VOLTAGE SOURCE CONVERTER TRANSMISSION …

1 VOLTAGE SOURCE CONVERTER TRANSMISSION TECHNOLOGIES. - THE RIGHT FIT FOR THE APPLICATION. Michael P. Bahrman Jan G. Johansson Bo A. Nilsson ABB Inc. ABB Utilities AB ABB Utilities AB. Abstract VOLTAGE SOURCE CONVERTER (VSC) VOLTAGE support combined with an energy storage SOURCE ). technology has been selected as the basis for several permit continuous and independent control of real and recent projects due to its controllability, compact reactive power . Reactive power control is also independent modular design , ease of system interface and low of that at any other terminal. Reactive power control can be environmental impact. This paper describes the used for dynamic VOLTAGE regulation to support the rationale for selection of VSC technology and the latest interconnecting ac system following contingencies.

2 This technical developments utilized in several recent capability can increase the overall transfer levels. Forced projects. commutation with VSC even permits black start, , the CONVERTER can be used to synthesize a balanced set of three Keywords VSC - HVDC IGBT - Valves PWM - phase voltages much like a synchronous machine. FACTS - STATCOM SVC Flicker power Quality 2. GENERAL SYSTEM CONSIDERATIONS. 1. INTRODUCTION. Dynamic Reactive power Compensation SVC vs. Traditional HVDC and FACTS installations have often STATCOM. provided economic solutions for special TRANSMISSION An SVC provides VOLTAGE regulation and dynamic reactive applications. HVDC is well-suited for long-distance, bulk- power reserve by means of thyristor-controlled reactors power TRANSMISSION , long submarine cable crossings, and (TCR) and thyristor-switched capacitors (TSC) for var asynchronous interconnections.

3 Static var compensators absorption and production respectively. A STATCOM. (SVC) provide a reserve SOURCE of dynamic reactive power accomplishes the same effect by using a VSC to synthesize a thereby raising power transfer limits. HVDC and FACTS. VOLTAGE waveform of variable magnitude with respect to the technologies permit transmitting more power over fewer system VOLTAGE as shown in Figure 1. Although both FACTS. TRANSMISSION lines. devices require filters which form an integral part of the net capacitive reactive power supply, the filters are usually a Deregulated generation markets, open access to larger part of the reactive power supply in an SVC. For very TRANSMISSION , formation of RTO's, regional differences in weak system applications, it is advantageous to have smaller generation costs and increased difficulty in siting new filters.

4 Often control of these FACTS devices is coordinated TRANSMISSION lines, however, have led to a renewed interest with mechanically-switched capacitor banks (MSC) to bias in FACTS and HVDC TRANSMISSION often in non-traditional their dynamic operating range and maintain reactive power applications. This is especially true with the lag in reserve margin. TRANSMISSION investment and the separation in ownership of generation and TRANSMISSION assets. HVDC and FACTS. The STATCOM branch offers both reactive power TRANSMISSION technologies available today offer the planner absorption and production capability whereas an SVC. increased flexibility in meeting TRANSMISSION challenges. requires separate branches for each. The STATCOM, especially when controlled with PWM, allows faster HVDC TRANSMISSION and reactive power compensation with response and thereby improves power quality.

5 This is very VOLTAGE SOURCE CONVERTER (VSC) technology has certain useful to mitigate flicker from disturbances caused by attributes which can be beneficial to overall system electric arc furnaces at steel mills. For normal ac network performance. HVDC Light and SVC Light technology disturbances where oscillatory modes usually do not exceed developed by ABB employs VOLTAGE SOURCE converters Hz, however, the additional bandwidth may provide no (VSC) with series-connected IGBT (insulated gate bipolar real system benefit. transistor) valves controlled with pulse width modulation (PWM). VSC converters used for power TRANSMISSION (or TSC branches than with STATCOM branches. Depending on the rating criteria, it can often be more economical to use a larger SVC especially considering the extra dynamic vars supplied at nominal VOLTAGE .)

6 With parallel shunt banks, the difference in the composite device V-I characteristics becomes less as illustrated in Figure 2. Finding the optimal FACTS solution must be evaluated together with the owner in each unique situation for the respective TRANSMISSION system. It is usually recommended to base the decision on a thorough system study. Ultimately, Figure 1. STATCOM it is the owner/user's task to define the system requirements for the FACTS device, and the supplier to determine the optimal solution. One of the main reasons for installing an SVC or STATCOM in TRANSMISSION networks is to increase the power transfer capability where limited by post-contingency DC TRANSMISSION System Current SOURCE VOLTAGE criteria or undervoltage loss of load probability. Converters vs. VSC. Determining the optimum mix of dynamic and switched Conventional HVDC TRANSMISSION employs line- compensation is a challenge.

7 Control systems are designed commutated, current- SOURCE converters requiring a to keep the normal operating point within the middle of the synchronous VOLTAGE SOURCE in order to operate. The SVC or STATCOM dynamic range. conversion process demands reactive power from filters, shunt banks, or series capacitors which are part of the If, after a disturbance, the FACTS device temporarily hits its CONVERTER station. Any surplus or deficit in reactive power capacitive limit, reactive power production will be must be accommodated by the ac system. This difference in decreased. For an SVC this decrease will be with VOLTAGE reactive power needs to be kept within a given band to keep squared, whereas with a STATCOM it will be with VOLTAGE . the ac VOLTAGE within the desired tolerance. The weaker the Most overhead TRANSMISSION VOLTAGE support applications system or the further away from generation, the tighter the require more dynamic capability on the capacitive side than reactive power exchange must be to stay within the desired on the inductive side.

8 It is more economical to do this with VOLTAGE tolerance. With 20 MVAR HF. VOLTAGE ( ). With 20 MVAR HF + 2x50 MVAR MSC Figure 3. VSC P-Q Characteristic VOLTAGE ( ). Proper control of the CONVERTER and its associated reactive power compensation allows the ac system VOLTAGE to be held within a fairly tight and acceptable range. Unlike a generator or static var compensator, however, a conventional HVDC CONVERTER cannot provide much dynamic VOLTAGE <--- Capacitive ------STATCOM Current ( )------- Inductive --->. support to the ac network. HVDC conversion technology using VOLTAGE SOURCE converters, however, can not only Figure 2. V-I Diagram 150 MVAR STATCOM control the power flow but also provide dynamic VOLTAGE regulation to the ac system. Figure 3 depicts the P - Q. characteristics for a VSC designed for HVDC TRANSMISSION .

9 Depending on the CONVERTER rating, series-connected IGBT. The capacitive limit is due to imposing a VOLTAGE limitation. valves are arranged in either a three-phase two-level or If the system VOLTAGE is reduced, this limit increases. The three-level bridge. In three-level converters, IGBT valves reactive power control range available depends on the active may also be used in place of diodes for neutral point power operating point. clamping. Each IGBT position is individually controlled and monitored via fiber optics and equipped with integrated anti- IGBT Valves parallel, free-wheeling diodes. Each IGBT has a rated TRANSMISSION Cable VOLTAGE of kV with rated currents up to 1500 A. Each Phase AC bus Reactor VSC station is built up with modular valve housings which are constructed to shield electromagnetic interference (EMI).

10 The valves are cooled with circulating water and water to air heat exchangers. PWM switching frequencies for the VSC. AC filters DC Capacitor typically range between 1-2 kHz depending on the CONVERTER topology, system frequency and specific application. Control System Figure 4. VSC TRANSMISSION The following factors make VSC-based TRANSMISSION attractive: Independent control of reactive and active power Reactive control independent of other terminal(s). Simpler interface with ac system Compact filters Provides continuous ac VOLTAGE regulation Figure 5. PWM Signal for Three-Level VSC. No minimum power restriction Operation in extremely weak systems No commutation failures Each VSC is effectively mid-point grounded and coupled to No restriction on multiple infeeds the AC bus via phase reactors and a power transformer with No polarity reversal needed to reverse power intermediary shunt AC filters.