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 1 2 For City Tee substations the primary feeder protection normally consists of feeder differential OC Relay between each substation along the circuit together with Tee point differential protection at each 3 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. Loop-Type s BSP 1 66kV Busbar 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 the BSP must coordinate with the 66kV bus Figure 2 Loop Type Substations 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 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 along with the BSP Distribution and Distribution feeders form three parallel paths between the BSP and the. 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. 11kV Busbar New 66kV feeders 11kV Busbar 220kV Busbar BSP 66kV Busbar 66kV Dist. 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. For a fault on the 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 end of the feeders, which in turn should trip before the 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 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. 1) The communications link would have to follow a separate route. 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 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. Ccts1& 2 BSP PS Comms route 66kV Dist. 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. Ccts 1&2 Cct 3 BSP Dist. St. Comms route BSP Cct 3 Cct 3 Dist. St. PS Comms route
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 City Tee feeders Ccts 1&2 Fwd Pickup BSP 66kV Dist. Cct 3 PS Loop Type feeder DOC Relay Blocking Signal Figure 5 Directional Comparison scheme blocking signals Channels communicate with each other over fibre 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 allows a permissive trip to occur, where both relays trip without any additional time delay. Near-instantaneous backup protection is provided to each cable in this way the only time delay being the relay s directional decision time plus the communication time delay. The IDMT directional overcurrent stages provide backup protection to the BSP and the in the event of a fault at either busbar not being cleared by the primary protection. Backup protection for busbar faults in the Distribution is provided by the IDMT stage of the directional overcurrent relays at the remote ends of the Distribution BSP and Distribution 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 have no instantaneous element, so it is not possible to coordinate the directional overcurrent relays on the Distribution outlets at the BSP and the with those relays. This problem can be solved by installing overcurrent protection which includes an instantaneous element. 2.3 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 and the Distribution 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 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. Fault on 66kV busbar of the BSP or the 11kV Busbar BSP 220kV Busbar 66kV Busbar 66kV Dist. 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. OC Relay Ethernet Switch 11kV Busbar DOC Relay 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 Directional Overcurrent relays on the and BSP feeders at the Distribution 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 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 and BSP to the distribution station.
Fault on City/Loop-type 66kV feeders from the Directional Overcurrent relay on the faulted feeder picks up and broadcasts GOOSE message. The bus coupler switches to its slower IDMT characteristic to coordinate with the feeder relay. Directional Overcurrent relays at the BSP and the Distribution switch to slower characteristic to coordinate with the slow bus coupler overcurrent characteristic at the. 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 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 to the 66kV Distribution remain in their slower IDMT characteristic in order to coordinate with the downstream relay that issued the GOOSE message. 11kV fault at the 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 (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 feeder directional overcurrent relays at the BSP and the 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 / the BSP / the Distribution (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 and the. If a 66kV busbar fault occurred at the BSP, the or the Distribution 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 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 and the 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, 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, the BSP and the Distribution 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 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 66kV bus, only the Distribution feeders at the BSP and 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 station would trip for a busbar fault in the BSP, Distribution or the 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 BSP and BSP Distribution 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 and the BSP Distribution cables as well as the busbars in each station. 66kV Bus coupler relays in the BSP and the 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 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. During a site survey, it was found that due to a lack of space in the bays of the indoor AIS compound at the, 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 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. 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. 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. This scheme could also only be implemented if the existing CTs on the feeders between the BSP, and Distribution 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. 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 or the Distribution 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.