STATCOM | Static Synchronous Compensator

Introduction

Static Synchronous Compensator was developed as an advanced static VAR compensator where a voltage source converter (VSC) is used instead of the controllable reactors and switched capacitors. Although VSCs require self-commutated power semiconductor devices such as GTO, IGBT,
IGCT, MCT, etc (with higher costs and losses) unlike in the case of variable impedance type SVC which uses thyristor devices, there are many technical advantages of a STATCOM over an SVC. These are primarily:

• (a) Faster response
• (b) Requires less space as bulky passive components (such as reactors) are eliminated
• (c) Inherently modular and relocatable
• (d) It can be interfaced with real power sources such as battery, fuel cell, or SMES (superconducting magnetic energy storage)
• (e) A STATCOM has superior performance during low voltage conditions as the reactive current can be maintained constant (In an SVC, the capacitive reactive current drops linearly with the voltage at the limit (of capacitive susceptance). Increasing the reactive current in a STATCOM under transient conditions is even possible if the devices are rated for transient overload. In an SVC, the maximum reactive current is determined by the rating of the passive components – reactors and capacitors.

A ± 80 MVA STATCOM using 4.5 kV, 3000 A GTO devices was developed in Japan in 1991. A ±100 MVA STATCOM, also based on GTOs (4.5 kV and 4000 A (peak turn off)) was commissioned in late 1995 at Sullivan substation of Tennessee Valley Authority (TVA) in the U.S.A.

The major objective of the prototype installation is to regulate the 161 kV bus voltage during daily load variations so that the duty of the tap changers on the transformer banks is minimized. (The failure of tap changers is a common problem when they are forced to act continuously). The STATCOM was originally called advanced SVC and then labeled as STATCON (STATic CONdenser).

Principle of Operation of STATCOM

A STATCOM is comparable to a Synchronous Condenser (or Compensator) which can supply variable reactive power and regulate the voltage of the bus where it is connected. The equivalent circuit of a Synchronous Condenser (SC) is shown in Fig, which shows a variable AC voltage source (E) whose magnitude is controlled by adjusting the field current.

Neglecting losses, the phase angle (δ) difference between the generated voltage (E) and the bus voltage (V ) can be assumed to be zero. By varying the magnitude of E, the reactive current supplied by SC can be varied when E = V and the reactive current output is zero. When E > V, the SC acts as a capacitor whereas when E < V, the SC acts as an inductor. When δ = 0, the reactive current drawn (I_r) is given by

I_r = V − E / X^‘

A STATCOM (previously called a static condenser (STATCON)) has a similar equivalent circuit to an SC. The AC voltage is directly proportional to the DC voltage (Vdc) across the capacitor (see Fig. which shows the circuit for a single phase STATCOM) If an energy source (a battery or a rectifier) is present on the DC side, the voltage Vdc can be held constant.

The self-commutated switches T₁ and T₂ (based on say GTOs) are switched on and off once in a cycle. The conduction period of each switch is 180◦ and care has to be taken to see that T₁ is off when T₂ is on and vice versa. The diodes D₁ and D₂
enable the conduction of the current in the reverse direction. The charge on the capacitors ensures that the diodes are reverse-biased.

Applications of STATCOM

STATCOM is an advanced type of SVC (based on VSC), and the motivations for the applications are similar to that of an SVC. A STATCOM has several advantages over a SVC, namely

• (i) Better performance under low voltage conditions,
• (ii) Faster response that is independent of the system conditions and
• (iii) Reduced space requirement (smaller footprint).

Also, a STATCOM can be built in a modular fashion which will reduce the engineering costs. The primary objective of applying STATCOM in transmission networks is the fast regulation of voltage at a load or an intermediate bus. This will also increase the power transfer capacity of the network and thus enhance ATC (Available Transfer Capacity).

The smaller footprint and faster response of a STATCOM provides an opportunity to import power (for an urban area) from distant economic generators while the voltage is stabilized with a local STATCOM (older, uneconomic generators have been used in urban areas primarily for voltage regulation). The use of multi-pulse and/or multilevel converters eliminates the need for harmonic filters in the case of STATCOM.

However, the costs increase not only due to the increased costs of magnetics and self-commutated devices (such as GTO thyristors) but also resulting from increased losses. (The total losses can vary from 0.5 to 1.0%). The new developments in power semiconductor technology are expected to bring down the costs and losses. The voltage and power ratings are expected to increase.

At present, the use of STATCOM in distribution systems has become attractive, not only for voltage regulation but also for eliminating harmonics and improving power quality. Utilizing an auxiliary controller with a Thevenin Voltage signal as input, it possible to damp both low-frequency oscillations and sub-synchronous oscillations. In the latter case, the objective is to provide adequate damping torque in the critical range of torsional frequencies.

Frequently Asked Questions (FAQs)

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