This document discusses DC load flow and short-circuit analysis. It describes why these studies are important for reliable DC systems and outlines the various models used to represent different DC system elements like batteries, chargers, loads, and branches in the studies. The purpose of DC load flow is to determine operating voltages and currents while DC short-circuit aims to find maximum fault currents to verify protection equipment ratings. Models include constant power, impedance and current loads as well as voltage and current sources for generators and converters.

1. © 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
DC Load Flow
DC Short-Circuit

2. Slide 2© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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)

3. Slide 3© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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:

4. Slide 4© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

5. Slide 5© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

6. Slide 6© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

7. Slide 7© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
LF -- Battery Model
• Constant voltage source behind battery
resistance .
• Active voltage source if Vbus < Vrate
• Inactive voltage source if Vbus > Vrate

8. Slide 8© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

9. Slide 9© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
LF -- Inverter Model
• Constant voltage source to AC system
• Constant power load to DC system

10. Slide 10© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

11. Slide 11© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

12. Slide 12© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

13. Slide 13© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
Constant Power Loads
• DC Motor
• DC Lumped Load (motor
load part)
• Inverter
• UPS

14. Slide 14© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
• Static Load
• DC Elementary
Diagram Editor
• DC Lumped Load
(static load part)
Constant Impedance Loads

15. Slide 15© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
• 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.
Constant Current Loads

16. Slide 16© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
LF -- Branch Model
• Cable
– Constant resistance at maximum operating
temperature
• Impedance
– Constant resistance

17. Slide 17© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

18. Slide 18© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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.

19. Slide 19© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

20. Slide 20© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
SC -- Charger Model
• Constant voltage source behind AC system
impedance (converted to DC side)
• Constant current source, Ksc * Ifl

21. Slide 21© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
SC -- UPS & Inverter Model
• UPS -- Constant current source
Ksc * Ifl
• Inverter -- Not considered in DC short-circuit
study

22. Slide 22© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
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

23. Slide 23© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
SC -- Load Model
• Motor – constant voltage behind transient
impedance
• Static load and ED load are considered in
calculation

24. Slide 24© 1996-2009 Operation Technology, Inc. – Workshop Notes: DC Load Flow/DC Short-Circuit
SC -- Branch Model
• Cable
– Constant resistance at minimum operating
temperature
• Impedance
– Constant resistance