Diesel Rotary Uninterruptible Power Supply – DRUPS are a synchronous generator, kinetic module/flywheel and diesel engine combined in one design. Electric power from the city network passing through a throttle powers the synchronous generator acting as an engine. In the customary mode, the generator fulfills the function of a stabilization and voltage filter: it liquidates harmonic distortions and short interruptions (to 50 ms), interfering pulses, mitigates voltage falls and leaps of the city network.
Dynamic UPSs are used in data centers, banks, hospitals, airports, as well as for building an uninterruptible power system of plants with conveyor lines where the load is reactive.
The value of load for most projects implemented using diesel rotary UPSs is usually in the range of from 1 MVA to several hundreds of megawatts. Besides, units with a power of about 2,5 MVA are used most often. A parallel connection of a required number of diesel rotary UPSs with 2N or N+1 reserving is used almost everywhere.
If the power of the data center is over 5 MW, it is feasible to use not a classical system consisting of a diesel generator, ATS, static UPS and a/c system, and diesel rotary UPS, this decision contains fewer components and does not require precise conditioning – input-outlet cooling.
One of the obligatory items of maintenance expenses in a data center is an uninterruptible power supply. Static (classical) UPSs consist of two components – the sources and batteries – and operate with diesel generators. But they have a shortage – limited service time of batteries (5 years).
Diesel generators used for dynamic UPSs differ from diesel engines used in diesel generators. As a rule, the manufacturers of diesel rotary UPSs manufacture only rotary power accumulators and carry out the final assembly and check of the unit.
MTU and Mitsubishi diesel generators are used. The engines manufactured by Mitsubishi under the order of Hitec Power Protection use a double starter, preheating and fuel feeding gravity system, a special coupling clutch freewheeling.
A kinetic module - a battery of kinetic energy consisting of two rotors, internal and external rotors - rotates on the same bent shaft with the synchronous machine. The internal rotor rotates at the same rate as the synchronous generator – 1500 rpm, the external one – about 3000 rpm (the internal rotor - 1500 rpm).
At the same time, the winding of the direct current internal rotor and the short-circuited winding of the external rotor engage in electromagnetic interaction. The kinetic model in the process of the normal operation of the system accumulates kinetic energy. In the case of a short (within 5 seconds) loss of external power supply or of its parameters going beyond the limits, the synchronous generator keeps rotating and powers the load at the expense of the transfer of the energy to it by the module. The magnetic field of the internal rotor of the kinetic module slows down the rotation of the external rotor and thanks to the electronic control unit, it automatically supports the frequency of the synchronous generator at the level of the required 1500 rpm.
If the loss of power lasts longer or power voltage changes more than 10%, a diesel engine turns on while the input breaker breaks, and the synchronous generator from the electrical machine transforms into a current generator.
Within this 5-second pause, the electromagnetic clutch activates, which provides the connection of the diesel engine and generator (the duration of the process – on average no more than 1 second). The diesel rotates at optimal or system speeds – 1500 and transfers the rotatory energy to the synchronous generator. The output voltage is removed from the windings of the generator and passes through a vital load. At this time, the kinetic packaged module accumulates energy returning to 3000 rpm. When the mains supply is restored, the synchronous generator switches to the synchronous engine mode.
In their DRUPS, manufacturers provided an additional mechanism for diesel engine start: if the engine does not come into operation, an electromagnetic clutch takes place (in 3–4 seconds) and, as a result, spinning of the starter. In this case, the engine is reliably jump-started, which increases the reliability of the startup.
The rotating parts of the kinetic energy accumulator and synchronous motor generator require bearing lubrication. In modern designs, this work is done without stopping the system, however, in some places, worn-out parts have to be replaced. And then decommissioning of direct rotary UPS is obligatory. Besides, inspection and maintenance of the genset are required once a year.
Starter batteries responsible for the start of the diesel genet, which have to work in heavy temperature conditions, claim constant attention.
Every 10 years, the complete replacement of the accumulating rotor of the kinetic storage unit is carried out. Over this period, bearings are replaced 2 times. This requires time-consuming and expensive work comparable with the engine overhaul – a range of work on balancing, measuring, setting and commissioning of the system.
To provide the operation of a dynamic UPS, the conditioning systems are not required; they charge fast (it takes several minutes to spin up the flywheel), that’s why they can withstand multiple power cuts within a small period.
In the case of multiple external power losses, diesel rotary UPS is a single option. The power of traditional static UPSs is often calculated in such a way as it is sufficient to maintain the operation of the equipment within 10-15 minutes. But if within a short period (less than an hour) electrical line voltage is lost several times in a row at least for 4-5 minutes, the batteries may not have time to charge. Because of their design features, this is not a threat to dynamic UPSs
The advantages of diesel rotary systems make themselves felt on powers approaching 1 MVA. Besides, this means not only and not just the power of IT equipment. Rather, on the contrary, dynamic systems can be effectively used in the industry, especially if the load is reactive or if short circuits in the system are possible (diesel rotary UPSs withstand the tenfold exceeding of short circuit current).
Comparing classical uninterruptible power systems consisting of 1 MVA power static UPSs with and 1,6 MVA diesel generator, to dynamic solutions for 1 MVA, experts note that starting from the 5th year of operation, the total price of owning dynamic UPS becomes less, and by the 10th year this UPS turns out to be 1,5 times more profitable.
For example, for data centers, 1 MVA of power is required to ensure power backup using two diesel rotary UPSs 1 MVA each (scheme 1+1). When the energy consumption of the data center starts increasing, an extra 1 MVA UPS will be required to maintain backup (scheme 2+1). In the case of a static UPS, the situation is different: they are available on the market in a wide range of power, that’s why this parameter may be increased gradually. The effectiveness of diesel rotary UPSs makes itself felt in the estimates of the Total cost of ownership for 7 years at a power exceeding 1 MW on the server load.
With no capital cost for batteries in the case of dynamic UPSs, the consumer should be ready for expenses to repair diesel engines, fuels, consumables.
Compared to classic UPSs, which provide backup with the help of a DG, diesel rotary UPSs require much more frequent starts of the diesel engine, since the time for supporting the load by kinetic energy accumulator devices is much less than that of batteries.
The frequent number of starts requires more diesel fuel, which requires additional storage tanks. Such storage tanks require enhanced security measures, such as the possibility to dump diesel fuel in the case of an accident.
System recovery time in case of failure: any failure in a static UPS can be repaired in 8 hours, and the minimum time for a complete recovery of a diesel rotary UPS is 24 hours.
At the same time, the probability of unexpected accidents when using diesel rotary UPSs is much higher compared to using batteries. This is because the chemical elements in the composition of the batteries and the ongoing chemical processes are largely controllable (for example, using special monitoring systems). Moreover, batteries are regularly changed during their operation. This allows you to take prophylactic actions in advance to prevent malfunctions. In the case of a diesel rotary UPS, a discontinuous occurrence of an accident is possible, since the behavior of the mechanical system is almost impossible to predict.
Since a dynamic UPS is a kinetic structure with many moving parts, it requires an even foundation and careful leveling during installation. Often, it is optimal to use a separate building for a diesel rotary UPS where should be crane beams for assembly and disassembly and maintenance of the UPS.
As distinct from static UPSs, diesel rotary ones are best used to power air conditioners. Air conditioning compressors are characterized by very high starting currents, which static uninterruptible power supplies cannot always provide.
Dynamic UPSs are made in the range of power from 100 to 3000 kVA:
Hitec Power Protection: 500 - 3000 кВА.
Euro-Diesel: No-Break KS5 200 - 1750 кВА, No-Break KS5-SB 200 - 2000 кВА.
Piller: Uniblock UBTD 420 - 2500 кВА.