Voltage control of multiple feeders by voltage regulator and instant DG

The main task for the electrical utilities is to supply satisfactory voltage levels to consumers. Voltage regulator is a device which regulates voltage profile according to the reference value of the regulating point. Voltage regulator is nothing but an auto transformer having tapings which is nearly close to reference value. The combination of line drop compensator with voltage regulator is already well established and in use for some times now (Gonen, 2007). To reduce loss as well as to improve the voltage profile of the system, research is going on optimal location and optimal tap position (Choi and Kim, 2001; Choi and Kim, 2001).

Also, distribution network changing rapidly in day to day life, like increasing load demand, reliability concerns and need to reduce the CO₂ emission. Now a day’s installation of DG in distribution network is increasing. While integrating DG with distribution network possess some new technical challenges and lot of benefits when it is integrating with distribution network (Ackermann et al., 2001; Pepermans et al., 2005). The limit of DG output is depending upon factors like thermal capacity, systems fault, protection issues, distribution system reverse power flow capabilities, and steady state voltage rise problem (Liewand and Strbac, 2002).

Hence by use of DG connected feeders there is no need of extra voltage regulator which reduces complexity. DG output is adjusted to that which the load on that feeder must be less than the allowable loading conditions.

The LDC having problem when DG is installed in the system, it will not work properly with the voltage regulator because LDC estimate voltage drop at the regulating point depending upon the local measurements. It does not consider the power injection at the regulating point by DG. For proper operation of voltage regulator when DG is installed, coordinated voltage schemes (or) active management schemes were proposed in literature.

The problem is considered as an optimization problem and according to the defined solution; the tap setting of voltage regulator is decided. The problem for optimal controlling of voltage regulator is that one voltage regulator is not sufficient for all loading conditions. Under some loading conditions, solution of the linear integer programming is not valid hence there is a tendency to control the voltage with extra voltage regulator. This is an extra complexity in the circuit as well as there is a need to solve the optimization problem again.

In this paper, we propose necessity of DG for optimal control of voltage regulator for multiple feeders. First, we take some constrained loading conditions and check whether there is solution or not. The problem is taken as optimization problem and the solution of that problem tells optimal tap setting of voltage regulator (Elkhatib et al., 2010). Solution gives integer number of tap settings required for optimal control of voltage. According to that voltage regulator set the voltage levels. If condition for feasible solution is not satisfied then there is a need of DG operation in that system. Now connect DG at that feeder which violates feasible solution. Forming voltage control problem as centralized optimization problem in the presence of DG was proposed (Elkhatib et al., 2011).

The voltage of different feeders has different loading conditions at different points is controlled by voltage regulator. The operation of voltage regulator is studied in this section.

3.1. Statement of the problem

Consider a system having multiple feeders with various lengths, loading conditions and various types of feeders. For proper tap position of voltage regulator control block is used in the Figure 2 for achieving the following objectives.

2.jpg

Figure 2. The system under study

(1) Maintain voltage of the regulating point nearer to reference value. If it is not possible then at least maintain given range for that feeder.

(2) Regulator bus permissible voltage range should not be violating when achieving (1).

(3) Voltage regulator no of steps should be in between available range when achieving (1).

3.2. Summarization of optimization problem

$\operatorname { Min } _ { T } \sum _ { n = 1 } ^ { N } \left( \mathrm { V } _ { \mathrm { reg } } - \left( \mathrm { V } _ { \mathrm { ref } } ^ { \mathrm { n } } + \mathrm { k } _ { \mathrm { n } } \mathrm { l } _ { \mathrm { n } } \mathrm { S } _ { \mathrm { n } } \frac { \mathrm { L } _ { \mathrm { n } } } { 2 } \right) \right) ^ { 2 }$         (4)

Sub to

 $\max _ { n } \left( V _ { \min } ^ { n } + k _ { n } l _ { n } S _ { n } \frac { L _ { n } } { 2 } \right) \leq V _ { r e g } \leq \min _ { n } \left( V _ { \max } ^ { n } + k _ { n } l _ { n } S _ { n } \frac { L _ { n } } { 2 } \right)$     (5)

$\frac { \max _ { n } \left( V _ { \min } ^ { n } + k _ { n } l _ { n } S _ { n } \frac { L _ { n } } { 2 } \right) - V _ { i n } } { V _ { t a p } } \leq T \leq \frac { \min _ { n } \left( V _ { \max } ^ { n } + k _ { n } l _ { n } s _ { n } \frac { L _ { n } } { 2 } \right) - V _ { i n } } { V _ { t a p } }$       (6)

$T _ { \min } \leq T \leq T _ { \max }$       (7)

$V _ { r e g } = V _ { i n } + T * V _ { t a p }$       (8)

$T _ { \max } = \frac { v _ { r e g } ^ { \max } - V _ { i n } } { V _ { t a p } } , T _ { \min } = \frac { v _ { r e g } ^ { \min } - V _ { i n } } { V _ { t a p } }$

Where,

$V _ { r e g }$→Output voltage of regulator

$V _ { r e f } ^ { n }$→n­­^th feeder reference voltage

k_n→n­­^th feeder K – factor (p.uvd / KVA. miles)

l_n→n­­^th feeder loading factor.

S_n→rated load of the feeder n.

L_n→Length between regulator & n­­^th feeder regulating point.

N→no. of feeders

V_in→input voltage to regulator.

T→tap position of the regulator

V_tap→One tap step corresponding voltage.

T_min→Min integer number of voltage regulator that does not exceed the voltage regulator bus range.

T_max→Max integer number of voltage regulator that does not exceed the voltage regulator bus range.

$V _ { r e g } ^ { \min }$→Min acceptable voltage of regulator bus.

$V _ { r e g } ^ { \max }$→Max acceptable voltage of regulator bus.

The term $k _ { n } l _ { n } S _ { n } \frac { L _ { n } } { 2 }$ is nothing but voltage drop of feeder n having length L_n with uniform distributed load of  L_nS_n having k-factor of  k_n by using integer programming solvers like branch and bound method we solve the above optimization problem.

The optimal tap setting can be chosen as:

Let $a = \frac { V _ { r e g } ^ { o p t i m u m } - V _ { i n } } { V _ { t a p } }$, then

$T _ { o p t i m u m } = \left\{ \begin{array} { c } { r \text {ound} ( a ) \text { if } T _ { \min } \leq \operatorname { round } ( a ) \leq T _ { \max } } \\ { \text {ceil} ( a ) \quad \text { if } \quad \operatorname { round } ( a ) \leq T _ { \min } } \\ { \text {floor} ( a ) \text { if } T _ { \max } \leq \operatorname { round } ( a ) } \end{array} \right.$

Consider T should be integer and it is greater than zero.

Then feasible condition for getting solution of optimization problem is

$\operatorname { ciel } \left( \frac { \max _ { n } \left( V _ { \min } ^ { n } + k _ { n } l _ { n } S _ { n } \frac { L _ { n } } { 2 } \right) - V _ { i n } } { V _ { t a p } } \right) \leq f \operatorname { loor } \left( \frac { \min _ { n } \left( V _ { \max } ^ { n } + k _ { n } l _ { n } S _ { n } \frac { L _ { n } } { 2 } \right) - V _ { i n } } { V _ { t a p } } \right)$

3.jpg

Figure 3. Recommended method

If feasible condition is not satisfied then there is no solution so we looking for extra voltage regulator otherwise we connect a DG for controlling of voltage by injecting the active power to the system.