HK Electric Investments and HK Electric Investments : Investigation Report of the Incident of a 275-kV Fault in HK Electric’s Power System on 19 April 2023

Incident of a 275-kV Fault in HK Electric's Power System

on 19 April 2023

INCIDENT INVESTIGATION REPORT

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The Hongkong Electric Company, Ltd. ("HK Electric"). All rights reserved.

Incident of a 275-kV Fault in HK Electric's Power System on 19 April 2023

Incident Investigation Report (Executive Summary)

Introduction

1. A 275-kV fault occurred at 00:45 hours on 19 April 2023 and caused a system- wide voltage dip. The fault was associated with Cyberport 275-kV Switching Station and the voltage dip resulted in power interruption to some customers four minutes later at 00:49 hours and the power restoration was initiated in 33 minutes at 01:22 hours. It was a very rare incident in HK Electric's system, the last power interruption incident of similar scale occurred in the year 1994 which was caused by an incident outside HK Electric system.
2. Electricity supply to about 44,000 customers scattered in HK Electric's supply areas was interrupted in this incident. This represented about 4% of the total loading at the time of the incident. The longest duration of power interruption was 48 minutes from 00:49 hours to 01:37 hours, by then the power supply to all affected customers was fully resumed.
3. The incident is HK Electric's first major power interruption in almost thirty years. We are taking the matter very seriously and have notified and reported the incident to the Director of Electrical and Mechanical Services. Our Company has immediately set up an Investigation Committee, led by Operations Director, to carry out investigation in relation to the incident, to report on the findings and to recommend improvement measures to prevent similar incidents from happening in the future. The Investigation Committee comprised the following 8 members:

Mr. Francis Cheng	Operations Director
Mr. S.T. Ip	General Manager (Transmission and Distribution(T&D))
Mr. F.S. Chow	General Manager (Generation)
Mr. C.S. Leung	Head of System Operations
Mr. Tony Yeung	Head of Construction & Maintenance (T&D)
Mr. T.F. Chan	Head of Operations (T&D)
Dr. C.L. Lee	Head of Operations (Generation)
Mr. Alex Ng	Group Legal Counsel

1.4 To keep our customers and the public informed, we had shortly after the incident provided various updates based on our knowledge at the time. From the investigation we have conducted, we now have a full picture that the incident occurred due to a series of events and a number of factors. A spare cable circuit was inadvertently energised during commissioning of refurbished switchgear at the Cyberport 275-kV switching station causing a fault at HK Electric's power supply system. A severe voltage dip of the system followed and affected the generating units at Lamma Power Station, necessitating a shutdown of two of the units and resulting in insufficient power generation to meet the load demand. The combination of these factors escalated into this power interruption.

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1. The incident is most regrettable, and we apologise again for the inconvenience caused to our customers. We will learn from this lesson and enhance our operations to sustain our commitment to delivering a reliable electricity supply to our customers.
2. Our report sets out the findings of our investigation, the causes of the incident, and the recommendations and improvement measures to be taken to prevent similar incidents from happening in the future.

What Happened

Cyberport 275-kV Switching Station and Spare Cable Circuits

1. Cyberport 275-kV Switching Station (CPX) at which the incident started is one of the 275-kV switching stations in HK Electric's transmission network for receiving power from Lamma Power Station (LPS). CPX was planned to be commissioned to meet the demand of the Cyberport development in 2002 using two 275-kV cable circuits in the vicinity. Following the installation and commissioning of the two Lamma 275-kV submarine cable circuits to CPX and subsequent network reconfiguration, two of the 275-kV cable circuits connected to CPX have become spare circuits and have been reserved for future use since 2009 as shown in Illustration 1 of the main report.
2. At the time of the incident, the 275-kV Gas Insulated Switchgear (GIS) of CPX was under commissioning after completion of the refurbishment of one of the switchgear sections as shown in Figures 1 and 2 of Appendix 2. This section of switchgear is connected to one of the two spare cable circuits (mentioned in 1.7) through a cable isolator. As part of the commissioning, all GIS switches that had been worked on, including the cable isolator (named L53A) connected to the spare cable circuit, had to be energised. According to our standard practice, such commissioning work has to be carried out at mid-night and only one in-service circuit is arranged to supply electricity for the commissioning as an additional precautionary measure in order to minimise possible impacts on both the system and our customers if something unforeseeable happened.

Three-phase Fault during Commissioning

1.9 The engineer in charge carried out the commissioning work according to the approved commissioning procedures at CPX. Another engineer with appropriate qualification was also deployed to CPX to double check the switchgear gas pressure and to carry out specific parts of the commissioning procedures not related to circuit energisation. The whole process was also supported remotely by an engineer in the System Control Centre at Ap Lei Chau, who was responsible for switching on and off the GIS during commissioning according to the site condition as confirmed by the engineer in charge as well as the approved commissioning procedures using remote control facility. At 00:45 hours on 19 April 2023, as per confirmation of the engineer in charge, the engineer at System Control Centre energised the cable isolator whereby a 275-kVthree-phase fault occurred and relevant Circuit Breakers (CBs) tripped correctly by backup protection system to isolate the fault. Subsequent investigation concluded that the spare 275-kV cable circuit connecting to the cable isolator with the other end

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of the cable capped inside Kai Lung Wan Tunnel portal was inadvertently energised causing the tripping.

Tripping of HK Electric-CLP Power Interconnector Circuits

1.10 HK Electric's power system has been connected with the power system of CLP

Power since 1981 through three cross-harbour interconnector circuits for providing emergency support to each other if and when needed. Immediately after occurrence of the fault, the three-phase-fault detection scheme in our system also operated automatically according to the design and disconnected all three interconnector circuits between HK Electric and CLP Power's power systems. This detection scheme is custom-built to prevent blackout of HK Electric's entire system due to possible severe power oscillation between the two power systems of HK Electric and CLP Power. After all three interconnector circuits were disconnected, emergency support from CLP Power was not available anymore.

Impact on the Power System

1.11 It was the first time that a 275-kVthree-phase-fault occurred in HK Electric's system, which gave rise to a very severe disturbance to the power system. The fault also caused a severe voltage dip and the system voltage had collapsed to nearly zero for about half a second, which was a very rare incident and caused considerable adverse impacts on the whole power system.

All the generating units at Lamma Power Station and their auxiliary equipment were also seriously affected by this unprecedented power disturbance. The total power generation fluctuated seriously in the first few seconds after the occurrence of the fault. The protection systems of the auxiliary and major equipment functioned to protect them from damage. For example, the lubricating oil pumps of some coal mills affected by the voltage dip were tripped by the undervoltage protection such that the cables and the motors of the pumps had been protected from overheating; and the moving parts of the coal mills had been protected from excessive wear by tripping the mills when the pressure of the lubricating oil dropped. These protection systems were designed for preventing damage of generating units during severe power disturbance rather than allowing them to ride through a system fault.

Shutdown of Two Generating Units and Activation of Under-frequencyLoad-Shedding

1.12 When the power generation of a power system cannot meet the load demand, the power system becomes unstable and the system frequency will drop. It is necessary to either increase the power generation or reduce the load demand to restore the system to normal. To tackle the situation of low system frequency, an under-frequency-load-shedding protection device is installed in each and every zone substation to disconnect a pre-defined amount of load automatically when the system frequency drops to a particular value in order to prevent a further decrease in system frequency, which would result in a system blackout if not arrested. There are six stages of under-frequencyload-shedding that operate at different values of system frequency. After the operation of the first stage, if the system frequency continues to drop even lower, stage 2 will be triggered and so on.

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