Replacing a distribution transformer has traditionally meant one thing: a planned outage. Customers — whether a residential block, a factory floor, or a hospital — simply wait while crews disconnect the old unit, install the new one, and restore supply. The typical window runs four to six hours, sometimes longer.
Chinese grid operators have developed a method that eliminates this outage entirely. By deploying a mobile box-type substation as a temporary parallel supply, the old transformer can be isolated and removed while connected loads never lose power. The new transformer is then installed and energized, and the mobile unit is quietly withdrawn. From the customer’s perspective, nothing happened.
This article documents how the method works, the specialist equipment it requires, and five real-world cases carried out across China between 2020 and 2026 — each one marking a regional “first” for the technique.
Why the Demand for Zero-Downtime Replacement Is Growing
Every transformer has a service life and a capacity ceiling. As electricity consumption climbs — driven by industrial expansion, EV charging infrastructure, and the electrification of heating — distribution transformers that were correctly sized a decade ago increasingly find themselves running at or above rated load.
When a transformer reaches the end of its service life or requires an upgrade in capacity, the conventional response has always involved a planned interruption. For residential areas, this is an inconvenience. For manufacturing facilities, it means halted production lines, scrapped batches, and contractual penalties. For hospitals, commercial data centers, and continuous-process industries, it is simply not acceptable.
The “zero-downtime” or “live replacement” approach directly addresses this constraint. It does not require any change to the underlying transformer technology being installed — standard oil-immersed or dry-type distribution transformers are used throughout. What changes is the sequencing of the operation, and the addition of one key piece of equipment.
The Core Method: Mobile Box-Type Substation as Temporary Bypass
The fundamental logic of the method is straightforward. Before the existing transformer is touched, an alternative supply path is established. A mobile box-type substation — a complete, self-contained transformer and switchgear assembly mounted on a road vehicle — is connected to the same distribution network, upstream of the transformer being replaced. Once the mobile unit is supplying the downstream load, the original transformer can be isolated, removed, and replaced without interrupting supply to any connected customer.
In practice, the operation proceeds through five distinct phases:
Phase 1 — Site Preparation and Mobile Substation Positioning
The mobile box-type substation vehicle is positioned at the work site. Depending on the local network configuration, the mobile unit may take its high-voltage supply from the same overhead line serving the existing transformer, or from an adjacent network point. Ground crews lay flexible HV and LV cables connecting the mobile unit to the network.
Phase 2 — Live Connection to the Overhead Network
This is the most technically demanding phase. Insulated aerial work platform vehicles (commonly called insulated bucket trucks or insulated boom trucks) lift crew members to the level of the overhead conductors. Working under full live-line conditions and wearing arc-rated PPE, the operators make the high-voltage connections that bring the mobile substation into the network.
Phase 3 — Load Transfer and Original Transformer Isolation
With the mobile substation now energized and supplying the load in parallel with the original transformer, the original unit is de-energized and isolated from the network. The switchgear operations required to achieve this — opening the LV distribution panel, operating the dropout fuse cutouts on the HV side — are carried out in a controlled sequence.
At this point, the original transformer is carrying zero load and has been fully isolated. The connected customers are being supplied entirely by the mobile substation. From their side, supply has been uninterrupted throughout.
Phase 4 — Transformer Removal and New Unit Installation
With the original transformer safely de-energized and isolated, the crane vehicle moves into position. The old transformer — typically an oil-immersed pole-mounted unit weighing several hundred kilograms — is unbolted from its mounting, lifted clear, and removed.
The new transformer — in most of these cases, a unit of substantially higher rated capacity than the one being replaced — is lifted into position, mechanically secured, and connected. HV and LV terminations are made to the network.
Phase 5 — New Transformer Energization and Mobile Substation Withdrawal
Once the new transformer has been installed, connected, and inspected, it is energized and brought into parallel operation alongside the mobile substation. Load is transferred back to the new permanent transformer, and the mobile substation is then isolated and withdrawn from the network. Live-line crews in the aerial platforms remove the temporary HV connections. Ground crews recover the flexible cables.
The operation is complete. The connected customers have been supplied without interruption throughout.
Five Real Cases: A Cross-Section of Chinese Practice
The following cases represent documented implementations of this technique across different provinces and network operators, spanning 2020 to 2026. Each was reported as a regional first at the time of implementation, reflecting how recently this capability has been spreading through China’s distribution grid operations.
Case 1 — Xinchang County, Zhejiang Province (December 2020)
Operator: State Grid Xinchang County Supply Company Location: Sanjiing Village, Xiaojiang Town, Xinchang County Original transformer: 100 kVA Replacement transformer: 400 kVA Customers served: 80+ residential households Operation duration: Approximately 2 hours
This case represents one of the earliest documented deployments of the mobile box-type substation technique for transformer replacement in Zhejiang Province. The existing 100 kVA unit had been running in an overloaded condition for an extended period, with intermittent faults affecting local residents. The work site presented additional complexity due to communications cables routed near the transformer mounting, which required careful planning to avoid contact during the crane operations.
The operation was completed in approximately two hours — a fraction of the time that a conventional planned outage and replacement would have required.
Case 2 — Shuangliu District, Chengdu, Sichuan Province (June 2023)
Operator: State Grid Chengdu Shuangliu Supply Company Location: Dajing Community, Jiujiang Street, Shuangliu District Original transformer: 50 kVA (oil-immersed, pole-mounted, in overloaded and leaking condition) Replacement transformer: 200 kVA Customers served: 100+ residential households Operation duration: Not reported Significance: First implementation of this technique by any district-level grid company in Sichuan Province
This case was notable for combining a genuine emergency condition — the original transformer had developed an oil leak due to sustained overloading and required urgent replacement — with a zero-downtime methodology. Rather than accept the inevitable outage that a conventional emergency replacement would have produced, the Shuangliu team deployed the mobile substation bypass approach.
The project was coordinated across three work groups: the live-line crew, the conventional maintenance crew, and the local supply station team.
Case 3 — Wuxi Economic Development Zone, Jiangsu Province (March 2023)
Operator: State Grid Wuxi Supply Company, No-Outage Operations Centre Location: Fenghua Line No. 21, Songxing Electronics Transformer Bay, Wuxi Economic Development Zone Network voltage: 20 kV (non-standard; most Chinese distribution networks operate at 10 kV) Customers affected if stopped: Industrial park including Japanese-owned electronics manufacturers, medical device manufacturers, 50+ additional users Operation duration: 5 hours Significance: First documented use of a 20 kV-capable mobile box-type substation in China; unit developed in-house by State Grid Wuxi
This case is technically the most complex of the five. The transformer bay operated at 20 kV rather than the standard 10 kV used across most of the Chinese distribution network. No commercially available mobile substation at the time was capable of operating at this voltage. The Wuxi No-Outage Operations Centre developed a purpose-built mobile unit with 20 kV / 10 kV intelligent switchable HV input and a 0.4 kV to 10 kV / 20 kV boost capability — described at the time as a domestic first.
The industrial park served included precision electronics manufacturers for whom even a brief interruption would have caused production disruption and potential equipment damage.
Case 4 — Yongding District, Fujian Province (April 2024)
Operator: State Grid Yongding District Supply Company Location: Shuanggu Road, Sanfeng Village, Chengjiao Town, Yongding District Original transformer: 200 kVA (10 kV, pole-mounted on double-pole structure) Replacement transformer: 400 kVA Customers served: 100+ residential households Operation duration: 5+ hours Significance: First implementation in Yongding District
The Yongding case is notable for the physical complexity of the installation. The original transformer was mounted on a double-pole structure — a heavier mounting arrangement requiring a larger crane radius and more precise lifts than a single-pole installation. The new 400 kVA oil-immersed unit was transported to site on a dedicated flatbed vehicle and lifted directly to the mounting height.
The entire bypass supply period — from the moment the original transformer was isolated to the moment the new unit was energized and the mobile substation withdrawn — was approximately five hours.
Case 5 — Qianyang County, Baoji, Shaanxi Province (April 2026)
Operator: State Grid Baoji Supply Company / State Grid Qianyang County Supply Company Location: Zhangjiatian Town, Qianyang County Original transformer: 100 kVA Replacement transformer: 200 kVA Customers served: 104 residential households Operation duration: ~8 hours (09:30 to 17:00) Significance: First implementation by State Grid Baoji Supply Company
This is the most recent of the five cases and introduces a specific technical refinement: rather than a simple parallel supply arrangement, the Baoji implementation used a synchronization device to manage the parallel connection of the generator vehicle with the grid supply. The synchronization device automatically verifies that phase sequence, voltage, frequency, and phase angle are within acceptable limits before closing the parallel connection — eliminating the risk of out-of-phase switching transients that could otherwise affect sensitive loads.
The case also represents a multi-discipline coordination: live HV line work, live LV work, and conventional ground operations running simultaneously across separate crews.
Equipment Requirements: What This Operation Needs
Every one of the five cases above required the same three categories of specialist vehicle on site:
1. Mobile Box-Type Substation Vehicle The central piece of equipment. A complete distribution transformer and associated MV/LV switchgear, mounted on a road-going vehicle chassis with flexible HV and LV cable connections. Operating voltage is typically 10 kV input / 0.4 kV output for standard distribution network work. The Wuxi case required a purpose-built unit capable of 20 kV input. Rated transformer capacity must be sufficient to carry the full load of the circuit being bypassed.
2. Insulated Aerial Work Platform (Insulated Bucket Truck) Required for all live-line connection work at the overhead conductors. Provides a fully insulated working platform that allows operators to work at distribution line height under energized conditions. Typically two units are deployed on larger operations to allow simultaneous work at both the HV and LV connection points.
3. Crane Vehicle Required for the physical removal of the old transformer and the installation of the new unit. Pole-mounted distribution transformers in the 100–400 kVA range typically weigh between 400 and 1,200 kg. A dedicated crane is essential; the geometry of a live overhead network does not allow the use of improvised lifting arrangements.
In the Yongding case, a fourth vehicle — a flatbed transport for the new transformer — was also present on site, bringing the total specialist vehicle count to four.
What This Means for Transformer Procurement
The adoption of live replacement methodology has a direct implication for transformer specifications. When the possibility of a zero-downtime replacement is on the table, the constraints that have historically forced operators to accept oversized transformers as a hedge against future planned outages are relaxed. Capacity upgrades can be carried out more frequently and in smaller increments, since each upgrade no longer carries the cost and disruption of a planned outage.
From a procurement perspective, this creates demand for a broader range of transformer ratings — particularly in the 200–630 kVA range where incremental capacity upgrades are most common — and for transformers that are compatible with the connection interfaces used by mobile substation equipment.
Conclusion
The zero-downtime transformer replacement method documented here is not experimental. It has been implemented by multiple State Grid subsidiary companies across China, at different network voltages, in residential, industrial, and mixed-use supply contexts. The technique is spreading because the economics are compelling: the cost of deploying specialist equipment for a few hours is substantially lower than the cost of a planned outage for most commercial and industrial customers.
The key enabler is the mobile box-type substation — a piece of equipment that the electrical industry has had available for decades, now being applied in a systematic way to one of distribution grid maintenance’s most disruptive routine tasks.
What this method does not require is any special modification to the permanent transformer being installed. The distribution transformer at the centre of the operation — whether oil-immersed or dry-type, whether 100 kVA or 630 kVA — is a standard unit. What changes is the operational context it enters: it must be ready to energise cleanly into a network that has been live throughout the replacement window, often within a tight physical envelope and on a schedule driven by the mobile substation’s available supply hours.
That operational context is increasingly the normal context. As live replacement methodology spreads through China’s distribution grid and begins to appear in international markets, the manufacturers who understand it — and who build transformers and mobile substation equipment to the connection interfaces and reliability standards it demands — are the ones who will be specified into these projects.
Zhongxin General (Sichuan Zhongxin General Electric Energy Co., Ltd.) manufactures oil-immersed and dry-type distribution transformers across the full range of ratings used in the cases documented here, from 100 kVA to well above 400 kVA, at standard 10 kV and non-standard voltage levels. Our prefabricated box-type substation products include mobile and transportable configurations suitable for use as temporary bypass supply in live replacement operations. For project enquiries, contact us via the form below or on WhatsApp.
