ETAP 5.
DC Load Flow
DC Short-Circuit
Copyright 2003 Operation Technology, Inc.
Why DC System Studies?
DC systems play an ever increasing role in:
• Substation and generating station instrumentation
and control systems (IEEE Std 946)
• Class 1E power system for nuclear power
generation station – providing power to reactor trip
system and engineered safety features
(IEEE Std 308)
• Provide back-up energy for emergencies in
hospital, generation station, production line, data
processing and communication, etc.
(IEEE Std 446 Orange Book)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 2
Why DC System Studies?
Due to the importance of DC systems,
it must be:
• Reliable – Having redundant sources and
can operate under single failure in the
system
• Adequate – Capable of providing sufficient
power at required system voltage in
specified time duration
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 3
Type of Studies
• Load Flow – Determine system operating
conditions and verify equipment continuous ratings
• Short-Circuit Calculation – Determine system SC
current and verify equipment SC capability
• Battery Sizing and Discharge Calculation (IEEE
Std 485) – Determine sufficient battery size to
provide required power in specified time duration
• Control System Diagram, Vd Calc – Determine
and verify sufficient voltage for control circuit
operation
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 4
DC System Elements
• DC System Source
– Charger/Converter and Battery
• DC System Loads
– Static Load, DC Motor, Lump Load , and DC Elementary
Diagram
• DC System Branch
– Cable and Impedance
• DC System Other Elements
– Bus and Node, DC Converter, DC Composite Motor, and
Inverter
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 5
Purpose of DC LF Study
• To determine system operating conditions --
bus voltages, branch flows, and power
provided by various sources
• To verify equipment continuous ratings
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 6
LF -- Battery Model
• Constant voltage source behind battery
resistance .
• Active voltage source if Vbus < Vrate
• Inactive voltage source if Vbus > Vrate
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 7
LF -- Charger Model
• Constant voltage source, if I < Imax
• Constant current source, if I > Imax,
injecting Imax into system
• Inactive if Vout < Vbus
• Constant power load to AC system
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 8
LF -- Inverter Model
• Constant voltage source to AC system
• Constant power load to DC system
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 9
LF -- UPS Model
• If auction diode option is checked, then DC
system provides power to UPS AC load if
Vdc > Vac.
• If auction diode option is not checked…
– and UPS AC input bus is energized, then UPS
becomes a source to DC system, similar to a
charger.
– and UPS AC input bus is not energized, then DC
system provides power to UPS AC load.
• Constant power load to DC system
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 10
LF -- DC Converter Model
• Constant voltage source to output bus, if
I<Imax
• Constant current source to output bus, if
I>Imax, injecting Imax into the bus
• Constant current load to the input bus
• Inactive if Vout < Vbus
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 11
LF -- Load Model
• Motor
– Constant P load, if Vmin <V<Vmax;
– Constant Z load, otherwise
• Static Load
– Constant resistance load
• Lumped Load
– Constant power, resistance, current load
• Elementary Diagram Load
– Constant resistance load
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 12
Constant Power Loads
• DC Motor
• DC Lumped Load (motor
load part)
• Inverter
• UPS
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 13
Constant Impedance Loads
• Static Load
• DC Elementary
Diagram Editor
• DC Lumped Load
(static load part)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 14
Constant Current Loads
• The current remains constant even if
the voltage changes.
• DC Constant current loads are used
to test Battery discharge capacity.
• DC Lump load can be used to
represent constant current load.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 15
LF -- Branch Model
• Cable
– Constant resistance at maximum operating
temperature
• Impedance
– Constant resistance
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 16
LF- Adjustments
• Cable Length Tolerance
– Positive adjustment for DCLF & DCBS
– Negative adjustment for DCSC
• Cable Resistance based on Temperature
– Copper / Aluminum
– Use Maximum Temperature for DCLF & DCBS
– Use Minimum Temperature for DCSC
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 17
Purpose of DC SC Study
• To determine short-circuit fault currents and
fault contributions from different sources.
– Maximum fault current
– System equivalent impedance at fault location
– Time constant for SC current rise
• To verify protective equipment capability.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 18
SC -- Battery Model
• Constant voltage source -- (Voc) behind
battery resistance
– Voc may be calculated based on battery specific
gravity (SG)
– Open Circuit Voltage (Voc) from library
– User-Defined value (% Vpc)
• Constant current source
– Ksc * One_Min_Discharge_Current
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 19
SC -- Charger Model
• Constant voltage source behind AC system
impedance (converted to DC side)
• Constant current source, Ksc * Ifl
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 20
SC -- UPS & Inverter Model
• UPS -- Constant current source
Ksc * Ifl
• Inverter -- Not considered in DC short-circuit
study
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 21
SC -- DC Converter Model
• Constant current source, Ksc * Ifl, to faults
on output side of the system
• Short-circuit contribution calculation stops at
DC converters
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 22
SC -- Load Model
• Motor – constant voltage behind transient
impedance
• Static load and ED load are considered in
calculation
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 23
SC -- Branch Model
• Cable
– Constant resistance at minimum operating
temperature
• Impedance
– Constant resistance
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 24
