Modeling and simulation of static compensator based on DQ model

Dissertation for Master's Degree in Electrical Engineering

Power Orientation

Abstract

Flexible power transmission systems known as FACTS[1] compensating devices are a modern tool to enhance control Adaptability and development of transmission capacity of power networks based on electronic power converters have been used in power systems during the last decade. In fact, FACTS systems are able to control the parameters and characteristics of transmission lines, such as series impedance, parallel impedance, phase angle, which act as the main limitation on the way to increase network capacity. The basic idea behind the concept of FACTS is to enable the transmission system by activating its elements and components. In fact, FACTS has a fundamental role in increasing the flexibility of power transmission and the security of the dynamic stability of power systems. In this thesis, the control structure of STATCOM (Static Reactive Power Compensator) is investigated. First, the STATCOM model is simulated based on the dq model. Then, with the appropriate fuzzy controller design, the reactive power management problem in the power network and the analysis of the STATCOM behavior during the fault are presented. The simulation results show that the proposed control method can control the reactive power in power networks under different operating conditions. gives Attention to voltage sags is primarily related to issues that affect several types of components: variable speed drives, process control components, and computers, which are problematic due to their sensitivity. Exciting capacitors and switching electronic loads also lead to short-term overcurrent. But because the length of overcurrent is so short, it does not lead to an effective voltage reduction.  These factors are not referred to as voltage drop and Lee classifies it as voltage cut or voltage transient. Voltage drop caused by short circuit and ground fault are the most important factors of equipment problems. In this chapter, the problem of voltage errors in distribution networks will be investigated first, and then considering the sensitivity of power electronic converters to this error, the control methods of power electronic converters will be investigated. Figure (1-1): An example of voltage sags and imbalances in the distribution network. It is from time. Effective voltage over a time window which is basically a time cycle and repeats itself for one or more times each cycle. According to figure (1-1), the voltage drop is the result of a ground-to-ground fault in an underground cable. The undervoltage as a result of the error leads to a decrease in the power transfer from the generator to the engine, the result of which is a decrease in the speed of the engine and an increase in the speed of the generator. These phenomena lead to limiting the error recovery time in transmission systems as well as the rules for connecting wind farms to the grid. Although protection methods have been suitable for consumers for many years, recently many issues and problems have arisen in relation to customers due to voltage dips. Basically, the necessary limits for stability in the distribution system are 70% voltage during one second. Many power electronic devices such as computers, process controllers and adjustable speed drives have a voltage drop of 85% for 45 seconds.

1-2 Characterization and indicators related to voltage drop

In order to describe the events related to voltage drop, processing should be done on the sampled voltage waveforms. In the IEC-61000-4-30 standard, two important characteristics, voltage and duration are defined as evaluating the quality of voltage dip. Both of these parameters are obtained from the effective voltage as a function of time.The importance of discussing the indicators of voltage drop and considering information related to measurement (power quality monitoring) and simulation. Measurement is a good way to evaluate the performance of a situation or a system. But measurements have limitations in predicting for year to year or situation to situation. A large number of monitoring devices for a long period of time are required to predict voltage sag performance. Random prediction methods are very suitable for performance prediction, for example, it is suitable for comparing different improvement methods. Usually, there are methods to reduce the effectiveness of equipment from voltage drops, some of which are listed below:

Reducing the number of errors

Speeding up the time to fix the error

Improving network design and performance

Using connection improvement tools: a very common method, connection A UPS or a constant voltage transformer is between the system and sensitive loads. For large loads, static compensators based on three-phase inverters are possible as a solution.

Improvement of components on the customer side: making equipment safe against all voltage drops is a solution to solve the problem, but it is not feasible for most equipment.

According to the references provided, there are ways to improve voltage quality. caused by outages and voltage drops are divided as follows:

reducing the number of short-circuit errors

reducing the time to fix the error

changing the system so that short-circuit errors lead to less severe events in the terminals of components or in communication with customers.

Improving the connection of components between sensitive devices and power supply

Improving equipment security

1-3 Reducing the number of short-circuit errors and its relationship with voltage imbalance

Reducing the number of short-circuit errors in a system not only reduces the frequency of collapse, but also reduces the frequency of imposed outages. Therefore, this method is an effective factor for improving the quality of nutrition, and many consumers consider it as an effective solution when voltage drops occur. Since the short circuit causes damage to the operating equipment and the network, therefore, many efforts have been made to reduce the frequency of the error as far as it is economically viable. Some of the methods used so far for this purpose are as follows:

Replacing overhead lines with underground cables

Using protective wires for overhead lines

Installing more protective wires to prevent errors during lightning

Increasing insulation levels

Increasing more inspection and maintenance periods

Decreasing the time to fix the error

Decreasing the time to fix the error does not reduce the number of events, but only reduces their intensity. Therefore, it does not affect the duration of outages and their number. By clearing the fault faster, it also does not affect the number of voltage dips, but it can reduce the duration of the dip. Maximum reduction in fault clearing time is achieved by using current limiting fuses. These fuses are able to clear the fault in half a cycle, so the voltage drop period rarely exceeds one cycle. In time transmission systems, fault elimination is often characterized by transient stability limitations.

1-4 The effect of changes in the power system on voltage imbalance

By making changes in the power system, the intensity of events can be reduced. Here the cost can be very high. Especially for transmission and sub-transmission voltage levels. The most important way to improve against outages is to install additional backup devices.