1. EEE812: ADVANCED POWER
SYSTEM PROTECTION
Distributed Generation: Impact on
Protection
Content prepared by Dr Campbell Booth
University of Strathclyde
2. Overview
Conventional distribution networks and protection -
summary
How distribution network are changing (active distribution
networks, DG, potential for islanded operation)
Potential issues for future networks
– Protection “blinding”, false tripping/coordination problems (several examples)
– DG ride-through
– Converter-interfaced sources
– Use of DC for distribution?
– Fault current limitation
Protection solutions?
3.
4. Distributed Generation Basics
Technologies
Small and Large scale combined heat and power units
Energy from waste units
Wind Farms
Tidal and wave energy units
Stand-by generators (diesel)
Types of generating units
Self-excited asynchronous generator
Mains-excited asynchronous generator
Power factor corrected asynchronous generator
Doubly fed induction generator (DFIG)
Synchronous generator
Inverter connected Synchronous Generator (Wind)
Inverter connected DC source (fuel cell, PV)
5. Distributed Generation Basics
Main reasons for Distributed Generation
Reduction of gaseous emissions (mainly CO2)
Diversification of energy sources
Ease of finding sites for smaller generators
Short construction times
Potentially reduced transmission losses
Increased efficiency with combined Heat and Power
(CHP) units
6. Conventional distribution networks
Operated radially
Designed for unidirectional power flow
11 kV
POWER FLOW DIRECTION
Protected with over-current protection relays, reclosers
and fuses
7. How distribution networks are changing
– Increase of distributed generation:
– Wind
– Hydro
– Biomass
– Photovoltaic
– Wave/Tidal
– others
– Introduction of network automation
– Connection of energy storage
8. Active distribution networks
Fault current magnitudes and directions becomes unpredictable,
potentially causing problems:
11 kV
– false tripping of feeders;
– lack of coordination between protection devices;
– other problems.
10. Protection of Distribution Networks
132/33kV
Distance, differential (some), overcurrent
11kV/415V
Overcurrent, reclosers, sectionalisers, fuses, RCDs
Remember, majority of faults transient – fuses should only operate if
fault is permanent
Typically, faults are isolated very quickly by reclosers, multiple
reclose attempts are attempted, and if fault is permanent and
downstream of fuses, fuses ultimately melt while system is in
reclosed state
Reclose is then successful
If permanent fault between recloser and fuse, then recloser will lock-
out after pre-defined number of attempts
Automatic sectionalisers/smart links sometimes used
12. Protection of distribution networks
(MV/LV)
Distribution network protection is
based on overcurrent protection,
reclosers and fuses (and
sectionalisers)
In rural overhead distribution
networks, >80% of faults are
temporary and auto reclosure
automation is adopted.
CBT1-11
CBT1-33
CBT2-11
CBT2-33
B33kV
B11kV
SpurA1
SpurA2
SpurA3
SpurA4
SpurA5
SpurA6
SpurA7
SpurA8
SpurB1
SpurB2
SpurB3
SpurB4
SpurB5
R-A R-B
PMAR-A
PMAR-B
Feeder
A
Feeder
B
SpurA9
SpurA10
SpurA1
13. Transient fault
Recloser will
successfully reclose
Permanent fault
Recloser will reclose
multiple times (with
variable delays before
re-opening) and fuse will
melt before max
reclosures attempted
Sectionalisers/“smart
links” may be used
instead of, or to “save”
fuses
Gers and Holmes
“Protection of
Electricity
Distribution
Networks”,
IEE Power &
Energy Series 47
Protection of distribution networks
(MV/LV)
14. Gers and Holmes
“Protection of Electricity
Distribution Networks”,
IEE Power & Energy Series 47
Protection of distribution networks
(MV/LV)
15. B
A C
Permanent Fault
PMAR
(fast/delayed
interruptions
and reclosures)
Sectionaliser
(counts number of
overcurrents/interruptions
- opens after certain number)
Fuse
IDMT
(with auto-reclose)
PMAR
Sectionaliser
Fuse
IDMT Start
Open
Count 1
1 “shot”
Reset
Open
Count 1
1 “shot”
Reset
Close
Count 1
2 “shots”
Start
Open
Count 2
2 “shots”
Reset
Open
Count 2
2 “shots”
Reset
Close
Count 2
melt
Reset
Close
Reset
melted
t
Fault inception
1
2
0
Protection of distribution networks
(MV/LV)
17. Protection of distribution networks
(MV/LV)
Sectionalisers may be used instead of or in
conjunction with fuses (“fuse savers”)
http://www.hubbellpowersystems.com/catalogs/switching/10D-Elec_Sect.pdf
19. B
A C
Operate (quickly)
Operate
(after a delay)
Don’t operate
V
V=IZ
With DG at B
No DG
DG fault
contribution
14MVA
Fault Behaviour – with DG
Source
(Grid)
11kV 30MVA Source (Zsource= j4.03W) j1W impedance to fault (ZAB=j0.2W)
20. Equivalent circuit – no DG
j4.03W
j0.8W
j0.2W
If Z from source to fault = j1W
(j0.2W for first feeder + j0.8 W for second
feeder):
Ifault= Igrid = Vph/Z = 6351/5.03 = 1263A
VB = 1263x0.8=1010V
VA = 1263x1=1263V
Grid
V
V=I/Z
With DG at B
No DG
B
A
21. Equivalent circuit – with DG
j4.03W
0.8W
j0.2W
j8.64W
Z from sources to fault =
j4.23//j8.64 + j0.8 = j3.64W
Ifault = Vph/Z = 6351/3.64 = 1745A
Igrid = (8.64)/(4.23+8.64) x1745
= 1171A
IDG = (4.23)/(4.23+8.64) x1745
= 574A
VB = 1745x0.8=1396V
VA = VB + (1171x0.2) =1630V
Grid DG
V
V=I/Z
With DG at B
No DG
B
A
22. Protection issues - “blinding”
t
If
Fault current as measured at
upstream relay with no
downstream DG
23. Protection issues - “blinding”
Under very high DG penetrations and very low grid infeed, infeed from
grid could be markedly reduced, therefore increasing risk of feeder
protection “blinding” (slow or non-operation of relay at A for backup
scenario in this case).
Problem? Probably not significant in interconnected system – but in
islanded mode?
t
If
Fault current as measured at
upstream relay with no
downstream DG
t
If
Fault current as measured at
upstream relay with significant
downstream DG
27. B
A C
Source
(Grid)
V=0
D
E
If protection at D is non-directional
overcurrent, then if contribution from
DG at E exceeds setting on D,
potential for mal-operation
In this case, if Ithreshold<500A at D,
potential for D to trip unnecessarily
Fault current=5000A
(4500+500)A
Infeed=500A
Z=0.6W
Z=0.8W
Infeed=
4500A
Protection issues – feeder protection
maloperation
30. 2
1 3
DG
If(at 2 and 3)
Protection issues - DG impact on
instantaneous (“high set”) protection
31. Use of directional relays
to provide correct protection
operation on parallel feeders
From NPAG: - chapter 9
Sensitive and fast acting
directional protection here
(Ithreshold=10% of rated
line current)
Looks “up”
into line
– prevents R2
operating
for this fault
Protection issues - DG impact on
directional protection
32. DG on load
side – R’2
(and possibly
R2) might
maloperate?
Protection issues - DG impact on
directional protection
33. DG on load side – R’1 R’2 (and possibly R1 and R2) might maloperate?
Even under load conditions – back-feed if DG>local load?
Protection issues - DG impact on
directional protection
34. Use of overcurrent
relays for protection
of ring mains
From NPAG: -
chapter 9
Fault as shown:
R5’ operates after 1.7
(R6’ after 2.1)
R2 after 0.5
(R3 after 0.9)
Protection
issues - DG
impact on
directional
protection
35. Additional DG
contribution may
result in
coordination
problems?
R1’ will operate
for this scenario?
Protection
issues - DG
impact on
directional
protection
Use of overcurrent
relays for protection
of ring mains
From NPAG: -
chapter 9
36. Impact on section
switches/fuses/…?
Blinding/maloperation/impact on automation/auto-
reclose/sectionaliser logic?
Possible problems if DG penetration/fault contribution
is high?
Performance in islanded mode – if permitted?
B
A C
PMAR Sectionaliser
Fuse
IDMT
