Historic factors and the fact that Japan is an archipelago consisting of four main islands and hundreds of smaller ones have contributed to what has become an unusual power grid considerably different from other developed countries.
Not only is the country’s grid divided into ten physically separate zones, but it’s also divided into two areas with different frequencies at which the grid operates.
One Country, Two Frequencies
Unlike other well-off countries in which the frequency at which their power grids operate were standardized over time, in Japan, it still is – and for the foreseeable future will be – split into two zones.
The eastern part of Honshu, including the Tokyo metropolitan area, and the entire island of Hokkaido use 50 Hz. The western part Honshu, including Nagoya and Osaka among other cities, as well as the islands of Kyushu, Shikoku, and Okinawa use 60 Hz.
The divide is accidental and traces its origins back to the 1890s when first generators were brought to Japan from overseas. German generators operating at 50 Hz were the first ones to make it into Tokyo. Around the same time, American generators operating at 60 Hz were brought to the largest city in the western part of the country, Osaka.
While the reasons behind the divide are historic, the consequences of it were most recently felt in 2011 when Japan was hit by the Great East Japan Earthquake.
With only 3 back-to-back HVDC frequency conversion stations in the country, portions of the country had to face blackouts. Not because of the lack of generation capacity, but because of the inability to transfer power quickly enough between the two systems.
Let’s talk about that more in a future article, though – and for now, let’s move on to the ten zones into which the country’s grid is divided.
From Hokuden in the North to Okiden in the South
In 2016, the electricity market in Japan was deregulated to allow for nationwide retail competition. However, besides being split into two separate areas with different frequencies, the grid is further physically divided into ten separate zones. Those zones follow the borders within which the 10 regional utilities used to have monopolies.
Similarly to the interconnections between the 50 Hz and 60 Hz areas, interconnections between nine of the ten grid zones exist but are fairly limited. The tenth one, Okinawa, is isolated since it powers an archipelago located hundreds of kilometers away from the four main islands of Honshu, Hokkaido, Kyushu, and Shikoku.
As one might expect, the TEPCO Power Grid covering the Tokyo metropolitan area among other places has, at 66,800 MW, the highest generation capacity of the ten. The Okinawa grid, on the other hand, only has a generation capacity of 2,140 MW and is not only the most isolated but also the smallest of the ten.
Below is a brief look at each of the ten zones including basic information like the incumbent utility, population, and generation capacity.
#1: Hokkaido
| Incumbent Utility | Hokkaido Electric Power Company |
| Population | 5.3M |
| Prefectures within Zone | Hokkaido |
| Major Cities within Zone | Sapporo, Hakodate |
| Frequency | 50Hz |
| Generation Capacity | 8,300 MW |
| Adjacent Zones | Tohoku |
#2: Tohoku
| Incumbent Utility | Tohoku Electric Power Company |
| Population | 11.2M |
| Prefectures within Zone | Aomori, Iwate, Akita, Miyagi, Yamagata, Fukushima, Niigata |
| Major Cities within Zone | Sendai, Niigata |
| Frequency | 50Hz |
| Generation Capacity | 16,800 MW |
| Adjacent Zones | Hokkaido, TEPCO, Chubu, Hokuriku |
#3: TEPCO Power Grid
| Incumbent Utility | Tokyo Electric Power Company |
| Population | 43.8M |
| Prefectures within Zone | Tokyo, Kanagawa, Saitama, Chiba, Tochigi, gunma, Ibaraki, Yamanashi, Shizuoka (east of Fuji River) |
| Major Cities within Zone | 23 Specials Wards of Tokyo (formerly Tokyo City), Yokohama, Kawasaki, Saitama, Chiba |
| Frequency | 50Hz |
| Generation Capacity | 66,800 MW |
| Adjacent Zones | Tohoku, Chubu |
#4: Chubu
| Incumbent Utility | Chubu Electric Power Company |
| Population | 17.1M |
| Prefectures within Zone | Aichi, Nagano, majority of Gifu, Mie, Shizuoka (west of Fuji River) |
| Major Cities within Zone | Nagoya, Shizuoka, Nagano |
| Frequency | 60Hz |
| Generation Capacity | 33,400 MW |
| Adjacent Zones | Tohoku, TEPCO, Hokuriku, Kansai |
#5: Hokuriku
| Incumbent Utility | Hokuriku Electric Power Company |
| Population | 3M |
| Prefectures within Zone | Toyama, Ishikawa, Fukui, part of Gifu |
| Major Cities within Zone | Kanazawa, Toyama |
| Frequency | 60Hz |
| Generation Capacity | 8,500 MW |
| Adjacent Zones | Tohoku, Chubu, Kansai |
#6: Kansai
| Incumbent Utility | Kansai Electric Power Company |
| Population | 20.7M |
| Prefectures within Zone | Kyoto, Osaka, Shiga, Nara, Wakayama, majority of Hyogo, parts of Fukui, Mie and Gifu |
| Major Cities within Zone | Osaka, Kobe, Kyoto |
| Frequency | 60Hz |
| Generation Capacity | 34,260 MW |
| Adjacent Zones | Hokuriku, Chubu, Shikoku, Chugoku |
#7: Chugoku
| Incumbent Utility | Chugoku Electric Power Company |
| Population | 7.4M |
| Prefectures within Zone | Hiroshima, Okayama, Tottori, Shimane, Yamaguchi, parts of Ehime and Kagawa |
| Major Cities within Zone | Hiroshima, Okayama |
| Frequency | 60Hz |
| Generation Capacity | 11,530 MW |
| Adjacent Zones | Kansai, Shikoku, Kyushu |
#8: Shikoku
| Incumbent Utility | Shikoku Electric Power Company |
| Population | 3.8M |
| Prefectures within Zone | Tokushima, Kochi, majority of Ehime and Kagawa |
| Major Cities within Zone | Matsuyama, Kochi |
| Frequency | 60Hz |
| Generation Capacity | 5,770 MW |
| Adjacent Zones | Kansai, Chugoku, Kyushu |
#9: Kyushu
| Incumbent Utility | Kyushu Electric Power Company |
| Population | 13M |
| Prefectures within Zone | Fukuoka, Saga, Nagasaki, Oita, Kumamoto, Miyazaki, Kagoshima |
| Major Cities within Zone | Fukuoka, Kita-Kyushu, Kumamoto |
| Frequency | 60Hz |
| Generation Capacity | 29,820 MW |
| Adjacent Zones | Chugoku, Shikoku |
#10: Okinawa
| Incumbent Utility | Okinawa Electric Power Company |
| Population | 1.4M |
| Prefectures within Zone | Okinawa |
| Major Cities within Zone | Naha, Okinawa City |
| Frequency | 60Hz |
| Generation Capacity | 2,140 MW |
| Adjacent Zones | None – isolated from main Japanese archipelago |
The Consequences of the Highly Fragmented Grid
The existence of these divisions affects the dynamics of the electricity supply system in a multitude of ways including:
- How the competitive retail and wholesale markets are regulated and how business is conducted within them
- The physical ability to move electricity from one region of the country to another when necessitated by natural disasters
The way the grid structure affects different aspects of electricity in Japan is a theme which will arise repeatedly as we dive deeper into various topics in the weeks and months ahead.
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