Expedite work on 330kV Anwomaso-Kintampo transmission line – GRIDCo report

The technical team constituted by Ghana Grid Company (GRIDCo) to investigate the cause of last Sunday’s nationwide blackout has recommended the expeditious completion of the ongoing work on the 330kV Anwomaso-Kintampo transmission line.

According to the team, the completion of the line will reduce loading on the Anwomaso 330/161 kV autotransformers and improve system stability during contingencies.

The team explained that the nationwide blackout came about because the “power system experienced a total collapse when the Prestea-Obuasi transmission line tripped on a fault while Bui was running in synchronous condenser mode.

“Loading on the Aboadze-Anwomaso 330 kV transmission line (TT8AW) increased causing overloading on the Anwomaso 330/161 kV transformers. The transformers tripped together with the TT8AW line and caused a severe surge on the NITS. This led to cascaded trips on other transmission lines and generating units in service, leading to a total system collapse at 14:04 h.”

Below is the full report from the technical team:

GHANA GRID COMPANY LTD

A REPORT ON THE TOTAL SYSTEM COLLAPSE THAT OCCURRED ON THE GHANA POWER SYSTEM ON SUNDAY MARCH 07, 2021

1. Introduction

The Ghana power system experienced a total collapse when the Prestea-Obuasi transmission line tripped on a fault while Bui was running in synchronous condenser mode. Loading on the Aboadze-Anwomaso 330 kV transmission line (TT8AW) increased causing overloading on the Anwomaso 330/161 kV transformers. The transformers tripped together with the TT8AW line and caused a severe surge on the NITS. This led to cascaded trips on other transmission lines and generating units in service, leading to a total system collapse at 14:04 h.

Restoration started immediately and by 18:30 h, supply had been restored to all customers.

2. Conditions prior to the collapse

Prior to the system collapse, the power system was in normal operating condition. The state of the generation and transmission system prior to the collapse was as follows:

i. Generation

Four (4) generating units were in service at Akosombo, three (3) units at Kpong GS, one (1) in synchronous condenser mode at Bui, three (3) units each at TAPCO and TICO, one (1) unit at KTPP, two (2) units each at Cenpower and Amandi, seven (7) units at Sunon Asogli and twenty-four (24) units at Karpower.

The total system generation was 2,315 MW.

ii. Transmission

All transmission lines were in service at the time of the incident except the Volta-Achimota transmission line No. 2 (V19H) which was out of service to provide safe working space to erect a tower for the new Volta-Accra East-Achimota transmission corridor upgrade project.

The voltage at Volta was 160 kV. The Ghana-Cote d’Ivoire interconnection was in service with inadvertent export of 39.9 MW to CIE. Power exports were as follows:

• SONABEL: 45.6 MW

• CEB: 67.7 MW

3. Sequence of events

At 13:46 h, the Prestea-Obuasi (P6B) transmission line tripped on a fault.

While the operator at SCC was making efforts to restore the line, loading on the Aboadze-Anwomaso 330 kV transmission line (TT8AW) increased to 450 MW. This caused overload on the Anwomaso 330/161 kV auto transformers (58T3 and 58T4).

At 13:48 h, 58T3 and 58T4 transformers tripped on overload, causing an opening of TT8AW line at Anwomaso end. The TT8AW line tripped at Aboadze end on overvoltage when the Anwomaso end opened.

The tripping of the P6B and TT8AW lines resulted in a surge that caused all generating units in service at TAPCO, TICO, Karpower, Cenpower and Amandi to trip.

At 14:04 h, all Akosombo and Kpong units tripped leading to a total system collapse.

4. Restoration

Restoration was started immediately after the collapse.

Akosombo unit No. 2 was tied to the system, followed by unit No. 4.

Supply was restored to VALCO, followed by Achimota, New Tema and Smelter II. Units were brought online at Karpower and Sunon Asogli and supply was restored to Accra East, Tafo, Nkawkaw, Anwomaso and Kumasi.

Supply was restored progressively as more generation came online and by 18:30 h, supply to all customers on the NITS had been restored.

5. Conclusion

The tripping of P6B line resulted in increased loading on the TT8AW line, leading to overload on the Anwomaso 330/161 kV auto transformers. The tripping of the Anwomaso 330/161 auto transformers together with the TT8AW line resulted in a major system disturbance that led to the total system collapse.

6. Recommendation

The ongoing work on the Anwomaso-Kintampo 330 kV transmission line should be expedited. The completion of the line will reduce loading on the Anwomaso 330/161 kV auto transformers and improve system stability during contingencies.