Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers. Kalyan K. Sen

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Figure 1-11 Transmission line Reactance Regulators: (a) Thyristor‐Controlled Series Capacitor (TCSC) and (b) Static Synchronous Series Compensator (SSSC).

      When discussing dynamic and transient events, mechanical LTCs react in ≈ 3–5 s (3,000–5,000 ms); TC LTCs react in < 1 s (1,000 ms) and IGBT‐based converters react in < 0.010 s (10 ms). These different technologies are referred to as slow, medium speed, and fast, respectively. Note that as the response time of a particular solution increases from slow using mechanical LTCs to medium speed using TC LTCs to fast using IGBTs, there is a corresponding increase in the solution’s life‐cycle costs (installation, operation, and maintenance), complexity, and impracticability of relocation. Other important features to consider are reliability, efficiency, component non‐obsolescence, and interoperability.

      For more than a century, the transmission line voltage magnitude has been regulated with transformers and tap changers. They are referred to, in this book, as the VRT in the form of a two‐winding transformer with galvanically isolated primary and secondary windings, called a Shunt–Shunt configuration, and an autotransformer with an electrical connection between the primary and secondary windings, called a Shunt–Series configuration. In both types of transformers, the line voltage is applied to the primary windings. In the two‐winding transformer, the full line voltage is induced in the secondary windings, whereas, in the autotransformer, only a fraction of the line voltage is induced in the secondary windings that are connected to the primary windings to produce the full line voltage. In both cases, the magnitude of the line voltage is regulated. The secondary voltage is varied with the use of LTCs. An LTC can step up/down the voltage without interruption of the load current. Both primary and secondary windings in the two‐winding transformer carry the full transmitted power. Both primary and secondary windings in the autotransformer carry only a fraction of the full transmitted power. Therefore, if the galvanic isolation is not needed, the rating of the transformer can be significantly reduced with a Shunt–Series configuration as compared to a Shunt–Shunt configuration. Regardless of which configuration is used, the voltages at the input (primary) and output (secondary) terminals of both a two‐winding transformer and an autotransformer are identical as discussed in Chapter 4, Section 4.1.

The primary reason for voltage regulation is due to the exchange of reactive power at the Point of Connection (POC) to the utility. However, a transformer neither generates nor absorbs reactive power. If a transformer delivers reactive power at one side (primary or secondary), it absorbs the same amount of reactive power on the other side (secondary or primary). Therefore, in the process of increasing voltage on the secondary side, it reduces voltage on the primary side. The opposite is true as well when, in the process of decreasing voltage on the secondary side, it increases voltage on the primary side. Figure 1-12 shows that a compensating voltage of ±15% of the natural primary voltage (V_sn) of 0.988 pu results in a secondary voltage (V_s′) in the range of 0.872 to 1.095 pu. In the process, the primary voltage (V_s) varies in the range of 1.022 to 0.945 pu. Therefore, a desired 15% change in voltage at the secondary terminal may result in a net 10.7% increase due to the reduction of voltage at the primary terminal and a net 11.6% decrease due to the increase of voltage at the primary terminal as discussed in Chapter 4, Section 4.1.

The indirect way to regulate the magnitude of the line voltage is to connect a reactor or a capacitor in shunt with the line. A shunt‐connected reactor absorbs reactive power from the line and lowers the line voltage, whereas a shunt‐connected capacitor raises the line voltage with its generated reactive power as discussed in Chapter 2. With a series‐connected switch, such as back‐to‐back thyristors (triac), whose duty cycle can be varied, the shunt reactor can be made to operate as a variable reactor, which is called a Thyristor‐Controlled Reactor (TCR). A Thyristor‐Switched Capacitor (TSC) connects fixed capacitors in a step‐like manner in shunt with the line through triacs. Therefore, a combination of the variable reactor and a parallel capacitor acts as a variable compensating reactor or capacitor, which is called SVC.