Power Generation

UPS, DUPS, and a lot of kinetic energy: How Dynamic Uninterruptible Power Supply systems work

Posted on 15 December 2020 by Lucie Maluck, Images by Kinolt

Rolls-Royce Power Systems now has Dynamic Uninterruptible Power Supply systems – UPS systems for short – in its product portfolio. The newly arrived systems are made by the Belgian company Kinolt that we took over last July.
Rolls-Royce Power Systems now has Dynamic Uninterruptible Power Supply systems – UPS systems for short – in its product portfolio. UPS technology probably sounds alien to anyone at the company who does not deal with it on a daily basis. After all, Rolls-Royce is mainly known for its mtu-brand engines that are also used in electrical generator sets. And it is equally well known that many customers use these generator sets to produce emergency power. The newly arrived systems are made by the Belgian company Kinolt – that we took over last July – and will be sold under the mtu brand name from next year on. Because of the growing request of energy and the increasing demand of protected power this exciting technology is essential for mission critical systems. And because these systems are dynamic i.e. work with rotating flywheel masses, they are referred to as Dynamic Uninterruptible Power Supply systems, or DUPS for short.

What Dynamic Uninterruptible Power Supply Systems do?

Dynamic UPS systems provide perfect conditioned electrical power to critical consumers. In normal operating mode i.e. when the public power grid is available, a choke – an electromagnetic coil is used – to eliminate current and voltage fluctuations that the power grid tends to produce. These voltage fluctuations are actually very frequent but are not perceived by 'normal' consumers. Today's sophisticated IT systems however are increasingly sensitive, and a power grid fluctuation can cause their function to be perturbed. When that happens in a hospital or data center or in factories that run very sensitive production processes, the consequences can be dramatic, and involve huge financial losses.  

How does a dynamic UPS system work? 

Kinolt's technology comprises a constantly rotating kinetic energy storage unit with flywheel, an mtu diesel engine and an alternator which, depending on the operating mode, also operates as an electric synchronous motor with its preferred compensation characteristics. A special control unit with the accompanying switchgear and a choke that acts as a line filter, complete the system. Power from the public grid drives the synchronous alternator, which acts as an electric motor in this operating mode. This in turn drives the flywheel in the kinetic energy storage unit which weighs tons and is designed as an electric machine with double rotor system. This serves as the 'energy accu' which stores energy and makes it available in the unlucky event of a power outage. 

What happens when the primary power source fails?

If the public power grid fails, the systems acts immediately and without interruption because it is already in operation anyway. Part of the energy stored in the flywheel is used to drive the generator, that starts producing electrical power. Meanwhile, another part of the stored rotational energy is used to facilitate very fast start-up of the diesel engine. Within the shortest possible time the engine accelerates to its rated power output and can now start producing electrical power via the generator and stabilize the flywheel. This naturally happens without any gap or interruption – hence the name 'Uninterruptible Power Supply'.  

Is another battery required?

No. With Kinolt's dynamic UPS technology, no battery is required. Previously, Rolls-Royce only had mtu standby generator sets in its portfolio for dealing with power outages. These require a certain amount of time to get going, however. To bridge that gap and ensure an uninterrupted flow of power, customers have often resorted to batteries. In Kinolt's system, however, no such battery is needed, since the energy in the kinetic energy storage unit can be made available immediately for generating power until the mtu diesel engine comes on-line. 

What happens precisely?

Well - here's an explanation for the expert or anyone wishing to become one: The flywheel – essentially a kinetic energy accumulator – forms the heart of the system. The flywheel consists of two rotating elements: an outer rotor running freely around an inner rotor which is driven by the main shaft and rotates at 1,500 rpm (50 Hz) or 1,800 rpm (60 Hz). It has two sets of windings: a three-phase alternating current (AC) winding and a direct current (DC) winding.

When the public power grid is available, the AC winding is energized to create a rotating magnetic field that drives the outer rotor at a maximum speed of 3,000 rpm. However, the relative speed between the inner and outer rings of the bearings is only 1,500 rpm, since the shaft itself rotates at 1,500 rpm. The external rotor stores kinetic energy advantageously, since the amount of kinetic energy increases by the square as the distance from the center of rotation increases.

In conditioning mode, when the mains supply is within tolerance, the synchronous machine is acting as a motor, driving the main shaft and thus the accu, storing kinetic energy in it. When the inner rotor rotates at 1500 rpm (50 Hz) or 1800 rpm (60 Hz), the outer rotor then rotates at maximum 3000 rpm. The kinetic energy is thus stored.

During a mains failure when the system is supplying emergency power, the DC winding is fed and the outer rotor is electrically coupled to the inner rotor by induction. Its kinetic energy is transferred to the inner rotor – driving it. This energy transfer is regulated by precise control of the current fed into the DC winding. The accumulator for the kinetic energy is an entirely brushless system.

During mains failure or voltage perturbation the stored kinetic energy is now transferred to the kinetic energy accumulator, which acts as a generator. This happens without any interruption to the user. Shortly after, the diesel engine starts and couples to the kinetic energy accu by latching the electromagnetic clutch. The energy is then relayed from the diesel engine to the synchronous machine and then to the loads.
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