The development of distributed energy, based on a high number of renewable sources, causes the fact that more and more power converters are connected to the power grid. The converters are interfaces between a parametrically unstable renewable source and the stable power grid .
During a converters’ work in such configurations there always appears a transient fault or a short time overload. These disruptive effects influence the distribution network, especially when distributed energy of great power is connected to the grid. One example of this state is the power electronics converter in phase failure work.
It can be a big source of disturbances and a danger for sensitive devices which work near the converter. An effective diagnosis, rapid security, reliability and software with the ability to recognise disturbances in the grid are the main aims for today’s power electronics.
This paper presents a model of a three-level NPC inverter (Neutral Point Clamped) with VOC control. The model was simulated in Matlab/Simulink environment. The research was made for the power electronics converters in phase failure work. The results of the research are emergency scenarios of switching on high-power converter branches and integrated IGBT protection, which is controlled by the DSP microprocessor system. The proposed protection is the next stage of the research. The main aim is improving the reliability of the converters and minimising their impact during faulty work on the power grid.
2. Phase failure work and its causes
A converters’ phase failure work is its emergency work. It is mainly due to a ground fault of the distributed network. Ground faults appear several times a year, usually because of weather factors like storms, catastrophic frost on power wires, trees fallen by the storm, etc. The causes of phase failure work can also be a short circuit in a wind power generator, a failure of photovoltaic panels, or human error.
Phase failure work is a danger for power converters. It can cause the next failure, which leads to the destruction of the inverter. To reduce the faults, it is necessary to design an advanced security system, which protects the sensitive semiconductor devices and eliminates phase failure work.
The basic protection of the thyristor power electronic system are fast fuses or short-circuit relays that can work with current transformers . Modern converters are built using fully controlled devices such as transistors IGBT . These devices are faster than thyristors and offer the possibility of a rapid and correct current control.
These features reduce the complexity of the control system and the dimensions of the inverter. On the other hand, the usage of power transistors in the inverter reduces the system’s short-circuit capability. The structure of the transistor with the insulated gate is softer. For this reason, systems of individual or group protection are used to shield the transistors. An example of such a protection system is shown in Fig. 1a.
Fig. 1a) The block scheme of an IGBT protection system, b) the model of an IGBT transistor used in the research
The system’s principle of operation shown in Fig. 1b is based on the voltage measurement on the transistor’s junction between the collector and the emitter (). The voltage suddenly increases during a short-circuit, which is caused by a high current flow through the internal resistance of the transistor . When the voltage exceeds a limit value , the comparator changes the logical state to the opposite value. When the comparator of the logical state is changed, the IGBT transistor turns off and the control system is blocked.
Based on the block scheme in (Fig. 1a) a numerical model of functional protection was developed, described in section 3.
3. The NPC power converter model working with the grid
The simulation model (Fig. 2) was designed in MATLAB/Simulink in order to analyse phase failure work. It is assumed that the converter meets the unusual requirements of work during dynamic states, typically for flexible AC transmission systems (FACTS) .
Fig. 2. A model presenting the organisation of individual sub-assemblies of the examined converter
This property is guaranteed by the control system whose construction is based on the Voltage Oriented Control (VOC). This algorithm comes from the field-oriented control (FOC) method, used in the drive system .
The parameters of the PI controller’s algorithm are chosen by analysis of the mathematical model and by simulation studies, on the basis of Ziegler-Nichols criterion. The model consists of a non-linear PID controller, called “Anti-Wind Up” . Thus the system with minimal overshoot has a faster response than the system with linear PI controllers.
The structure of the Inverter-control block consists of the synchronisation system – a converter with the network, called Phase Locking Loop (PLL). It is responsible for the correct synchronisation of the inverter with the power grid . The PLL algorithm is based on a modified method of synchronously rotating reference frame – Synchronous Reference Frame (SRF) . The algorithm is chosen by the criterion of maintaining synchronism in the disturbance states. Synchronisation is very important for the converter’s work and its correctness has a huge impact on the quality of the research.
Signals generated by the FOC algorithm are applied to the spatial modulator SVM (Space Vector Modulator) . It generates a sequence of PWM signals for the converter, which consists of twelve IGBTs.
When the simulation starts, the circuit breaker is opened and when the inverter is synchronised with the network, it closes automatically. The study was conducted with the idea of taking into account most of the possible scenarios of emergency work. Events are based on deliberate control blocking or parameter modification, such as voltage for individual transistors.
A model power converter is connected to a low voltage power system by a switch. The most important parameters of the model are shown in Tab. 1.
Tab. 1. The parameters of the low voltage power system modelled
The power converter’s model is a simulation program with the copyright simulation model of the IGBT. The IGBT model includes effects such as saturation and the rise and fall time of the current. The voltage drop on the collector-emitter junction (Fig. 1b) is very important because of the short-circuit protection which is applied to the logical protection system, shown in Fig. 3.
Fig. 3. The full protection model of a high power IGBT transistor
The main task of the protection system is to detect a transistor’s saturation and turn off the short circuit. The output signal “error detection” is transmitted to the other logical structures to turn off the other transistors immediately and to block the phase failure work. A protection system against losing the control voltage and active Miller capacitance is also added .
The logical model is the basis of programming the TMS320F28069 DSP processor (from Texas Instruments). In the future, the logical model will include additional elements of short circuit detection:
- a precisely controlled time delay of 2–12 ms to avoid unnecessary activation of protection system while switching the transistor
- a graded system of switching the transistor, designed to provide protection against overvoltage coming from the parasitic inductance of the DC converter’s circuits.
Assumptions for designing the laboratory model are defined as follows:
- protection of the IGBT during transient states of the power grid
- minimising the impact of the inverter’s internal short-circuit on the distribution network
- estimation of the semiconductor temperature and blocking the gate signal until the transistor recovers its short-circuit capacity
- gradual turning on and off of the transistor to reduce EMI disturbances
- blocking an uncontrolled switching of the transistor’s gate caused by Miller capacitance
- improving the reliability of the converter systems.
4. The study of the phenomena of phase failure work
Phase failure work is an emergency status which occurs when the voltage on the damage phase is close to zero. This condition can appear in the following cases:
- open-circuit in one phase of the inverter
- short-circuit of the converter’s branch
- control failure of the IGBT
- phase-to-ground fault near the inverter.
The series of measurements were simulated during periodic faults. Research of the periodic faults during the stable work of the grid was simulated to show whether the converter is able to work properly after faults and what is the role of the control system in this case. The simulation model is shown in Fig. 2. The selected simulation results are presented in the waveforms in Fig. 4–9. A failure occurs in 0.4 s and lasts for 0.1 s in each of the tests.
Fig. 4. Open-circuit of the phase: a) the output current in the inverter, b) the output current in the generator
Fig. 5. Open-circuit of the inverter phase: a) the output current in the inverter, b) the output current in the load
Fig. 6. Short-circuit in the inverter phase: a) the output current of the inverter, b) the output current at the generator terminals
Fig. 7. Short-circuit in the inverter phase: a) Id current of the inverter, b) voltage on the load
Fig. 8. A short-circuit of the low half bridge: a) the output current of the converter, b) the voltage fault impact on the grid
Fig. 9. A short-circuit of the low half bridge: a) Id current of the converter, b) the voltage in the input of the filters
4.1. Open-circuit in one phase of the inverter
The output current wave in inverter and generator, during the open circuit in one phase, is shown in Fig. 4a and 4b. Before the fault, the converter covers 75% of the power load. In 0.4 s of simulation time, the converter reduces the current injected to the network. There are high harmonics in the current wave, which impact on the load’s voltage (Fig. 5b). The chosen protection system does not react to the fault.
4.2. Short-circuit of the converter’s branch
A phase-to-ground fault of the load is simulated near the inverter (Fig. 6). Due to the loss of power in one of the phases, the inverter partly lost synchronisation with the grid. This is indicated by the current rise above the reference value (Fig. 5a and 6a). The inverter has lost controllability and it is a serious threat for the loads in the power grid. The voltage on the load is deformed, and the converter’s work failure results in an unacceptable value of the nominal voltage in the power grid (Fig. 7b). As a result, the VOC control is not coping with the highly unbalanced load occurring close to the inverter. In such cases, it is recommended to turn off the inverter immediately .
It should be mentioned that the simulation of the phase inverter fault is carried out with a locked protection. When the protection is enabled, it turns off the inverter in the first half-period of time after the fault occurs.
4.3. Control failure of the IGBT
The next disturbance is the short-circuit in the low half-bridge. The result is shown in Fig. 8. This kind of interference can be simulated by numerical software. In each of the upper half-bridge switching cycles a through current flows, and its value can run to hundreds of Amperes. The cause of this situation is the large capacity of the low impedance capacitors on the
If the protection system does not turn off the faulty circuit within 50 ms, the half bridge will be damaged or the semiconductors can explode.
The half bridge fault does not make a substantial impact on the power grid. The current feedback detects the energy flow from the inverter to the power grid, like in Fig. 9a, and reduces the duty cycle to a minimum. The deformation of the output voltage wave is huge due to a half-bridge damage (Fig. 10b). The internal short-circuit of the inverter is not dangerous for the network. When the failure occurs, the short-circuit current is limited by the inductance of the coupling choke. Next, the fuse installed on the DC bus should blow or the system (Fig. 1) should turn off the other transistors of the inverter. The speed of the short-circuit protection allows to avoid a through short-circuit.
This paper presents an investigation of phase failure work and its impact on the transient parameters of the three-level grid converter, which integrates renewable sources with the power grid. The research was done for two conditions: first when the cause of the phase failure work comes from the inverter, and second when it comes from the power grid. Particular attention was paid to the amplitude and dynamics of the current, the possibilities of protecting the inverter from damage and returning to stable work after the faults are fixed.
The paper confirms that phase failure work is an emergency mode and it is dangerous for the inverter. On the basis of simulation results it was concluded that protracted operation of the inverter is unacceptable during phase failure work. Even a temporary failure of the inverter causes changes in the current and voltages of high-speed rise and high amplitude. This runs the risk of a permanent damage to the inverter’s components. The inverter does not meet the standards concerning the limits of high harmonics injected into the power grid and in some cases it loses controllability. To prevent incidents like these, the semiconductor devices should be equipped in the protection systems described in the second part of the paper.
Phase failure work due to faults from the power grid is not so dangerous for the components of inverters and allows a return to the stable work of the system after the faults are fixed.
The results of the simulation are the basis of a fast logical system whose main task will be diagnosing the fault and protecting the power transistors. Future study also includes a control system protecting the inverter itself and the integrated the power grid against phase failure work, as well as a control structure that allows to work with a highly unbalanced system load.