1.The role of SVG
Static Var Generator is a representative power electronic device, which includes three functional modules: sensing module, control calculation module and compensation output module. Its working principle is to use the current information collected by external CT, and analyze the current through the control chip, such as PF, S, Q, etc. On this basis, the inverter circuit is used to compensate it and output it to the output end.
Static Var Generator uses the self-commutating static reactive power generator composed of IGBT (IGBT), and connects it to the power grid through the reactor, so that the voltage amplitude and phase of AC end can be reasonably adjusted, or the current of AC end can be directly regulated. Quickly absorb or release the required non-functional quantity to achieve rapid and dynamic adjustment of reactive power. This method can track both the pulse current of pulsed load and the harmonics.
2. Difference between SVG and SVC
(1)SVG
According to the actual situation, a new reactive power compensation method, voltage compensation and current compensation, is proposed. The basic principle of SVG is to use a reactor or parallel commutator circuit to adjust the phase and amplitude of the output voltage on the AC side, or directly control the current on the AC side, so as to achieve the effect of reactive power compensation.
(2)SVC
It is a kind of thyristor as a solid switch, by adjusting the capacity of capacitors and reactors in the power grid, to achieve the adjustment of power grid admittance. According to the difference of control object and control mode, it can be divided into static reactive power compensation device (FC+ TCR) and TCR (MSC) composed of thyristor control reactor (TCR) and thyristor switching capacitor (FC+ TCR).
3. The advantages of SVG and SVC comparison
The output current of STATCOM is independent of voltage, and it is characterized by constant current source and has a wider operating range. The essence of SVC is impedance compensation, and the output current and voltage are linear. Therefore, when the system voltage is low, the same capacity STATCOM can provide greater compensation capacity than SVC
STATCOM has a faster response time than SVC, so it is more suitable for suppressing voltage flicker. STATCOM response time is less than 10 ms, while SVC response time is generally 20 to 40 ms. STATCOM from rated capacitive reactive power to rated inductive reactive power (or vice versa) can be completed within 1 ms, this response speed is fully capable of compensating the impact load;
The bridge circuit of STATCOM uses multiplicity technology, multilevel technology or PWM technology to eliminate the lower frequency harmonics, and reduce the higher frequency harmonics such as 7 and 11 to an acceptable degree. SVC itself needs to produce a certain amount of harmonics, such as the 5 and 7 characteristic harmonics of TCR type are relatively large, accounting for 5% to 8% of the fundamental wave value. Other high order harmonics such as SR, TCT and so on also produce 3, 5, 7, 11, etc., which brings many difficulties to the filter design of SVC system.
Under fault conditions, STATCOM has better control stability than SVC. SVC uses a large number of capacitor reactors, and when the capacity of the external system is comparable to the capacity of the compensation device, SVC will produce instability. STATCOM is not sensitive to external system operating conditions and structural changes.
STATCOM occupies a smaller area and has lower cost than SVC with the same capacity. Because STATCOM uses DC container to store energy, the capacitor volume can be reduced, and the reactive power can be controlled smoothly without shunt reactor, so the installation size is greatly reduced. 12)STATCOM can provide active power within a certain range to reduce active power shock. SVC can only provide reactive power and does not provide active power.
