Simulation Research of Current-modulated Tripole DC Transmission System

Jing Wu1, Jialiang Wen1, Jun Wen2, Yu Wang1, Chang Peng1, Rui Wu1 1. China Electric Power Research Institute (CEPRI)

Haidian District, Beijing 100192, China wujing@epri.sgcc.com.cn

2. North China Electric Power University (NCEPU) Haidian District, Beijing 102206, China

Abstract—This paper introduced a method of converting AC transmission lines to tripole DC using current modulation. This method cannot only make full use of existing lines and equipment, but also improve power capacity and system performance. So it is available in both technology and economy. This paper also analyzed the principles of current modulation. Moreover, tripole DC transmission system is simulated by PSCAD/EMTDC. The simulation results showed that tripole DC transmission scheme is available by using current modulation.

Keywords-tripole DC, current modulation, PSCAD/EMTDC model, simulation analysis

I. INTRODUCTION As the rapid development of electric power industry and the

continuous growth in electrical loads, transmission network is becoming one of the limiting factors. The reasons are: 1) energy sources and electrical loads are distributed reversely in China, so high power long distance transmission is needed; 2) the existing transmission network cannot meet the demand of electrical loads; 3) because of the limited land resources, application of new transmission corridor is becoming more and more difficult. Therefore, conversion of AC transmission lines to DC has been a consideration recently. As tripole DC transmission scheme have important economic and operational advantages over conventional bipole DC scheme. Those advantages, while project-specific, may include lower cost, higher reliability, and higher overload capacity. The system can be achieved with conventional, commercially available, well-proven equipment. So tripole DC transmission scheme is a better choice.

The tripole DC transmission system is a special system. It can be regarded as a coordinated combination of conventional bipole and monopole systems. And comprehensive simulation of the tripole DC transmission system using PSCAD/EMTDC shows it can be achieved by current modulation.

II. PRINCIPLES OF TRIPOLE DC TRANSMISSION SCHEME Figure 1 shows the configuration of tripole DC

transmission system. Tripole DC transmission system uses the three phase positions of original AC system as a positive pole (pole 1 in figure 1), a negative pole (pole 2 in figure 1) and a

modulation pole (pole 3 in figure 1). Poles 1 and 2 are equivalent to a conventional bipole DC transmission system, while pole 3 is equivalent to a monopole DC transmission system with current reversing capacity. The rectifier side and inverter side of these two systems are both fed into the same commutation bus.

Figure 1. The configuration of tripole DC transmission system

Current reversing capacity can be achieved either by installation of anti-parallel valves or installation of anti-parallel thyristors within same valve.

Figure 2. The principles of current modulation

978-1-4577-1600-3/12/$26.00 © 2012 IEEE

For example, at any instant in time, the polarity of pole 1 is positive and the polarity of pole 2 is negative all the time. And poles 1 and 2 alternately carry the higher level current Imax and the lower level current Imin, while pole 3 alternately relieve a portion of the current from pole 1 and then from pole 2 as shown in figure 2.

In figure 2, the higher level current can be above the rating of the line conductor and other series-path equipment so long as that pole’s rms current is 1.0 p.u. of continuous rating. Assuming the duration of Imax and Imin are equal:

0.1)21

min2

ax2 =+ II m（

(1)

As pole 3, the modulating pole, relieves more and more of the current from poles 1 and 2, that pole reaches full rating when

0.1minax =− IIm (2)

Solving the quadratic derived from (1) and (2) shows:

37.0137.1

37.12

31

min

max

=−=

=+=

I

I

(3)

A. Modulation Ratio The ratio of Imax to Imin is defined as M, the modulation

ratio. In the above example, M=1.37/0.37=3.73. Here, the system’s transmitting power is maximum. In the limit, when M=1, the current in pole 3 is zero and pole 1 and pole 2 are acting as bipole.

B. Transmission Power and Line Losses Power is proportional to average current. Figure 2 shows

that at any instant in time, the total tripole power )1(P minmaxtri ++= IIV which, according to (3) and

assuming 1 p.u. voltage, gives a value of 3.74. on the same basis the power for a bipole, using just two conductors would be 2.0. Thus the tripole/bipole power ratio is 3.74/2.0=1.37. The tripole carries 37% more power using all three conductors then the bipole would using just two.

At any instant in time in figure 2, tripole losses RII )1(L 22

min2

maxtri ++= =3.0R, where R is the total dc resistance of one conductor. Bipole losses at full current are 2.0R on the same basis. Tripole losses are 1.5 times bipole losses or about 10% higher on a per kW basis.

C. The Period of the Higher/Lower Level Current Cycle Conductor temperature will rise almost linearly when Imax

is applied on either pole 1 or 2. The period of the higher/lower level current cycle (defined as T0) must be short enough to keep conductor temperature within tolerable limits. Usually, T0 is four or five minutes for overhead lines, and it is longer for cables.

D. Transitional Ramps The inset in fig. 2 shows that current ramps can be modest

in slope, e.g. a short period T (one or two seconds) can be allowed to reverse voltage on pole 3 all with continuous total power. Since T would be very short compared to the period of current reversal, it will have minimal effect on thermal duties.

III. SIMULATION OF CURRENT MODULATION

A. Simulation Model According to the configuration of tripole DC transmission

system in figure 1, the PSCAD/EMTDC model is built, as shown in figure 3. This model is based on the Cigre Benchmark model for HVDC controls. The basic parameters are as follows. Pulse number is 12, rating voltage is 500kV, DC line inductance is 0.5968H, DC line resistance is 2.5 Ω , inverter resistance is 9.522Ω, rectifier resistance is 21.4245Ω. Reactive power compensator is fixed capacitors. Filter is damped filter. Moreover, pole 3 uses a configuration of anti-parallel valves.

Figure 3. the PSCAD/EMTDC model of tripole DC transmission system

B. Control Methods The control methods of pole 3 are similar to that of poles 1

and 2. Rectifier is operating in constant current control. Inverter is operating in constant extinction angle and constant current control with VDCOL and current deviation control. The details of control methods are described as follows.

• Rectifier

Rectifier is operating in constant current control. The input is the deviation of current order and measured current. The deviation is delivered to PI controller to output the ignition angle order α. Control diagram as shown in figure 4. Pole 1’s current order is Imin (0.37p.u.) in the first half cycle and Imax (1.37p.u.) in the second half cycle. Pole 2’s current order is opposite to pole 1’s. Pole 3’s current order is the difference of poles 1 and 2’s current order. And when pole 1’s measured current is larger than pole 2’s measured current, the ignition signal is sent to the anti-parallel valve of pole 3, otherwise the ignition signal is sent to the other valve of pole 3.

Figure 4. constant current control diagram of rectifier

• Inverter

(1) Constant Extinction Angle Control

The minimum value of extinction angel in one cycle is set as the measured extinction angle γ. Here, extinction angle γ order is 20o. The angle of maximum deviation is 31o, i.e. the maximum angle of current deviation control can reach 51o. Control diagram as shown in figure 5.

Figure 5. constant extinction angle control diagram of inverter

(2) Constant Current Control

Inverter’s constant current control is the same as rectifier’s. The input is current order and measured current. But inverter’s current order Idi,ref is smaller than rectifier’s current order Idr,ref. Usually, the current margin ΔI is 0.1 p.u.. Control diagram as shown in figure 6.

Figure 6. constant current control diagram of inverter

(3) Low-voltage Current-limiting Unit (VDCOL)

When DC voltage or AC voltage drops to a specified value, VDCOL will limit the DC current order. The parameters of Pole 1’s or 2’s and pole 3’s VDCOL are different. DC operation voltages of pole 1 or 2 are 0.8 and 0.4 p.u., DC fixed currents of pole 1 or 2 are 0.3 and 1.37 p.u.. DC operation voltages of pole 3 are 0.9 and 0.4 p.u., DC fixed currents of

pole 3 are 0.55 and 1.0 p.u.. Control diagram and waveform as shown in figure 7.

(a) VDCOL control diagram

(b) waveforms

Figure 7. VDCOL control diagram and waveforms

(4) Current Deviation Control

In order to transit between constant extinction angle control and VDCOL control smoothly, current deviation control is introduced. The method is increasing extinction angle properly based on the difference of current rating and measured current. If extinction angle reaches the maximum value, constant current control is started. Control diagram and waveform as shown in figure 8.

(a) current deviation control diagram

(b) waveform

Figure 8. current deviation control diagram and waveform

C. Analysis of Simulation Results Set the period of the higher/lower level current cycle

T0=2s, i.e. the higher level current and lower level current’s duration time are both 1s. Set the time of current ramp t=0.1s, i.e. the time of current changing from the higher level to the lower level (or from the lower level to the higher level) is 0.1s. By simulation analysis, we can get the following current waveforms.

(a) pole 1’s current waveform at rectifier

(b) pole 2’s current waveform at rectifier

(c) pole 3’s current waveform at rectifier

(d) pole 1’s current waveform at inverter

(e) pole 2’s current waveform at inverter

(f) pole 3’s current waveform at inverter

Figure 9. simulation current waveforms of tripole DC transmission system

From the simulation results, we can see that the current in pole 1 is + and the current in pole 2 is - all the time. And these two poles alternately carry the higher level current Imax (1.37p.u.) and the lower level current Imin (0.37p.u.), while pole 3 alternately relieves apportion of the current from poles 1 and 2. Thus, the sum of the currents at any given time is zero, i.e. there is no earth current. Current modulation is achieved.

Therefore, we can get the following conclusion that tripole DC transmission scheme can be achieved by current modulation.

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This paper introduced the principles of current modulation using in the tripole DC transmission system. And by the means of PSCAD/EMTDC simulation software, we built tripole DC transmission system’s model. Then, we simulated and analyzed its current modulating method. Thus the feasibility of tripole DC transmission system is proven. This paper also shows that converting AC transmission lines to tripole DC is available and this method has a great application prospect in the future grid construction.

REFERENCES [1] Lionel O. Barthold, Hartmut Huang. “Conversion of AC Transmission

lines to HVDC using Current Modulation”. Inaugural IEEE PES 2005 Conference and Exposition in Africa. Durban, South Africa. 11-15 July, 2005.

[2] Lionel O. Barthold. “Technical and Economic Aspects of Tripole HVDC”. International Conference on Power System Technology, 2006.

[3] Lionel O. Barthold, H. K. Clark, D. Woodford. “Principles and Applications of Current-Modulated HVDC Transmission Systems”. IEEE T&D Conference Dallas, Tex. May 21-26, 2006.

[4] L. O. Barthold, Dale E. Douglass, Dennis A. Woodford. “Maximizing the Capability of Existing AC Transmission Lines”. CIGRE report B2-109, 2008.

[5] A. Edris, L. O. Barthold, D. A. Douglas. “Upgrading AC Transmission to DC for Maximum Power Transfer Capacity”. IEEE 2008.

[6] A. Edris, L. O. Barthold. “Evaluating conversion of ac power transmission lines to DC”. CIGRE Canada Conference on Power Systems Winnipeg, 2008. IEEE 2008.

[7] L. O. Barthold, R. Adapa, Harrison Clark, Dennis Woodford. “System advantages of conversion ac transmission lines to dc”.

[8] L. O. Barthold. “Current Modulation of Direct Current Transmission Lines”. U.S. Patents 6,714,427B1, Mar.30, 2004.

[9] D. A. Woodford. “Transmission Upgrading with Synchronous DC Links”. Proceedings of the Future of Power Delivery in the 21st Century, EPRI Publication TR－109806, May 1998.

[10] P. Naidoo, D. Muftic, D. Ijumba. “Investigations into the Upgrading of Existing HVAC Power Transmission Circuits for Higher Power Transfers using HVDC Technology”. Inaugural IEEE PES 2005 Conference and Exposition in Africa. Durban, South Africa, 11－15 July 2005.

[11] Orzechowski. “Analysis of possible enhancement of transmission capacity while converting 220 kV alternating current overhead lines into direct current lines”. CIGRE report B4－105, 2004.

[12] “Guide for Upgrading Transmission Systems with HVDC Transmission”. CIGRE Brochure Ref.127, C14, WG 14.11, 1998.

[13] R. Adapa, L. Barthold, D. Douglass, D. Woodford. “Technical and Economic Incentives for AC to DC Line Conversion”. CIGRE B2－203, Paris, 2010.