1. Lecture on EE 4223 High Voltage DC and Flexible AC Transmission
Md. Alamgir Hossain
Assistant Professor
Dept. of Electrical and Electronic Engineering
Khulna University of Engineering & Technology 1

2. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
The HVDC converter operates as a source of current harmonics on the AC-side and voltage harmonics on the DC-side. Since excessive current harmonic results in voltage distortion, additional losses, overheating, and harmonic interference, it needs to be limited.
The quality of converter power is normally evaluated in terms of the following performance parameters: harmonic factor (HF), total harmonic distortion (THD) and distortion factor (DF).
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

3. Harmonics of HVDC
whereV1 is the RMS value of the fundamental component and Vn is the RMS value of the nth harmonic component.
IEEE standard 519 harmonic limits
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

4. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

5. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

6. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
Characteristic Harmonics
With zero commutation reactance the ideal current waveform for a star/star-connected converter transformer is defined as follows:
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

7. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
Ideal Phase Current Waveforms on Primary Side
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

8. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
The Fourier series for this current waveform for Y-Y is
Similarly, the Fourier series for this current waveform for Δ-Y is
The current in a 12-pulse converter system is
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

9. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

10. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Harmonics of HVDC
Non-Characteristic Harmonics
In HVDC systems, the sources of the non-characteristic AC-side harmonics are:
Unbalanced AC system, and converter transformer impedances are not exactly equal in the three phases.
DC may be modulated from another converter station in the case of a rectifier–inverter link.
The firing angle control systems often gives rise to substantial errors in their implementation.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

11. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
AC Side Filters
The total size of all the branches of a filter is determined by the reactive power requirements of the harmonic source.
Typical specified factors to be considered:
Total voltage distortion
Telephone influence factor
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

12. AC Side Filters
Fig. Equivalent circuit for a harmonic source, filter and AC system
The parallel filter is used frequently since it offers amore economic option as compared to the serial filter, and it can even compensate for the reactive power of the fundamental frequency.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

13. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Shunt Passive Filter
The shunt passive filter has the following problems:
The source impedance, which is not accurately known and varies with the system configuration, strongly influences filtering characteristics of the shunt passive filter. The shunt passive filter acts as a sink to the harmonic current flowing from the source. In the worst case, the shunt passive filter falls in series resonance with the source impedance. At a specific frequency, an anti-resonance or parallel resonance occurs between the source impedance and the shunt passive filter, which is the so-called harmonic amplification.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

14. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Shunt Active Filter
A voltage source converter is used as the shunt active power filter. This is controlled so as to draw or supply a compensating current from or to the utility, such that it cancels current harmonics on the AC side i.e. this active power filter generates the nonlinearities opposite to the load nonlinearities.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

15. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Shunt Active Filter
This is the basic principle of shunt active power filter to eliminate the
current harmonics and to compensate the reactive power.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

16. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
HVDC Converters
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

17. Voltage Source Converter(VSC)
The main advantages of VSC converters, compared with line-commutated converters) (LCCs) are summarized as:
Active and reactive power can be controlled independently
Capable of generating leading or lagging reactive power independently of the active power level
If there is no transmission of active power, both converter stations operate as two independent static synchronous compensators (STATCOMs) to regulate local AC network voltages.
Harmonic filters are at higher frequencies and therefore have low size, losses and costs.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

18. Voltage Source Converter
VSC Operating Principle
The conduction in solid-state switches is unidirectional, anti-parallel diodes are connected across them, to ensure that the bridge voltage has only one polarity, while the current can flow in both directions.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

19. Voltage Source Converter
There are four possible current paths in a single phase two-level converter
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

20. Voltage Source Converter
The main switches can be turned on and off in any desired pattern; however, immediately before one switch is turned on, the opposite switch must be turned off, as their simultaneous conduction would create a short circuit of the DC capacitors.
 In practice…
When the upper switch is turned off, the reactance of the AC circuit maintains the present current, thus the diode in parallel with the lower switch turns on. As a result, the output voltage changes from plus to minus Vd/2, i.e. the polarity reversal has been initiated by turning off the upper switch.
Thus there is no need to turn on the lower switch immediately following the turn-off of the upper one.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

21. Voltage Source Converter
Power Transfer Control
If the VSC is connected to a passive network on the AC side, the power flows from DC to AC. But, for active AC system, the power can be made to flow in either direction.
The active and reactive power exchanges (P and Q) between two active sources are expressed as
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

22. Voltage Source Converter
V2=V1 No current and hence no exchange of reactive power
V2>V1 VSC acts as a capacitor and supplies reactive power to AC system
V2<V1 VSC acts as an inductor and receives reactive power from AC system
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

23. Voltage Source Converter
Active and reactive power limits determined by the maximum allowable valve current and maximum allowable DC voltage respectively on the storage capacitor. For a given AC system voltage, the DC voltage rating is determined by the maximum AC output voltage that the converter must generate to provide the maximum required reactive power.
Converter Components
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

24. Voltage Source Converter
Converter Components
DC Capacitor:
Stabilize the DC voltage. The size of the DC capacitor reduces with increasing switching frequencies.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

25. Voltage Source Converter
Coupling Reactance
The VSC is not directly connected to the AC supply when the latter is a strong system. In such a case, some coupling reactance is needed between the system busbar and the converter terminals that reduce the fault and harmonic current as well as stabilizes the AC current. Besides, a high frequency blocking filter between the converter and the interface transformer is placed for economical operation.
High Voltage IGBT Valve
The valves of are either turned off (i.e. non-conducting) or saturated (i.e. completely turned on). To withstand the very high voltage ratings of HVDC transmission, the converter valves comprise many series-connected IGBTs.
The anti-parallel Diodes
The anti-parallel diodes in the VSC bridge constitute an uncontrolled bridge rectifier and have to be designed to withstand the fault-created stresses.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

26. Voltage Source Converter
Single-Phase Full-Wave Bridge Converter
The dc voltage is converted to ac voltage with the appropriate valve turn-on, turn-off sequence
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

27. Voltage Source Converter
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

28. Voltage Source Converter
If S1 and S2 are turned on simultaneously, the DC voltage Vdc appears across the AC load. While turning on S3 and S4 simultaneously, the negative voltage−Vdc appears across the load. Four diodes clamp the load voltage to within the DC supply voltage rails (−Vdc to Vdc).
Following figure shows the phase-to-zero, phase-to-phase and AC current waveforms assuming Vdc=200V. An inductive load is assumed and therefore AC current is lagging the AC voltage.
The RMS AC voltage is
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

29. Voltage Source Converter
The instantaneous AC voltage can be expresses as a sum of fundamental and odd harmonic voltages by using the Fourier series:
Forn= 1, the fundamental RMS voltage is:
The power-balance equation.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

30. Voltage Source Converter
Three-Phase VSC
Figure: Three legged conventional VSC converter
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

31. Voltage Source Converter
Figure: VSC converter interfaced with delta-star transformer.
The conduction angle of each switch is 180 , which is modulated in such a way that the two switches on the same converter leg do not conduct simultaneously (to prevent a DC short circuit).
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

32. Voltage Source Converter
If the converter operates as a rectifier with unity power factor, only the diodes conduct the current, whereas during inverter operation with unity power factor only the turn-off devices conduct. By appropriate control of the turn-on and turn-off switching, a three-phase AC waveform is produced at the AC output.
There are, however, two important differences between CSC and VSC. One is the duration of the valve conducting period, which in the case of the VSC is 1800 instead of the 1200 generally adopted for the CSC configuration.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

33. Voltage Source Converter
Figure: Switch firing signals for 180 conduction method.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

34. Voltage Source Converter
Figure : Converter line-to-line voltages for 180 conduction.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

35. Voltage Source Converter
The frequency of AC voltage can be controlled by changing the period between pulses, whereas the phase shift of AC voltage can be controlled directly by varying the phase displacement of the pulses.
The instantaneous line-to-line voltage can be expressed using the Fourier series as follows:
The line-to-line RMS voltage is:
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

36. Voltage Source Converter
For n= 1, the line to line RMS fundamental voltage is:
Example: 14.1, 14.2
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

37. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Multi-Level VSC
To obtain high-quality AC voltage or current with a two-level converter requires high switching frequencies (1–2 kHz). Such high frequencies, combined with high voltages in HVDC applications, cause significant switching losses, device/valve voltage rating constraints and high electromagnetic interference (EMI).
Multilevel converters use an array of modules to build the required AC voltage level from a number of individual DC power supplies. The DC voltage sources are typically formed using capacitors and a charge-balancing scheme is used to maintain the capacitor voltages constant.
A two-level inverter generates an AC-phase voltage with either of the two voltages (½Vdc or−½Vdc). The three-level inverter generates three voltage levels (½Vdc,−½Vdc or 0), while the five-level inverter generates five voltage levels.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

38. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Multi-Level VSC
Figure. One phase leg VSC with: a) two levels; b) three levels; and c) five-levels.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

39. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Multi-Level VSC
As the number of levels increases, the AC voltage appears closer to the sinusoidal waveform and the total harmonic distortion (THD) reduces. The most utilized multilevel topologies are:
Diode clamped converter;
Flying capacitor converter;
H-bridge cascade converter;
Modular multilevel converter.
The attractive features of multilevel converters are:
reduces THD and lower dv/dt,
reduces switching losses,
able to increase voltage and power level
inherent reliability
Multilevel converter disadvantages involve:
control become complex
multiple DC voltage and balancing capacitor voltage required
conduction losses increase
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

40. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Multi-Level VSC
Sa1, Sa2 ON
Sa3, Sa4 ON
Sa2, Sa3 ON
Neutral Point Clamped Multilevel Converter
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

41. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Multi-Level VSC
The switching rule is that only two valves that are directly connected can be switched ON at any one time i.e. Sa1 & Sa2, or Sa2 & Sa3, or Sa3 & Sa4. Switching on Sa1 & Sa2 connects the ac terminal AC to the positive terminal DC voltage resulting 1/2Vdc, Sa2 & Sa3 connects to the zero potential i,e 0V point via the clamping diodes, Sa3 & Sa4 connects to the negative terminal of DC voltage resulting -1/2Vdc.
For a general (n+ 1) level diode clamped multilevel converter, n storage capacitors are required. Some practical difficulties of this converter can be summarized as:
It requires high-speed clamping diodes
For more than three levels, the clamping diodes are subjected to
different voltage stress.
Charge balance of DC capacitor need to maintain.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

42. Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET
Multi-Level VSC
Figure: Three-level flying capacitor converter.
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

43. Voltage Source Converter
The converter leg provides a three-level phase to zero AC voltage, which isVa0={0,½Vdc or−½Vdc}. The operating principle is illustrated for leg a:for voltage level ½Vdc, switches Sa1 and Sa2 are ON; for a 0 level, switches Sa1 and Sa3 or Sa2 and Sa4 are ON; and for a−½Vdc level, switches Sa3and Sa4 are ON. Capacitor Ca therefore introduces negative voltage in series with either C1or C2..
Self Study: Schematic diagram of half-bridge and full-bridge MMC cell. Some important operational advantages of full-bridge over half-bridge MMC.
and of
Ahmed, Khaled_ Jovcic, Dragan-High voltage direct current transmission _ converters, systems and DC grids-Wiley (2015)
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

44. Multi-Terminal HVDC(MTDC)
Multi-Terminal HVDC systems connect more than two converters together providing additional reliability and compensate for the loss of any single converter of the system. Typically, one of the converters regulates the DC voltage and the others converters control the power flow.
A multi-terminal HVDC distribution configuration is perfectly suited to the connection of DC output types of power sources (such as photo -voltaics) and of DC input loads (such as IT-related equipment)
Figure. Multi-terminal HVDC System-Three Terminals
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET

45. Multi-Terminal HVDC(MTDC)
Md. Alamgir Hossain, Assistant Professor, Dept. of EEE, KUET