the speed of the prime mover of the
generator which would be running at a speed lower than a synchronous value. Diesel
generator feeders are tripped when such conditions occur to prevent equipment damage. If
tripped, the diesel generator could be restarted once it is confirmed that it is the external
abnormal factors that caused an underfrequency condition.
33. Undervoltage Alarm/ Trip
Undervoltage conditions will prevail if the supply voltage goes below a set value (e.g. 20%
below the nominal value). The undervoltage conditions are monitored by IEDs using voltage
inputs from the VTs.
Motor feeders are tripped when such conditions occur to prevent equipment damage. The
relevant motor protection IED will send Undervoltage Trip signal for any undervoltage
condition detected on the feeder concerned leading to its circuit breaker opening. If tripped,
the motor could be restarted once it is confirmed that it is the external abnormal factors (e.g.
cable fault) that caused an undervoltage condition.
In case of incomer and interconnector schemes, only Undervoltage Alarm will be issued to
the DCS to be recorded in the sequence of events. This will help with diagnosis of any
abnormal situation leading to an undervoltage condition upstream to the bay concerned.