What two 150 MVA transformers do at the Kwara substation

The Kwara 330/132/33 kV transmission substation, delivered by International Consolidated Contractors Offshore SAL for Nigeria’s Rural Electrification Agency, uses two 150 MVA power transformers to move electricity from the 330 kV transmission system into lower-voltage networks. Together they provide 300 MVA of installed transformation capacity — but their role involves more than simply reducing voltage.

The two largest pieces of equipment at the station do not generate electricity.

They do not store it either.

Their job is to change the electrical conditions under which power moves through the network.

Electricity arriving at the facility is carried at 330 kV, a voltage suited to moving bulk power efficiently across long distances. Before that electricity can continue through regional and lower-voltage networks, it must be transformed.

At Kwara, that responsibility belongs to two power transformers, each rated at 150 MVA and designed to operate across the 330 kV, 132 kV and 33 kV levels. Together, they form the central transfer point within the facility.

What does a 150 MVA transformer mean?

A 150 MVA transformer is designed to handle up to 150 megavolt-amperes of apparent power under its specified operating conditions.

MVA is a measure used to describe the capacity of transformers and other alternating-current equipment. It combines two components:

• real power, measured in megawatts
• reactive power, measured in megavolt-amperes reactive

The actual amount of useful power transferred in megawatts depends partly on the power factor of the connected system. A transformer rated at 150 MVA should therefore not automatically be described as a 150 MW transformer.

Its rating defines the total electrical loading it can handle within its design limits, including both real and reactive components.

With two units installed, the station has a combined transformation capacity of 300 MVA. That figure describes the total installed capacity of both transformers. It does not mean they must always operate at their maximum ratings.

Loading changes according to electricity demand, the availability of connected lines and the condition of the wider network.

Why electricity must be transformed

High-voltage transmission exists because moving electricity over long distances is more efficient when voltage is high and current is comparatively low.

Electrical losses in conductors increase with current. By raising the voltage, the network can transfer large amounts of power while reducing the current required for the same power level. This makes 330 kV suitable for bulk transmission.

The same voltage is not practical for every part of the network. Equipment designed for 330 kV requires large clearances, extensive insulation and substantial switching infrastructure. It would be inefficient and costly to connect every regional or local network directly at that level.

The Kwara transformers create the transition. They receive electricity from the 330 kV side and make it available at 132 kV and 33 kV for onward movement through other parts of the power system.

How a transformer changes voltage

A power transformer transfers electricity between separate windings through a magnetic field.

Alternating current flows through the high-voltage winding and creates a changing magnetic field inside the transformer’s core. That magnetic field induces voltage in the other windings. The relationship between the number of turns in the windings determines how the voltage changes.

The 330 kV winding contains the arrangement needed for the incoming transmission voltage. The 132 kV and 33 kV windings provide lower-voltage outputs.

There is no direct metallic connection carrying current from one winding to another in the usual transformer arrangement. The transfer occurs through electromagnetic induction.

This gives the transformer two functions at once:

• it changes voltage
• it provides electrical separation between the connected systems

The frequency of the electricity does not change. Only the voltage-current relationship is transformed.

What happens to current when voltage falls?

When a transformer reduces voltage, the current available on the lower-voltage side generally increases for the same amount of transferred power, allowing for transformer losses and system conditions.

This relationship explains why different voltage levels use equipment of different sizes and ratings. On the 330 kV side, large quantities of power can be moved with comparatively lower current. On the 132 kV and 33 kV sides, the same transferred power results in higher current.

The lower-voltage windings, busbars, switchgear and outgoing connections must therefore be designed to carry those current levels safely.

The transformer does not multiply energy. It exchanges higher voltage and lower current for lower voltage and higher current.

Why Kwara uses two transformers

A single 300 MVA transformer could theoretically provide the same total nameplate capacity as two 150 MVA units. But the operational behaviour would be different.

With two transformers, the substation has two independent transformation paths. This allows operators to distribute loading between the units. It also creates more options during maintenance.

A transformer requires regular inspection, testing and servicing. If the entire facility depended on one unit, taking that transformer out of service would remove the station’s only transformation path.

With two units installed, one transformer can potentially remain in operation while the other is isolated, provided the remaining transformer can support the required load safely.

This does not mean the substation retains its full 300 MVA capacity when one unit is unavailable. The operating capacity would be reduced.

Network operators would need to consider:

• the remaining transformer’s 150 MVA rating
• current demand
• ambient and equipment temperatures
• power factor
• connected network conditions
• protection limits
• acceptable short-term loading

The benefit is not unlimited backup capacity. It is the ability to continue operating at a reduced level instead of losing the complete transformation function.

Sharing the load

When both transformers are available, their load can be shared.

For two transformers to operate in parallel, their electrical characteristics and operating settings must be compatible. Their voltage ratios, phase relationships and impedance characteristics must support stable load sharing.

If the units are not properly coordinated, one transformer may carry more than its intended portion of the load.

The protection and control systems monitor the condition of each unit. Operators can review measurements such as:

• voltage
• current
• real and reactive power
• transformer loading
• oil and winding temperature
• tap position
• alarms
• protection status

The objective is not necessarily to force both transformers to carry exactly identical loads at every moment. It is to keep each unit within its acceptable operating range while meeting network demand.

The third winding matters

The Kwara transformers are identified as 330/132/33 kV units. This means they connect three voltage levels rather than only two.

A two-winding transformer might connect a high-voltage network directly to one lower-voltage system. A three-winding arrangement allows one transformer to provide connections at two lower voltage levels.

At Kwara, the 132 kV winding supports regional power transfer. The 33 kV winding provides another path towards lower-voltage networks or station-related requirements, depending on the approved connection arrangement.

This design makes the transformer more than a simple step-down device between two fixed points. It becomes a junction between three parts of the electricity network.

Power flows through the transformer according to system configuration, connected demand and operating requirements.

The transformer bay controls the connection

Each of the two Kwara transformers has an associated 330 kV transformer bay.

The transformer bay contains the equipment required to connect the unit to the high-voltage busbar and disconnect it when necessary. This includes switching, measurement, protection and isolation equipment.

The bay allows one transformer to be removed from service without automatically disconnecting the other.

During normal operation, the circuit breaker remains closed and allows current to flow. If the transformer develops a serious fault, the protection system can instruct the breaker to open. Disconnectors can then create visible isolation after the current has been interrupted.

The transformer bay establishes a controlled electrical boundary around each unit.

What could go wrong inside a transformer?

A large power transformer operates under continuous electrical, thermal and mechanical stress. Possible abnormal conditions include:

• internal winding faults
• insulation failure
• overheating
• excessive current
• abnormal pressure
• oil-related problems
• faults at bushings or external connections
• cooling-system failure

These conditions do not all require the same response. Some may generate an alarm so operators can investigate. Others require the transformer to be disconnected immediately.

Protection systems are selected to identify different types of abnormal behaviour. Differential protection, for example, compares electrical measurements from different sides of the transformer’s protected zone. If the current entering and leaving the zone differs beyond expected limits, the system may identify an internal fault.

Other protection functions can respond to overcurrent, abnormal temperature, pressure changes or equipment-specific conditions.

The purpose is to separate a serious transformer fault from the rest of the network before it causes greater damage.

Why transformers produce heat

No transformer is perfectly efficient. Although most of the incoming electrical power is transferred to the lower-voltage side, a small portion becomes heat.

Heat is produced through electrical resistance in the windings and magnetic losses in the transformer core. The amount of heat increases with loading. If the temperature rises too far, transformer insulation can deteriorate.

Cooling is therefore part of the transformer’s operating system. Large power transformers commonly use insulating oil to carry heat away from the core and windings. The heated oil moves towards cooling equipment, where the heat is released before the oil returns to the main tank.

Temperature monitoring helps operators understand whether the transformer is operating within safe limits.

Loading decisions cannot be separated from thermal conditions. A transformer may be electrically capable of carrying a load, but temperature still determines whether that loading can be sustained safely.

Transformer oil does more than cool

The oil inside a large transformer performs two principal duties. It helps remove heat. It also provides electrical insulation.

The windings and other internal components operate at high electrical stress. The insulating oil helps prevent unwanted electrical discharge between energised parts.

Its condition is therefore important. Moisture, contamination or degradation can reduce insulation performance.

Oil testing can provide information about the internal condition of the transformer. Certain gases produced within the oil may indicate overheating, electrical discharge or other developing problems.

Monitoring allows maintenance teams to identify some issues before they lead to complete failure.

How voltage is adjusted while the transformer operates

Electricity-system voltage changes as demand and network conditions change. A transformer therefore needs a way to adjust its voltage ratio within a controlled range.

This function is typically provided by a tap-changing mechanism. A tap changer alters the effective number of turns in part of the transformer winding. Changing the winding ratio changes the output voltage.

Where an on-load tap changer is installed, adjustments can be made while the transformer remains energised and carrying load. This allows operators or automatic control systems to maintain voltage within the required range without shutting down the unit for every adjustment.

The transformer may appear mechanically static from the outside. Inside, its operating ratio can be changed to respond to conditions on the network.

What happens when one transformer trips?

Suppose protection detects a serious fault in one of the Kwara transformers. The associated circuit breakers must open and isolate the unit from the connected voltage levels.

The affected transformer becomes unavailable. The second transformer may remain in service if it is healthy and the fault does not affect a shared part of the substation.

Operators must then assess the new condition. They need to determine:

• how much load the remaining transformer is carrying
• whether any load must be transferred elsewhere
• whether demand needs to be reduced
• whether the busbar arrangement remains secure
• whether the faulted transformer can be inspected safely
• what caused the protection operation

The station’s installed capacity has fallen from 300 MVA to a maximum nameplate capacity of 150 MVA while one unit is unavailable. This makes operating decisions more constrained.

The second transformer provides continuity, but not unlimited replacement.

Maintenance begins before failure

Power transformers are expected to operate for many years, but their reliability depends on maintenance and condition monitoring. Routine activities may include:

• visual inspection
• checking for oil leaks
• temperature review
• oil sampling
• insulation testing
• bushing inspection
• cooling-system checks
• protection testing
• tap-changer maintenance
• verification of alarms and monitoring devices

Maintenance is easier to plan when two units are available. One transformer can be isolated while the other continues supporting part of the network, subject to operating conditions.

This does not eliminate the effect of maintenance. It gives system operators more control over when and how that effect is managed.

How SCADA sees the transformers

The Kwara substation’s automation and SCADA systems provide operators with information about the transformers and their associated bays. Instead of relying only on local inspection, authorised operators can monitor important conditions from the control environment.

Information may include:

• transformer loading
• voltage and current
• active and reactive power
• circuit-breaker positions
• alarms
• temperature indications
• protection operations
• tap position
• auxiliary-system status

SCADA creates visibility. Protection systems create automatic response. The transformer itself performs the physical transfer of power between voltage levels. All three are required for safe operation.

A transformer that cannot be observed is difficult to manage. A transformer that cannot be disconnected is dangerous. A transformer that cannot transfer power is simply an expensive stationary object.

Direct answer: what do the two Kwara transformers do?

The two 150 MVA transformers at the Kwara 330/132/33 kV transmission substation receive electricity from the 330 kV transmission system and transfer it to the 132 kV and 33 kV networks. Together, they provide 300 MVA of installed transformation capacity.

Using two transformers allows:

• load to be shared between units
• one unit to be isolated for maintenance
• reduced service to continue when one transformer is unavailable, where network conditions permit
• separate protection and control of each transformation path
• electricity to be supplied at both 132 kV and 33 kV

They are the equipment through which bulk transmission power becomes usable by the next layers of the electricity network.

How the transformers were delivered

International Consolidated Contractors Offshore SAL delivered the Kwara substation for the Rural Electrification Agency as an engineering, procurement and construction project. The work included the supply, installation and commissioning of the two 150 MVA, 330/132/33 kV power transformers and their associated equipment.

That equipment had to be integrated with:

• the four 330 kV line bays
• the two transformer bays
• the high-voltage busbar
• circuit breakers and disconnectors
• protection and control systems
• SCADA and telecommunications
• auxiliary AC and DC supplies
• foundations and oil-management infrastructure
• the substation earthing system

The facility was commissioned on 9 January 2025. By that stage, the transformers had to operate not as isolated machines, but as the central transfer points within a complete substation.

Two transformers, two kinds of capacity

The most obvious capacity at Kwara is numerical. Two transformers. 150 MVA each. 300 MVA combined.

The less obvious capacity is operational. The ability to divide load. The ability to isolate one unit. The ability to perform maintenance without automatically removing both transformation paths. The ability to connect one high-voltage transmission system to two lower-voltage networks.

One transformer changes voltage. Two transformers change what the substation can do when conditions change.