Distributed generation is increasing on distribution networks, changing them from purely radial structures with unidirectional power flows. This presents challenges for protection schemes which were designed for conventional networks. Issues include protection devices not operating or operating incorrectly due to changes in fault current magnitudes and directions. Solutions involve advanced protection techniques like directional relays and coordination of protection elements. High distributed generation penetration could potentially cause blinding of upstream protection devices or unwanted tripping of generators during faults.

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?

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.

9. Islanding operation
– Reduced fault levels
– Changed fault current direction
– System control?
11 kV

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

11. B
A
t
I
t
Decreasing Fault Current
I
Fault 1
Fault 1
tF1
Fault 2
tF2
Fault 2
tF2
Fault 2
Relay 1 Relay 2
Source
Protection of Distribution Networks
(HV/MV)

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)

16. B
A C
PMAR Sectionaliser
Fuse
IDMT
PMAR
Sectionaliser
Fuse
IDMT Start
Open
Reset
Open
Reset
Close
Reset
Close
t
Fault inception
Protection of distribution networks
(MV/LV)
Transient Fault

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

18. B
A C
Source
(Grid)
Operate (quickly)
Operate
(after a delay)
Don’t operate
V
V=IZ
11kV 30MVA Source (Zsource= j4.03W) j1W impedance to fault (ZAB=j0.2W)
Fault Behaviour – no DG

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

24. B
A C
Source
(Grid)
V=0
Infeed=5000A
(4500+500)A
Infeed=
4500A
Infeed=500A
0.6W
0.8W
D
E
Protection issues – generator tripping
on undervoltage

25. G59 – DG interface protection settings

26. B
A C
Source
(Grid)
V=0
Fault current=5000A
(4500+500)A
Infeed=
4500A
Infeed=500A
Z=0.6W
Z=0.8W
D
E
VA=VD=5000x0.6=3000V
(less than 50% nominal)
VED=500x0.8=400V, therefore
VE=3400V
So if protection at A does not operate
within 0.4-0.5s, chance of DG at E
tripping, unnecessarily, on
undervoltage – also slight chance of
relay at D operating if threshold
violated
Protection issues – generator tripping
on undervoltage

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

28. Protection issues - DG impact on
instantaneous (“high set”) protection

29. 2
1 3
Operate (time delay)
Operate? Operate (instant)
If
Protection issues - DG impact on
instantaneous (“high set”) protection

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