Analysis of Misoperations of Protection Schemes in the Nordic Grid - 1st of December 2005

Authors: Jan Åge Walseth, Jan Eskedal and Øyvind Breidablik, Statnett SF, Norway

The Nordic transmission system is designed for a maximum production loss of 1200 MW. The operation of the grid outside design limits led to unwanted triggering of a second system protection scheme, which should have led to shedding of another 1000 MW of production from the Nordic grid. Fortunately this system protection scheme also malfunctioned.

This article discusses the results of the investigation of this event, addressing the following items:

1. Analysis of the event from pendeling (10-20 Hz) recordings of power and frequency

2. Causes for misoperations that occurred during the event

3. Restorative actions by dispatchers

4. Impact of the event on design and implementation of system protection schemes in the Nordic grid

The electrical grid in Norway is a part of the Nordel electrical system, which encompasses the electrical grids of Norway, Sweden Finland and eastern Denmark. The Nordel grid and market is in balance with regard to energy and has an installed capacity of 89000 MW and a maximum load of 69000 MW. The production is 53% hydroelectric and the rest is thermal/ nuclear.

The main bottleneck in the Nordel grid is also shown in Figure 1. This bottleneck is between most of the hydroelectric power plants (northwest), and the thermal, and nuclear production (southeast). The main consumption is also southeast of the main bottleneck.

In Norway two system protection schemes have been implemented to deal with problems with high production northwest of the main bottleneck. These system protection schemes are implemented in the following manner: An area is connected to the rest of the power system trough a predefined bottleneck. If transfer on the bottleneck exceeds a predefined limit, the power system dispatcher sensitizes the system protection scheme. If the bottleneck is further restricted due to a disturbance (i.e. a set of predefined breaker operations) production within the area is automatically tripped by the system protection scheme. The system protection scheme may also introduce a net split. These system protection schemes are described in some detail further on, and affected areas are shown in  Figure 1.

System protection scheme “Nordland”

The brown-shaded area (northern Scandinavia) in Figure 2 contains nearly 15% of the installed hydroelectric capacity (~6000 MW) in the Nordel grid. At the same time the consumption is relatively low. If one of the big PTC’s out of the brown-shaded area (420 kV through Northern Sweden or 300 kV through Middle Norway) is disconnected, or if there is a grid split inside the area the net power flow on the remaining PTC’s must be restricted to keep the PTC’s in service. If all production within the area is disconnected from the grid, this will also be higher than the dimensioning production loss (1200 MW) for the Nordel grid.

If system protection scheme “Nordland” is sensitized it will work in the following manner:

  • Shed production. Up to 1200 MW of production may be shed
  • The system protection scheme may also disconnect the northernmost part of Norway from the main Nordel grid. In this manner even more production is shed

This part of the system protection scheme will only be activated when there is a production surplus in the northernmost part of Norway.

If the northernmost part of Norway is islanded by the system protection scheme, the Regional Control Centre Northern Norway has the task of getting  the island in balance with regard to production and consumption before this part is reconnected to the Nordel grid.

System protection scheme “Østland”

The main load center in Norway is in the eastern part around Oslo, with nearly half of Norway’s population (circle in Figure 2). There might be a problem with power transfer capacity either to the Oslo area, or from Norway to Sweden in cases where there is a high transfer from west to east over the main Nordel bottleneck. In these cases system protection scheme “Østland” will be sensitized. This protection scheme will, in cases of outages or overload on central lines in the Oslo area, shed production (up to 1200MW) on the west coast of Norway (ellipsis in Figure 2). This production shedding will lighten the load on the remaining lines in the Oslo area.

Sequence of Events 15:02 1st of December 2005

The situation 1st of December in the Nordel grid was characterized by high hydroelectric production northwest of the main Nordel bottleneck (Figure 4). System protection scheme “Nordland” was sensitized due to high production in northern Scandinavia (2300 MW out of the area). Both the net split and production shedding functions were activated.

In addition, the system protection scheme “Østland” was activated because of high transfer from Norway to Sweden (2100 MW). The situation was in no way unusually strained for this time of the year. Control Center Northern Norway had allowed relay testing on a line differential protection on 420 kV. Control Center Southern Norway had a major upgrade on the SCADA system, which caused a considerable delay on both measurements and indications.

In Northern Sweden (Porjus hydroelectric power station) a 420 kV reactor was to be disconnected. At 15:02:33 the breaker command was issued but one phase failed.

The 420 kV busbar differential protection in Porjus did clear the fault, thereby shutting off one of the main PTC from Northern Scandinavia trough Sweden. The disturbance led to trigging of the “Nordland” system protection scheme. The planned remedial actions should have led to instantaneous shedding 1030 MW of production from the Nordel grid. However the remedial actions in Norway were delayed. This was the main cause for the consequences of the disturbance.

On the Swedish side the system protection did work as intended, and shed 600 MW of production. The remaining 1700 MW of surplus production was pressed southwards trough the grid in Norway. This led to overload on the remaining lines, and a 220 kV line against Sweden tripped after 800 ms and two 300 kV lines against Middle Norway tripped after 850 ms (Figure 5).

The total production loss from the main Nordel system was now 2450 MW as some hydroelectric production in Sweden went out due to the initial fault in Porjus. In addition, 150 MW of load went out due to the voltage dip when the grid was divided in two parts. The production surplus in the islanded area led to a rapid frequency rise within the area, and the loss of production from Nordel led to the greatest recorded frequency deviation for the main Nordel system (0.8 Hz).

After 2.2 seconds the grid split function of the system protection “Nordland” operated and the area was split into two subareas, hereafter called Middle Norway and North Norway (see Figure 6). Both areas had a surplus of production and the frequency rise continued in both areas. Due to over-frequency, 300 MW of production were disconnected in Middle Norway, and 187 MW were disconnected in North Norway.

After 3.3 seconds, the production shedding of system protection “Nordland” worked, and 300 MW of production were shed from North Norway. North Norway, which originally had a production surplus of 430 MW at nominal frequency, now had a deficit of 40 MW (not considering the reduced production due to over-frequency and generator droop curves). Hence the frequency in North Norway went down below 48,5 Hz, and automatic under-frequency load shedding at 48,5 Hz took out 128 MW of load. Figure 3 shows the frequencies for the Nordel area, Middle and North Norway shown for the first 25 seconds of the fault.

The frequency drop in the main Nordel grid as Middle and North Norway was disconnected, led to that spinning reserve in Nordel being activated. This automatic production rise in the remaining system was divided between the Nordel countries as such: Sweden 600 MW, Finland 450 MW, HVDC tie-lines to other grids 480 MW (Russia, Central Europe), and Norway 1050 MW (Figure 7). Nearly all of the increased production in Norway was on west coast.The situation led to an increased power flow of ~1200MW from west to east in Southern Norway. 400 MW of these MW’s covered internal Norwegian power deficit, and the rest (700 MW)  covered the main Nordel bottleneck to cover up the Swedish power deficit.

The extra 700 MW on the PTC between Southern Norway and Sweden led to the system protection scheme “Østland” triggering due to overload on one of the lines in the corridor. This system protection should have led to production shedding of 1150 MW on the west coast of Norway. The system protection scheme triggered, but fortunately it did not work as intended and no production was shed.

In the best scenario the production shedding would have led to under-frequency load shedding of 2400 MW of load, and in the worst scenario the production shedding would have led to breakdown in the Nordel grid. In the main Nordel grid the situation led to further manual activation of 2000MW of production (fast reserve) to get the situation under control.

After 15 minutes Middle Norway was phased in to the main Nordel grid and after 24 minutes North Norway was phased in the main Nordel grid. A problem in the rebuilding of the grid was that both producers and consumers were unaware of what caused the shedding of load or production. Hence both load and production was put in service in the areas without concession from the Regional Control Centers.

In total the disturbance led to a loss of load of 250 MW (70MWh). In addition the system protection schemes were deemed unfit, until the reasons for misoperation were corrected. This had consequences for the capacities on the PTC’s which again led to costs of ~4,2 mill Norwegian kroner. The reasons for misoperation are discussed below.

Findings after disturbance

To slow reaction time on system protection “Nordland”: This system protection scheme was designed in 2001 and put into service. In one of the hydroelectric power stations where the response should have been shedding of 300 MW of production, the system protection scheme was realized through software in the bay nodes. When the system protection scheme was tested in 2001 the response times were found adequate. However between 2001 and 2005 the power station owner had changed their SCADA system, and communication software for the SCADA system had been added to the bay nodes, thereby slowing down the cycle times of the nodes considerably.

More stations were found where the system protection was realized trough software in bay nodes and/or station computers, and this design principle was deemed unusable as response times could not be guaranteed. In the future system protection in the Nordel grid shall be realized through discrete components with guaranteed response times, even if the configuration of the rest of the station control equipment changes.

In short, a system protection scheme should have as high reliability as the protection. As a consequence of this, the relay department is now made responsible for the implementation and commissioning of the system protection schemes, and new demands with respect to communication and implementation are enforced. System planning is still responsible for system protection scheme algorithm, setting and sensitizing.

Misoperation of system protection “Østland”: The triggering of the system protection scheme was as intended. However, it can be discussed if an overload situation should have led to triggering of the system protection scheme. In the current case where the power system was outside design limits (the system is designed for a maximum loss of production of 1200 MW, the loss was 2450MW) which led to more spinning reserve than designed for, being brought into action on the west coast of Norway. In this way the power system was saved from heavy under-frequency load shedding or breakdown. After the disturbance the overload limit of the system protection scheme was set to a higher value, so high that in the current case it would not have triggered.

The reason that the triggering did not lead to production load shedding can be found in Figure 8. The clamps on the photography were forgotten open after a test of the system protection scheme. As a consequence new testing procedures for protection schemes were implemented in Statnett:

  • System planning decides when to test, and what shall be tested
  • Before a test a detailed test plan will be issued and approved by system planning

In this way Statnett hopes to avoid similar situations like the one in Figure 8.

Other findings

The disconnection of hydroelectric generators due to over-frequency in North Norway was unnecessary for machine protection and eventually led to under-frequency load shedding. Statnett has now specified for producers that over-frequency production shedding is unwanted, and shall only be used where it is necessary for machine protection.

Under-frequency load shedding was also in action during this disturbance. There were more cases of utilities connecting shed load without the concession of the Regional Control Center. In the worst case this may lead to a blackout. Statnett has repeated the rules for bringing shed load back into service for the utilities. A recent hurricane (with production shortage) has shown that the utilities are aware of the rules now.

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BeijingSifang June 2016