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Active Power Filters

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SAHF working principles

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 SAHF functional diagram 


◆Filtering principles

SAHF conducts real-time collection of current signal through external CT, separates the harmonics part through internal detection of circuit, and generates the compensating current which is opposite to the equal harmonics phase in the system by means of IGBT power converter, to filter the harmonics.

SAHF outputting compensating current is subject to harmonic content in the system, thus avoiding the occurrence of overcompensation. In addition, SAHF has function of internal overload protection. When harmonic content in the system is greater than filter’s capacity, SAHF can automatically limit its output within 100% rated capacity to avoid occurrence of filter overload. 

Principles for reactive power compensation 

By setting the parameters, users can make SAHF conduct dynamic reactive power compensation while filtering the harmonics. 

According to reactive power of the system, SAHF generates capacitive or inductive fundamental current through IGBT power converter to realize dynamic reactive power compensation. The compensation target value can be set on SAHF operation panel. Overcompensation will not be occurred and compensation will be stepless and not generate surge current impact against load and power grid. 


SAHF internal control principles


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SAHF internal control schematic diagram


As shown in Figure 2-2, after turning on the disconnecting switch and in order to prevent the capacitor of DC bus from instant impact after charging, SAHF firstly charges the capacitor of DC bus by using soft start resistance and this process will hold tens of seconds. When the Udc of bus voltage reaches the preset value, main contactor will be closed. As a power storage component, DC capacitor supplies power for outwardly outputting the compensating current through IGBT inverter and internal reactor. Through external CT, SAHF conducts real-time collection of current signal and transmits the signal to signal conditioning circuit, then transmits FPGA to the controller, which will separate fundamental wave elements and extract all harmonics; the controller will take the difference value which is obtained by contrasting between the collected harmonics elements and the compensating current generated by  SAHF as the real-time compensating signal and output it to the driving circuit. IGBT converter will be triggered to inject the compensating current for harmonics to the power grid, achieving the function of harmonics filtering.

APFs characteristics of dynamic state and steady state
Especially speaking of SAHF is its excellent characteristics of dynamic state and steady state. The following figure shows SAHF300A-400-3L
APF’s dynamic state characteristics when the load is changed and steady state characteristics when fully loaded.

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Features of SAHF series 

1.Have the ability to simultaneously filter second – fiftieth harmonics 

2.Response time is less than 0.1ms and full compensation time for step-changed harmonics is less than 10ms.

3.Adopt 5.7 inches Chinese color touch screen with high resolution 

4.Have the function of removing three-phase unbalance 

5.Have the function of automatic current limiting and free from overload

6.Be able to conduct standby function management 

7.Be able to only implement filtering or simultaneously implement filtering and reactive power compensation, as well as set the priority of functions 

8.Filter outputting adopts LCL filtering structure. This will not only ensure that higher harmonic generated by IGBT high frequency switch will not be injected into power grid when SAHF makes compensation to obtain a better compensation effect, but also it is applicable for any resistance of on-site power grid system. It can also prevent from resonance to guarantee the safety of device. 

9.Adopt Spartan3adsp FPGA produced by Xilinx Company for centralized control. FPGA clock frequency can reach up to 200MHz. Inside FPGA clock are 84 hardware DSP units with DSP parallelly running. Its arithmetic speed is far higher than that of a single DSP control mode, along with small communication delay and quick response speed. This is good for adopting more advanced control algorithm to make SAHF SAHF achieve more excellent filtering and compensating effect. 

10.SAHF APF’s can allow 8 devices to be connected parallelly and only one FPGA is used for centralized control to overcome the shortcomings such as communication delay among several controllers and mutual interference. This enhances SAHF compensation functions and improves the reliability of system.

11. SAHF APF’s adopt strategy of closed-loop control and combine with the original self-adapting control algorithm for current average. Because switch frequency controlled by current average is constant, they overcome shortcomings of traditional hysteretic current control such as a wide range of output spectrum, difficult for filtering, interference from high frequency harmonic to power grid, etc., which are caused by switch frequency, to make SAHF achieve excellent filtering function.   

12.Simple design and model selection. Unnecessary to thoroughly analyze the power grid and only require to measure harmonic current.

13.Carry out fault logging and event recording, as well as be able to inquire about history records. 


Product model

Product model and its meanings
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Model for SAHF series products 

【Note】

1. Series code: SAHF series
2. Mode of connection: 3L stands for three-phase three-wire system and 4L stands for three-phase four-wire system
3. Rated voltage: 
400V
4. Rated current: 50~350A

【Example】:
SAHF300A-400-3L refers to SAHF series low-voltage APF's with rated voltage 400V and rated compensating current 100A. Its mode of connection is three-phase three-wire system. 
In actual practice, according to users’ requirements, separately SAHF series capacities or carry out integrated application by combining
with tuning and compensating device. 

For product model and corresponding voltage level, rated compensating current, mode of power distribution, see Table 2.

Table 2   List for 400V SAHF/4L APF’s Models

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Table 3   List for 400V SAHF/3L APF’s Models 

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Table 4   List for 690V SAHF/3L APF’s Models 

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Product selection and ordering


1. Scope of application 

a)Applicable for harmonic suppression in low-voltage system.

b)As for situation in which harmonics are serious and reactive power changes a lot, it’s recommended to use pure SAHF-type compensating devices. 

c)As for situation in which harmonics are serious while reactive power changes a little, it’s recommended that SAHF devices are used to administrate harmonics and parallel capacitor bank are used to compensate power factor. 

2. Ordering instruction

a)Please inform the specification, model No. and quantity of the required product.

b)Offer the rated capacity of transformer, requirements for compensating system, system operating voltage, active power, power factor and load current harmonic data. 

c)Inform in detail the main load characteristics and harmonic content. If load harmonic content can not be analyzed or impossibly detected, our company can supply harmonics precision instruments for testing and professional software for analysis.

d)Appearance and size for the body of compensating box will be designed according to users’ requirements (such as HNS, GCS, GCK, GGD, etc.).

e)If needed to connect with other switch boards, system diagram, box type, body size, color, installation location, etc., shall be supplied. 

3.Installation

Please comply with installation conditions as shown in Table 5.


Table 5 Installation conditions


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