Milestone-Proposal:Sakuma Frequency Converter Station, 1965
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To the proposer’s knowledge, is this achievement subject to litigation? No
Is the achievement you are proposing more than 25 years old? Yes
Is the achievement you are proposing within IEEE’s designated fields as defined by IEEE Bylaw I-104.11, namely: Engineering, Computer Sciences and Information Technology, Physical Sciences, Biological and Medical Sciences, Mathematics, Technical Communications, Education, Management, and Law and Policy. Yes
Did the achievement provide a meaningful benefit for humanity? Yes
Was it of at least regional importance? Yes
Has an IEEE Organizational Unit agreed to pay for the milestone plaque(s)? Yes
Has the IEEE Section(s) in which the plaque(s) will be located agreed to arrange the dedication ceremony? Yes
Has the IEEE Section in which the milestone is located agreed to take responsibility for the plaque after it is dedicated? Yes
Has the owner of the site agreed to have it designated as an IEEE Milestone? Yes
Year or range of years in which the achievement occurred:
1965
Title of the proposed milestone:
Sakuma Frequency Converter Station, 1965
Plaque citation summarizing the achievement and its significance; if personal name(s) are included, such name(s) must follow the achievement itself in the citation wording: Text absolutely limited by plaque dimensions to 70 words; 60 is preferable for aesthetic reasons.
The Sakuma Frequency Converter Station, completed in 1965 by Electric Power Development Co., Ltd. (J-POWER), enabled bidirectional power exchange between Japan’s 50 Hz eastern grid and 60 Hz western grid through a 300 MW back-to-back HVDC link. Initially using mercury-arc valves and later upgraded to thyristors in 1993, the station greatly improved grid reliability and interconnection. It marked a significant technological milestone in Japan’s national power infrastructure development.
200-250 word abstract describing the significance of the technical achievement being proposed, the person(s) involved, historical context, humanitarian and social impact, as well as any possible controversies the advocate might need to review.
The Sakuma Frequency Converter Station, developed and commissioned in 1965 by Electric Power Development Co., Ltd. (J-POWER), was Japan’s first large-scale facility to connect the country’s eastern 50 Hz power grid with the western 60 Hz grid. This pioneering project used a back-to-back high-voltage direct current (HVDC) converter rated at 300 MW and ±125 kV to enable stable, bidirectional power exchange between the asynchronous grids.
The original system employed mercury-arc valves arranged in a twelve-pulse configuration using two six-pulse bridges. Supporting infrastructure included smoothing reactors (0.12 H) and harmonic filters (5th, 7th, 11th, 13th orders) to ensure power quality. The AC systems on both sides were connected via specially designed transformers with delta and wye windings. In 1993, the converter station was upgraded to solid-state thyristor valves, improving efficiency, reliability, and ease of maintenance while preserving the original facility structure.
As a critical link in Japan’s power system, the station enhanced national grid stability, facilitated energy sharing, and laid the foundation for future HVDC applications in the country. The successful implementation of this project by J-POWER marked a transformative step in Japan’s power engineering and infrastructure development. The Sakuma Frequency Converter Station is thus proposed as a significant IEEE Milestone for its technical and societal impact.
IEEE technical societies and technical councils within whose fields of interest the Milestone proposal resides.
IEEE Power & Energy Society
IEEE Power Electronics Society
In what IEEE section(s) does it reside?
IEEE Nagoya Section
IEEE Organizational Unit(s) which have agreed to sponsor the Milestone:
IEEE Organizational Unit(s) paying for milestone plaque(s):
Unit: IEEE Nagoya Section
Senior Officer Name: Hideyuki Uehara
IEEE Organizational Unit(s) arranging the dedication ceremony:
Unit: IEEE Nagoya Section
Senior Officer Name: Hideyuki Uehara
IEEE section(s) monitoring the plaque(s):
IEEE Section: IEEE Nagoya Section
IEEE Section Chair name: Hideyuki Uehara
Milestone proposer(s):
Proposer name: Chiaki Ishikawa
Proposer email: Proposer's email masked to public
Proposer name: Kouji Takahashi
Proposer email: Proposer's email masked to public
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Street address(es) and GPS coordinates in decimal form of the intended milestone plaque site(s):
Describe briefly the intended site(s) of the milestone plaque(s). The intended site(s) must have a direct connection with the achievement (e.g. where developed, invented, tested, demonstrated, installed, or operated, etc.). A museum where a device or example of the technology is displayed, or the university where the inventor studied, are not, in themselves, sufficient connection for a milestone plaque.
Please give the details of the mounting, i.e. on the outside of the building, in the ground floor entrance hall, on a plinth on the grounds, etc. If visitors to the plaque site will need to go through security, or make an appointment, please give the contact information visitors will need.
Are the original buildings extant?
Details of the plaque mounting:
How is the site protected/secured, and in what ways is it accessible to the public?
Who is the present owner of the site(s)?
What is the historical significance of the work (its technological, scientific, or social importance)? If personal names are included in citation, include detailed support at the end of this section preceded by "Justification for Inclusion of Name(s)". (see section 6 of Milestone Guidelines)
Social and Historical Significance
Bridging Japan's East-West Power Divide
Since the Meiji Era, Japan's electric power grid has been uniquely divided: 50 Hz in the east and 60 Hz in the west. This legacy, originating from the early adoption of different foreign generator technologies in Tokyo and Osaka, persisted for over six decades. By the 1960s, the dual-frequency problem had become a serious limitation to efficient national power distribution. The Sakuma Frequency Converter Station, completed in 1965, was Japan’s first large-scale solution to this fundamental issue.
By enabling a seamless 300 MW transfer of power between the two frequency systems using advanced HVDC back-to-back conversion, Sakuma effectively unified the operational capabilities of the nation’s eastern and western grids. For the first time, Japan could balance supply and demand across regions regardless of frequency difference.
📷 [Insert diagram showing East (50 Hz) and West (60 Hz) Japan with Sakuma connecting them – possibly adapted from “第1図 関連系統図”]
Immediate Technical and Operational Benefits
Sakuma introduced a level of operational flexibility that was previously impossible. Traditional methods for frequency conversion—using dual-frequency generators—were costly, time-consuming, and limited to only a few hundred megawatts. Sakuma's HVDC-based system could switch power direction in seconds, with no need for synchronous generator adjustment. This allowed rapid response to regional imbalances and reduced the required spinning reserve capacity across the national grid.
For instance, simulations from the era showed that with Sakuma in operation, frequency drops in the Tokyo region (due to generation loss, such as a 340 MW outage at Yokosuka Thermal Plant) could be halved. This helped prevent blackouts and significantly improved system resilience.
📊 [Consider a chart showing comparative frequency drop simulations with/without Sakuma – based on your cited data]
Pioneering High-Capacity Frequency Conversion
At the time of its commissioning, Sakuma was the largest frequency converter station in the world using mercury-arc rectifiers. It served as a proof of concept for HVDC back-to-back conversion, employing innovative design techniques like 12-pulse rectification, water-cooled cathode systems, and dual-cycle transformers with minimized stray capacitance. These technical choices greatly influenced future converter stations in Japan and internationally.
Furthermore, Sakuma was constructed with acoustic and electromagnetic compatibility in mind—a notable achievement given its proximity to residential areas. Its pioneering double-soundproof transformer housing became a standard for urban power facilities.
🏗️ [Include a photo of the facility or a technical schematic – e.g., “第3図 変換所全景” or “第4図 機器配置図”]
Long-Term Impact on Japan's Energy Infrastructure
Sakuma laid the foundation for all subsequent East-West HVDC links in Japan, including those at Shin-Shinano and Higashi-Shimizu. More importantly, it established the concept of cross-frequency grid balancing as a standard practice. This became especially critical during national emergencies, such as the 2011 Great East Japan Earthquake, when frequency conversion links helped transfer electricity from western Japan to support the power-deprived east.
The principles developed at Sakuma continue to influence modern power grid interconnection and the integration of renewable energy sources, which often require flexible and decoupled frequency operation.
A Technological and Strategic Milestone
More than just a technical innovation, the Sakuma Frequency Converter Station represented a national strategic breakthrough. It addressed a century-old infrastructure divide, enhanced energy security, and positioned Japan at the forefront of HVDC technology development.
Its legacy continues not only in its physical operation but in the design philosophies and system architectures of modern power systems worldwide.
Photo 1 Sakuma Frequency Converter Station
Technical Design and Innovation of Sakuma Frequency Converter Station
Overview of Purpose and Location
The Sakuma Frequency Converter Station was purpose-built to interconnect Japan’s 50 Hz and 60 Hz power grids, which are separated by a central boundary across Honshū. Located roughly 1 km from Sakuma Power Plant in Shizuoka Prefecture, the facility was designed to transmit up to 300 MW at ±125 kV DC between two 275 kV AC systems. Work began in April 1962, with commercial service planned for November 1965. A system diagram (Figure 1) illustrates its integration with the eastern (50 Hz) lines toward Tokyo and western (60 Hz) lines toward Nagoya.
(Include Figure 1: Regional grid interconnection diagram.)
Why HVDC Linking Beats Frequency Switching
Before Sakuma, sharing large generation between grids required switching existing plants from one frequency to another — a slow, disruptive process taking minutes to over ten minutes, and restricted by operational constraints. In contrast, the HVDC link allows instantaneous power transfers of up to 300 MW, fully DC-controlled, avoiding these shortcomings.
Simulations indicated national reserve capacity savings of approximately 290 MW by 1964 and 380 MW by 1968 thanks to this interconnection. Furthermore, if a large generator suddenly trips (e.g. a 340 MW drop at Yokosuka Power Plant), frequency deviation in a standalone 50 Hz grid would reach up to −0.83 Hz; with DC interconnection, deviation reduced to about −0.35 Hz — dramatically improving system stability.
Core Converter Design and Layout
Sakuma houses two converter poles, each rated 150 MW across ±120 kV DC. Each pole employs paired mercury-arc valve bridges for 50 and 60 Hz sides, sharing one bypass valve and bypass disconnector. Arranged in series with a DC smoothing reactor, a single pole outputs ±125 kV. Valve groups V1 and V3 form one bridge, and V2 and V4 the other; the mid-point between the reactors remains at zero potential.
(Add a one-line schematic diagram, Figure 2: Single-line converter arrangement.)
AC output from Sakuma is exclusively supplied at 60 Hz through an on-site 4500 kVA transformer, with emergency supply from the plant’s 3.3 kV distribution in case of station blackout.
(Include Figure 3: Site photo, and Figure 4: Equipment layout.)
Mercury-Arc Valves and Cooling Features
The mercury-arc valves operate with six-phase Graetz configurations at 1200 A and 125 kV nominal. Each valve unit measures approximately 3.4 × 3.2 × 1.2 m and weighs around 4 tonnes. Internally, they maintain a vacuum under 0.001 mmHg, sustained by a mercury vapor pump and auxiliary vacuum tank.
Unlike the England–France DC link, Sakuma uses water-cooling for the cathode tank and air-cooling for the anode section. Purified water (>10 kΩ·cm) circulates at 32 ± 1 °C; bypass valves are cooled at 40 ± 2 °C. Anode air is blown from the converter hall to hold temperature between 75–110 °C, while ambient air in the hall is maintained between 0–30 °C.
CR dividers ensure uniform voltage distribution along the electrodes during off periods
→ (include diagram of valve and cooling).
Converter Transformers & AC Side Protection
Each pole uses transformers in a double-bank Y–Y/Y–Δ configuration, delivering 12-pulse conversion to reduce harmonics. These transformers, sizable for their era, were designed to withstand up to 20 A of DC excitation resulting from frequency interactions between the two systems. To manage noise (estimated at ~93 dB at the transformer, reduced by 10–15 dB due to DC excitation) and protect nearby residences, a dual sound-proof tank was installed within a concrete acoustical building, with separate oil–water cooling.
AC side filters and surge arrestors were modeled to withstand impulse voltages up to 1.8–2 times normal system voltage. Surge arresters were rated at 650 kV and above to protect equipment during switching and conversion operations.
DC Smoothing Reactors & Overvoltage Control
High-inductance DC reactors—1 H at 100 A, and 0.4 H above 500 A—were installed in series with each converter pole to smooth ripple and support commutation stability. They are insulated to 750 kV BIL to handle transient over voltages. Noise from these reactors was studied and filtered in design to minimize impact.
Transient damping RC snubber circuits (45,000 pF & 2.4 kΩ) were placed in parallel with each valve to suppress switching oscillations during commutation.
Control, Protection & Instrumentation
The converters operate under constant-current control. The rectifier side (forward converter) adjusts the valve firing angle α to track a current reference. The inverter (reverse converter) includes a fixed margin (~100 A) to ensure reliable commutation. Tap-changers on the AC side maintain α within 10°–20°, and inverter margin angles at approximately 17° (50 Hz) or 19° (60 Hz).
One-pole operation is possible during faults using a single bypass bridge; two-pole operation (12-pulse) is preferred for reduced harmonics. Step-control allows adjustable power flow up to 300 MW in either direction, with emergency support settings triggered in <0.3 s.
Monitoring systems include current, voltage, angle, and frequency instruments. Protection covers back-arc detection, current imbalance, over-current, grounding faults, and auxiliary systems (cooling, vacuum pumps). Inverter failure protocols move current to bypass and restore operation after fault clearance.
Radio-Frequency Interference Mitigation
Mercury-arc valve switching generates rapid voltage changes, producing RF emissions via dipole radiation and current loops. Measurements predicted ~75 dBμV/m noise without shielding. To address this, the valve hall uses a shielded building attenuating 60–70 dB, reducing emissions to 5–15 dB above background. A “blocking room”—a filter enclosure between the hall and switchyard—further attenuates loop emissions using block filters and aluminum mesh.
These measures positioned Sakuma at the forefront of RF mitigation technology in HVDC stations, providing valuable insights for future installations.
Design Highlights Snapshot
- 12‑pulse, ±125 kV DC link rated for 300 MW
- Mercury‑arc valve bridges with innovative water‑cooled cathodes
- Dual sound‑insulated transformer housings and filtering to protect nearby residents
- Advanced valve control ensuring reliable commutation across frequency boundaries
- Comprehensive protection schemes for arcs, imbalances, and auxiliary failures
- Cutting‑edge RFI shielding and noise suppression, validated both by modeling and measurement
What obstacles (technical, political, geographic) needed to be overcome?
Obstacles (technical, political, geographic) needed to be overcome
Technical Challenges
High‑Capacity Frequency Conversion Design
Japan’s dual-frequency power system—50 Hz in the east and 60 Hz in the west—operated at separate ultra‑high‑voltage grids. By the early 1960s, the 50 Hz grid had grown to approximately 11 GW of capacity, and the 60 Hz network to 16 GW. To bridge these systems, the Sakuma station required a ±125 kV, 300 MW DC link composed of two 150 MW converter poles.
This was unprecedented in Japan. The design had to address:
- Bidirectional power flow between frequency zones.
- High-voltage insulation, ensuring safe operation at extreme voltages.
- Thermal, acoustic, and electromagnetic constraints on large-scale equipment.
To achieve this, the converter utilized series-connected dual poles, each comprising one 50 Hz and one 60 Hz mercury-arc set, sharing a bypass rectifier and switch. This design halved the required bypass equipment and simplified control compared to separated stations.
Figure insertion suggestion: Single-line converter diagram (Figure 2).
Equipment Engineering: Rectifiers, Transformers, Reactors
Mercury‑arc Rectifiers:
Each rectifier weighed ~4 tonnes and measured 3.4 × 3.2 × 1.2 m. Featuring six-phase Graetz wiring and four anodes in parallel, they operated at 1,200 A and 125 kV DC. To maintain vacuum below 0.001 mmHg, a mercury pump and auxiliary vacuum tanks were required, along with portable pumps for regular maintenance.
Innovative cooling employed airflow for the anodes, but pure water circulated through the cathode tank at 32 ± 1 °C, enabling stable operation.
Image suggestion: Photo or schematic of rectifier hall and cooling setup.
Transformers:
The converters used Y–Y and Y–Δ windings to create a 12‑phase system, reducing harmonics while maximizing transformer utilization. Engineers had to manage stray DC currents (~20 A) from frequency interactions and maintain transformer impedance that was low enough for efficient regulation but high enough to suppress fault currents.
Because of DC magnetization, noise levels could reach 93 phon without mitigation. Designers enclosed the transformers in double-shell tanks, housed them in concrete acoustic buildings, and used separate oil-water cooling to reduce noise to ≤ 45 phon, matching local zoning limits.
DC Reactors:
The DC reactors played a pivotal role in smoothing current oscillations during commutation. Designed as air-gap iron-core reactors, they provided 1 H inductance at 100 A and 0.4 H at 500 A+, with insulation rated at BIL 550 kV to ground and 750 kV between terminals. These heavy, precise reactors ensured current stability and suppressed dangerous transients.
Control, Protection, and RFI Suppression
Harmonic and Transient Suppression:
Commutation events generate high-frequency transients. To damp these, each rectifier group included a 45 nF capacitor in series with a 2.4 kΩ resistor across the mercury arc tubes. Scaled laboratory models and simulations guided the design, ensuring effective damping and mitigating over voltages.
Overvoltage Protection:
Lacking external DC lines, the station had no conventional lightning channels. Engineers modeled surge events and used 266 kV surge arresters with protective discharge thresholds set at ≥ 650 kV, per JEC standards. These were tested on-site during trial runs to verify reliability.
Radio-Frequency Interference (RFI) :
Mercury-arc switching produced rapid voltage spikes (dv/dt), radiating electromagnetic noise through both direct emissions and current loops. Mitigation included:
- Thick metallic shielding around rectifier halls and buildings, offering 60–70 dB reduction from baseline ~75 dB emissions.
- An intermediate “blocking room” between the main hall and outdoor switchyard, filled with chokes and filters to block broadcast-band interference.
These were among the first major RFI solutions in high-voltage DC systems, effectively reducing emissions below ambient noise levels.
Diagram suggestion: RFI shielding layout (Figure 15).
Control Systems:
The station employed a constant-current control regime, adjusting firing (commutation) angles on both rectifier and inverter sides, along with tap-changing on transformers, to maintain steady power. Reverse power transfer was executable within 0.3 seconds, using emergency control switches.
Protective Systems:
Protection encompassed:
- Reverse-arc detection via AC/DC current comparison to isolate faulty converter groups and switch to bypass rectifiers.
- Guarding against faults, earth-leaks, and over currents.
- Auxiliary power redundancy: pumps, fans, and biasing systems had backup supply and would trip operations if voltage fell below 60%.
These systems collectively ensured operational safety and continuity.
Geographic and Site-Specific Constraints
Site Selection and Land Use
Located 1.7 km from the Sakuma hydroelectric plant in Shizuoka Prefecture, the converter station used twin-bundled 275 kV transmission lines, enabling straightforward AC linkage with local grids. Topographic challenges included rugged terrain, earthquake resilience, and secure foundations capable of supporting heavy machinery.
A meticulously designed site plan (see Figure 4) orchestrated the layout of equipment, access roads, and outdoor busbars while minimizing footprint and ensuring safety.
Environmental and Noise Considerations
The station was near quiet residential zones with ambient noise ~45 phon. Without intervention, plant noise could exceed regulatory thresholds, so engineers adopted:
- Architecturally designed transformer buildings for noise absorption.
- Enclosures around reactors and cooling units.
- Quiet HVAC systems and precise placement of mechanical equipment.
Measured sound at the station boundary reached just 45 phon or less, meeting strict local standards.
Infrastructure Integration with Local Grid
A short high-voltage line linked the converter with the Sakuma plant. Only one AC interrupter was needed (at the power plant end), thanks to the short distance. The plant provided 60 Hz internal power to the converter via a 4,500 kVA auxiliary transformer, with backup supplied at 3.3 kV—ensuring uninterrupted operations during faults or outages.
Political and Institutional Challenges
Inter-Utility Coordination and Governance
The Sakuma project required cooperative oversight from regional utilities (east/west) under the national Electric Power Council. A special Sakuma Frequency Converter Subcommittee with member representatives from all impacted utilities was established to manage:
- Construction planning and testing.
- Operational protocols.
- Emergency and fault response agreements.
- Long-term dispatch and pricing coordination.
This committee was critical in ensuring product-level decisions were consented across all stakeholders.
Regulatory and Policy Alignment
The concept of linking asynchronous grids via DC conversion devices—without separate transmission lines—was new to Japan and rare globally. The project required drafting new technical standards, defining safety protocols, tariff structures, and dispatch frameworks. Regulators had to balance innovation support with consumer protections and operational reliability.
Trial Operations and Operational Standards
Originally slated for November 1965, construction finished early, and pre-commissioning started in May 1965. Trial runs affected national grid stability, requiring careful planning, phased testing, and committee approval. The subcommittee oversaw test scenarios, grid disturbances, protection responses, and measured functional criteria before declaring full operational status.
Summary of Overcome Obstacles and Solutions
Obstacle Category: Key Solutions Implemented
High-power frequency conversion: Series dual-pole design with shared bypass, reducing equipment and control complexity
Stray DC currents & transformer noise: Special windings, acoustic enclosure, impedance tuning, and environmental testing
Harmonic/Transient Overvoltage: RC dampers, surge arresters ≥650 kV, validated via lab and field testing
RFI Emissions: Enclosures, conductive shielding, blocking-room with filters—reducing radiated noise by 60–70 dB
Control & Power Reversal Speed: Constant-current control, emergency switches enabling ±300 MW reversal within 0.3 s
Auxiliary Power Reliability: Dual-source redundancy; automatic shutdown at <60% voltage
Inter-utility Coordination: National-level committee; harmonized operations, emergency planning, dispatch agreements
Site Constraints (noise, terrain): Custom-built enclosures, HVAC design, compact site layout, environmental compliance
Impact and Legacy
Operational Efficiency and Grid Stability
With 300 MW transfer capacity and rapid reversal, Sakuma eliminated the need for “bridge-generation” from rapid-response fossil-fuel plants. Load studies indicated savings of 290 MW in 1964 and 380 MW by 1968 in spinning reserve capacity. During a simulated collapse of a 340 MW Yokosuka plant, frequency deviation reduced from 0.33–0.83 Hz down to 0.14–0.35 Hz thanks to the DC link.
Technological Innovation and Knowledge Transfer
The project pioneered:
- Integrated converter stations without dedicated DC lines.
- Simplified controls via series dual poles and shared bypass.
- Enhanced EMI/RFI control in HVDC environments.
- Real-world data on controlling advanced rectifiers and reactors.
These innovations shaped Japan’s HVDC strategy and influenced global installations.
Policy Implications for Future Systems
The Sakuma success strengthened interregional grid planning frameworks and policies supporting cross-system power exchange. It set national standards in engineering, safety, and commercial design for future HVDC and frequency-conversion projects.
Visual References (to Insert)
(1) system Interconnection Diagram (Figure 1): illustrating 50 Hz/60 Hz converter link.
(2) Single‑Line Converter Diagram (Figure 2): showing rectifier, bypass, and reactor layout.
(3) Site Layout Plan (Figure 4): detailing equipment placement.
(4) Transformer Enclosure Drawing or Photo: highlighting acoustic measures.
(5) RFI Shielding Diagram (Figure 15): showing enclosure and filter implementation.
(6) Site Photographs:
- Overview of converter station (Figure 3).
- Rectifier hall interior.
- Blocking-room or EMI filter detail.
Conclusion
The Sakuma Frequency Converter Station represents a landmark achievement in site-specific engineering, grid reliability, and inter-utility cooperation. By addressing immense technical obstacles—such as high-capacity mercury-arc conversion, control systems, EMI shielding, and noise mitigation—alongside geographic limitations and political coordination, the project became a model for future synchronous grid interconnections. Its contributions continue to resonate in the broader narrative of electrical power systems evolution.
What features set this work apart from similar achievements?
What features set this work apart from similar achievements?
Overview of the Sakuma Frequency Converter Station
The Sakuma Frequency Converter Station, located in Tenryu Ward, Hamamatsu City, Shizuoka Prefecture, is Japan’s first commercial frequency converter facility. Commissioned in 1965, it was originally built to transfer 300 MW of power and has since been upgraded to a maximum transfer capacity of 600 MW. It plays a critical role in connecting the eastern and western Japanese power grids, which operate at different frequencies—50 Hz in the east and 60 Hz in the west. The facility uses high-voltage direct current (HVDC) technology, employing a back-to-back conversion method that temporarily converts alternating current (AC) into direct current (DC), then reconverts it to AC at the target frequency.
Key Technical Features and National Importance
What makes the Sakuma station particularly unique is its role in addressing Japan’s frequency-split problem, a legacy of the country’s historical use of both German- and American-made generators in the early 20th century. Unlike most HVDC systems worldwide, Sakuma is designed not primarily for long-distance bulk power transmission, but for frequency conversion between asynchronous systems within a single country. This capability enables flexible power interchange between eastern and western Japan, especially during times of peak demand, system failure, or natural disasters—thus significantly contributing to the stability and resilience of Japan’s overall power system.
Comparison with European HVDC Systems
In contrast, most of Europe operates under a unified 50 Hz synchronous grid managed by ENTSO-E (European Network of Transmission System Operators for Electricity). As such, Europe generally does not require frequency conversion infrastructure for internal transmission. However, HVDC systems are still used extensively across borders—for instance, in submarine interconnectors such as NorNed (Netherlands–Norway) and BritNed (UK–Netherlands). These links improve system stability, allow efficient energy trade, and manage geographic separation. Importantly, while these systems use HVDC technology, they do not serve the same purpose as Sakuma, which directly addresses intra-national frequency incompatibility.
Comparison with North American Systems
North America, particularly the United States and Canada, offers a more comparable case. The continent is divided into multiple large synchronous zones—such as the Eastern Interconnection, Western Interconnection, and the Texas (ERCOT) system—that are not synchronized with each other. To connect these asynchronous grids, back-to-back HVDC converter stations are commonly employed, much like the Sakuma station. For example, facilities such as the Miles City HVDC Tie and the Rapid City DC Tie in the U.S. perform frequency decoupling and allow power flow between independent systems. However, the scale of these systems can be significantly larger, sometimes transferring several gigawatts across vast distances.
A Unique Solution to a Unique Problem
While Sakuma shares technological similarities with certain North American HVDC systems, it remains unique in its geopolitical and historical context. Japan, despite its relatively compact geographic size, maintains two entirely separate frequency standards—a situation not found elsewhere in industrialized nations. Sakuma is a direct response to this rare infrastructure challenge, and its design reflects a focus on bridging this internal divide rather than exporting or importing electricity internationally. It is one of the few HVDC installations in the world whose primary purpose is frequency conversion rather than long-distance transmission or international interconnection.
Conclusion
The Sakuma Frequency Converter Station represents a technically sophisticated and nationally critical solution to Japan’s dual-frequency power grid. While it shares some operational features with HVDC systems in Europe and North America, its primary role and the context of its implementation are quite distinct. Understanding Sakuma in light of international comparisons helps highlight both the common engineering principles and the unique structural challenges that have shaped power system development across regions.
Why was the achievement successful and impactful?
Why was the achievement successful and impactful?
Bridging Japan’s Dual-Frequency Grid
The Sakuma Frequency Converter Station, completed in 1965, represented a critical advancement in Japan’s power infrastructure by directly linking two major power grids operating at different frequencies—50 Hz in eastern Japan and 60 Hz in the west. Until its construction, power exchange between these regions was limited to cumbersome and capacity-restricted generator switching systems. Sakuma introduced a pioneering high-voltage direct current (HVDC) back-to-back converter system, enabling stable and rapid energy transfer between the two regions. This landmark achievement ensured grid stability, enhanced energy security, and became a foundational model for future frequency converter stations in Japan.
Technical Innovation in HVDC Integration
Sakuma was Japan’s first large-scale HVDC frequency conversion system and the world's first to use water-cooled mercury arc valves in a back-to-back configuration for frequency conversion. The station was designed with a 300 MW transfer capacity using ±125 kV DC voltage—an ambitious specification for the time. The system featured complex converter transformers using Y-Y and Y-Δ 12-phase configurations, precise firing angle control mechanisms, and high-speed valve switching. By utilizing series-connected 50 Hz and 60 Hz rectifier groups, the station reduced the number of bypass elements and simplified the control system. These innovations demonstrated Japan’s engineering excellence in adapting HVDC technologies to unique frequency coordination challenges.
Enhancing Grid Stability and Efficiency
Prior to Sakuma, energy transfers between the two frequency regions were slow and operationally constrained. The HVDC link allowed for rapid and stable power interchange with no mechanical switching, significantly improving operational flexibility. This provided critical benefits: (1) reduced the need for large reserve generation capacity—by approximately 290 MW in 1964 and 380 MW by 1968—and (2) enabled mutual support in emergencies. For example, simulations in 1964 showed that frequency dips following a 340 MW loss in the 50 Hz system could be halved through the Sakuma link, mitigating cascade failures and blackouts.
A Foundation for Future Development
The success of the Sakuma Frequency Converter Station laid the groundwork for subsequent frequency converters at Shin-Shinano and Higashi-Shimizu, forming a robust national grid interconnection. Its influence extended internationally, offering a practical model for linking asynchronous networks. Moreover, the knowledge gained from controlling high-voltage mercury arc rectifiers, filtering harmonics, and managing complex control dynamics advanced the state of the art in HVDC systems. These innovations remained in use for decades and evolved into modern thyristor-based systems.
Legacy of Engineering Excellence
Sakuma’s construction and operation were a product of meticulous engineering, combining advanced power electronics, custom-designed transformers, and rigorous system simulations. At the time, few global precedents existed for such an undertaking. Its success demonstrated Japan’s capability to tackle complex grid challenges with elegant and forward-thinking solutions. As such, Sakuma remains not just a technical milestone, but a symbol of postwar industrial progress and international engineering leadership.
Supporting texts and citations to establish the dates, location, and importance of the achievement: Minimum of five (5), but as many as needed to support the milestone, such as patents, contemporary newspaper articles, journal articles, or chapters in scholarly books. 'Scholarly' is defined as peer-reviewed, with references, and published. You must supply the texts or excerpts themselves, not just the references. At least one of the references must be from a scholarly book or journal article. All supporting materials must be in English, or accompanied by an English translation.
Bibliography
Reference
[1] Susumu Kuwahara; “Sakuma Frequency Converter Station”, Vol. 85, No. 925, pp.1625–1634, The Journal of the Institute of Electrical Engineers of Japan, 1965.
[Translation to English: 1. Introduction]
1. Introduction
Japan’s electric power system is divided at the central region of Honshu Island, with the eastern side standardized at 50 Hz and the western side at 60 Hz. Within each of these two regions—excluding Hokkaido—all areas are interconnected via extra-high voltage transmission systems. By the end of 1965, the system capacity is expected to reach approximately 11,000 MW for the 50 Hz system and about 16,000 MW for the 60 Hz system.
The Sakuma Frequency Converter Station was constructed by the Electric Power Development Company (J-Power) with the goal of directly interconnecting these two frequency systems using direct current (DC) technology. The facility is outlined as follows:
Location: Sakuma Town, Iwata District, Shizuoka Prefecture (near Sakuma Power Station)
Output Capacity: 300 MW
AC Voltage: 275 kV
DC Voltage: ±125 kV
Construction Start: April 1962
Planned Operation Start: November 1965
Figure 1: Related System Diagram
The converter station is located approximately 1 km from the Sakuma Power Station and is connected to it via a 1.7 km-long 275 kV AC transmission line (with one line each for 50 Hz and 60 Hz on the same towers). From there, it links to the major power grids of Tokyo and Nagoya via the existing Sakuma East and West trunk lines. These system connections are shown in Figure 1.
The benefits of interconnecting the two frequency systems through the Sakuma Frequency Converter Station can be summarized as follows:
(1) Although switching between power plants designed for the two different frequencies (dual-cycle plants) is possible at a capacity of about 300 to 400 MW, such switching typically takes several minutes—up to around 15 minutes—and imposes operational constraints on the system. In contrast, DC interconnection avoids these drawbacks. Additionally, as thermal power development is rapidly progressing and the proportion of advanced large-scale thermal power within total generation continues to grow, relying solely on dual-cycle plant output for power interchange is becoming quantitatively insufficient.
(2) Calculations on the reduction of operating reserve have shown that, on a national scale, approximately 290 MW of reserve could be saved in 1964 and about 380 MW in 1968.
(3) In the event of a sudden loss of generation in one frequency system, emergency support from the other system via the frequency converter can significantly mitigate frequency drops in the affected system and help prevent cascading failures. For example, simulations for the 1964 system show that, in the event of a 340 MW outage at the Yokosuka Thermal Power Station in the 50 Hz system, frequency drop would be 0.33 Hz at peak load and 0.83 Hz under light load conditions (40% load). However, with DC interconnection, these drops would be limited to 0.14 Hz and 0.35 Hz respectively.
[2]
[3] Susumu KUWAHARA and Tatsuya TAKENOUCHI; “General Aspect of Sakuma Frequency Converter Station of The Electric Power Development Co.”, Mitsubishi Denki Giho, November 1965.
Media:Mitsubishi Denki Giho_1.pdf
[Abstract]
High voltage DC transmission technology was first developed in Sweden. In the year 1961 tie-lines of 160MW were laid over the Anglo-French Channel for commercial operation. The Electric Power Development Company, realizing that the DC transmission technique is applicable to frequency conversion equipment, set out to construct a Sakuma Frequency Converter Station for the purpose of tying via direct current the eastern power system at 50 cycles and the western at 60 cycles, the difference of which is a fatal drawback in this country. The station is slated to enter into commercial operation in the fall. The apparatus installed are unprecedented in this country and also the largest as AC to DC conversion equipment in the world. Description is made herein on DC machines as the major topics.
[4] Shozo TSUKAMOTO・Masao YANO・Kazuo SUZUKI・Teruo AIYAMA; “Pure Water-Cooling System in Sakuma Converter Station of the Electric Power Development Co.” , Mitsubishi Denki Giho, November 1965.
Media:Mitsubishi Denki Giho_2.pdf
[Abstract]
Temperature control is an important problem of a mercury rectifier because its dynamic characteristics are influenced by the temperature of the tank. Especially a high voltage high power converter requires an extremely narrow limitation on the optimum temperature of the cooling water, which is ±1°C in the case of Sakuma Converter Station. Moreover, for a high voltage converter, insulation with cooling water poses another problem. To reduce the length of the water insulating pipe and to prevent the contamination of it, cooling water of high purity is a vital requisite. This article reports on the pure water-cooling system for a high voltage static frequency changer, briefing the system meeting the demands.
[5] “Ryohei TAMURA・Kunikazu SAKATA”; “368 MVA, 353 MVA Transformers for Sakuma Frequency Converter Station ofthe Electric Power Development Co.” , Mitsubishi Denki Giho, November 1965.
Media:Mitsubishi Denki Giho_3.pdf
[Abstract]
In 1958 3,000 kW 20 kV DC transmission equipment was manufactured by Mitsubishi and supplied to connect Kyushu Takero Substation to Futago Substation of the Mitsubishi Mining Co. Takashima mine. Only actual results of commercial operation with regard to DC transmission were made available there. But it is a matter of regret that the engineering achievement on the DC transmission has been left neglected since then. In the meantime, increase of recent power demand has come to dictate the tying of the power systems of different frequency from the viewpoint of the business operation in a broad range. This has resulted in the installation of frequency converters by the Electric Power Development Company at the site of Sakuma. Transformers to be used for the purpose have been manufactured by Mitsubishi, being of unparalleled capacity in the world.
[6] Ikuo YAMADA・Kimiharu OKAMOTO:, “Analysis of Abnormal Phenomena in the Sakuma Frequency Converter of the Electric Power Development Co.” , Mitsubishi Denki Giho, November 1965.
Media:Mitsubishi Denki Giho_4.pdf
[Abstract]
Numerous abnormal phenomena are involved in high voltage high power frequency converters. Analysis has been made on arc-back and short-circuit current in DC excited mercury rectifier with the Sakuma installation. It is learnt that the current referred to enlarges with the diminution of R/X and of control angle α. It also becomes larger arc-back are producing δq, approaches the spot right after the commutation. By taking R/X, α and θq as input and the waveform and magnitude of arc-back current as output, a computing program has been developed. The DC excitation of transformer is considered due to the slip of ripple current of different frequency systems fed, and to irregularity of control angles of forward and reverse of the converter. In the Sakuma installation, the latter predominating and needs its diminution for the prevention of DC excitation.
[7] Takeshi MORI, Shigeo NISHINA and Hisashi NAGAMACHI: “AC Circuit Protection and Switchboards in Sakuma Frequency Converter Station of the Electric Power Development Co.” , Mitsubishi Denki Giho, November 1965.
Media:Mitsubishi Denki Giho_5.pdf
[Abstract]
Sakuma frequency converter station receives electric power from Sakuma power station over a 275 kV transmission line and operates to convert power at 50～60 cycles. On the extra high voltage side of the main transformers for use in conversion are set up filters to absorb higher harmonics for the protection of the systems. On the other hand, power for the control of station service auxiliary machines is obtained by stepping down the high voltage power to 3 kV through delta connection of tertiary windings of the main transformer on the 60-cycle system. The power is connected to auxiliary circuits and also tied to Sakuma power station. Of the control apparatus installed their AC switchboard equipment with AC protective devices as the major assembly is built and delivered by Mitsubishi.
[8] Yoshiyuki Nakamura; “Special Feature, Energy and Energy Facilities: Overview and Structure of the Sakuma Power Station and the Sakuma Frequency Converter Station”. pp. 33-38, Construction Planning for Execution, April 2012.
[Translation to English: Chapter 6]
6. Construction of the Sakuma Frequency Converter Station
The Sakuma Frequency Converter Station is referred to as “Sakuma FC,” with "FC" standing for "Frequency Converter." The Sakuma FC was completed and began operation on October 10, 1965. With its commissioning, Japan’s power grid—previously divided between 50 Hz in eastern Japan and 60 Hz in western Japan—became interconnected. The operation of Sakuma FC significantly accelerated the development of wide-area power system operation.
The reason Japan's electrical frequencies are split—50 Hz in the east and 60 Hz in the west—dates back to the late 19th century. In 1895 , Tokyo Electric Light Company imported generators from AEG in Germany (50 Hz), while Osaka Electric Light Company imported generators from General Electric in the United States in 1897, which operated at 60 Hz. Each company subsequently built thermal power plants using their respective equipment.
The difference in frequencies brought about numerous inefficiencies, such as the need for separate reserve power in each region, the inability to transfer electricity between regions, and differences in electrical equipment standards. There was an attempt to unify the frequencies shortly after World War II, but due to the unexpectedly rapid pace of post-war economic recovery, unification was not achieved.
The idea of interconnecting the two frequency systems began in the fall of 1958 with a study tour on wide-area power system operation. During this visit, attention was drawn to a project between the United Kingdom and France to link their power grids via high-voltage direct current (HVDC) transmission, involving the purchase of 160,000 kW of equipment from a Swedish company. This inspired the proposal to use similar frequency conversion equipment in Japan.
In April 1961, the Central Research Institute of Electric Power Industry established the "Interconnection of Two Frequency Systems Committee," chaired by Professor Setsuo Fukuda of the University of Tokyo, to study the technical aspects of the project.
Land preparation for the site began in January 1963. The first shipment of equipment from Sweden arrived at Nagoya Port in April 1964, and construction progressed rapidly. The station began operation on October 10, 1965.
The main specifications of the Sakuma Frequency Converter Station are as follows:
- Approved Output: 300,000 kW
- Converter Type: Mercury-arc rectifier
- Voltage × Current: 125 kV × 1,200 A
- Capacity per Pole × Number of Poles: 150,000 kW × 2 poles
- Number of Units per Group × Number of Groups: 6 units × 4 groups
- Start of Operation: October 10, 1965 (Showa 40)
- Note: On June 11, 1993, operation began using newly replaced thyristor valves.
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