Читать книгу Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers - Kalyan K. Sen - Страница 21

In the early 1990s, there was a renewed interest to experiment with novel power electronics VSCs‐based PFCs due to the availability of high‐power, forced‐commutated, semiconductor switches, such as 4500 V, 4000 A‐rated gate‐turn‐off (GTO) thyristors. A new definition, namely Flexible Alternating Current Transmission Systems (FACTS), was proposed as “alternating current transmission systems incorporating power electronic‐based and other static controllers to enhance controllability and increased power transfer capability.” The purpose of the FACTS solution was to completely overhaul and improve the power delivery techniques that were developed, since the introduction of free flow of electricity in the late 19^th century. Under this program, Westinghouse installed a ±100‐MVA‐rated STATCON (STATic CONdenser as discussed in the literature) at the Tennessee Valley Authority’s (TVA) Sullivan substation in Tennessee, USA. This new equipment was fundamentally different from the conventional thyristor‐controlled SVC. This equipment was called STATCON because its steady‐state output characteristics are similar to those of the rotating synchronous condenser. This was the world’s first commercial installation of a STATCON, which was later renamed as STATCOM. This project was based on a previously demonstrated project that was documented in a report, titled 1 MVAR Advanced Static VAR Generator Development Program, ESEERCO Report, EP 84‐30, April 1987. The TVA‐STATCOM demonstrated (i) realization of a ±100 MVA, 161 kV‐rated VSC, based on a harmonic‐neutralization technique that did not require the use of any filter at the output of the VSC, (ii) viability of GTO‐based VSCs at high power for transmission applications and (iii) fast control response of a VSC‐based compensator as shown in Figure 1-13. This STATCOM was built using MSDOS (Microsoft Disk Operating System), which became obsolete before its installation in 1995 due to the arrival of MS Windows operating system in 1993. The STATCOM was retired from service due to component obsolescence. This demonstrates the consequential risks associated with the new power electronics technologies in regard to the life expectancy under utility conventional standards. Rapid advances in semiconductor technology and computer operating systems do not align with the utility business model and quickly outpace the utility adoption standards. This is a prime example of technology obsolescence that should be considered when making a technical evaluation.

Figure 1-13 Response time of the first commercial STATCON for 100 Mvar capacitive step (left) and 100 Mvar inductive step (right) (field performance) (Westinghouse).

The capability of providing a 100‐Mvar step change in 2 ms by a VSC‐based STATCOM, using forced‐commutated GTOs, was a major improvement in response time than what was obtained in a naturally commutated, thyristor‐based SVC. The SVC uses thyristors, which have a natural transport delay of half of a power cycle, meaning when a thyristor turns on in a positive half cycle, it turns off naturally in the next negative half cycle following the zero‐crossing of the voltage. Figure 1-14 shows a two‐month voltage profile without an SVC (left) and with an SVC (right). This is a significant improvement over no compensation. The experience of the last five decades has shown that the response time of several cycles using an SVC is quite adequate in most utility applications. This is primarily why the higher cost of the VSC technology with component obsolescence issues have prevented its widespread use/adoption in utility applications.

Figure 1-14 Voltage profile without an SVC (left) and with an SVC (right) (field performance) (Barot et al. 2014).

However, the fast response from a VSC may be just the right solution to address various issues in addition to var compensation, such as

 Unbalanced voltage

 Harmonic voltage and current

 Voltage spikes

 Voltage flicker.

One such application is shown in Figure 1-15 where a voltage source (V_source) supplies power to an electric arc furnace load through a network whose impedance is represented as Thèvenin impedance (Z_Th). The bus voltage (V_plant) at the input of the plant is stepped down twice through, first, the Main Transformer and, then the Arc Furnace Transformer.

Figure 1-15 A single‐line diagram of an electric arc furnace.

Figure 1-16 Instantaneous plant input bus voltage (v_plant) and plant current (i), drawn by a typical electric arc furnace (field performance) (Sen 2015).

A typical instantaneous plant input bus voltage (v_plant) and current (i), drawn by an electric arc furnace in three phases (A, B, and C) without any compensation, are shown in Figure 1-16. During the operation of the arc furnace, the random nature of the load current (i.e. electric arc) creates proportionally rapid and random voltage changes across Z_Th. When this random voltage across Z_Th is subtracted from the regulated source voltage (V_source), a varying voltage, known as voltage flicker, is created at the input voltage bus of the plant. However, the fast‐acting STATCOM is capable of providing a unique solution to reduce this voltage flicker by supplying the fluctuating active power (Figure 1-19) and reactive power (Figure 1-18) needs of the rapidly‐changing load of the electric arc furnace. Since a typical load is more inductive than capacitive in nature, the compensation scheme is designed to be more capacitive than inductive in nature. Since STATCOM operates both in capacitive and inductive modes in near‐equal ranges, a STATCOM, combined with an optional Fixed Capacitor (FC) may be an appropriate solution in this application. In 1998, Westinghouse installed a +140/–20‐MVA‐rated power electronics VSC‐based shunt compensator, consisting of a ±80‐MVA‐rated STATCOM and a +60‐MVA‐rated FC at a steel plant as shown in Figure 1-17.

The voltage (V_comp) at the compensation bus is regulated to maintain the voltage flicker at an acceptable level. The control philosophy in this project is given by the following equations:

(1‐6)

Figure 1-17 Use of a STATCOM, combined with a Fixed Capacitor (FC) as an arc furnace compensator.

Figure 1-18 Instantaneous reactive power drawn by the furnace and supplied by the compensator (STATCOM and FC) (field performance) (Sen 2015).

where

 Putil is the active power supplied by the utility

 Pfurn is the active power used by the arc furnace

 Pcomp is the active power exchanged by the compensator

and

(1‐7)

where

 Qfurn is the reactive power used by the arc furnace

 Qcomp is the reactive power exchanged by the compensator.

As shown in Figure 1-18, the compensation scheme supplies nearly all the instantaneous reactive power (q) drawn by the furnace.

This preferred solution for a utility application is to use a larger‐than‐usual‐sized DC capacitor, which provides a sink and a source of fluctuating active power for the load, thereby leveling off the active power drawn from the utility source. The instantaneous active power (p) drawn by the compensator, which is composed of a STATCOM and a FC, and the furnace and supplied by the utility are shown in Figure 1-19. The negative active power drawn by the compensator indicates that a part of the furnace power is supplied by the compensator. If the size of the DC capacitor was further increased to support a larger amount of fluctuating active power, the active power drawn from the utility could be made equivalent to a fixed resistive load.

A comparison of voltage flicker (termed as Level) without and with a compensation, provided by the STATCOM and the FC, is shown in Figure 1-20. The flicker measurements without and with a compensation show that the STATCOM was designed correctly to reduce the flicker to a desired level of below 0.25% at 75 MW of load. Therefore, it was not commercially needed to increase the size of the DC capacitor any further. Even though this is the most useful application of a VSC‐based STATCOM, it was also retired from service due to component obsolescence within a few years after commissioning and a less perfect, but adequate solution – naturally commutated, thyristor‐based SVC was installed in its place.

Figure 1-19 Instantaneous active power drawn by a compensator (STATCOM and FC) and a furnace and supplied by a utility (field performance) (Sen 2015).

Figure 1-20 Flicker measurements without and with a compensation provided by a STATCOM (field performance) (Sen 2015).

The first generation of FACTS Controllers consisted of GTO‐based VSCs. Since the commissioning of the world’s first commercial STATCON at TVA in 1995, nine GTO‐based VSCs were built. During the same time, several IGBT‐based VSCs were also built. During the past two decades, the IGBT technology has advanced by several generations. So, what about the GTO technology? Unfortunately, it is no longer manufactured. While the IGBT is the industry workhorse at the present time, the future trend is to develop wide bandgap‐type Silicon Carbide (SiC)‐ and Gallium Nitride (GaN)‐based switches due to the several attractive attributes: higher‐temperature operation, smaller turn‐on and turn‐off times that result in a lower switching loss, smaller gate‐drive circuit, smaller snubber circuit, lower cooling requirement, and so on. So how can one source spare parts for the GTO‐based FACTS Controllers? That is not an easy question to answer. Most of the installations with GTO‐based VSCs were dismantled after a short life span, which was unforeseen at the inception of FACTS development. Based on the facts about FACTS Controllers of the last three decades, component obsolescence is the primary reason for a 10–15 years of life span of power electronics‐based VSCs.

In this context, one might ask why not use the latest high‐power electronics switches to replace the aging outdated switches? The answer is simple – it is impossible. Even though there is no available switch, which can be considered as a perfect switch, meaning zero forward voltage across the switch during conduction and zero transition time from on‐to‐off and vice versa, the snubber circuit, gate‐drive circuit, and cooling requirement vary from a VSC that is made with one type of switch to another VSC that is made with a different type of switch. This fact alone forces the aging VSCs to retire when spare parts are unavailable.

Moreover, the control cabinet becomes completely outdated in a decade or so, requiring an upgrade. There is no initiative to keep any commercially used, legacy power electronics system alive after it passes its natural longevity. These facts need to be taken into account to calculate the true cost of power electronics VSC‐based FACTS Controllers. In fact, the same is true for any power electronics‐based system, including an IBR.

Figure 1-21 Single‐line diagram of a Dynamic Voltage Restorer (DVR).

Even with the abovementioned challenges about the power electronics VSC‐based solutions, there are applications where inverters are just the right solutions. One such application is the Dynamic Voltage Restorer (DVR). The purpose of a DVR, as its name suggests, is to restore the voltage of a critical load if a sag or swell occurs on any phase of the critical load, such as plants where good voltage regulation is essential to avoid a shutdown of production. DVR is a flexible voltage compensator that is needed for sensitive loads that requires a good voltage quality. IEEE 1159 defines a voltage sag as “the decrease in the rms voltage level to 10% – 90% (1–90% for EN 50160 standard) of nominal, at the power frequency for durations of ½ cycle to one (1) minute.” Note that the equivalent terminology from the IEC is “dip.”

Figure 1-21 shows a single‐line diagram of a DVR that feeds a critical load, which is located on a power distribution line that is one of several lines supplied by a transmission line through a stepdown transformer. The heart of a DVR is a DC–AC converter (also called an inverter or VSC), which is normally bypassed with a thyristor‐based bypass‐switch. The DC capacitor is trickle‐charged through a stepdown transformer, a rectifier, and a DC–DC converter.

During a voltage sag or swell due to a short‐circuit fault, lightning or capacitor bank switching on an adjacent distribution line, the bypass‐switch opens and a voltage that is generated by the VSC is placed in series with the load through a coupling transformer to restore the load’s nominal voltage while the conventional system protection equipment clears the fault.

Since a DVR is installed upstream of a sensitive load, it senses the sagged or swelled voltage on its input terminals. The DVR control system is such that it continually compares the input voltage waveform with an internal reference voltage signal and determines the proper voltage that must be added or subtracted to restore the nominal voltage at the load terminals. Figure 1-22 shows a sag correction by a DVR during a field test. The sag occurred in one phase, which was corrected within a few ms.

A DVR is generally designed to store a fixed amount of energy that can be used for voltage compensation during a designated amount of time, for example, 30 fundamental frequency cycles during a voltage sag or swell. This designated amount of time may be adequate to protect a sensitive load, such as a semiconductor manufacturing plant from a voltage sag or swell. However, in some applications where the input voltage to a critical load requires a continuous regulation within a specified range, the guaranteed voltage support for 30 fundamental cycles may be a limitation. This deficiency can be mitigated by a more general controller, which was designed in the 1980s and called an Active Power Line Conditioner (APLC). This electronic circuit topology was patented by Westinghouse in 1987 (U.S. patent number 4,651,265, titled “Active Power Conditioner System”). The APLC introduced the concept of a common DC link between two forced‐commutated switch‐based VSCs, which are connected back‐to‐back at their common DC link. One VSC is connected in shunt and another is connected in series with the line that supplies a load. These VSCs exchange active power (P_link) between the shunt‐ and series‐connected VSCs for continuous regulation of line voltage in distribution‐level applications as shown in Figure 1-23a.

Figure 1-22 Sag correction by a DVR (field performance) (Sen 2015).

The APLC extends the concept of an autotransformer, which is also a Shunt–Series configuration, meaning one unit is connected in shunt and the other unit is connected in series with the line. The APLC configuration, shown in Figure 1-23a, is identical to a stepdown autotransformer that supplies power to a load on the low‐voltage side. The major difference between an APLC and an autotransformer is that the Shunt and the Series Units in an autotransformer exchange active power as well as reactive power. However, in an APLC, only active power is exchanged between the Shunt and the Series Units, since reactive power cannot flow through the common DC link capacitor. The same Shunt–Series VSCs concept was used later in the 1990s in the design of the Unified Power Flow Controller (UPFC) for regulation of line power in transmission‐level applications as shown in Figure 1-23b.

The lessons learned from the installations of the first‐generation FACTS controllers, such as ±160 MVA‐rated UPFC at American Electric Power (AEP), ±100 MVA‐rated CSC at New York Power Authority (NYPA), and ±80 MVA‐rated UPFC at Korea Electric Power Corporation (KEPCO), are that FACTS controllers have limited applications due to their high life‐time costs, which include installation, operation, and maintenance (specialized equipment and trained labor). The main feature of very fast (millisecond‐range) response time, offered by the power electronics inverter‐based FACTS controllers is not needed in most utility applications. In search for the right PFC at an affordable price, the Shunt–Series configuration is used to create the Sen Transformer, which can provide a solution to meet the majority of power flow control needs for the utilities worldwide.

Figure 1-23 (a) Basic circuits for Active Power Line Conditioner and (b) Unified Power Flow Controller.