Coordinated Voltage Control of Active Distribution Network

This paper presents a centralized coordinated voltage control method for active distribution network to solve off-limit problem of voltage after incorporation of distributed generation (DG). The proposed method consists of two parts, it coordinated primal-dual interior point method-based voltage regulation schemes of DG reactive powers and capacitors with centralized on-load tap changer (OLTC) controlling method which utilizes system’s maximum and minimum voltages, to improve the qualified rate of voltage and reduce the operation numbers of OLTC. The proposed coordination has considered the cost of capacitors. The method is tested using a radial edited IEEE-33 nodes distribution network which is modelled using MATLAB.


Introduction
Large-scale development of DG, will bring problem to traditional distribution network such as bidirectional power flow and voltage off-limit.Traditional distribution network is designed based on the assumption that power flow is unidirectional and the network does not contain any controlled resources.Voltage control is mainly through OLTC and shunt capacitors in substation.These devices are run locally in most distribution network, without any communication or coordination with each other [1] [2].In [3], the author proposed a simple automatic voltage control method, which controls local reactive power output of DG, reducing active power output of DG as well.These local control are usually restricted to one bus voltage, relying only on local measurements.
In fact, information and controllable equipment of all network can be used basing on more and more convenient measurement and communication system.In [4], the author has proposed a coordinated voltage control method of substation OLTC and reactive output of DERs based on maximum and minimum voltage of the network.As the number of controllable components increase, the difficulty of determining control rules becomes increasingly complex.In this paper, a centralized coordinated voltage method has been proposed including coordination between primal-dual interior point method-based voltage regulation schemes of DG reactive powers and capacitors, and centralized on-load tap changer (OLTC) controlling method which utilizes the system's maximum and minimum voltages, to ensure a stable and economical distribution network voltage.The proposed method consists of two parts: primal-dual interior point method-based voltage regulation schemes of DG reactive powers and capacitors, centralized on-load tap changer (OLTC) controlling method which utilizes the system's maximum and minimum voltages proposed in [5].

Objective function
Objective function consists of three parts, 1) Minimizing extreme value of voltage Coordination between optimizing schemes and centralized OLTC control is also reflected by this sub-objective function: where where v Z is proportionality coefficient; 2) Minimizing use amount of reactive power of DGs DGs' reactive power are finite: where is maximal reactive power of DG i, and ,where DGi P is active power of DG; max Limiting the frequency of switching capacitor is intended to ensure the normal life of the compensating device and reduce the workload of operation and maintenance: where N C is the total number of nodes with shunt capacitor bank; q Z is the cost factor of each switching operation of capacitor; j q ' is the change in the number of compensation capacitor of node j before and after each optimization.In general, v Z and QDG Z are all set to 1.

Constraints
1) Power flow constraints ( , ) 0 G x u (5) where x and u respectively is state variable and control variable.
2) Constrains of reactive power of DGs where j q is compensation capacity of shunt capacitor bank in node j; ,max j q is total capacity of shunt capacitor bank in node j; N C is the total number of nodes with shunt capacitor bank.

4) Constrains of voltage magnitude
The node voltage amplitude must be controlled at rated voltage of 7% ~ 7%.From a mathematical point of view, optimizing schemes are multi-objective nonlinear programming, and mixed-integer programming.

Algorithm
Primal-dual interior point method is used to deal with mixed integer optimization.Continuous control variables are reactive power output of DGs, discrete control variables is tap position of OLTC and numbers of groups of shunt capacitor banks switching on.

Evaluation Index
Comprehensive evaluation index is used to evaluate the proposed control strategy: For further analysis and comparison, average offset dimension of voltage of each sample point is: is absolute value of offset dimension of voltage of node j at sampling point i.

Simulation result
The simulation takes modified IEEE-33 system.Rated voltage is 12.66kV.Intermittent solar energy can significantly change the system voltage profile and is usually located at the end of feeder.So 4 solar-based DGs are connected at different positions.The total number of tap positions of substation OLTC is 19.The time for voltage control is a typical 24-hour day, control interval is 15 minutes, and the total number of sampling is 96.The topological graph of IEEE-33 system is shown in figure 1. Curves of power of residential loads and solar-based DG during a day are shown in figure 2. Node 2, 15, 31 respectively are installed with shunt capacitor bank, each group of capacity is 200 kvar, and number of groups are 10, 3, and 3, respectively.The capacity of four DG is 3 MVA.Range of power factor are set to 0.95 lagging to 0.95 leading.To easily test effect of proposed coordinated voltage control scheme, assume the distributed photovoltaic DGs adopt decoupling control of active and reactive power.Figure 3 shows system's maximum and minimum voltages under two cases: Case 1: Power factors of all DGs are unit, numbers of all shunt capacitor bank are zero.OLTC, shunt capacitor banks and reactive power of DGs are not controlled.
Case 2: Tap position, shunt capacitor bank and reactive power of DGs is all controlled using proposed coordinated voltage control scheme.Active power of DGs output is same as case 1.On the other hand, reactive power of DGs is adjustable according to constraints shown in formula (6).Shown in figure 3 and 4, system's maximum voltage is off-limit of 1.07 p.u. during about 12:00 to 16:00 when DGs have greater active power output, system's minimum voltage is off-limit of 0.93 p.u. during about 20:00 to 22:00 without any control; Proposed coordinated control scheme have solved the off-limit problem of voltage successfully, remarkably reduced average offset dimension of voltage specially in both overvoltage and under-voltage.Proposed method is contrasted by centralized voltage control method without capacitors.Proposed centralized coordinated voltage control scheme has considered the effect of shunt capacitor banks because shunt reactive power compensation can help improve under-voltage.Figure 5 shows the tap position of both scheme.The proposed scheme has decreased daily tap operation from 9 to 7 taps/day.Comprehensive evaluation index of all three cases respectively are 0.0270, 0.0260 and 0.0257 shown in table 1, both centralized and proposed centralized coordinated voltage control scheme can reduce offset dimension of system voltage, effect of proposed method this paper is better specially in undervoltage result from heavy load and light DG output.

Conclusion
This paper presents a centralized coordinated voltage control scheme suited for active power distribution network.Once appears off-limit voltage, the reactive power of DG and shunt capacitor bank is optimized using primal-dual interior point method first of all, coordinated with substation OLTC by setting specific objective function then, to provide a more relaxed operation of the OLTC which can increase its life time; Especially in the optimization part, the proposed coordination has considered the cost of capacitor and incorporated it in whole coordinated control, reducing voltage excursion of active distribution network and decreased operation of OLTC facing heavy loading.
upper and lower limit of feeder voltage, eg 1.07 p.u. and 0.93 p.u.; and minimum voltage of the network measured.
voltage of DG; DGi pf is power factor of DG; DGi S is apparent power of DG; g V is voltage of system power supply; X represents the total reactance of the DG interfacing transformer and filter in case of inverter-based DG or it represents the synchronous reactance in case of synchronous machine-based DG.For inverterbased DG, max DGi V relies on the dc-link voltage, while for synchronous machine-based DG max DGi V depends on the field voltage.3)Minimizing the cost of using shunt capacitors ,

Figure 2 .
Figure 2. Power of residential loads and solar-based DG

Figure 3 .
Figure 3. System's maximum and minimum voltages; (a) Without control, (b) Proposed coordinated control