Medium Voltage Diesel Generator

Large facilities usually distribute power at medium voltage (MV). A medium voltage diesel generator system is a generator installation built to deliver power directly at that medium voltage level. Rather than generating at low voltage (400 V) and carrying it over long distances, running the diesel generator at medium voltage means lower losses and thinner cables.

Medium voltage projects are always engineered to order. Power, voltage level, number of sets and protection requirements are assessed together, and the generator sets, switchgear and, where needed, the transformer are designed as one system.

Two basic configurations

Medium voltage generator systems use one of two configurations:

• Sets with medium voltage alternators: The alternator generates directly at medium voltage. The set has no separate step-up transformer.
• Low voltage set + step-up transformer: A standard 400 V generator set is used, and its output is raised to medium voltage by a step-up transformer.

	Set with MV alternator	400 V set + step-up transformer
Generator output	Directly at medium voltage	400 V, raised to medium voltage by the transformer
Transformer	Not required	Step-up transformer required
Set selection	Configured with a medium voltage alternator	Selected from standard catalogue sets
Supplying 400 V loads	Separate step-down transformer required	400 V loads can be supplied ahead of the transformer
Suitable projects	Very high power, sites with MV distribution	Connecting existing 400 V sets to MV, mixed loads

Which configuration is suitable is decided at the design stage, based on total power, grid voltage level, distribution distance and the site’s existing infrastructure.

Why medium voltage?

For the same power, current falls as voltage rises. In practice this means:

• Lower losses: Voltage drop and heat losses are reduced over long transmission distances.
• Smaller cables and busbars: Instead of the many parallel cables and large busbars needed for currents of thousands of amperes at 400 V, smaller cross-sections are used.
• Compatibility with site infrastructure: On sites supplied at medium voltage, the generator connects to the existing MV distribution system; there is no need to install a separate generator at every transformer substation.
• Parallel operation: Several generators can run in parallel on a medium voltage busbar via a synchronization panel.

Synchronized operation at medium voltage

A high power demand is often met not by a single generator but by several sets running in parallel on a medium voltage busbar. The synchronization controllers match the voltage, frequency and phase angle of the sets, connect them to the busbar and share the load. When the load rises an additional set comes online; when it falls a set goes offline.

This arrangement also provides redundancy: while one set is under maintenance or has failed, the others continue to supply critical loads. Detailed information on parallel operation is in our article on generator synchronization and paralleling.

Protection and operation

In medium voltage systems the generator becomes part of the site’s medium voltage distribution system. The protection scheme is therefore considered together with the site’s existing protection:

• Circuit breaker and protection relays: Protections such as overcurrent, earth fault and reverse power are set according to the site’s short-circuit level.
• Earthing: The earthing method of the generator star point is chosen to suit the site’s medium voltage earthing system.
• Interlocking: The interlocking between the mains and generator circuit breakers is designed to prevent incorrect connections.
• Monitoring: The operating status, alarms and measurements of the sets are monitored from the panel and, where needed, remotely.

After commissioning, periodic maintenance and load tests keep the system ready to run at any time.

Which power ratings?

The sets used in medium voltage systems are selected from the high-power diesel generators in our product catalogue. In our catalogue, sets up to 1,100 kVA are supplied with a soundproof canopy, and sets above 1,100 kVA (1,250–4,125 kVA standby; Baudouin, Cummins and Perkins engines) as containerised generators.

You can browse the high-power sets by rating:

• 1000 kVA generator and 1250 kVA generator
• 1500 kVA generator and 2000 kVA generator
• 2500 kVA generator, 3000 kVA generator and 4000 kVA generator

For projects that need high power from two synchronized sets on a single base frame, see our dual generator sets.

Applications

• Large industrial plants and businesses in organised industrial zones
• Data centres and large telecoms centres
• Hospital and university campuses
• Mining sites and off-grid energy projects
• Shopping centres and large commercial buildings supplied at medium voltage
• Cogeneration plants

Design steps

1. Needs analysis: Total power, load profile, duty (standby or prime) and critical loads are defined.
2. Configuration: The MV alternator or 400 V set + step-up transformer configuration, the number of sets and the redundancy arrangement are decided.
3. Switchgear and protection: Synchronization, breakers and protection are designed around the site’s MV distribution system.
4. Installation location: Placement of the sets indoors, in canopies or in containers, the fuel system and the exhaust outlet are planned.
5. Testing and commissioning: Every generator is tested at full load in our factory before shipment, and the system is commissioned on site with system tests.

Information needed for a quotation

• Total power demand and duty
• The site’s medium voltage level and frequency (50 Hz or 60 Hz)
• Number of sets and the need for parallel operation with the grid
• Distance between the generator and the MV distribution point
• Installation location and site conditions (ambient temperature, altitude)

With this information our engineers prepare the right configuration for you. Request a quote for your project; for installation see installation and commissioning, and for the operating period our generator service.