ABB UniPack-S Steel Compact Substation

ABB UniPack-S Steel Compact Secondary Substation: Design, Standards, and Applications

As the global distribution network transitions from overhead lines to underground cabling, Compact Secondary Substations (CSS) have become the core nodes of modern urban and rural distribution networks. ABB, as a leader in the field of power technology, has launched the UniPack-S series of steel compact secondary substations to address multiple challenges such as renewable energy access, industrial power supply reliability, and urban infrastructure expansion with lightweight, high safety, and modular design. This article is based on ABB's official product manual "UniPack-S Catalog" (catalog number 1VPD11001A0409 Rev. A), comprehensively reviewing the design philosophy, technical standards, structural characteristics, core equipment configuration, and smart grid compatibility of the product, providing in-depth technical references for power system planners, engineers, and operation teams.

Application scenario coverage: from renewable energy to transportation infrastructure

The original design intention of UniPack-S is to adapt to harsh environments and provide flexible layouts. Its application scenarios cover four major fields:

1. Renewable energy and distributed power generation

With the increasing demand for distributed generation and operational efficiency, the distribution network is rapidly evolving. UniPack-S is designed specifically for grid connection of renewable energy sources such as wind and solar. Its sturdy steel casing can adapt to harsh environments such as deserts, high humidity coastal areas, and high salt spray. The internal layout of the substation can be customized according to equipment types and application requirements, supporting bidirectional power flow (from the power generation end to the grid, or from the grid to the load).

2. Industrial power supply

Industrial users have extremely high requirements for power supply reliability, operational efficiency, and power quality. ABB prefabricated substations can achieve single supplier delivery, significantly shorten on-site construction period, reduce on-site resource occupation, and lower project risks. UniPack-S provides precise control over energy costs and helps businesses meet sustainable development goals through low environmental impact design.

3. Public utilities

The existing power grid is facing increasing pressure on electricity demand and must provide stable and sustainable power supply through radial or ring network connections. UniPack-S provides a type tested solution that supports remote monitoring and switch operation, achieving equipment protection and selective protection coordination. Its pre design package can meet various protection, metering, and automation requirements in the distribution network.

4. Infrastructure

For special applications such as railways, electric vehicle charging stations, and electric bus infrastructure, UniPack-S can be equipped with specialized equipment (such as DC traction switchgear, high protection level low-voltage cabinets, etc.) to meet the unique needs of rail transit and electric transportation.

Design Evolution of Compact Secondary Substations: From Concrete to Steel

The CSS market was born during the period when distribution companies shifted from overhead lines to underground cables. Traditional large-scale step-down substations (with overhead lines) have become impractical, and customers need a compact device that can be quickly installed on site, minimizes civil work, and has equivalent functionality.

2.1 Limitations of Concrete CSS

Early CSS often used on-site construction or prefabrication of concrete. Concrete has weather resistance, but it is heavy and has high transportation and lifting costs. With urban expansion, CSS installation points are gradually moving away from concrete factories, leading to a significant increase in transportation costs. Meanwhile, rural areas require lighter and cheaper solutions.

2.2 Advantages of Steel CSS

ABB has developed steel shell CSS, which achieves the same functionality as concrete but significantly reduces weight. Steel CSS is convenient for long-distance transportation and lifting, and does not require heavy equipment on site. Its effective ventilation design and different IP rating options make it suitable for both cold and hot climates. The highest anti-corrosion level of the shell can reach C5 (according to ISO 12944), and it can be installed in corrosive environments such as coastal industrial areas. The "S" of UniPack-S stands for Steel.

2.3 Security and operational convenience

UniPack-S has passed the highest safety standard tests in the GB and IEC series. The standard CSS design ensures the highest level of safety for the public and operators through internal arc classification IAC-AB testing. Due to its lightweight and durability, rural electrification supply has become easier. In addition, modern smart grids support remote monitoring and control, reducing on-site access frequency, so equipment needs to be maintenance free as much as possible - the design of UniPack-S meets this requirement.

International standards followed: from IEC 61330 to IEC 62271-202 ed. 2

Before 1995, there was no unified international standard for prefabricated CSS. In 1995, the International Electrotechnical Commission (IEC) released the first CSS standard IEC 61330 (formerly IEC 1330). This standard is based on IEC 298 (Metal Enclosed Switchgear) and specifies the rated values, design and operating rules, type testing, and routine testing requirements. It covers cable connected substations with transformer capacity not exceeding 1600kVA and significantly improves the internal arc safety level.

Since 2001, IEC has been unifying the relevant standards for medium voltage switchgear, adopting the IEC 62271 numbering system. In 2006, the standard for prefabricated substations was upgraded to IEC 62271-202, based on IEC 61330 and IEC 62271-200. This standard focuses on MV/LV or LV/MV CSS with rated voltage of 1kV to 52kV, frequency of 50/60Hz, cable connection, internal or external operation, and public accessibility.

In March 2014, the second edition of IEC 62271-202 was released, with major modifications including:

Update the temperature rise test method;

Consider electromagnetic field assessment;

Add optional tests;

Consider the impact of the product on the environment;

The internal arc test requirements are adapted to IEC 62271-200:2011;

Assessment requirements for increasing the pressure release volume under the floor.

3.1 Relevant standard system

All internal components of CSS must undergo type testing in advance according to their relevant standards, including:

IEC 62271-1 (General Specification for High Voltage Switchgear)

IEC 62271-200 (AC metal enclosed switchgear)

IEC 62271-201 (Insulated enclosed switchgear)

IEC 62271-202 (High Voltage/Low Voltage Prefabricated Substations)

IEC 60076 (Power Transformers)

IEC 61439 (Low Voltage Switchgear)

IEC/TR 62271-208 (Electromagnetic Field Technical Report)

IEC 60529 (Protection level)

IEC 62262 (Mechanical Shock Protection)

IEC 60721 (Classification of Environmental Conditions)

3.2 Classification of Type Tests

According to IEC 62271-202 ed. 2, type tests are divided into three categories:

Mandatory type tests: dielectric test, temperature rise test, short-time withstand current and peak withstand current test of main and grounding circuits, verification of protection level, calculation or testing of mechanical loads (snow, wind, impact), verification of auxiliary and control circuits.

Mandatory type tests when applicable: internal arc fault test (if IAC level is required), EMC test (complete CSS is usually not required as MV and LV equipment have been tested separately).

Optional tests: Sound level test (agreed upon with the manufacturer and user), electromagnetic field measurement or calculation.

The internal arc test is very important to ensure the safety of operators (IAC-A level) and the public (IAC-B level) when internal arcs occur under normal operating conditions.

Detailed explanation of UniPack-S structural design

4.1 Construction Composition

The UniPack-S casing is an above ground building installed on site foundations (ABB can provide additional or integrated foundations). Non walk-in is used for external operations, and there is an operating channel within the walk-in station.

4.2 Materials and Corrosion Protection

The standard shell is made of galvanized steel plate, which has higher corrosion protection than painted black steel. Galvanized metal parts are pre treated and further coated with pure polyester powder coating. According to ISO 12944, surface treatment corresponds to corrosion grade C4 H. Hot dip galvanized steel sheet ensures long-term mechanical life.

4.3 Ventilation system

Natural and effective ventilation is achieved through the air inlet on the door or ventilation wall components. Hot air is expelled from the top ventilation louvers. ABB has designed an efficient ventilation system that provides enclosures with low temperature ratings (10K or 20K) to enhance transformer performance and lifespan. For hot climates or continuous heavy-duty transformers, it is strongly recommended to use low-temperature grade enclosures. ABB can conduct thermal simulation as required to optimize the lifespan of transformers.

4.4 Roof

UniPack-S provides separate roofs for MV switchgear room, LV distribution panel room, and transformer room. The roof of the transformer is detachable, making it easy to install and maintain the transformer. The roof has two slopes: standard 6 degrees and 18 degrees for heavy snow weather. The roof design can withstand a minimum uniformly distributed load of 250kg/m ².

4.5 Doors

The door is made of pre galvanized steel plate, with a sturdy structure to prevent unauthorized entry, providing high IP rating, and protecting the public during internal arcing. Standard stainless steel hinges with high corrosion resistance and bending rigidity. According to the variant, use 2-point or 3-point locking (3-point locking provides better functionality and safety). The transformer room door is also equipped with safety barriers to remind operators to follow safety measures and prevent accidental entry. The walk-in CSS can be equipped with an internal door handle and an emergency push rod lock.

4.6 Basic Types

UniPack-S offers three basic types:

Galvanized steel foundation - requires a separate oil sump.

Concrete slab foundation - with independent oil collection pits, can be directly installed on the ground.

Compact foundation - integrated oil sump, can be directly installed on the ground.

All foundation types are designed to withstand the weight of the substation and can be lifted by a crane with four anchor points without deformation.

4.7 CSS Types

Walk in: Provides a service area independent of external conditions.

N-type (Non walk in): Optimize footprint.

Compact: Non walk-in compact version.

4.8 Installation and Advantages

The factory delivers a tested solution, and only external cables need to be connected on site. No complex civil engineering is required, just excavate the foundation pit and place the substation. The advantages include:

High security design protects equipment, personnel, and the environment;

The internal arc test complies with IEC 62271-202;

Compartment design and lockable doors;

Factory delivery reduces on-site installation time;

Ready to connect to SCADA or smart grid;

High corrosion resistance;

Lightweight and easy to transport;

Include oil collection pits to prevent transformer oil leakage and pollution;

A wide product portfolio covers various applications from power generation to secondary distribution.

Standard layout and parameters

UniPack-S offers multiple standard layouts, named Mercury, Terra, Mars, Pluto, Saturn, Luna. The main technical parameters are as follows:

Maximum transformer capacity: 3500 kVA (oil transformer)

Rated voltage: up to 36 kV

Short time withstand current of grounding network: maximum 21 kA/1s

Transformer room IP protection level: minimum IP23D, optional IP35/IP45

MV/LV compartment IP rating: IP43 minimum, optional IP54

Shell thermal rating: 10K or 20K

LV rated voltage: up to 800V

LV rated current: up to 5000A

Short term withstand capacity of LV busbar: up to 66 kA/1s

The size of each compartment is determined based on the selected equipment (such as SafeRing, SafeLink, NAL, UniSec, etc.).

Detailed explanation of core equipment

6.1 Medium voltage switchgear: SafeRing/SafePlus

SafeRing is a ring main unit used for secondary distribution networks, offering 10 configurations and suitable for 12/24kV distribution networks. Scalable and compatible with SafePlus. SafeRing is a fully sealed system, with a stainless steel air box containing all live parts and switch functions. The constant atmospheric conditions ensure high reliability and personnel safety, and are almost maintenance free.

Main modules:

C module (cable switch)

De module (direct cable connection with grounding switch)

F module (switch fuse combination)

V module (vacuum circuit breaker)

Optional modules (SafePlus): including busbar grounding, busbar segmentation, metering module, etc.

Example of rated value (12kV):

Rated current: 630A (C/F module), 200A (V module)

Power frequency withstand voltage: 28kV

Lightning impulse withstand voltage: 95kV

Short time withstand current: 16kA/1s or 21kA/3s

SafeRing comes standard with grounding switch, mechanical interlock, operating handle, padlock facilities, cable conduit, etc. Optional remote control monitoring unit, motor operation, auxiliary contacts, capacitive voltage indication system, short circuit indicator, etc.

6.2 Medium voltage switchgear: UniSec

UniSec is ABB's air insulated switchgear, which complies with the LSC2A-PM (switch isolating switch) classification according to IEC 62271-200. Constructed with pre galvanized metal sheet, each unit includes busbar room, cable room, etc. All live parts shall be air insulated and equipped with SF switches - isolating switches, withdrawable vacuum or SF6 circuit breakers. Provide multiple functional units: SDC (Switch Isolating Switch Unit), SFV (Switch Fuse+Measurement), SBC (Circuit Breaker+Switch Isolating Switch), etc. The rated voltage is 12-24kV, the main busbar current is 630/800/1250A, the maximum short-time withstand current is 25kA/3s, and the maximum internal arc withstand current is 25kA/1s.

6.3 Transformer

One or two oil immersed or dry-type transformers can be installed indoors, with a maximum single capacity of 3500kVA. Natural ventilation cooling is adopted. The dimensions of the transformer room are provided in the standard layout data sheet. Customized layouts can be provided to accommodate higher capacities. The oil collection pit can accommodate 100% transformer oil. ABB can also provide substations without transformers, and on-site installation instructions can be found in the installation manual.

6.4 Low Voltage Switchgear (LVS)

LVS is the most customized component in CSS. ABB divides it into three levels:

LVS 1: Wall mounted, suitable for small capacity and basic incoming and outgoing line needs. Bus length 5/8/12/16/20 modules (1 module=100mm). The rated current depends on the type of busbar: DIN busbar 1000-2500A, Kabeldon busbar 400-1600A, Z-type busbar 630-1900A. The incoming line can use a fuse type isolation switch (back connection) or an OETL load switch/IsoMax MCCB. The outgoing line can use fuse switches (63-630A), air circuit breakers (Emax/Emax2), and molded case circuit breakers (Tmax).

LVS 2: Frame structure (1.5mm galvanized steel), with DIN rails on the top for measuring equipment, cover plate RAL 7035. Similar to LVS 1 but supports higher current and advanced incoming line protection.

LVS 3: Independent (2mm galvanized steel frame), with the incoming line located above the busbar, can be configured with ACB (Emax2) up to 3150A. The busbar length is 12/16/20 modules, and the rated current is up to 5000A (DIN busbar). There is space at the top to install incoming circuit breakers, streetlight equipment, measuring equipment, etc.

All LVS comply with IEC 61439-1, and the system grounding can be TN-C (4-wire, PEN grounded or insulated) or TN-S (5-wire, N and PE separated).

6.5 Selection of Switchgear

Tmax molded case circuit breaker: up to 1600A, fixed, plug-in or withdrawable versions, high breaking capacity, remote control and bus communication.

Emax 2 air circuit breaker: up to 6300A, fixed or withdrawable, with a breaking capacity of 42-200kA, supporting Modbus, Profibus, DeviceNet, Modbus TCP, Profinet, EtherNet/IP, and IEC 61850 communication.

OT series isolating switch: compact design, AC-23A full current rating, up to 690V/1000V, fast switching mechanism, no need for capacity reduction.

InLine vertical fuse switch: used for DIN NH fuses, 160-1250A, maintenance free, supports electronic fuse monitoring (EFM) and remote indication, with simple selective calculation (upper and lower current ratio of 1.6:1).

Kabeldon fuse system: Insulated busbar system (400-2500A), including connectors, isolating switches, switches, fuse switches, MCCBs, etc., with short assembly time and compact design.

Smart grid compatibility

ABB has defined four levels of CSS automation solutions:

Level 1: Basic monitoring, including overall CSS monitoring, low-voltage side current/voltage/energy measurement.

Level 2: Add control of medium and low voltage primary equipment on the basis of Level 1.

Level 3: Add precise current, voltage, and energy measurement on the medium voltage side based on Level 2.

Level 4: Adding the protection function of incoming and outgoing circuit breakers on the basis of Level 3 is the most complete technical solution.

The benefits of automation include improving power supply quality, reducing the frequency and duration of power outages, enhancing voltage quality, improving operational efficiency and network stability, improving operation and maintenance tools, and reducing travel to difficult to reach locations.

Environmental Protection and Recycling

The UniPack-S series is produced according to ABB's strict quality and environmental procedures and is certified by ISO 9001 and ISO 14001. ABB is committed to developing and supplying products that have minimal environmental impact, are safe to use, recyclable, reusable, or disposed of safely. The research and development goal is to generate sustainable technologies, systems, and products.