CONTROL OF EHV DC SYSTEM(Control Characteristics)
Natural Voltage Characteristic (NV) and the Constant Ignition Angle (CIA) control
The Natural Voltage Characteristic corresponds to zero delay angle α=0.This has the characteristic equation given by Vd = (V0-3wLc/Id).
The Constant Ignition Angle control is a similar characteristic which is parallel to the NV characteristic with a controllable intercept V0 cos α.These are shown in fig11.18
The Inverter is usually operated at constant extinction angle. This has the characteristic equation given by Vd=(v0cosδ-3wLc/Id). This is shown in figure 11.19
In a d.c. link it is common practice to operate the link at constant current rather than at constant voltage. Ofcourse, constant current means that current is held nearly constant and not exactly constant.
In constant current control, the power is varied by varying the voltage. There is an allowed range of current settings within which the current varies.
Full Characteristic of Convertor
The complete characteristic of each convertor has the N.V. characteristic and equipped with C.C. control and the C.E.A control.This is shown in figure11.20 for a single convertor
The constant current controller adjusts the firing angle (α)so that a current will maintained even for short-circuits on the d.c. line. The C.C. control is present in the inverter too, although the inverter is not usually operated in that region. The rectifier is normally operated in the C.C. region while the inverter is operated in the C.E.A. region.
Because the thyristors conduct only in one direction, the current in a convertor cannot be reversed. Power reversal can only be obtained by the reversal of the direct voltage (average value) Vd.
For inversion to be possible, a high value of inductance must be present, and the delay angle α>90 since Vd changes polarity at this angle. The theoretical maximum delay for inversion would occur at α=1800.Thus it is common practice to define a period of advance from this point rather than a delay from the previous cross-over as defined for rectification. Thus we define β =∏-α as the ignition angle for inversion or the angle of advance.Similarly the extinction angle is also defined as δ = ∏ -w.The definition of the commutation angle γ is unchanged.
It must be noted, that unlike with rectification which can be operated with α =0,inversion cannot be carried ot with β+0,since the mninimum angle required for deionisationo of the arc and gaininig control grid
Thus we have the practical relationship δ<β<∏/2 practical values lies between 10 and 80
Compounding of Convertors
Figure 11.21 shows a system of 2 convertors, connected by a hvdc link. Both convertors are provided withCEA and CC control so that either can work as a rectifier or an invertor. The compounded characteristics are shown in figure 11.22.
The margin setting Idm between the current setting Ids for the invertor and for the rectifier is usually kept at about 10% to 20% of the current setting. The setting of the convertor operating as rectifier is kept higher than the setting of that as invertor by the margin setting Idm.
The usual operating point for power transfer is the intersection of the CC control of the rectifier and the CEA control of the invertor. (For comparison, the characteristics of convertor B has been drawn inverted). It must also be ensured by proper tap changing that the N.V. characteristic of the convertor operating in the rectification mode is higher than the C.E.A. characteristic of the invertor, as Vo of the two ends are not necessarily equal.Figure 11.21
With convertor A operating as rectifier, and convertor B operating as invertor, the steady state current under allcircumstances will remain within the upper limit (Ids + Idm) and the lower limit Ids. That is, the system directcurrent will not change by more than Idm under all operating conditions. By reversing the margin setting Idm,that is making the setting of convertor B to exceed that of A, power flow can be automatically reversed.Convertor B will then operate as a rectifier and A as an invertor. The reversal of power occurs as a result of the reversal of polarity of the voltage.
Classification of d.c. links
D.C. links are classified into 3 types:-
- Monopolar links
- Bipolar links
- Homopolar links.
In the case of the monopolar link there is only one conductor and the ground serves as the return path. The linknormally operates at negative polarity as there is less corona loss and radio interference is reduced. Figure11.23 (a) shows a monopolar link.
The bipolar links have two conductors, one operating at positive polarity and the other operating at negativepolarity. The junction between the two convertors may be grounded at one or both ends. The ground does not normally carry a current. However, if both ends are grounded, each link could be independently operated whennecessary.
The homopolar links have two or more conductors having the same polarity (usually negative) and always operate with ground path as return.
Harmonics and Filters
As was mentioned earlier, the harmonics present on the a.c. system are (6k±1). Thus the a.c. harmonic filters are tuned to the 5th, 7th, 11th, and 13th harmonics to reduce the harmonic content in the voltages and currents in the a.c. network to acceptable levels. Higher harmonics would not penetrate very far into the a.c. system. The harmonics are mainly present in the a.c. current as the a.c. voltage is heavily dependant on the a.c. system itself.The Harmonics present on the d.c. side are mainly on output voltage. These are in multiples of 6 as the waveform repeats itself 6 times. The d.c. current is smoothed by the smoothing reactors.
The hvdc system has a lower line cost per unit length as compared to an equally reliable a.c. system due to the lesser number of conductors and smaller tower size.
However, the d.c. system needs two expensive convertor stations which may cost around two to three times the corresponding a.c. transformer stations. Thus hvdc transmission is not generally economical for short distances, unless other factors dictate otherwise.
Economic considerations call for a certain minimum transmission distance (break-even distance) before hvdc can be onsidered competitive purely on cost.Estimates for the break even distance of overhead lines are around 500 km with a wide variation about this value depending on the magnitude of power transfer and the range of costs of lines and equipment. The break even distances are reducing with the progress made in the development of converting devices.
Q.What are the kinds of d.c links?
Q.Explain Natural Voltage Characteristic (NV) and the Constant Ignition Angle (CIA) control?
Q.Write a short note on harmonics and filter?
Q.Describe the full characteristics of converter?