by Rene Troost, CIGRE B5 chair of The Netherlands
Friday, September 2, 2022. The day, many people of our community went home from the CIGRE 2022 Session in Paris. At this moment, an impressive disturbance happened in Flevoland, The Netherlands. This article describes the main storyline, based on the public post-mortem analysis report.
Introduction: The electricity grid in the Netherlands is being greatly expanded to facilitate the energy transition. One of these new high-voltage substations is the 150kV Dronten Olsterpad substation. At approximately 15:05h on Friday, September 2, 2022, a short circuit occurred at this substation. Normally, short circuits are switched off withing a fraction of a second. However, due to a confluence of circumstances, during this incident the short circuit was not disconnected until after approximately 4.5 minutes.
The electric arc resulting from the short circuit caused a lot of damage in the substation and, as a result of the long duration of the short-circuit current, parts of the high-voltage grid were severely overloaded. The conductors of the feeding 150kV circuit between the 150kV Lelystad substation and the 150kV Dronten Olsterpad substation have consequently overheated, causing the conductors to lengthen (thermal expansion). Smoke also came from combustion of the grease contained in these conductors. This thermal expansion caused the conductors of this 150kV circuit sagged so much that at the location where the 150kV circuit crosses the railway, the distance to the catenary of this became too small, and a second short circuit occurred. This short circuit between the 150kV grid and the 1.5kV railway overhead line caused major damage to the rail infrastructure.
For several minutes, there was a voltage drop in the high-voltage network in the Flevopolder and parts of the high-voltage grid were switched off, interrupting the supply of energy to customers. In the grids of the DSOs Liander and Stedin, voltage drops, and voltage peaks were recorded. Because of the low-hanging conductors, some roads, including the A6 highway and N309, were temporarily closed. The incident did not result in any physical injuries.
Analysis: Figure 1 shows overview of the relevant part of the high-voltage grid in the Flevopolder area with the 380kV substations and connections in red and those at the 150-kV level in blue.
On September 2, 2022, work was carried out at Dronten Olsterpad substation to complete the work, that had been successfully carried out on a part of this station the previous day. After completion of these work, an earthing switch was switched on at the energized 150kV circuit Lelystad – Dronten Olsterpad. This initiated the short circuit at the Dronten Olsterpad substation. This short circuit is referred to in the timeline in Figure 3 referred to as short circuit (I). This circuit was energized because during the commissioning procedure on the day before the disturbance a voltage test was successfully performed. But after completion of this procedure the circuit could no longer be de-energized in the usual manner. It was not possible to switch the circuit breaker (CB) off from the grid control center.
On September 2, technicians tried to solve this problem. Because of this, at 15:04h (40 seconds before the earthing switch on the opposite end was swithed on), technicians tried to locally control the CB. To locally control the CB, a local/remote switch has been switched to ‘local,’ which interrupts all commands, including trip commands. This switch has not been switched back to its original position, resulting in not tripping the CB after the earth fault (at 15:05h) at the opposite end (Dronten Olsterpad).
Normally, the Breaker Failure (BF) protection shall switch off all surrounding CBs as a backup protection scheme. Due to a undiscovered wiring fault in 150kV substation Lelystad, the BF protection didn’t even clear the short circuit. (see Figure 2).
Because of the failure of the protection system, as described above, a short circuit current of approximately 8.5kA flowed through the connected high-voltage components for an extended period of time. The conductors in the 150kV overhead line Lelystad – Dronten Olsterpad have reached a temperature of more than 300 °C, where 80 °C is allowed. This extreme temperature increase has led to significant thermal expansion and consequently the lengthening of the conductors, which consequently hung significantly lower than permitted.
The distance between the conductors of this circuit and the overhead line of the railway thereby became so small that a new short circuit occurred. This short circuit is referred in Figure 3 as short circuit (II). Because this second short circuit was closer to the feeding substation Lelystad, the short-circuit current increased to more than 15kA. This eventually triggered the protections of surrounding stations and power plant and resulted in a tripped and cleared fault after 4 minutes and 22 seconds. This was just 5 seconds before the short circuit have been manually cleared by operators of the national grid control center, see also Figure 3.
Summary & Conclusions: The short circuit was caused by an earthing switch that was incorrectly routinely turned on at substation Dronten Olsterpad on the energized circuit Lelystad – Dronten Olsterpad. The fault wasn’t cleared properly. Three forms of electrical protection failed to work properly:
1) The CB trip circuit of the affected overhead line was blocked because the local control switch, which had been turned 40 seconds before the incident.
2) There was an undiscovered wiring fault in the breaker failure protection circuit that prevented this protection scheme to act as designed. The wiring fault had not been discovered due to quality procedures not fully implemented.
3) The feeder bay protections didn’t recognize the short circuit within the protected area. Only after the short circuit with the overhead line of the railway had occurred, the protections of the feeder bays became active. It also should be noted that some protection relays were not configured in accordance with the protection setting policy.
Rene Troost is graduated as an Electrical Engineer. In 2014, Troost joined Stedin, the DSO for the South-West area of The Netherlands, including the Port of Rotterdam. He is currently responsible for substation automation policy in Stedin. He chairs the Dutch CIGRE B5 study committee.