Hvdc r6a65

HVDC

Tri Desmana Rachmildha

Sistem AC Kekurangan • Elemen induktif dan kapasitif membatasi kapasitas saluran • Arus charging • Drop tegangan akibat induktansi

• Skin effect • Kendala koneksi langsung 2 sistem • Akibat perbedaan frekuensi • Akibat perbedaan sudut dan urutan fasa

• Kendala keseimbangan fasa • Ketidakseimbangan tegangan • Ketidakseimbangan arus (beban)

Charging current and voltage drop

Sejarah HVDC

In 1941, the first contract for a commercial HVDC system was signed in : 60 MW were to be supplied to the city of Berlin via an underground cable of 115 km length. The system with ±200 kV and 150 A was ready for energizing in 1945. It was never put into operation.

History  First commercial application of HVDC between Swedish mainland and the island of Gotland in 1954.  Underwater link of 90 km and 20 MW.  After the advent of thyristor convertor, New Brunswick and Quebec 320 MW back-to-back DC interconnection commissioned in 1972.  With reduced size, cost and improved reliability of power electronic converters, has made HVDC transmission more widespread.  In North America, total HVDC transmission capacity in 1987 was 14,000 MW.

Sistem transmisi DC Kelebihan Teknis • Memungkinkan koneksi sistem yang berbeda • Frekuensi, tegangan, fasa

• Tidak ada drop tegangan akibat induktansi • Arus charging pada steady state adalah nol • > 40 km, lebih disukai menggunakan HVDC

• Tidak ada skin effect • Mudah dalam pengendalian daya aktif

Sistem transmisi DC Kelebihan Ekonomis • Yang diperhitungkan : • AC vs. DC station terminal costs • AC vs. DC line costs • AC vs. DC capitalised value of losses • Jarak break-even : 500 km – 800 km, tergantung berbagai faktor

Sistem transmisi DC Isu lingkungan dll • Positif : • Lahan yang digunakan lebih sedikit • Yang harus diperhatikan: • Gardu konverter yang ‘bising’ • Kompatibilitas elektromagnetik • Penggunaan tanah atau laut sebagai return path untuk operasi monopolar

Typical transmission line structures for approx. 1000 MW

Ekonomis

Advantages In a number of applications HVDC is more effective than AC transmission. Examples include: Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and ) Long power transmission without intermediate taps, for example, in remote areas Power transmission and stabilization between unsynchronized AC distribution systems Connecting a remote generating plant to the distribution grid Reducing line cost: 1) fewer conductors 2) thinner conductors since HVDC does not suffer from the skin effect Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies Synchronize AC produced by renewable energy sources

Disadvantages  The disadvantages of HVDC are in conversion, switching and control.  Expensive inverters with limited overload capacity  Higher losses in static inverters at smaller transmission distances  The cost of the inverters may not be offset by reductions in line construction cost and lower line loss.  High voltage DC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and wear would be too great to allow reliable switching.

Tipe transmisi HVDC Umum

- Arus dan aliran daya dikendalikan dengan cara mengatur beda tegangan dan polaritasnya

Back to back converter

Tipe Transmisi HVDC

Monopolar Long-Distance Transmission

Menggunakan elektroda

Menggunakan metallic return path (saluran, grid dll)

Bipole Long Distance Transmission Ground return path

Menggunakan ground return path

Bipole Long Distance Transmission Metallic return path - Untuk jarak yang tidak begitu jauh - Jika tidak diperkenankan menggunakan elektroda

Menggunakan metallic return path

Teori konverter Rectifier (penyearah) Penyearah : - Dioda - Thyristor (terkendali) Penyearah thyristor terkendali dapat menghasilkan tegangan - Positif ( < 90o) - nol ( = 90o) - Negatif ( > 90o)

Penyearah 12 pulsa Memiliki harmonik yang lebih rendah di sisi ac

Multiple Bridge Converters •

Two banks of transformers, one connected in Y-Y and the other Y- are used to supply each pair of bridges

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The three-phase voltage supplied at one bridge is displaced from the other by 30 degrees

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These AC wave shapes for the two bridges add up to produce a wave shape that is more sinusoidal than the current waves of each of the 6-pulse bridges

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This 12-pulse arrangement effectively eliminates 5th and 7th harmonics on the AC side. This reduces the cost of harmonic filters

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This arrangement also reduces ripple in the DC voltage

Control of HVDC Systems • Objectives of Control • •

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Efficient and stable operation Maximum flexibility of power control without compromising the safety of equipment

Content • •

Principle of operation of various control systems Implementation and their performance during normal and abnormal system conditions

Basic principles of control Direct current from the rectifier to the inverter

Power at the rectifier terminal

Power at the inverter terminal

Basic means of control • Internal voltages, Vdorcos and Vdoicos, can used be controlled to control the voltages at any point on the line and the current flow (power) • This can be accomplished by: • Controlling firing angles of the rectifier and inverter (for fast action) • Changing taps on the transformers on the AC side (slow response)

• Power reversal is obtained by reversal of polarity of direct voltages at both ends

Basis for selection of control • Following considerations influence the selection of control characteristics: • Prevention of large fluctuation in DC voltage/current due to variation In AC side voltage • Maintaining direct voltage near rated value • Power factor at the receiving and sending ends should be as high as possible

Control implementation • Tap changer control • It is used to keep the converter firing angles ( and ) within the desired range • They are sized to allow for minimum and maximum steady state voltage variation

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Current limits: 

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Maximum short circuit current is limited to 1.2 to 1.3 times normal full load current to avoid thermal damage to equipment Minimum current limit is set to avoid ripple in the current that may cause it to be discontinuous or intermittent

Minimum firing angle limit: 

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In case of a DC fault, the inverter station may switch to rectification mode. This would result in reversal of power flow To prevent this, the a minimum value for firing angle is set

Control implementation • Power control • To transmit a scheduled power, the corresponding current order is determined by:

Iord=Po/Vd 

Bridge/converter unit control 

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Determines firing angles and sets their limits

Pole control 

It coordinates the conversion of current order to a firing angle order, tap changer control and other protection sequences

Multiterminal HVDC network • Successful application of two-terminal DC systems led to the development of multiterminal networks • There are two possible connection schemes for MTDC systems: • Constant voltage parallel scheme • Constant current series scheme

HVDC dengan Multilevel converter