ETAP 5.0
ETAP 5.0
Copyright 2003 Operation Technology, Inc.
DC Load Flow
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)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 3
Why DC System Studies?
• 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
Due to the importance of DC systems,
it must be:
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 5
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
Purpose of DC LF Study
• To determine system operating conditions
--bus voltages, branch flows, and power
provided by various sources
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 7
LF -- Battery Model
• Constant voltage source behind battery
resistance .
• Active voltage source if Vbus < Vrate
• Inactive voltage source if Vbus > Vrate
LF -- Charger Model
• Constant voltage source, if I < Imax
• Constant current source, if I > Imax,
injecting Imax into system
• Inactive if Vout < Vbus
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 9
LF -- Inverter Model
• Constant voltage source to AC system
• Constant power load to DC system
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.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 11
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
LF -- Load Model
• Motor
– Constant P load, if Vmin <V<Vmax;
– Constant Z load, otherwise
• Static Load
– Constant resistance load
• Lumped Load
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 13
Constant Power Loads
• DC Motor
• DC Lumped Load (motor
load part)
• Inverter
• UPS
• 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 15
• 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.
LF -- Branch Model
• Cable
– Constant resistance at maximum operating
temperature
• Impedance
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 17
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
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 19
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
SC -- Charger Model
• Constant voltage source behind AC system
impedance (converted to DC side)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit Slide 21
SC -- UPS & Inverter Model
• UPS -- Constant current source
Ksc * Ifl
• Inverter -- Not considered in DC short-circuit
study
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 23
