APPLICATION OF DC TRANSMISSION

APPLICATION OF DC TRANSMISSION 

The detailed comparison of AC and DC transmission in terms of economics and technical performance, leads to the following areas of application for DC transmission :

1. Long distance bulk power transmission
2. Underground  or  underwater cables 
3. Asynchronous interconnection of AC systems operating at different frequencies or where independent control of systems is desired 
4. Control and stabilization of power flows in AC ties in an integrated power system .

 The first two applications are dictated primarily by the economic advantages of DC transmission, where the concept of breakeven distance is important. To be realistic, one must also assign a monetary value for the technical advantages of DC (or penalty costs for the drawbacks of AC ). The problem of evaluation of the economic benefits is further complicated by the various alternatives that may be considered in solving problems of AC transmission - phase shifters, static var systems, series capacitors, single pole switching etc.

                  The technical superiority  of DC transmission dictates its use for asynchronous inter connections, even when the transmission distances are negligible. Actually there are many 'back to back' DC links in existence where the rectification and inversion or carried out in the same converter station with no DC lines.The advantage of such DC links liec in the reduction of the overall conversion costs and improving the reliability of DC system.

                The alternative to DC ties may require strengthening existing AC network near the bounder of the two systems. This cost can be prohibitive if the capacity of the tie required is moderate compared to the size of the systems inter connected.

           In large inter connected systems, power flow in AC ties (particularly under disturbance conditions)  Can be uncontrolled and leed to over load and stability problems thus endangering systems security. Strategically placed DC lines can overcome this problems requires detailed study to evaluate the benefits. 

        

             Presently the number of DC lines in a power grid is very small compared to the number of AC lines. this indicates the DC transmission is justified only for specific application. although advances in technology  and introduction of multi - terminal DC (MT DC) systems are expected to increase the scope of applications of DC transmission  it is not anticipated  That AC grid will be replaced by DC power grid in future. there are two major reasons for this.Firstly , the control and protection of MT DC systems is very complex and the inability of voltage transformation in DC network imposes economic penalties. Secondly, the advances in DC technology have resulted in the improvement of the performance  of AC transmission, through introduction of static var systems, static phase shifters, etc.     

   

        The rate of growth of DC transmission was slow in the beginning. in over 16 years, only 6000 MW of DC systems were installed using mercury arc valves. The introduction of thyristor valves overcome some of the problems of system operation mainly due to the arc backs in mercury arc valves. Since then, the rate of growth of DC transmission capacity has reached an average of 2500 MW/years

Sl.No	HVDC - Systems	Transmission distance KM			Rated VoltagekVxNo.of circuits	Nominalcapacity MW	Max continuouscapasityMW	Commissioning date	Comments
		overhead line	cable	Total
A	Mercury-arc-Valve Systems
1	Gtland-Swedish Mainland	0	96	96	150	30	30	1954/70
2	Cross Channel 1 (GB-F)	0	65	65	±100	160	160	1961
3	Volgograd-Donbass (SU)	470	0	470	±400	720	720	1962-65
4	Konti-skan (DK-S)	95	85	180	250	250	275	1965
5	Sakuma (J)	_	_	_	125×2	300	300	1965
6	New Zeland (NZ)	570	39	609	±250	600	600	1965
7	Sardinia-Italian Mainland	292	121	413	200	200	200	1967
8	Vacounver pole I (CDN)	totla 41	total 33	74	±260	312	312	1968/69
9	pacific Intertie (US)	1362	0	1362	±400	1600	1600	1970
10	Nelson River Bipole I (CDN)	890	0	890	±450	1620	1669	1973-77
11	King snorth (GB)	0	82	82	±266	640	640	1974
B	Thyristor Valve Systems
12	Eel River (CDN)	_	_	_	80×2	320	350	1972	Asynchronous Tie
13	Skagerrak (DK-N)	113	127	240	±250	500	510	1976/77
14	David A.Hamil (USA)	_	_	_	50	100	110	1977	Asynchronous Tie
15	Cabora Bassa - Apollo (MOC-ZA)	1414	0	1414	±533	1920	1920	1977/79
16	Vancouver Pole 2 (CDN)	total 41	total  33	74	-280	370	476	1977/79
17	Square Butte (US)	749	0	749	±250	500	550	1977
18	Shin-Shinano (J)	_	_	_	125×2	300	300	1977	50/60 Hz  Tie
19	Nelson River Bipole 2 (CDN)	930	0	930	±250	900	1000	1978
20	Cu (Underwood Minneapolis)(us)	710	0	910	±400	1000	1100	1979
21	Hokkaido-Honshu (J)	124	44	158	250	300	300	1979/80
22	Asaray (PY-BR)	_	_	_	26	50	_	1981	50/60 Hz  Tie
23	EPRI Compact Station (USA)	_	0.6	0.6	100/400	100	_	1981
24	Vyborg (USSR-Finland)	_	_	_	±85×3	170	_	1982	Asynchronous Tie
25	Inga Shaba (ZAIRE)	1700	0	1700	±500	560	_	1982
26	Dumrohr (A)	_	_	_	±145	550	633	1983	Asynchronous Tie
27	Gotland 2-Swdish Mainland	7	91	98	150	130	165	1983
28	Eddy Co. (USA)	_	_	_	82	200		1983	Asynchronous Tie
29	Itaipu (BR)	783/806	0	783/806	±300	1575		1984
30	Chateauguary (CDN)	_	_	_	140	1000		1984	Asynchronous Tie
31	Itaipu(BR)	783/806	0	783/806	±600×2	6300		1985-87
32	Oklaunion (US)	_	_	_	82	200		1984	Asynchronous Tie
33	Pacifik Intertie	_	_	_	±500	400		1985
34	Wien Sud-Ost (A)	_	_	_	145	550		1987	Asynchronous Tie
35	Corsica Tap (F)	_	_	_	200	50		1986
36	Greece-Bulgaria	_	_	_	NA	300			Asynchronous Tie
37	Madawaska (CDN)	_	_	_	144	350		1985	Asynchronous Tie
38	Miles City (US)	_	_	_	82	200		1985	Asynchronous Tie
39	Walker Co. (US)	256	0	256	±400	500-1500		1985	Asynchronous Tie
40	Cross Channel 2 (GB-F)	0	72	72	±270×2	2000		1985/86
41	Kanti-Skan2(DKS)	95	85	160	250	270		1988/89
42	Ekibastus-Centre (USSR)	2400	0	2400	±250	6000		1985-88
43	Store Baelt (DK)	35	30	55	280	350		1989-90
44	Skagerrak (DK-N)	113	127	240	300	320		1988-89
45	Intermountain (US)	794	0	794	±500	1600		1987
46	Liberty Mead (US)	400	0	400	±364/±500	1600/2200		1989-90
47	Nelson River Biipole 3	930	0	930	±500	2000		1992/97
48	Chicoasen (MEX)	720	0	720	±500	900/1800		1985/90
49	Yukatan-Mexico City
50	Quebec-New England	175/375		175/375	±450	690/2070		1986/92
51	Des Cantons-Camerford	175		175	±450	690		1986
52	Sidney (US)	_	_	_		200		1986	Asynchronous Tie
53	Black Water (US)	_	_	_	56	200		1985	Asynchronous Tie
54	Highgate (US)	_	_	_	56	200		1985	Asynchronous Tie
55	SACOI-2 (Italy)				200	300		1989
56	Pacific IntertieII (US)				±500	1100
57	Gezhouba-Nan Qiao (China)	1080	_	1080	±500	1200		1987-91
58	Rihand-delhi (India)	1000	_	1000	±500	1000		1987
59	Uruguaiana (BR-Argentina)					50		1986/87	Asynchronous Tie
60	Camerford-Sandy Pond	200				1400		1990
61	vvVindhyachaI (India)	_	_	_	70	250×2		1988	Asynchronous Tie
62	Gotland 3-Swdish Mainland	_	98	98	150	130	165	1987
63	South finland East Sweden	35	185	220	350	420		1989/90