1, the role of SVG
SVG is a typical power electronic equipment, which is composed of three basic functional modules: detection module, control operation module and compensation output module. Its working principle is to detect the current information of the external CT system, and then analyze the current information, such as PF, S, Q, etc., through the control chip; Then the controller gives the compensated drive signal, and finally the inverter circuit composed of the power electronic inverter circuit sends out the compensated current.
The SVG static reactive power generator consists of a self-commutating bridge circuit composed of a turn-off power electronic device (IGBT), which is connected to the power grid in parallel through the reactor, and the amplitude and phase of the output voltage on the AC side of the bridge circuit can be properly adjusted, or the current on the AC side can be directly controlled. Quickly absorb or emit the required reactive power to achieve the purpose of rapid dynamic adjustment of reactive power. As an active compensation device, it can not only track the impulse current of impulse load, but also track and compensate the harmonic current.
2. Differences between SVG and SVC
SVG is the abbreviation of StaticVarGenerator, which means static reactive power generator. SVC is the abbreviation of StaticVarCompensator, which means reactive power compensator
(1) SVG
It can be divided into voltage type and current type, which can provide both lagging reactive power and leading reactive power. Simply put, the basic principle of SVG is that the self-commutating bridge circuit is connected to the power grid through the reactor or directly in parallel, and the phase and amplitude of the output voltage on the AC side of the bridge circuit are properly adjusted, or the current on the AC side is directly controlled, so that the circuit can absorb or emit reactive current to meet the requirements and achieve the purpose of power reactive compensation.
(2) SVC
It is a typical power electronic device for reactive power compensation, which uses thyristor as a solid-state switch to control the capacity of the reactor and capacitor in the access system, thereby changing the admittance of the transmission system. According to the different control objects and control methods, it is divided into the static reactive power compensation device (FC+TCR) used with thyristor control reactor (TCR) and thyristor switching capacitor (FC) and the device used with TCR and mechanical switching capacitor (MSC).
Reviews: SVG is the main equipment to adjust the system voltage, I think its core is the self-commutating bridge circuit, through IGBT (fan in accordance with the component) control to achieve the current change of the self-commutating bridge circuit, and the self-commutating bridge circuit generally has a number of power units (currently not clear) in series organization, forming a star-shaped connection. A compensating current is generated to adjust the bus voltage.
3. Advantages of SVG compared with SVC
STATCOM has the following advantages compared with synchronous modulation camera and SVC device:
1) Using digital control technology, the system has high reliability, basically does not need maintenance, and can save a lot of maintenance costs;
2) The performance of improving the transient stability of the system and damping the oscillation of the system is much better than that of the traditional synchronous modulation camera;
3) The control is flexible, the adjustment speed is faster, the adjustment speed is wide, and the response speed can be adjusted continuously and quickly under the inductive and capacitive operating conditions.
4) static operation, safe and stable, no large rotating equipment like the adjusting camera, no wear, no mechanical noise, will greatly improve the life of the device, improve the environmental impact;
5) The capacitor capacity requirements are not high, can save the conventional device in the large inductance and large capacitance and huge switching mechanism, so that STATCOM small volume, low loss;
6) Small connection reactance. The connection reactance of STATCOM to the power grid is to filter out the higher harmonics existing in the current, and to connect the converter to the power grid, so its inductance is far less than the inductance required to compensate SVC devices such as TCR with the same capacity
7) 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
8)STATCOM has a faster response speed than SVC, so it is more suitable for suppressing voltage flicker. STATCOM response time is less than 10ms, while SVC response time is generally 20-40ms. STATCOM from rated capacitive reactive power to rated inductive reactive power (or vice versa) can be completed within 1ms, this response speed is fully capable of compensating the impact load;
9) 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.
10) 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.
11)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.
Comparative analysis of SVG and SVC reactive power compensation devices
Compared to SVC, SVG is superior to SVC in the following ways:
First, different working principles
(1)SVC can be regarded as a dynamic reactive power source. According to the needs of access to the grid, it can provide capacitive reactive power to the grid, it can also absorb the excess inductive reactive power of the grid, the capacitor bank is usually connected to the grid as a filter bank, it can provide reactive power to the grid, when the grid does not need too much reactive power, these excess capacitive reactive power, by a parallel reactor to absorb. The reactor current is controlled by a thyristor valve group. By adjusting the thyristor trigger phase Angle, the effective value of current flowing through the reactor can be changed, so as to ensure that the reactive power of SVC in the grid access point can just stabilize the voltage of the point within the specified range, and play the role of reactive power compensation.
(2)SVG takes high-power voltage-type inverter as its core. By adjusting the amplitude and phase of the output voltage of the inverter, or directly controlling the amplitude and phase of the current on the AC side, it can rapidly absorb or emit the required reactive power and realize the purpose of rapid dynamic adjustment of reactive power.
Second, the response speed is fast
General SVC response speed is 20-40ms; The response speed of SVG is no more than 5ms, which can better suppress voltage fluctuation and flicker. Under the same compensation capacity, SVG has the best compensation effect for voltage fluctuation and flicker.
Third, low voltage characteristics are good
SVG has the characteristics of a current source, and the output capacity is little affected by the bus voltage. The lower the system voltage, the more the need for dynamic reactive power regulation voltage. SVG has good low-voltage characteristics, and the output reactive current has no relationship with the system voltage, so it can be regarded as a controllable and constant current source. When the system voltage is reduced, it can still output rated reactive current and has strong overload capability. While SVC is an impedance characteristic, the output capacity is greatly affected by the bus voltage, the lower the system voltage, the output reactive current capacity is proportionally reduced, and there is no overload capability. Therefore, the reactive power compensation capability of SVG is independent of the system voltage, while the reactive power compensation capability of SVC decreases linearly with the decrease of the system voltage.
Fourth, the operation safety performance is improved
SVC uses thyristor to adjust the reactance and multiple sets of capacitors as the main means of reactive power compensation, which is very prone to resonant amplification, resulting in safety accidents. When the system voltage fluctuates greatly, the compensation effect is greatly affected and the running loss is large. SVG supporting capacitors do not need to set filter banks, there is no resonance amplification phenomenon, SVG is an active compensation device, is the use of turn-off device IGBT current source device, thus avoiding the phenomenon of resonance, operation safety performance is greatly improved.
Fifth, harmonic characteristics
SVC uses thyristor to control the equivalent fundamental impedance of the reactor, which is not only greatly affected by the harmonics of the system, but also produces a large number of harmonics by itself, so the harmonic content generated by SVC must be filtered by a filter bank. SVG adopts three-level single-phase bridge technology, single-phase can output 5-level voltage waveform, and adopts the pulse modulation method of carrier phase shift, which is not only less affected by system harmonics, but also can suppress system harmonics. Compared with SVC, SVG greatly reduces the harmonic content in the compensation current by adopting multiple, multilevel or pulse width adjustment techniques.
Sixth ,Small footprint
With the same compensation capacity, the footprint of SVG is reduced by 1/2 to 2/3 compared to that of SVC. Because SVG uses fewer reactors and capacitors than SVC, it greatly reduces the size and footprint of the device. The reactor in SVC is not only relatively large in size, but also takes into account the installation interval between each other, and the overall footprint is larger.
To sum up, SVG reactive power compensation device can greatly improve the power quality of the power grid due to the advantages of fast response speed, low harmonic content and strong reactive power regulation ability, and has become the development direction of reactive power compensation technology.