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Posted on 25 February 2019

Power Electronics for Medium-Voltage Applications

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While the development and production of power electronics systems in the industrial voltage range between 400Veff and 660Veff are regarded as state of the art, at voltage levels above problems occur which are not expected in this form. They begin with a voltage level of 1000Veff and aggravate of course when voltage levels between 3000Veff and several 10,000V are intended to be reached.

By Werner Bresch, GvA

In this context it is rather strange that it is less the power semiconductors used which presents the problems, but rather the lack of knowledge and understanding of the general behaviour of power semiconductors in medium-voltage applications. It is also the lack of availability of suitable auxiliary materials such as clamping devices, coolers, insulation materials, auxiliaries such as triggers, auxiliary voltage supplies for drivers or controllers as well as sensors and the lack of understanding and knowledge of how to transfer them to a suitable and sensible mechanical set-up. The worst thing is that the abovementioned materials and auxiliaries are not simply available to purchase at the nearest specialist electronics store. They simply do not exist!

Insulation coordination in medium-voltage applications

Irrespective of the circuit topology used, for example 3-level, multilevel, cascaded systems or direct series connection, special demands must be placed on the insulation in medium-voltage applications. Air gaps and creepage paths with the corresponding levels of contamination as well as air humidity and perhaps condensation have to be prevented. As the voltage values rise, the creepage resistance of the insulating materials used becomes of ever greater importance. Frequently, additional requirements are placed on these insulating materials with regard to compressive creep strength, tensile strength, shear strength, for example for clamping devices, and resistance to oil when they are used under oil etc. The difficulty often resides in combining the required properties.

Another aspect is the dielectric strength, which is determined essentially through the choice of creepage paths and air gaps, the insulating materials used (CTI value) and not least by the geometric shape of the current carrying metallic parts. Before the disruptive discharge over an air gap to the chassis on earth potential occurs, there is a partial discharge. This takes place on sharp-edged live parts such as heat sinks or clamping devices on account of a local field rise. This external partial discharge forms on the surface of the mechanical structure of the power electronics and spreads out into the surrounding air space. In darkened rooms, this is easy to identify by way of a weak bluish glow. Another indication that a partial discharge is taking place is an ozone odour which is typical of it. Although the bluish glow may look visually beautiful, it causes lasting and irreversible damage to the insulation of the system. Another typical characteristic of a partial discharge is that there is a voltage value where the partial discharge begins (inception voltage). If the voltage is reduced significantly below this value, the partial discharge will stop or expire (extinction voltage). If the voltage rises significantly above the value of the inception voltage, a disruptive discharge of the voltage to earth will occur.

The insulation voltage specified in the data sheet as test verification and as an aid in deciding whether the power electronics component or power electronics module may be used for the specified operating voltage is not very helpful. The specified insulation voltage value merely tells us that the component has been checked for this voltage and there was no disconnection failure shut down of the test equipment. It does not state that there was no partial discharge during the insulation voltage measurement and therefore no pre-damage to the insulation path occurred. The following insulation voltage measurements may therefore be repeated only with a reduced test voltage. It makes much more sense to specify a partial discharge voltage and at this point to specify the partial discharge extinction voltage. The partial discharge voltage, i.e. the voltage level at which the partial discharge begins, is higher and will not be reached if the test voltage is not increased above the level of the partial discharge extinction voltage. The ageing of the insulation path is therefore avoided. What has been described above applies very generally and relates essentially to auxiliary materials and auxiliaries such as heat sinks, clamping devices, mechanical fixtures, trigger modules, auxiliary voltage supplies, wiring networks, sensors, coolants (air, water, oil) etc.

Power semiconductors in medium-voltage applications

Generally, all known types of power semiconductors can be used in medium-voltage applications. These are predominantly components such as IGBTs in 3-level or multi-level circuit topologies, cascaded systems, but also in direct series connection. However, in many cases IGCTs, GTOs (transparent emitters), thyristors and also diodes (avalanche diodes) are used in the circuit topologies mentioned above.

However, power semiconductors of whatever type used in a mediumvoltage application - that is a whole different world!

IGBTs in medium-voltage applications

Appreciated and well-known properties of IGBTs can no longer be utilised, but other properties of bipolar power semiconductors which have almost been forgotten suddenly become very interesting. For example, the short-circuit recording through desaturation, which is easy to implement, and independent switch-off by the IGBT driver can no longer be utilised in medium-voltage applications. The short circuit would have to be recorded externally and reported to the CPU, and the IGBTs would have to be switched off selectively in the correct sequence within 10us. A real challenge. What is possible, however, is crowbar protection, as is usually implemented when using bipolar power semiconductors. If IGBT modules are used, the insulation voltage may no longer be sufficient. The coolers then need to be insulated. In the event of a fault, IGBT modules present problems because plasma discharge may be expected. Plasma discharge in mediumvoltage applications is an unpleasant thing. The approach to solving this problem is the use of pressure-contact IGBT housing technologies. In conjunction with the wider air gaps and longer creepage paths required for medium voltage, this leads to longer commutation paths affected by leakage inductance, which entails a reduction in the switching speed and thus a reduction in the clock frequencies on account of increased switching losses. We are then no longer talking about a clock frequency of a few kHz, we are talking at best about a few hundred Hz. The short-circuit protection philosophy needs to be completely reconsidered, as does the safety concept in relation to redundancy in the event that a component fails (is a component short-circuited or is it no longer conductive). Increased effort is also required in respect of the triggering, monitoring, status feedback, particularly if a direct series circuit is implemented with IGBTs. In this case, it is also sensible to make dedicated selections.

IGCTs, GTOs, thyristors, diodes in medium-voltage applications

The use of bipolar components is often equated to using an obsolete technology. However, in medium-voltage applications these components have a charm all of their own. The switching behaviour of these components is controlled by di/dt reactors and therefore defined very precisely. If at all, leakage inductances play only a secondary role. Air gaps and creepage paths are therefore easier to set. The achievable clock frequencies (IGCT, GTO) are roughly identical to those of the IGBTs (of the same voltage class). The coolers must be constructed so as to be isolated as they are directly loaded with potential. Usually, components are used in hermetically sealed, pressure-contact housings (disc cells). Plasma discharge is unlikely because in the event of a fault the component goes into a short circuit (conductive). This means that safe further operation in the event of a fault is exclusively an issue of redundancy (single, dual, multiple redundancy). The components have a very high surge current capacity in comparison with IGBTs over a relatively long period of time of 10ms (factor of 1000 in comparison with IGBTs). In the case of turn-off bipolar power semiconductors, this leaves plenty of time to safely record an overcurrent externally and to turn it off within the turn-off limit current. There remains the option of an additional crowbar triggering in order to prompt upstream overcurrent protective devices to turn off the system. Although this method is primitive, it is effective. Here too, increased effort is required in respect of triggering, monitoring, status feedback, particularly if a direct series circuit is implemented with IGCTs, GTOs, thyristors and diodes. In this case, it is also sensible to make dedicated selections.

Wiring

If power semiconductors are connected directly in series, particular attention must be paid to ensuring an even static and dynamic voltage sharing. A static voltage sharing is forced by a voltage divider resistor in parallel with each component which is connected in series. The dynamic voltage sharing can be achieved by means of an RC element in parallel with each component. In the case of IGBTs increased effort in relation to triggering, monitoring, status feedback and communication within the series circuit is necessary. In addition, overvoltage limiters may also be employed. It is definitely sensible to select and pair the components in view of those parameters which are relevant to series circuits. This may reduce the costs for protective circuits significantly. From a voltage level of 20kV and up, parasitic leakage capacitances to ground may cause blocking voltage unbalance. The size of the parasitic leakage capacitance and therefore of the voltage unbalance depends on the zero voltage capacitance of the power semiconductor, the distance between the live parts and the chassis earth as well as the dielectric between them (air, oil). An additional suppressor capacitor in close proximity parallel with the power semiconductor solves this problem.

Mechanical configuration

Gradually a multistage complex mechanical structure develops consisting of a heat sink, power semiconductor, clamping units if necessary, protective circuit, driver circuits, sensors, communication and feedback electronics and feeder units for each of these stages. As has already been outlined, the procurement of parts for a single stage operating on the industrial network 400Veff/660Veff would not be difficult. The individual components are designed for this. If they are connected in series in order to move on to the medium-voltage range, it is apparent that the stages constructed in this way are not suitable and cannot simply be upscaled to use at a higher voltage level. Neither the insulation strength nor the form of the individual components within the stage are suitable for this purpose. Attention must therefore be given beforehand to assessing the suitability of the materials employed for use at a higher voltage level. For example, sharp edges should be avoided on live metallic parts and, if necessary, a homogeneous distribution of the electric field should be ensured through the use of field shielding rings around each stage of the series circuit.

Components for medium-voltage applications

For medium-voltage applications, GvA Leistungselektronik GmbH has developed a series of „ready-to-use“ components which can be used in a scaleable fashion in the form of a modular system.

Inductive power supply system (IPSS)

The inductive power supply system (IPSS) is particularly suitable for supplying power to trigger modules such as thyristors or IGBTs in medium-voltage applications, regardless of whether they are in 3-level or multi-level configuration, in cascaded systems or direct series connection. The supply system can also be used, for example, to supply power to measurement and sensor systems, auxiliary electronics etc. at a high voltage level. The IPSS consists of 3 individual function blocks, the basic unit (BU), the current loop (CL) and the decoupling unit (DCU).

The basic unit (BU)

The BU is the primary-side part related to the chassis earth of the IPSS and is supplied with a DC voltage between 24Vdc and 110Vdc. The large input voltage range of the BU enables supply both from a PLC and by means of other standardised DC voltage sources (48Vdc, 60Vdc, 108Vdc). The required voltage level depends directly on the number of decoupling units that need to be supplied. The BU generates an impressed trapezoidal alternating current with a frequency of 21kHz at the output. The amplitude of the alternating current on the output side and its frequency is independent of the level of the input voltage.

The current loop (CL)

The BU supplies a secondary alternating current of 10Aeff as standard. The wire cross-section must be dimensioned in accordance with this. Since in medium-voltage applications the decoupling units are at high potential, particular attention must be paid to the insulation strength of the current loop CL. The insulation strength and the partial discharge strength are determined mainly by the quality of the current loop insulation. The maximum length of the current loop is 6m as standard and is therefore sufficient for most applications. Larger current loop lengths are possible on request.

Inductive Power Supply System (IPSS) with BU, CL and DCU

The decoupling unit (DCU)

The decoupling unit uses a toroidal transformer through which the conductor of the current loop is guided. There are various versions of the toroidal transformer with a different number of windings for different secondary output voltages. As standard, the decoupling unit is available with 12Vdc, 15Vdc and 24Vdc. The output voltage is controlled and is independent of the level of the output current or loading. The maximum output current of a decoupling unit is 1000mA in the standard version.

Block Diagram of IPSS

System properties of the IPSS

The IPSS inception voltage between the input of a BU and the output of a decoupling unit may be up to 12kVeff. The same applies to the voltage difference between the outputs of two decoupling units. The insulation voltage is up to 17kVeff (partial discharge inception voltage) or 13kVeff (partial discharge extinction voltage). The insulation test voltage is 24kVeff for 1 minute. Higher insulation voltage values are available on request.

Thyristor trigger unit (TTU) for the electrically isolated triggering and monitoring of high-power thyristors

The trigger unit has been developed specifically to cater for the needs of high-power thyristors used in medium-voltage applications and allows the safe electrically isolated triggering and monitoring of these elements. The IPSS, which has already been outlined, may be used as the power supply unit. Its insulation coordinates will then be relevant for the thyristor series circuit to be created. The trigger module generates a gate trigger current at the thyristor with a gate trigger current rise of up to 2.5A with a subsequent back porch trigger current of 500mA. Therefore the trigger unit is suitable both for the reliable triggering of thyristors with high di/dt loads, as occur for example in pulser or crowbar switches, and for applications with a large inductive load. Furthermore, the trigger unit has an integrated emergency trigger function and protects the thyristor from overhead ignition when the permitted operating voltage is exceeded. In addition, the trigger unit permits the recording of the anode voltage and the cooler temperature. This offers lots of different options for monitoring, detecting a faulty power semiconductor element and voltage zero-crossings and, last but not least, fault states in cooling or unpermitted overload situations. The fault signals are transmitted via fibre-optic cables in a potential-free way.

Complete power stages for medium-voltage applications

The power stages for vast variety of circuit topologies listed below are „ready-to-use“ power electronics units. However, they are not finished devices in the sense of a device with a controller and the corresponding software. This is the responsibility of our customers and in many cases it is their core area of expertise. Therefore, GvA Leistungselektronik GmbH does not appear as a competitor in the sense of finished devices in any case. We manufacture and supply power electronics modules.

Medium-voltage thyristor pulser bidirectional; W1C 12kV / 32kA

This pulser switch consists of a discrete mechanical unit consisting of 8 thyristors connected anti-parallel in series. It is equipped with the trigger module and the IPSS auxiliary voltage supply as well as a protective network for static and dynamic voltage sharing. The highest permitted connection voltage is 12kVeff, and the highest permitted pulse current is up to 32kA with a pulse duration of up to 100ms. The switch is air-cooled (self-ventilated) and therefore one of the weakest versions in terms of continuous current-carrying capacity. The mechanical design is such that much higher current carrying capacities can be achieved by using forced ventilation or liquid cooling and by using larger disc-type thyristors. The status monitoring and error feedback signals are transmitted via fibre-optic cables in a potential-free way, as described above under trigger module.

Thyristor trigger unit (TTU)

Block diagram of TTU

Medium-voltage thyristor switch W1C 24 kV / 600A

This single-phase alternating current switch consists of a discrete unit comprising 18 double thyristor modules connected in series. It is equipped with the trigger module and the IPSS auxiliary voltage supply, as well as a protective network for static and dynamic voltage sharing. The highest permitted connection voltage is 24kVeff, the highest permitted current is up to 600Aeff. The switch is liquidcooled. The mechanical design is such that larger thyristor modules may be used and thus much higher current carrying capacities can be achieved. The status monitoring and error feedback signals are transmitted via fibre-optic cables in a potential-free way, as described above under trigger module.

Medium-voltage IGBT DC switch E1C 16kV /10A

This 16kVdc IGBT switch was already described in detail in Bodos Power Systems edition ZKZ 64717 12-12 issue December 2012. In order to avoid any unnecessary repetition, we refer interested readers to this edition of Bodos Power Systems.

Universal 3-level IGBT inverter stack IO6 C 8060 W 073

This IGBT 3-level inverter power unit is used universally, for example as a drive inverter or as a medium-voltage active filter. It is fitted with 1200A / 3300V IGBTs, IGBT drivers with status sensors, DC filter, current and voltage sensors and is equipped for liquid cooling. The maximum output voltage is 2kVeff, the maximum output power is 2MVA.

Thyristor Pulser Bidirectional W1C 12kV / 32kA

Block diagram of medium voltage switch W1C 24kV / 600A

……..and other solutions for medium-voltage applications

Over the past 20 years during which GvA Leistungselektronik GmbH has existed, many power electronics units for medium-voltage applications have been developed and commissioned for in some cases very exotic applications. In order to keep the scope of this publication to a sensible level, they are only outlined in brief below:

  • Uncontrolled rectifiers 20kVdc / 40kVdc to 100kVdc, 40Adc.
  • Three-phase contactless IGCT medium-voltage switch, 12kveff, 1600Aeff, water-cooled.
  • Various IGCT 3-level inverters with different power semiconductor equipment up to 4.2kVeff output voltage, 3MVA output power, air and water-cooled.
  • IGCT traction chopper 4kVdc, 600Adc, 1000Hz, oil-cooled, two IGCTs connected directly in series.
  • IGCT 3-level inverter phase branches output voltage 12kVeff, output current 600Aeff, four IGCTs connected directly in series per switching function (16 x IGCTs per 3-level inverter phase branch)
  • Thyristor, IGCT, GCT pulser switch in various voltage and current classes.

IGBT DC Switch, E1C 16kV /10A with IPSS, IGBT driver, senses, diagnostic, communication interface and potential free feedback.

Universal IGBT 3-Level Inverter Stack

IGCT 3-Level-Inverter-Stack

Don’t be afraid of power electronics in the medium-voltage ranges

In the context of the applications outlined above and the diversity in the use of a wide range of different power semiconductors and circuit topologies, GvA Leistungselektronik GmbH boasts a broad wealth of experience of using power semiconductors and power electronics output stages in the medium-voltage range. Over the years, the IPSS, trigger modules and IGBT controllers described above have emerged as core components, which are now offered as individual components or in combination with power semiconductors as power electronic stacks within the field of power electronics. Our customers receive system components, which have been tried and tested many times, and they can benefit from this product`s because the time to market for a new development in power electronics in the mediumvoltage range is shortened significantly.

Contact: info@gva-leistungselektronik.de

 

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