ICCO Delivers the Nnewi 800 MVA Transmission Substation

International Consolidated Contractors Offshore SAL designed and built the Nnewi 330/132/33 kV transmission substation in Anambra State for the Transmission Company of Nigeria, under a contract awarded in May 2021. The facility entered commercial operation on 1 June 2023 with four power transformers totalling 800 MVA, switchyards at three voltage levels, fixed and variable reactor facilities and a fully automated control system.

The defining feature of the Nnewi transmission substation is not a single transformer or switchyard. It is the number of options built into the facility.

Electricity can enter through multiple 330 kV line bays. It can be transformed through two 300 MVA autotransformers and two 100 MVA power transformers. It can be directed through 330 kV, 132 kV and 33 kV switchyards. Voltage conditions can be managed through fixed and variable reactor facilities. Protection, automation and telecommunications systems monitor the entire arrangement.

The result is a substation designed around capacity, flexibility and control.

Four transformers, two different duties

The Nnewi substation contains four major power transformers. Two are rated at 300 MVA, 330/132/33 kV. The other two are rated at 100 MVA, 132/33 kV. Together, they provide 800 MVA of installed transformation capacity.

The larger 300 MVA units receive power from the 330 kV transmission level and reduce it for movement through the 132 kV and 33 kV networks. The two 100 MVA transformers operate between 132 kV and 33 kV, adding further capacity for power to move from the regional transmission system into lower-voltage networks.

This arrangement gives the substation more than one transformation path. Rather than depending on a single pair of units to perform every function, the facility can manage different voltage transitions through equipment designed for separate roles.

That matters during periods of high demand, planned maintenance or equipment unavailability, when operators have more options for managing load and organising the flow of electricity through the station.

The transformers are equipped with cooling systems, on-load tap changers, protection, control and auxiliary systems. On-load tap changers allow transformer voltage ratios to be adjusted while the units remain energised, which helps operators maintain the required output voltage as network conditions change.

Three switchyards on one site

The Nnewi project includes complete switchyards at 330 kV, 132 kV and 33 kV. Each performs the same broad function at a different level of the electricity network: it receives connections, directs power and allows individual lines or equipment to be isolated.

The ten 330 kV line bays give the station multiple high-voltage connection points, and the transformer bays connect the two 300 MVA autotransformers to the 330 kV system.

At 132 kV, the sectionalising bays allow the busbar to be divided into separate operating sections. This gives operators greater flexibility when carrying out maintenance or responding to a fault: one part of the switchyard can be isolated while another section remains available. At 33 kV, the bus coupler allows separate busbar sections to be connected or divided as required.

Across all three voltage levels, the switchyards give the facility a wide range of possible operating configurations.

Why the bays matter

A substation bay is more than a physical space between steel structures. It is a controlled connection containing the equipment needed to switch, isolate, measure and protect a line, transformer or reactor.

The project included disconnect switches, grounding switches, post insulators, surge arresters and instrument transformers across the 330 kV, 132 kV and 33 kV systems.

Circuit breakers interrupt current when a fault occurs or equipment must be removed from service. Disconnect switches provide visible isolation after the breaker has interrupted the current. Grounding switches connect isolated equipment safely to earth during maintenance. Current and voltage transformers provide measurements to protection relays, meters and control systems. Surge arresters protect equipment against sudden voltage increases caused by lightning or switching events.

A large substation can only operate safely when these devices respond in the correct order. The bay layout gives each connection its own switching and protection arrangement, allowing faults to be isolated without unnecessarily removing unaffected equipment from service.

Voltage control beyond the transformers

The Nnewi substation includes both fixed and variable reactor facilities. A 75 MVAr shunt reactor provides a fixed level of reactive-power absorption, and two variable line reactors can operate between 25 and 62 MVAr.

High-voltage transmission lines can generate excess reactive power, particularly when they are lightly loaded, which can raise voltage beyond the desired operating range. Reactors absorb part of that reactive power.

The variable units can adjust their response as network conditions change, giving operators more precise control than a fixed reactor alone. This becomes especially useful in a station connected to several transmission circuits, where the amount of voltage support required may differ according to which lines are energised, how heavily they are loaded and how power is moving through the wider network.

Connecting Nnewi to the existing grid

The project also included 330 kV line-in, line-out connection works.

A line-in, line-out arrangement — often shortened to LILO — allows an existing transmission line to be brought into a new substation and then returned to the wider route. Instead of constructing an entirely separate long-distance line, the existing circuit is diverted through the new facility.

The Nnewi connection works included line-entry gantries, transmission-line structures, terminations, Optical Ground Wire interfaces and other associated equipment. The gantries provide the physical transition between the overhead line and the substation equipment. The line terminations connect the incoming conductors to the switchyard. OPGW interfaces carry the fibre-optic communication channels associated with the transmission line into the station’s protection and telecommunications systems.

This connection work placed the Nnewi substation within the operating network rather than leaving it as a standalone installation.

A digital substation architecture

The physical equipment at Nnewi is supported by an integrated Substation Automation System. The project included:

• SCADA

• remote terminal units

• protection systems

• control and metering

• an IEC 61850 communication network

• telecommunications systems

• an interface with the National Control Centre

IEC 61850 provides a standard framework for communication between intelligent electronic devices within a substation. Protection relays, bay-control units, meters and other equipment can exchange information over the station network rather than depending entirely on conventional point-to-point wiring.

SCADA gives operators a view of the site’s operating condition. It displays breaker and disconnector positions, transformer loading, voltage measurements, alarms and other information, and it allows authorised control commands to be issued from the station or a remote control centre.

The National Control Centre interface extends that visibility beyond Nnewi, allowing the facility to be monitored as part of the wider transmission system.

Protection begins with system studies

Protection settings cannot be selected in isolation. They depend on how current and voltage are expected to behave across the network during normal operation and during faults.

The project included power-system studies covering load flow, short-circuit conditions, grid-code compliance, protection coordination, relay settings and fault levels.

Load-flow studies examine how electricity is expected to move through the system under different operating arrangements. Short-circuit studies calculate the current that could flow during faults, helping engineers select equipment ratings and protection settings. Protection-coordination studies determine how relays and breakers should operate in sequence, with the objective of isolating the smallest affected section while keeping the rest of the network available.

These studies turn a collection of protection devices into a coordinated defence system.

The systems that remain active during a fault

The substation includes complete auxiliary AC and DC systems, covering station-service transformers, uninterruptible power supplies, battery banks, battery chargers, diesel generators, HVAC systems and other supporting facilities.

Battery-backed DC power is essential because breakers, relays, alarms and communication equipment must continue working during a fault or loss of normal station supply. The uninterruptible power system supports sensitive equipment that cannot tolerate interruption, and diesel generators provide another level of backup for essential station services.

HVAC systems maintain operating conditions inside control, relay and battery rooms, where heat, dust and humidity can affect equipment performance.

These auxiliary systems are rarely the largest items on site, but they allow the substation to remain controllable when the external network is under stress.

Protection beyond the electrical fault

The Nnewi facility also includes security, fire-detection and equipment-protection systems. CCTV and access control help manage movement around the site, while fire-detection and firefighting installations address risks within the control buildings and around major electrical equipment.

The transformers and reactors are supported by explosion-prevention and fire-protection systems. Large transformers contain substantial volumes of insulating oil, and under severe internal fault conditions rapid pressure increases can create the risk of tank rupture and fire. Explosion-prevention systems are designed to detect and respond to those conditions before they escalate.

Oil-containment facilities, drainage, perimeter fencing and lightning protection form part of the wider site-safety arrangement. The protection philosophy therefore extends beyond electrical relays: it includes people, equipment, buildings and the physical environment of the station.

Building the site around the equipment

The Nnewi substation was developed with the complete civil and structural infrastructure required for operation, including reinforced-concrete foundations, control buildings, cable trenches and ducts, internal roads, drainage, transformer bund walls, oil-containment facilities, earthing grids, perimeter fencing, and steel gantries and busbar supports.

The foundations had to support transformers, switchgear, reactors and steel structures with different loading requirements. Cable trenches created organised routes for protection, control, metering and communication cables. Transformer bund walls and oil-containment facilities were designed to control the spread of insulating oil in the event of leakage or equipment failure. The earthing grid provided a controlled path for fault current while helping maintain safer voltage conditions across areas accessible to personnel.

Every major electrical system depended on this physical infrastructure being completed accurately before installation and testing could progress.

From individual equipment to one operating station

International Consolidated Contractors Offshore SAL was responsible for bringing the complete facility into operation. The work covered engineering, procurement, civil construction, transformer installation, switchyards, reactor facilities, line connections, automation, protection, telecommunications, security systems and auxiliary services.

Factory acceptance tests were carried out before major equipment was delivered. Site acceptance testing then confirmed that installed equipment, wiring, protection settings and communication systems operated correctly within the completed arrangement. Pre-commissioning checks were followed by commissioning, reliability testing, operator training and preparation of as-built documentation.

On 1 June 2023, the Nnewi 330/132/33 kV transmission substation entered commercial operation.

That date matters less than what follows it. The final test of a substation begins after commissioning: the station must continue to switch lines, transform power, manage voltage and respond to faults through different network conditions over time.

At Nnewi, that responsibility is shared across four transformers, three switchyards, multiple reactor facilities and a digital control architecture designed to make them function as one grid node.