Grammar: The Future Simple Tense

Word List:

1. fuel cell	топливный элемент, топливная батарея
2. photovoltaics	фотоэлектричество, фотоэлектрическая энергетика
3. renewable energy sources	возобновляемые источники энергии
4. static var compensation	компенсация статической реактивной мощности
5. a modular, scaleable power electronics technology	технология создания модульных блоков силовой электроники
6. real and reactive power flow	активная и реактивная составляющие мощности
7. medium voltage adjustable speed motor drives	запуск моторов с регулировкой скорости при среднем напряжении
8. multilevel converter	многоуровневый конвертор (преобразователь)
9. a high voltage dc back-to-back intertie	непосредственное соединение двух высоковольтных конверторов постоянного тока друг за другом
10. harmonic filtering	фильтрация гармоник
11. fast response	быстродействие
12. dynamic voltage restoration	восстановление (стабилизация) действующего напряжения
13. utility interface	промежуточное звено

Multilevel Converters as a Utility Interface for Renewable
Energy Systems

Electric power production in the 21^st Century will see dramatic changes in both the physical infrastructure and the control and information infrastructure. A shift will take place from a relatively few large, concentrated generation centers and the transmission of electricity over mostly a high voltage ac grid to a more diverse and dispersed generation infrastructure that also has a higher percentage of dc transmission lines.

Some of the distributed generation power sources that are expected to increase greatly their market share of the total power produced in the US and abroad include renewable energy sources such as photovoltaics, wind, etc. Fuel cell technology is also nearing the development point where it could start to supply a significant share of the power needs. The advent of high power electronic modules has also encouraged the use of more dc transmission and made the prospects of interfacing dc power sources such as fuel cells and photovoltaics more easily attainable. A modular, scalable power electronics technology that is ideal for these types of utility applications is the transformerless multilevel converter.

The use of a multilevel converter to control the frequency, voltage output (including phase angle), and real and reactive power flow at a dc/ac interface provides significant opportunities in the control of distributed power system.

Additional applications of multilevel converters include such uses as medium voltage adjustable speed motor drives, static var compensation, dynamic voltage restoration, harmonic filtering, or for a high voltage dc back-to-back intertie.

Because distributed power sources are expected to become increasingly prevalent in the near future, the use of a multilevel converter to control the frequency and voltage output (including phase angle) from renewable energy sources will provide significant advantages because of its fast response and autonomous control. Additionally, multilevel converters can also control the real and reactive power flow from a utility connected renewable energy source. These power electronic topologies are attractive for continuous control of system dynamic behavior and to reduce power quality problems such as voltage harmonics, voltage imbalance, or sags.

Focused Practice

I. Answer the following questions:

1. What will electric power production in the 21^st Century see?

2. Where will a shift take place?

3. What do some of the distributed generation power sources include?

4. Why will the use of a multilevel converter to control the frequency and voltage output from renewable energy sources provide significant advantages?

5. Are there any additional applications of multilevel converters?

6. What uses do they include?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Multilevel Converters.

Unit 63

Grammar: The Passive Voice

Word List:

1. power rating	номинальная мощность
2. sampled	дискретный
3. PWM-pulse width modulation	широтно-импульсная модуляция
4. a multiple	сборка
5. a voltage-source inverter	преобразователь как источник напряжения
6. a current-source inverter	преобразователь как источник тока
7. diode-clamped inverter	диодно-связанный инвертор
8. flying capacitor inverter	инвертор с дополнительными конденсаторами
9. staggering	разброс (параметров)
10. ripple	пульсации
11. bridge converter	мостовой преобразователь

Bandwidth Considerations for Multilevel Converters

A multilevel converter has a multiple of the usual six switches found in a three-phase inverter. The main motivation for such converters is that voltage [in a voltage-source inverter (VSI) and current in a current-source inverter (CSI)] is shared among these multiple switches, allowing a higher converter power rating than the individual switch volt-ampere (VA) rating would otherwise allow. This sharing is achieved by summing the outputs of several two-level converters with transformers or inductors, or direct series connection, or by more complex topologies such as the diode-clamped inverter and the flying capacitor inverter.

Another secondary, but very important advantage is the extra degrees of switching freedom that the multiple switches permit. Each switch still has the same limited switching frequency, but by staggering the switching instants of the individual switches, the overall switching frequency of the multilevel converter effectively becomes a multiple of that of the individual switches. A further gain comes since we switch between multiple voltage levels at this higher frequency rather than two, so the switching harmonics appear at a higher frequency and a lower level. Overall converter input and output ripple is much reduced, and less filtering is required. This is an important advantage in a high-power converter where the switching frequency is low and filtering is expensive.

Many different approaches to multilevel control have been published. The diode-clamped topology imposes restrictions on allowed switch states and requires further control to maintain the auxiliary capacitors at their correct voltages. As a consequence, the control and modulation of these converters are treated as one whole problem.

Naturally sampled synchronous pulsewidth modulation (SPWM) and uniformly sampled SPWM are easily implemented, produce good results for moderate switch frequencies, and are easily extended to multibridge converters.

In naturally sampled PWM, the input waveform is naturally sampled by the carrier wave. The natural PWM modulator is quite simply implemented in analog hardware. In uniformly sampled PWM, the input signal is regularly sampled at the beginning of each switch cycle before being compared with the triangle waveform. This approach is easily implemented with a microcontroller.

Focused Practice

I. Answer the following questions:

1. How many switches does a multilevel converter have?

2. What is the main motivation for such converters?

3. How is the sharing achieved?

4. What does the overall switching frequency become?

5. How does it come that it becomes like that?

6. Where do the switching harmonics appear?

7. Have there been any approaches to multilevel control published?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Motivations for multilevel converters.

Unit 64