Project 'DC Industry' - Part 3
Protection and switching technology in the DC grid
DC networks instead of the conventional AC networks in the factory - this is the goal behind the DC Industry project. The third part of the series of articles on 'DC industry' deals with the structure and function of the circuit breakers required for this.
A good 20 companies and research institutes are working together in the DC Industry project to develop an open DC concept for industrial production facilities. This industrial DC grid is connected to the AC grid via a central, bidirectional and regenerative rectifier. The nominal voltage on the DC side is 650 V for regulated rectifiers (active front end) and 540 V for unregulated rectifiers. Applications and devices on the DC grid that form a logical unit are each assigned to a load zone - for example a group of drives, feeders, storage units or energy generators. Each load zone is connected to the DC grid via a so-called DC branch, which combines the functions of charging, monitoring, communication, fault protection and disconnection.
Figure 1: Overview of the industrial DC grid with load zones (gray) and the DC feeders (yellow).
© DC industryTo date, the application partners Daimler, Homag and KHS have already set up four model applications in which the concept is being validated and tested. Around 50 DC feeders in four performance classes (44/100/200/400 A rated current) are required to protect these systems. In addition to the classic functions of circuit-breakers - i.e. conducting current, measuring current, switching and isolating - these feeders have other tasks to perform, especially in the model systems implemented:
- Measuring the voltage between DC plus and DC minus at the input and output terminals
- Switching off at voltages outside the defined voltage band
- Indication of the device status and some measured values on the display
- Pre-charging of the capacities in the load zones when first switched on
- Communication of the measured values and the status of the feeder
Current measurement and protection must function bidirectionally - this is necessary due to the distributed storage in the load zones, which also discharge their energy back into the fault location in the event of a fault in another load zone or on the DC bus. Due to the large number of capacitors, the fault current in the DC grid increases very quickly in the event of a short circuit - possibly to several 10 A/µs. Fast detection and disconnection of the short-circuit current is therefore essential to protect the system and the semiconductors in the rectifiers and inverters. Purely mechanical state-of-the-art circuit-breakers with a switch-off time of several milliseconds are not suitable in such DC grids, as they cannot prevent other load zones from being affected in the event of a fault. For this reason, hybrid switches and semiconductor switches are used in the DC industry. Both solutions switch off the short-circuit current so quickly that thermal effects on the supply lines to the load zone or on the fault location do not play a role in the dimensioning of the circuit breakers.
Structure and function of the circuit breakers
Semiconductor switches are equipped with power semiconductors that can be switched off, for example IGBTs. Figure 2 shows the basic structure with bidirectional semiconductor modules and a varistor (overvoltage arrester) connected in parallel in the protected pole. The shunt is used for current measurement. Isolation relays in both poles ensure isolation when switched off - they are de-energized by the varistor after the current is interrupted by the semiconductor modules and the energy in the system is dissipated. The advantage of semiconductor switches is that they only allow a very small amount of energy (joules of heat) to pass into the system in the event of a fault - in DC industry applications with a rated voltage of 650 V(DC), this is typically less than 100 A²s in the event of a short circuit. The short current flow time of less than 100 µs is essential for this.
In hybrid switches, a mechanical contact is connected in parallel to the semiconductor modules, which carries the rated current and opens extremely quickly in the event of a fault - typically in less than 0.5 ms. Figure 3 shows the design of the hybrid switch: a coil is arranged in series to limit the rapid current rise in the event of a short circuit. This coil reduces the current rise and thus gives the mechanical contact time to open without being overloaded.
Preloading, communication and connectivity
The DC feeder therefore decouples a load zone from the next higher level - for example from the main DC grid - and optionally includes a mechanical disconnection point so that the load zone can be safely disconnected from the main DC grid for maintenance work, for example. Conceptually, it is not specified on which connection side of the DC feeder the mechanical disconnection point is located. As generators or storage units may also be located within a load zone, the generally applicable safety rules for working on electrical systems must be observed. For maintenance/repair work, a disconnector in accordance with IEC 60947-3 is required in the DC feeder of each load zone. It guarantees galvanic isolation and must have two poles.
The DC grid with its load zones is pre-charged in several levels as shown in Figure 4. First, the 1st level - the feed-in devices and, if applicable, the storage system - is pre-charged and thus the unloaded DC grid. All load zones are separated. The 2nd level with the load zones is pre-charged (DC bus / 1st hierarchy level) with a time delay and triggered by the voltage rise of the DC grid voltage with a waiting time. If further load zones are connected below a load zone, their precharging (3rd level) is again staggered. The staggered precharging should take a maximum of ten seconds per level.
The DC feeders in Figure 4 are shown in simplified form in Figure 5 and optionally contain a two-pole disconnector - if necessary for maintenance purposes, for example - which can also be designed as a manual switch. In the following considerations, it is always regarded as already closed, unless otherwise stated. The pre-charging switch SV is closed to pre-charge the downstream DC system or the assigned load zones. This starts precharging via the resistor RV. As soon as pre-charging is complete, the pre-charging resistor is bypassed via the electronic switch SE and SV is then opened. Alternatively, SE can be designed as a hybrid switch as described above. Different implementation variants are possible for precharging in the DC branch (see Fig. 5):
- Precharging via the resistor RV. The resistor RV and the DC contactor SV are assigned to the switching and protection device and are controlled by it. Alternatively, precharging can be carried out via a buck converter. The advantage of this is that fewer losses occur and even very large capacitances can be charged with limited current. This can be realized either
- via an additional IGBT SV and the choke L V or
- using the existing IGBT SE1 and a combination of choke L V with bypass switch SÜ.
The precharging of the DC system can always be carried out by a higher-level control system. In this case, the controller determines a sensible switch-on sequence for the switches and the individual pre-charging and bypass switches of the load zones are controlled directly. However, it is to be expected that there will also be DC grids without superimposed control. For this reason, a self-sufficient pre-charging concept is described below, in which the logic is stored in the DC feeder and which can also run without control communication with the grid management. Such a solution represents the minimum functionality and can be extended or replaced by the grid management system. The internal parameters and status variables in the DC branch are communicated via Ethernet in order to make automation-related decisions.
The preloading concept is only explained here using variant a). However, variants b) and c) can also be used in the load zones. Instead of simply closing SV, a current is set in LV in cycles. As an example, in an unloaded load zone after a maintenance operation, connection A of the DC feeder is connected to the DC grid and connection B to the load zone as shown in Figure 6. The load zone is initially discharged and the loads are not in operation. During the charging process, all capacities of the components are charged simultaneously.
The DC feeder waits for a voltage rise in the DC grid (UA). If this voltage UA exceeds the value U1, the pre-charging process is started after T1 with S V = 1. If the voltage difference UA-UB falls below ≤ ΔU within T2, the switch SE closes after the time T2 has elapsed. If the voltage does not fall below ΔU or if an excess temperature of R V occurs, the pre-charging process is aborted (SV = 0) and an error is output. If pre-charging is successful, switch SV opens after time T3.
Interchangeability of the connection technology a must
For future connectivity, the interchangeability of the connection technology used for the DC industry in particular must be fulfilled (definition according to IEC in IEV 581-24-03). This means that the shape, dimensions, function and technical properties of the connection technology on the drives, switchgear, supply devices and infrastructure components of a power and installation class (hybrid or pure DC power) from different sources must be identical. In other words, it must be possible to replace a connector from manufacturer A - regardless of whether it is freely configurable, with pre-assembled cables or integrated in the device - with a connector from manufacturer B while retaining the functionality of the overall connection in all its technical properties (electrical, mechanical and climatic).
With regard to the infrastructure connectors for hybrid cables in particular, the project partners are endeavoring to keep the number of variants to a minimum by means of a scalable or modular concept. The infrastructure connectors are used where energy (main and auxiliary power), Ethernet-based data and signals (such as STO - Safe Torque Off) are distributed outside the control cabinet or on the control cabinet wall. Examples include wall bushings, outlet (distribution box), wallbox (charging station) and FieldBox.
In addition to defining general connection requirements, unauthorized disconnection of plugs and sockets must be prevented for the DC industry. In the case of simple unlocking (level 1), unintentional disconnection of the connectors is prevented. The extended unlocking enables deliberate unlocking by hand (level 2) or with a standard tool (level 3). Finally, level 4 provides for a further extension of the unlocking process, for example using a key or a special tool; or, in the case of automatic locking, the opening of the connector is authorized by a defined state.
In addition to the DC feeders presented in this article, a large number of classic switching devices for DC applications such as contactors or load-break switches are already available on the market today. This means that industrial production systems based on a DC network can be implemented in the near future.
Authors:
Dr. Hartwig Stammberger is responsible for technology and simulation of power distribution components at Eaton Industries;
Wolfgang Veit is responsible for the Industry/Mechanical Engineering & Switchgear Construction segment support at ABB Stotz-Kontakt.
Olaf Grünberg is Technology Developer Electronics and DC Industry Coordinator at Weidmüller Interface.
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