Butsev I.V.
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Features of power supply systems using Diesel Dynamic Uninterruptible Power Supplies (DDIBP)
In the following presentation, the author will try to avoid marketing clichΓ©s and will rely solely on practical experience. The test subjects will be HITEC Power Protection's DDIBP.
DDIBP installation device
The DDIBP device, from the point of view of an electromechanic, looks quite simple and predictable.
The main source of energy is a Diesel Engine (DD), with sufficient power, taking into account the efficiency of the installation, for long-term continuous power supply to the load. This accordingly imposes rather stringent requirements on its reliability, readiness for launch and stability of operation. Therefore, it is completely logical to use ship DDs, which the vendor repaints from yellow to its own color.
As a reversible converter of mechanical energy into electrical energy and vice versa, the installation includes a motor-generator with a power exceeding the nameplate capacity of the installation to improve, first of all, the dynamic characteristics of the power source during transients.
Since the manufacturer claims uninterrupted power supply, there is an element in the installation that supports the power supply of the load during transitions from one operating mode to another. This purpose is served by an inertial drive or an induction clutch. It is a massive body rotating at high speed and accumulating mechanical energy. The manufacturer describes his device as an induction motor inside an induction motor. Those. There is a stator, an outer rotor and an inner rotor. Moreover, the outer rotor is rigidly connected to the common shaft of the installation and rotates synchronously with the motor-generator shaft. The inner rotor is additionally untwisted relative to the outer one and is actually a drive. To provide power and interaction between the individual parts, brush assemblies with slip rings are used.
To ensure the transfer of mechanical energy from the DD to the rest of the installation, an overrunning clutch is used.
The most important part of the installation is the automatic control system, which, by analyzing the operation parameters of individual parts, has an impact on the management of the installation as a whole.
Also, the most important element of the installation is the reactor, a three-phase choke with a winding tap, designed to integrate the installation into the power supply system and allows relatively safe switching between modes, limiting equalizing currents.
And finally, auxiliary, but by no means secondary subsystems - ventilation, fuel supply, cooling and gas exhaust.
Operating modes of the DDIBP installation
I think it would be helpful to describe the various states of the DDIBP installation:
- operating mode OFF
The mechanical part of the installation is not moving. Power is supplied to the control system, the DD preheating system, the floating charge system for starter batteries, and the recirculation ventilation unit. After preheating, the unit is ready to start.
- operating mode START
When the START command is given, the DD is started, which spins the external rotor of the accumulator and the motor-generator through the overrunning clutch. As the DD warms up, its cooling system is activated. After reaching the operating speed, the internal rotor of the drive begins to spin up (charge). The charging process of the drive is indirectly judged by the current it consumes. This process takes 5-7 minutes.
In the presence of an external power supply, it takes some time for the final synchronization with the external network, and when a sufficient degree of commonality is reached, the installation is connected to it.
DD reduces the speed and goes into a cooling cycle, which takes about 10 minutes, followed by a stop. The overrunning clutch disengages and further rotation of the unit is supported by a motor-generator with simultaneous compensation of losses in the drive. The unit is ready to power the load and goes into UPS mode.
In the absence of external power supply, the unit is ready to power the load and its own needs from the motor-generator and continues to operate in the DIESEL mode.
- operating mode DIESEL
In this mode, the energy source is DD. The motor-generator rotated by it feeds the load. The motor-generator as a voltage source has a pronounced frequency response and has a noticeable inertia, reacting with a delay to sudden changes in the load. Because the manufacturer completes the units with marine DD operation in this mode is limited only by fuel reserves and the ability to maintain the thermal regime of the unit. In this operating mode, the sound pressure level in the vicinity of the unit exceeds 105 dBA.
- UPS operating mode
In this mode, the external network is the source of energy. The motor-generator, connected through the reactor both to the external network and to the load, operates in the mode of a synchronous compensator, compensating the reactive component of the load power within certain limits. In general, the installation of a DDIBP connected in series with an external network, by definition, degrades its characteristics as a voltage source, increasing the equivalent internal impedance. In this mode of operation, the sound pressure level in the vicinity of the installation is about 100 dBA.
In case of problems with the external network, the unit is disconnected from it, a command is given to start the DD and the unit switches to the DIESEL mode. It should be noted that the start of a constantly heated DM occurs without load until the speed of the DM shaft of the remaining parts of the installation is exceeded with the overrunning clutch closing. The typical start-up and exit time of the operating revolutions of the DD is 3-5 seconds.
- BYPASS mode
If necessary, for example, during maintenance, the load power can be transferred to the bypass line directly from the external network. Switching to the bypass line and back occurs with an overlap in the response time of the switching devices, which makes it possible to avoid even a short-term loss of load power. the control system seeks to maintain the in-phase output voltage of the DDIBP installation and the external network. In this case, the operating mode of the installation itself does not change, i.e. if the DD was working, then it will continue to work or the installation itself was powered from an external network, then it will continue.
- operating mode STOP
When the STOP command is given, the load power is switched to the bypass line, the power supply to the motor-generator and storage is interrupted. The unit continues to rotate by inertia for some more time and after stopping it goes into the OFF mode.
DDIBP connection diagrams and their features
Single installation
This is the easiest way to use an independent DDIBP. The installation can have two outputs - NB (no break, uninterrupted power supply) without interruption of power supply and SB (short break, guaranteed power supply) with short-term interruption of power supply. Each of the outputs can have its own bypass (see figure 1.).
Ris.1
The NB output is usually connected to a critical load (IT, refrigeration circulation pumps, precision air conditioners), and the SB output is connected to a load for which a short interruption of the power supply is not critical (refrigeration system chillers). In order to prevent a complete loss of power supply to the critical load, the switching of the output of the installation and the bypass circuit is carried out with overlap in time, and the fault currents are reduced to safe values ββdue to the complex resistance of a part of the reactor winding.
Particular attention should be paid to the power supply from the DDIBP of a non-linear load, i.e. load, which is characterized by the presence of a noticeable amount of harmonics in the spectral composition of the consumed current. Due to the peculiarities of the operation of the synchronous generator and the connection scheme, this leads to a distortion of the voltage shape at the output of the installation, as well as the presence of harmonic components of the current consumed when the installation is powered from an external AC voltage network.
Below are images of the form (see Fig. 2) and harmonic analysis of the output voltage (see Fig. 3) when powered from an external network. The harmonic distortion coefficient exceeded 10% with a modest non-linear load in the form of a frequency converter. At the same time, the unit did not switch to diesel mode, which confirms that the control system does not monitor such an important parameter as the harmonic distortion coefficient of the output voltage. According to observations, the level of harmonic distortion does not depend on the power of the load, but on the ratio of the powers of the non-linear and linear loads, and when tested for a pure active, thermal load, the voltage shape at the output of the installation is really close to sinusoidal. But this situation is very far from reality, especially when it comes to powering engineering equipment that incorporates frequency converters, and IT loads that have switching power supplies that are not always equipped with a power factor corrector (PFC).
Ris.2
Ris.3
In this and subsequent schemes, three circumstances attract themselves:
- Galvanic connection between the input and output of the installation.
- The imbalance of the phase load from the output falls on the input.
- The need for additional measures to reduce the harmonics of the load current.
- The harmonic components of the load current and the distortion caused by transients flow from the output to the input.
Parallel circuit
In order to power up the power supply system of the installation, DDIBP can be connected in parallel, connecting the input and output circuits of individual installations. At the same time, one must understand that the installation loses its independence and becomes part of the system when the conditions of synchronism and in-phase are met, in physics this is denoted in one word - coherence. From a practical point of view, this means that all installations included in the system must operate in the same mode, i.e., for example, the variant with partial operation from DD and partial from the external network is not allowed. The bypass line in this case is created common to the entire system (see Fig. 4).
With this connection scheme, there are two potentially dangerous modes:
- Connection of the second and subsequent installations to the output bus of the system in compliance with the conditions of coherence.
- Disconnection of a single unit from the output bus in compliance with the conditions of coherence until the opening of the output switches.
Ris.4
The emergency shutdown of a single installation can lead to a situation where it starts to slow down, and the output switching device has not yet opened. In this case, in a short time, the phase difference between the installation and the rest of the system can reach emergency values, causing a short circuit mode.
You also need to pay attention to load balancing between individual installations. In the equipment considered here, balancing is carried out due to the falling load characteristic of the generator. Due to its non-ideality and non-identity of the characteristics of the installation instances, the distribution between the installations is also uneven. In addition, when approaching the maximum load values, distribution begins to be affected by such seemingly insignificant factors as the length of the connected lines, the points of connection to the distribution network of installations and loads, as well as the quality (transitional resistance) of the connections themselves.
It must always be remembered that DDIBP and switching devices are electromechanical devices with a significant moment of inertia and appreciable values ββof the reaction delay time to control actions from the automatic control system.
Parallel circuit with βmediumβ voltage connection
In this case, the generator is connected to the reactor through a transformer with the appropriate transformation ratio. Thus, the reactor and switching machines operate at the "medium" voltage level, and the generator operates at the level of 0.4 kV (see Fig. 5).
Ris.5
With this use case, attention must be paid to the nature of the final load and its connection scheme. Those. if the final load is connected through step-down transformers, it must be borne in mind that the connection of the transformer to the mains is, with a high degree of probability, accompanied by the process of remagnetization of the core, which in turn causes an inrush current consumption and, consequently, a voltage drop (see Fig. 6).
Sensitive equipment may not work properly in this situation.
At least the fast lighting flashes and the default motor frequency converters restart.
Ris.6
Schematic with "split" output bus
In order to optimize the number of installations in the power supply system, the manufacturer proposes to use a βsplitβ output bus scheme, in which the installations are parallel both in input and output, with each installation individually connected to more than one output bus. In this case, the number of bypass lines must be equal to the number of output busbars (see Fig. 7).
It must be understood that the output buses are not independent and are galvanically connected to each other through the switching devices of each of the installations.
Thus, despite the manufacturer's assurances, this circuit is a single power supply with internal redundancy, in the case of a parallel circuit, having several galvanically connected outputs.
Ris.7
Here, just as in the previous case, it is necessary to pay attention not only to load balancing between installations, but also between output buses.
Also, some customers categorically object to the supply of "dirty" food, i.e. bypass, to the load in any operating mode. With such an approach, for example, in data centers, a problem (overload) on one of the beams leads to a system crash with a complete shutdown of the payload.
The life cycle of DDIBP and its impact on the power supply system as a whole
We should not forget that DDIBP installations are electromechanical devices that require careful, if not more, reverent attitude and periodic maintenance.
The maintenance schedule includes decommissioning, shutdown, cleaning, lubrication (once every six months), as well as loading the generator for a test load (once a year). It usually takes two working days to service one installation. And the lack of a specially designed circuit for connecting the generator to the test load leads to the need to de-energize the payload.
For example, let's take a redundant system of 15 parallel-operating DDIBPs connected via βmediumβ voltage to a double βsplitβ bus in the absence of a dedicated circuit for connecting a test load.
With such initial data, to service the system for 30 (!) -ti calendar days in the mode every other day, it will be necessary to de-energize one of the output buses to connect the test load. Thus, the payload power availability of one of the output rails is -0,959, and in fact even 0,92.
In addition, the return to the regular payload power supply scheme will require the inclusion of the required number of step-down transformers, which, in turn, will cause multiple voltage dips in the entire (!) System associated with the remagnetization of the transformers.
Recommendations for the use of DDIBP
From the foregoing, a disappointing conclusion suggests itself - at the output of the power supply system using DDIBP, high-quality (!) Uninterrupted voltage is present when all of the following conditions are met:
- External power supply has no significant disadvantages;
- The system load is constant in time, active and linear in nature (the last two characteristics do not apply to data center equipment);
- There are no distortions in the system caused by switching of reactive elements.
In summary, the following recommendations can be made:
- Separate power supply systems for engineering and IT equipment, and divide the latter into subsystems to minimize mutual influence.
- Dedicate a separate network to provide the ability to serve a single installation with the ability to connect an outdoor test load with a capacity equal to a single installation. Prepare for these purposes a platform and cable management for connection.
- Constantly monitor the load balance between power busbars, individual units and phases.
- Avoid the use of step-down transformers connected to the DDIBP output.
- Thoroughly test and record the operation of automation and power switching devices in order to collect statistics.
- To verify the power quality of the load, test installations and systems using a non-linear load.
- When servicing, disassemble starter batteries and test them individually as despite the presence of the so-called equalizers and the backup launch panel (RSP), due to one faulty battery, the DD may not start.
- Take extra care to minimize load current harmonics.
- Document the sound and thermal fields of installations, the results of vibration tests for a prompt response to the first manifestations of various kinds of mechanical problems.
- Avoid long downtime of installations, take measures to evenly distribute the motor resource.
- Complete the installation with vibration sensors to prevent an emergency.
- If sound and thermal fields change, vibrations appear, extraneous odors, immediately take the units out of operation for further diagnostics.
PS The author will be grateful for feedback on the subject of the article.
Source: habr.com