EVALUATION OF ALTERNATIVE BACKUP PROTECTION SCHEMES ON A 66KV
DISTRIBUTION NETWORK
F. Malone*, P.D. Doyle
†
*ESB International, Ireland fergus.malone@esbi.ie
†
ESB International, Ireland paul.d.doyle@esbi.ie
Keywords: Overcurrent Protection, Coordination, IEC 61850, Directional Comparison
Abstract
Directional and non directional overcurrent protection are well established methods of providing either
primary or backup protection on distribution networks. Compared to unit protection which only protects
a specific piece of equipment, such as a transformer or feeder, overcurrent protection has the
advantage of being able to provide protection to large sections of network. To do this the overcurrent
protection on a network must be coordinated. On radial distribution systems this can be easily
achieved, however, on a meshed or looped system it is not always possible to achieve the required
coordination. For this reason overcurrent protection is sometimes unsuitable for certain applications.
This paper looks at a section of 66kV distribution network for which overcurrent protection is
unsuitable. The inadequacy of the standard protection scheme for the network in question is explained
and possible alternatives discussed. The paper then goes through the rationale for selecting the most
suitable alternative protection scheme for the section of network in question.
1
Introduction
The 66kV distribution system under examination has two standard types of 66kV substations, “City
Tee Type” and “Loop Type”. These standard types of 66kV substations are described in the next
section.
City Tee Type
City Tee substations are formed when three 66kV
feeders from a 220kV/66kV Bulk Supply Point (BSP)
substation feed directly into three 66kV/11kV
transformers in a 66kV substation. A Tee point is
located on each feeder before entry to the
66kV/11kV transformers. From these three Tee
points another three 66kV/11kV transformers are
fed in the next substation. Again at the next
substation there will be Tee point connections to the
final substation on that section of 66kV network.
There are no 66kV busbars in City Tee substations.
The City Tee substations layout can be seen in
Figure 1.
BSP 66kV Busbar
Station 1
Station 2
For City Tee substations the primary feeder
protection normally consists of feeder differential
OC Relay
between each substation along the circuit together
Station 3
with Tee point differential protection at each
substation. Backup protection is provided using
Figure 1 – City Tee Type Substations
overcurrent protection located at the feeder outlet at
the BSP. This overcurrent protection must
coordinate with the 66kV bus coupler overcurrent protection in the BSP and also with the 11kV
overcurrent protection on the 66kV/11kV transformers in each 66kV substation.
Loop Type
Loop type substations are substations which have
their own 66kV busbar, which is fed by two
separate feeders. One of these two 66kV feeders
will be connected directly onto the 66kV busbar in
a BSP while the other will connect to a 66kV
busbar in another Loop Type substation. The
second 66kV feeder at this Loop Type station will
then connect to a second BSP. The Loop Type
substation layout can be seen in Figure 2.
BSP 1 66kV Busbar
Loop-Type
Stations
For Loop Type substations the primary feeder
protection normally consists of feeder differential
between each Loop Type substation and each
DOC Relay
BSP along with feeder differential between the two
Loop Type substations. Backup protection is
provided using directional overcurrent protection
located at both ends of each of the 66kV feeders.
BSP 2 66kV Busbar
The directional overcurrent protection located at
Figure 2 – Loop Type Substations
the BSP must coordinate with the 66kV bus
coupler overcurrent protection in the BSP, the
66kV overcurrent protection on the feeder between the two Loop Type substations and also with the
66kV overcurrent protection on the 66kV/11kV transformers in the 66kV substation. The directional
overcurrent protection at the Loop Type substations on the feeders connected to the BSPs can be set
to trip very quickly as these feeders would never carry load current towards their respective BSPs, as
there is always a normally-open point at some location between the two BSPs.
Section of 66kV Network under Examination
Before the introduction of a 220kV transmission
network the 66kV network was the main
transmission network for the system in question. As
such all power stations were connected onto the
66kV network. Gradually over time most power
stations were transferred to the 220kV network.
However, the section of network under examination
still has a power station connected at 66kV. In
recent times load demands in the region required a
connection onto the 220kV system which came in
the form of a 220kV BSP. To increase operational
flexibility it was decided to link the Power Station to
the BSP via two 66kV feeders. As a result the 66kV
network in question is quite unusual. This network
can be seen in Figure 3.
As can be seen in Figure 3, the two 66kV feeders
between the BSP and the Power Station along with
the BSP – Distribution Station and Distribution
Station – Power Station feeders form three parallel
paths between the BSP and the Power Station.
Also, all three parallel paths are fully cabled and are
quite short in length. It is not possible to coordinate
the directional overcurrent protection on the feeders
as for a fault on any particular feeder the directional
overcurrent protection on all three circuits will pick
up as the fault current splits between the three
parallel paths.
220kV Busbar
BSP
66kV Busbar
11kV Busbar
66kV Dist.
Station
New 66kV
feeders
Power
Station
11kV Busbar
OC Relay
DOC Relay
Figure 3 – 66kV Network under examination
Another problem is trying to coordinate the 66kV directional overcurrent protection with the 66kV bus
coupler protection at both the BSP and the Power Station. For a fault on the Power Station 66kV
busbar, the local 66kV bus coupler overcurrent relay should trip before the directional overcurrent
protection at the BSP end of the feeders, which in turn should trip before the BSP bus coupler
overcurrent protection. Likewise for a fault on the BSP 66kV busbar, the local bus coupler overcurrent
relay should trip before the directional overcurrent protection at the Power Station end of the feeders,
which in turn should trip before the Power Station bus coupler overcurrent protection. It is not possible
to achieve both of these requirements simultaneously.
In addition to this problem, the 66kV bus coupler protection in the Power Station is quite slow as a
result of coordinating with slow outgoing 66kV feeders. Due to high short circuit levels on the 66kV
system the timer of the bus coupler protection at the BSP cannot be increased due to the limitations of
the switchgear duty. This also limits the ability to time-grade the protection on this section of network.
For these reasons directional overcurrent protection is unsuitable as backup protection on this section
of the 66kV network.
2
Alternative Backup Protection Schemes
As a result of the issues outlined above it was decided to investigate alternative backup protection
schemes for the 66kV network in question. These alternative protection schemes are outlined below.
2.1
Duplicate Feeder Differential
The first alternative backup protection scheme considered was a duplicate feeder differential scheme.
This would require a replication of the feeder differential scheme currently installed as primary
protection but with some important
alterations.
BSP
1) The communications link would have to
follow a separate route.
66kV Dist.
Station
2)
An
alternative
protection
relay
manufacturer to that of the primary
differential scheme would have to be used.
Figure 4 shows suggested routes for the
fibre-optic links between the duplicate
differential relays for each feeder. It is worth
noting that both the BSP – Power Station
circuits 1 & 2 are buried in the same trench,
so in order to prevent a common mode of
failure the communications channels for the
backup protection scheme should follow the
circuit 3 route via the 66kV Distribution
Station.
Power
Cct 3
Power
Ccts
1&2
Power
Station
Ccts1& 2
BSP – PS
Comms route
Cct 3
BSP – Dist. St.
Comms route
Cct 3
Dist. St. – PS
Comms route
Figure 4 – Backup Protection Communication Channels
A duplicate feeder differential scheme will
only provide backup protection for the 66kV
feeders. As a result the backup busbar protection for each station would be compromised. Without
directional overcurrent relays on the feeders the backup busbar protection for either station would be
dependant on the coupler and source transformers or generators tripping out in the remote station.
2.2
Directional Comparision Overcurrent
The next scheme examined was a directional comparision overcurrent scheme. This scheme uses
directional overcurrent relays on both ends of each feeder, similar to the original backup protection
proposed on the network. In addition, the two directional overcurrent relays at each end of the feeder
communicate with each other over fibre
BSP
optic channels. A fault is determined to be
internal to the feeder if the relays at each
end pick up in the forward direction. This
66kV Dist.
allows a permissive trip to occur, where both
Station
relays trip without any additional time delay.
DOC Relay
Near-instantaneous backup protection is
Power
Blocking
provided to each cable in this way – the only
Power
Cct 3
Signal
Ccts
time delay being the relay’s directional
1&2
decision time plus the communication time
delay.
PS
The IDMT directional overcurrent stages
provide backup protection to the BSP and
the Power Station in the event of a fault at
either busbar not being cleared by the
Figure 5 – Directional Comparison scheme blocking signals
primary protection. Backup protection for
Channels
busbar faults in the Distribution Station is
provided by the IDMT stage of the directional overcurrent relays at the remote ends of the Distribution
Station – BSP and Distribution Station – Power Station feeders. An extra measure is required to
ensure selectivity for faults on the “Loop Type” 66kV feeder (as shown in Figure 5) in the event of that
feeder’s unit protection failing to clear the fault. As this is a “Loop Type” feeder, there is no
instantaneous overcurrent stage enabled on the directional overcurrent relay. It is not possible to
coordinate the IDMT stage of this relay with the directional overcurrent relays on circuits 1, 2 & 3, so a
blocking signal is required to be sent to the appropriate relays in the event of the “Loop Type” feeder
relay picking up for a fault in the forward direction. In the case of the “City Tee” 66kV feeders, it is not
possible to coordinate the directional overcurrent relays on circuits 1, 2 and 3 with the “City Tee”
directional overcurrent relays for downstream 11kV faults, so a blocking signal is also required from
each of these relays in the event that they pick up in the forward direction. A simplified schematic of
these blocking signals is shown in Figure 5. The existing overcurrent relays on the two 66kV
transformer feeders at the Distribution Station have no instantaneous element, so it is not possible to
coordinate the directional overcurrent relays on the Distribution Station outlets at the BSP and the
Power Station with those relays. This problem can be solved by installing overcurrent protection which
includes an instantaneous element.
City Tee
feeders
2.3
Fwd Pickup
Loop
Type
feeder
IEC 61850 Blocked Overcurrent Scheme
The final scheme which was looked at was a blocked overcurrent scheme utilising the IEC 61850
communication protocols [1]. This scheme involves connecting IEC 61850 compatible relays in each
station via CAT-5 ethernet cable to their local Ethernet switch. The Ethernet switches are linked via
fibre optic channels between the stations. This allows each relay to send and receive GOOSE
(Generic Object Oriented Substation Event) messages. A GOOSE message can be sent by any relay
to all the other Ethernet-connected devices in all three stations to indicate the pickup of a specific
directional element. Each device that receives the message will be programmed to block the
appropriate element, switch between tripping characteristics or ignore the message altogether, as
appropriate. This system enables selective tripping for all faults on the 66kV network in question.
The following describes the operation of the system for different fault scenarios:
Fault on 66kV feeders between the BSP, the
Power Station and the Distribution Station
•
BSP
Directional Overcurrent relays on each
end of the faulted feeder pick up in the
forward
direction
and
broadcast
GOOSE messages to that effect.
•
Directional Overcurrent relays on
parallel feeders are blocked from
tripping.
•
Bus coupler overcurrent relays in the
BSP and the Power Station switch to
slower IDMT characteristic as the fault
is confirmed to be not on the busbar.
•
Tripping is accelerated for the relay at
each end of the faulted feeder.
220kV Busbar
66kV Busbar
66kV Dist.
Station
11kV
Busbar
Power Station
Fault on 66kV busbar of the BSP or the Power
Station
•
•
•
Bus coupler overcurrent relay picks up
for the fault and no blocking signal is
received from a relay on any busbar
outlet. So the bus coupler relay remains
in its fast IDMT characteristic and trips
the coupler.
The overcurrent relay on any HV
transformer/generator connected to the
faulted busbar section trips, along with
any feeder carrying fault current
connected to the same section.
11kV Busbar
OC Relay
DOC Relay
Ethernet Switch
Cat 5 Cable
Fibre Optic link
Figure 6 – IEC61850 Blocked Overcurrent Scheme
Channels
The healthy bus section(s) and feeder(s) remain intact and load can still be fed from the
healthy bus sections.
Fault on 66kV busbar of the 66kV Distribution Station
•
Directional Overcurrent relays on the Power Station and BSP feeders at the Distribution
Station pick up in the reverse direction and broadcast GOOSE messages.
•
Directional Overcurrent relays on parallel feeders are blocked from tripping.
•
Bus coupler overcurrent relays in the BSP and the Power Station switch to their slower IDMT
characteristic as the fault is confirmed not to be on the busbar.
•
Tripping is accelerated for the relays at each end of the feeders from the Power Station and
BSP to the distribution station.
Fault on City/Loop-type 66kV feeders from the Power Station
•
Directional Overcurrent relay on the faulted feeder picks up and broadcasts GOOSE
message.
•
The Power Station bus coupler switches to its slower IDMT characteristic to coordinate with
the feeder relay.
•
Directional Overcurrent relays at the BSP and the Distribution Station switch to slower
characteristic to coordinate with the slow bus coupler overcurrent characteristic at the Power
Station.
•
The BSP bus coupler switches to its slower overcurrent characteristic to coordinate with the
feeder relays.
•
The Directional Overcurrent relay on the faulted feeder clears the fault and nothing else trips
on the network.
Fault on the outgoing 66kV feeders from the 66kV Distribution Station
•
66kV Directional Overcurrent relay on the faulted feeder picks up and broadcasts a GOOSE
message
•
Directional overcurrent relays on the feeders from the BSP and Power Station to the 66kV
Distribution Station remain in their slower IDMT characteristic in order to coordinate with the
downstream relay that issued the GOOSE message.
11kV fault at the Power Station or the BSP (not cleared by 11kV protection)
•
66kV Overcurrent relay on the 66/11kV transformers picks up in the IDMT stage.
•
This overcurrent relay is not linked via IEC61850 so no GOOSE message is sent.
•
Bus coupler overcurrent relay remains in its fast IDMT characteristic but this is set to
coordinate with the 66/11kV transformer overcurrent relay so fault is cleared selectively.
11kV fault at 66kV Distribution Station (not cleared by 11kV protection)
•
66kV Overcurrent relay on 66/11kV transformers picks up in the IDMT stage.
•
This relay is not linked via IEC61850 so no GOOSE message is sent.
•
Distribution Station feeder directional overcurrent relays at the BSP and the Power Station
pick up, but due to their high pickup current they coordinate with the 66/11kV transformers
overcurrent relays for 11kV faults.
Fault on HV side of the 66/11kV transformers at the Power Station / the BSP / the Distribution Station
(not cleared by differential protection)
•
The instantaneous element of the 66kV Overcurrent relay on the 66/11kV transformer picks up
and trips straight away, clearing the fault.
3
Discussion
This section discusses the technical merits of each scheme along with the cost and physical
installation requirements.
3.1
Duplicate Feeder Differential
Backup protection for the 66kV cables is instantaneous with the duplicate differential scheme.
However, it would not be practical to install duplicate busbar differential protection for the 66kV
busbars at the BSP, the Distribution Station and the Power Station. If a 66kV busbar fault occurred at
the BSP, the Power Station or the Distribution Station and primary protection failed the 220kV
transformers at the BSP and the generators at the power station would have to be disconnected by
overcurrent protection in order to clear the fault. Due to the low probability of busbar faults, this may
be acceptable as long as the fault clearance time is quick enough to avoid any damage to equipment.
However, the short circuit duty was calculated for 66kV busbar faults and it was found that for certain
faults at the Distribution Station 66kV busbars, the short circuit withstand of the busbar was
exceeded. It may be possible to ensure the short circuit duty is kept within the busbar ratings by
changing the protection settings on the existing protection in downstream substations. This is currently
being investigated. Also the CT cores being used for the backup protection scheme may not meet the
criteria for use in a differential scheme so may have to be replaced.
Equipment Required
•
8x Differential Relays
•
CTs on 8 bays for feeder differential protection. (depending on testing / suitability of existing
relays)
Installation
•
Installing CTs on each end of the 66kV cables between the BSP, the Distribution Station and
the Power Station would require outages of each 66kV feeder. The outages should be
possible to organise due to the three parallel circuits connecting the BSP and the Power
Station, however replacing the GIS CTs at the BSP (if the existing overcurrent CTs are
unsuitable for differential protection) would not be as straightforward.
•
Commissioning of the scheme would be relatively straightforward.
3.2
Directional Comparision Overcurrent
Backup protection for the cables is similar in performance to the duplicate differential scheme, offering
near-instantaneous backup protection [2]. Unlike the duplicate differential scheme, the directional
overcurrent relays can also provide extra protection to the busbars at the Power Station, the BSP and
the Distribution Station in the event of the busbar differential protection failing to clear a fault.
The benefits of this backup busbar protection are:
Busbar faults are cleared faster resulting in a reduced short circuit duty. This prevents any risk of
damage to the busbar in the event of the busbar differential protection failing.
For a busbar fault in the Power Station or the BSP, the three 66kV branches are disconnected. This
allows the coupler closest to the faulted busbar section to trip and only the transformer/generator(s)
connected to the faulted busbar section to trip, allowing some load to continue to be fed. For a fault at
Distribution Station 66kV bus, only the Distribution Station feeders at the BSP and Power Station will
be disconnected. This is a more selective busbar protection than that offered by the duplicate
differential scheme. With the duplicate differential scheme, both transformers in the BSP and
generators in the Power station would trip for a busbar fault in the BSP, Distribution Station or the
Power Station that was not cleared by primary protection.
Equipment Required
•
3 Teleprotection devices
•
3x Numerical Directional Overcurrent relays.
•
2x Numerical Overcurrent Relays.
Installation
•
With no new instrument transformers required and only 5 relay replacements, the
implementation of this scheme would be the least disruptive to the network. Commissioning
would be relatively simple.
•
An advantage of this scheme is that it makes use of the existing DOC relays that are already
installed on the Power Station – BSP and BSP – Distribution Station feeders as well as the
associated instrument transformers.
3.3
IEC 61850 Blocked Overcurrent Scheme
This scheme offers fully selective backup protection to the BSP to Power Station and the BSP –
Distribution Station – Power Station cables as well as the busbars in each station. 66kV Bus coupler
relays in the BSP and the Power Station could also be coordinated with the feeder relays so the load
disruption would be minimised for busbar faults that are not cleared by primary protection.
Equipment Required
•
16x IEC61850 compatible Directional Overcurrent relays
•
VTs on 5 Power Station generator transformer bays
•
3x Ethernet hubs
Installation
This scheme would require a directional overcurrent relay on every outlet from the busbar in each
station to determine whether or not a fault lies on the busbar. This requires VTs to be installed on the
generator bays in the Power Station. During a site survey, it was found that due to a lack of space in
the bays of the indoor AIS compound at the Power Station, the installation of VTs would not be
practical.
Aside from the high cost of the equipment required and the complex commissioning associated with
this option, the lack of space to install VTs in the generator bays at the Power Station means that this
protection scheme is not a viable option.
4
Conclusions
Duplicate Differential
The duplicate differential protection scheme does not offer acceptable backup protection for the
busbars in the BSP and Distribution Station. Slow fault clearance times for backup protection could
potentially cause damage to the GIS equipment at the BSP or the indoor AIS equipment at the Power
Station. However this may be possible to rectify pending a review of downstream substation protection
with a view to speeding up the backup protection in the BSP and the Power Station.
This scheme could also only be implemented if the existing CTs on the feeders between the BSP,
Power Station and Distribution Station were tested and deemed suitable for differential protection.
Pending those two criteria being fulfilled, the duplicate differential scheme would be a simple and
robust option.
IEC61850 blocked overcurrent scheme
Implementation of the IEC61850 blocked overcurrent scheme would not be practical due to the
requirement for VTs in the generator transformer bays at the Power Station. The cost of installing and
commissioning this scheme would also be high due to the quantity of equipment required and the
complexity of setting up the scheme.
Directional Comparison Overcurrent scheme
The Directional Comparison Overcurrent scheme is a viable option for the following reasons:
•
It offers instantaneous and selective backup protection for cable faults.
•
Clearance time for busbar faults is reasonably fast with no risk of the short circuit withstand
rating being exceeded on any busbar.
•
In the event of a busbar fault at the BSP, the Power Station or the Distribution Station where
primary protection fails, the load and source transformers/generators on the healthy busbars
can remain connected.
•
This scheme can be implemented without installing any new instrument transformers. It also
uses standard teleprotection equipment which should be relatively straightforward to
commission.
References
[1]
Hakala-Ranta, Rintamäki, Starck. “Utilizing possibilities of IEC 61850 and GOOSE”, CIRED
2009, Paper 0741 (2009).
[2]
D. Tholomier, S. Richards, A. Apostolov, “Which one is better – Line Differential or Directional
Comparison?”, DPSP 2008.