1. Hao Zhai, Hao Yi, Zhirong Zeng, Zhenxiong Wang, Feng Wang, Fang Zhuo
A network-wide harmonic predictive control method for SAPF based on online harmonic impedance measurement
Hao Zhai, Hao Yi, Zhirong Zeng, Zhenxiong Wang, Feng Wang, Fang Zhuo
State Key Laboratory of Electrical Insulation and Power Equipment, Xi‘an Jiaotong University
Xi’an, , China
Shunt active power filters (SAPF) generally compensate harmonic current of centralized non-linear loads, which may cause undesired economical burden for networks with distributed harmonic sources. This paper proposed an SAPF predictive control method based on online impedance measurement which can suppress the network-wide harmonics systematically in an optimized way.
Key words: «Active Filter», «Impedance Measurement», «Power Quality», «Harmonics».
Modeling analysis for SAPF compensating network-wide harmonics
Network harmonic analyzing modeling is the basis of network-wide harmonic suppression design, where the choice of non-linear load model is one of the key issues which affect the accuracy and complexity. Three typical models of nonlinear loads are compared as follows:
Constant current source model
To use this model, the typical harmonic current spectrum of each non-linear load should be acquired in advance, thus the harmonic current of nonlinear loads can be obtained based on the information of fundamental power flow. The influence of network-side harmonic voltage is ignored in the model, so the accuracy cannot be ensured when the network-side voltage is distorted evidently.
Norton equivalent circuit model
The model shows better precision as it uses a harmonic current source paralleled with impedance at each order to simulate the external characteristics of network-side harmonic voltage and current.. The model parameters will change with the operating point of the load.
Coupled admittance matrix model
The relationship between network-side harmonic voltage and current at different orders of some typical nonlinear loads (2/3-phase rectifiers, TCRs) can be established by coupled admittance matrix parameters, as shown below:
The model shows very high precision as it bases on the working principle of power electronic converters and can take harmonic coupling relations of all typical frequencies into account.
Because the accuracy of the first model is poor under highly distorted conditions and the matrix model has too many parameters, so Norton model is utilized in this paper, and impedance measurement are applied to update the model parameters. For an m-node balanced network with distributed nonlinear loads, the relationship between voltage and current : consider an SAPF at node q, the harmonic voltage change will be
In order to predict the harmonic voltage change, the key is to measure
harmonic transfer impedances.

2. Simulation verificationto
Predictive control method of SAPF for network-wide harmonic suppression
System introduction
Some measuring devices are installed at the nodes to be compensated, by which the real time network parameters can be obtained online.
A predictive controller is added to the SAPF which will take the place of traditional current instruction generator, and it can measure impedance online by the obtained information and apply them in the optimization of SAPF instruction.
2. Description of the predictive controller
Step1:calculate harmonic transfer impedances:
Step 2: generate the optimized current instruction of SAPF
The harmonic voltage can be predicted as
And the objective function (OF) will be:
Finally the new current instruction is:
The current instruction is shown in each rectangle box and updated with an interval of T.
Green lines: the process of collecting information
Blue lines: optimization calculation
Fig.1 System structure
Fig.2 System timing diagram
Simulation verification
A 7-node 3-phase system is built in PSCAD/EMTDC to test the proposed method. There are 3 uncontrolled rectifiers and 3 linear loads in the network. An SAPF is installed at node 7 to compensate the network-wide harmonic at 5th.
Table I Simulation parameters
Grid voltage(v)
Line impedance
240(rms)
∠0。 Z1 Z2 Z3 Z4 Z5 Z6 Z7
0.1Ω
0.05Ω
0.03Ω
0.06Ω
0.04Ω
0.02Ω
0.07Ω
Load1
Load2
Load3
DC-side of
Rectifier 1
Rectifier 2
Rectifier 3
1.2Ω
1.6Ω
2.0Ω
1mH+1Ω
1.5mH+1.5Ω
2mH+0.5Ω
Fig.3 Seven-node system
Fig.4 Topology of SAPF
To test the effectiveness of the method for a load varying network, the DC side resistance of rectifier 1 is changed at 0.9s.
Fig.5 Voltage amplitude by traditional method
Fig.6 Voltage amplitude by the proposed method
Fig.7 Instruction and real output of SAPF current by the proposed method
Fig.8 OF on different conditions (Change the SAPF installation point and discuss its effect)
Conclusion
A predictive control method of SAPF is proposed in this paper, which can compensate the network-wide harmonics in an optimized way. Compared with traditional load compensation method, the proposed method is more economical for network-wide harmonic suppression, and its compensation performance is less sensitive to the installation point of SAPF.