Development of DC systems in the late 19th century
AC solutions came somewhat later
But AC systems (50 or 60 Hz) became the standard in the world  Simple transformation between different Voltage levels  Short circuit current interruption
Limited interest for DC in the second half of the 20th century
Increased interest in the last years  Renewable energy sources  Offshore solutions

1. The Webinar video is available here:
https://register.gotowebinar.com/recording/4525984979424798721

2. Protection and local control of HVDC grids
Kees Koreman chairman JWG B4/B5.59
Webinar 15 November 2018

3. Introduction and positioning of the work
Main components and rationale for meshed HVDC grids
Functional requirements on protection
Short circuit phenomena
Short circuit limiting techniques
Protection system components
Protection system overview
Conclusions
Table of contents
2

4. Introduction and
positioning of the work
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
3

5. Introduction
Development of DC systems in the late 19th century
AC solutions came somewhat later
But AC systems (50 or 60 Hz) became the standard in the world
Simple transformation between different Voltage levels
Short circuit current interruption
Limited interest for DC in the second half of the 20th century
Increased interest in the last years
Renewable energy sources
Offshore solutions
4

6. 5

7. Positioning of the work
Cigre B4 gained interest and started a WG B4.52
who finished their work in 2013 (TB 533)
A new WG was formed together with SC A3
JWG A3/B4.34 on DC switching equipment (TB 683)
Five areas of further work were defined
WG B4.56 on connection agreements (TB 657)
WG B4.57 on simulation models (TB 604)
WB B4.58 on load flow control (TB 699)
WG B4.60 on reliability design (TB 713)
JWG B4/B5.59 on protection (TB 739) this work
6

8. Main components and
rationale for meshed DC
grids
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
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9. Some definitions for meshed grids and protection
Meshed system (IEV reference 131-13-16)
A set of branches forming a loop and containing only one link of
a given co-tree
Loop (IEV reference 131-13-12)
A closed path passing only once through every node in the path
Protection system (IEV reference 448-11-04)
An arrangement of one or more protection equipments, and other
devices intended to perform one or more specified protection
functions
8

10. Considered HVDC system designs
Consisting of two poles of opposite polarity with the same
voltage amplitude
Symmetrical monopolar system
Full bipolar system with or without metallic return
(Asymmetrical monopoles possible)
9

11. Converter topologies
Multi modular half-bridge converter
Cascaded two level converter
Module output voltage 0 or Vcap
In case of a DC pole to pole fault
Uncontrolled current path possible
Diode parallel to IGBT T2
Fault current cannot be blocked
AC circuit breaker needs to operate
Non fault current blocking converters
10

12. Converter topologies
Full-bridge converters less used
Module output voltage 0, Vcap or - Vcap
Uncontrolled current path possible
Allways incorporates the module
capacitor
Fault current blocked by converter
operation
AC circuit breaker operation not required
Fault current blocking converters
11

13. Developing grids
Full scale HVDC meshed grid will not emerge directly
Development through connection of individual point-to-point
connections
Adding converters and lines/cables
Develop multi terminal systems
Developing into a fully meshed grid
Possibly by using DC-DC Converters
12

14. Kees Koreman| October 2018
13

15. Functional requirements
on protection
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
14

16. Objectives and Requirements for Protection
Large similarity with protection in AC grids
Ensure Human safety
Fast fault clearance
Minimise impact on the system
Minimise stress on components
Protection philosophy requirements
Reliability
Speed
Sensitivity
Selectivity
15

17. HVDC equipment constraints
Semiconductors and power electronics
Low overcurrent widthstand capabilities
Fast temperature rise
Breakdown leads to permanent damage
Transformers and connections
Similar to HVAC systems
Overcurrents 5 – 10 times rated current
Widthstand time hundreds of ms
Prevent permanent damage
16

18. HVDC grid constraints
Full description derived by WG B4.56
Worst case unbalance
Sectionalised HVDC grid
Voltage profiles apply on non-faulted sections
Balanced pole voltages
Overvoltage profiles to be developed
Faulted sections need to be isolated
Tripping time considered
Brochure 657 gives 10 – 15 ms
Recent developments 3 – 6 ms
Maximum allowable Voltage profile
17

19. 18

20. DC grid protection philosophies
The entire HVDC grid is seen as a single protection zone
The entire zone is switched off
By the AC breakers in all converter bays
By current control in full bridge converters
By HVDC circuit breakers in the converter connection points
The faulted section is identified and disconnected after fault
clearance with mechanical or high speed switches
Non-selective fault clearing strategy
19

21. DC grid protection philosophies
The entire HVDC grid is divided into several protection zones
The faulted zone is isolated from the other zones
By HVDC circuit breakers in the connection points
By DC-DC converters between the zones
By current limiters (SFCL) between the zones
The faulted section is identified and disconnected after fault
clearance with mechanical or high speed switches
Partial selective fault clearing strategy
20

22. DC grid protection philosophies
Each branch in the HVDC grid is a separate protection zone
The faulted branch is isolated from the other zones
By HVDC circuit breakers in the faulted branch
The faulted branch is immediately identified
Full selective fault clearing strategy
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23. Kees Koreman| October 2018
Non-selective fault clearing strategy
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24. Kees Koreman| October 2018
Partial selective fault clearing strategy
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25. Kees Koreman| October 2018
Full selective fault clearing strategy
24

26. Short circuit phenomena
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
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27. Fault current development
Open circuit faults can be detected by:
Redistribution of load currents
Power flow no longer matches the scheduled power flow
Focus of the brochure is on short circuit faults
Current development is highly dependent on the earthing
concept
High impedance earthing
Low impedance earthing
Earthing concept less dominant when pole-to-pole faults occur
26

28. Fault current development
Fault example with low impedance earthing
27

29. Fault current development
28

30. Fault current increase
General rules cannot be given
Factors to consider:
Transmission line type
Fault resistance
DC side inductance
DC side capacitance
Interruption instant
AC system strength
Converter topology
Detailed simulations are required
29

31. Short circuit limiting
techniques
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
30

32. Introduction
Invisible to the grid during normal
conditions
Act instantaneously after a fault
Significant impedance during faults
Out of action after clearance
Capability to act multiple times
31
Characteristics of current limiters

33. Limiting the rate-of-rise of fault currents
Currently HVDC circuit breakers have moderate interrupting
capability
Current interruption level 10 – 20 kA
Operating time 3 – 5 ms
Limiting the rate-of-rise of current will help
DC reactors
Superconducting fault current limiters
Time to reach the maximum current will increase
32

34. Limiting the overcurrent magnitude
The HVAC grid is the source delivering the current
Impedance between the fault and the AC sources is
determining
Transformer impedance (0,1 - 0,2 pu)
Converter impedance (< 0,1 pu)
Additional measures are required
AC resistance
DC resistance
DC – DC converters
HVDC circuit breakers
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35. DC system earthing
High impedance earthing low impedance earthing
34

36. DC system earthing
35

37. DC system earthing
Conclusions
Both principles can be used in a HVDC meshed grid
High impedance earthing reduces the fault current but increases
the over-voltages on the non-faulted pole
Monopolar systems can be attached in both principles
The recommendation is to implement the low impedance
earthing given the anticipated size of the meshed HVDC grid
36

38. Protection system
components
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
37

39. Protection system algorithms
Many HVAC based algorithms are also applicable for
HVDC applications (with minor modifications)
DC overcurrent protection
Distance and differential protection
Under Voltage and Voltage unbalance protection
Specific DC algorithms to increase selectivity and
speed
Travelling wave protection
Voltage and/or current derivative protection (ROCOV
and/or ROCOC)
Simulation in the HVDC grid prepared by WG B4.57
38

40. Fault current distribution
This example shows the pole to earth steady
state fault current distribution in the reference
grid during a fault in the HVDC cable
between bus 1 and bus 3
DC overcurrent protection will not lead to a
fully selective operation
Alternative algorithms are needed
39

41. Rate-of-Change of Voltage (ROCOV)
Derivative of the voltage is determined directly from the
measured voltages
Selectivity is improved with the addition of series reactors
between the station and the measuring point
Additional advantage is the reduction of the rate of rise of the
fault current
40

42. Rate-of-Change of Voltage (ROCOV)
Effect of series reactor
41

43. Rate-of-Change of Voltage (ROCOV)
Calculation of
ROCOV in the connection point
ROCOC in the connections
Sign of ROCOV and ROCOC
Different indicates the faulted
section
Equal indicates fault outside
zone
Selective detection improved
combining ROCOV and ROCOC
42

44. Preliminary conclusions
regarding algorithms [1]
Best performance is obtained by rate of
change protection algorithms
Voltage change is the best indicator
Current change can assist to increase
selectivity
Other algorithms are not selective enough
but can be used to protect equipment by
offering delayed protection functions
43

45. Preliminary conclusions
regarding algorithms [2]
Utilisation of ROCOV only relies on local
measurements
Telecommunication between stations is not
required. Communication delay has not effect
on tripping time
Telecommunication can be supporting for
back-up protection functions. Delayed
tripping in case of a breaker failure
44

46. Measuring equipment
Large bandwidth is required
High pass characteristic sufficient
Current measurement
Direct shunt
Zero flux
Optical
Rogowski coil
Voltage measurement
Resistive divider
RC divider
Essential for correct operation of ROCOV and ROCOC
45

47. Acting equipment
Impressive development DC circuit-breakers
Various principles
Semiconductor breaker
Hybrid circuit-breaker
Resonant circuit-breaker
Mechanical with active current injection
Excellent overview is given in TB 683
Upscaling to 500 +kV possible
46

48. Operating principle hybrid CB
Commutation
Branch
Energy Absorption
Branch
Low
Voltage
Switch
Primary Branch
Source
Voltage
Current
Contact
Voltage
47

49. Operating principle hybrid CB
Commutation
Branch
Energy Absorption
Branch
Low
Voltage
Switch
Primary Branch
Source
Voltage
Current
Contact
Voltage
Low Voltage
Switch Open;
Commutation
Branch Closed
Fault
Detection
48

50. Operating principle hybrid CB
Energy Absorption
Branch
Low
Voltage
Switch
Primary Branch
Commutation
Branch
Source
Voltage
Current
Contact
Voltage
Commutation
Branch
Conduction
Low Voltage
Switch Open;
Commutation
Branch Closed
Fault
Detection
49

51. Operating principle hybrid CB
Commutation
Branch
Energy Absorption
Branch
Low
Voltage
Switch
Primary Branch
Source
Voltage
Current
Contact
Voltage
Commutation
Branch
Conduction
Low Voltage
Switch Open;
Commutation
Branch Closed
Fault
Detection
Mechanical
Switch Open
50

52. Operating principle hybrid CB
Commutation
Branch
Energy Absorption
Branch
Low
Voltage
Switch
Primary Branch
Source
Voltage
Current
Contact
Voltage
Commutation
Branch
Conduction
Low Voltage
Switch Open;
Commutation
Branch Closed
Fault
Detection
Mechanical
Switch Open
Energy
Absorption
Commutation
Branch OFF
51

53. Fault localisation
Fast fault localisation is needed to avoid long outage times
With underground cables
With submarine cables
(overhead lines)
Accuracy in the localisation limits the required amount of spare
cable
Repair strategies need to be in place
52

54. Basics for localisation
Offline measurements
TDR
Murray bridge
Direct analysis of fault voltages and
currents
Cigre WG B1.52 gives further analysis
53

55. Protection system
overview
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
54

56. Point – to – point protection overview
55

57. Connection point protection overview
56

58. Conclusions
Protection and local control of HVDC grids
Webinar Cigre 15 November 2018
57

59. Conclusions [1]
Technology has moved fast in the last years
Measuring techniques are available
Specific algorithms are tested and available
HVDC circuit breakers are available
ROCOV and ROCOC algorithms improve selectivity
No use of telecommunication between the stations
A series reactor further improves selective operation
50 or 100 mH in each pole is sufficient
Regarding the technology
58

60. Conclusions [2]
Three strategies are available
Non selective fault clearing
Partial selective fault clearing
Full selective fault clearing
An advise cannot be given at this stage
Depending on the size of the HVDC grid
Economical analysis needed
Detailed simulation studies are essential
Regarding the application of DC grids
59

61. Conclusions [3]
The technology for HVDC grids is available: ready for use!
Lines and cables
Converters and stations
DC – DC converters
DC current interruption facilities
Protection and control algorithms
It allows for the development of a meshed DC grid
Link to promotion movie: https://youtu.be/lBnWEK9IUn4
60

62. Copyright © 2018
This tutorial has been prepared based upon
the work of CIGRE and its Working Groups.
If it is used in total or in part, proper
reference and credit should be given to
CIGRE.
Disclaimer notice
“CIGRE gives no warranty or assurance
about the contents of this publication, nor
does it accept any responsibility, as to the
accuracy or exhaustiveness of the
information. All implied warranties and
conditions are excluded to the maximum
extent permitted by law”.
Copyright &
Disclaimer notice
61