Effective Location of Distributed Generators based on Voltage Stability

centralising generation units through interconnected transmission lines. The distribution network is considered to be a passive network, where the power is flowing in a single direction. Due to the high voltage ratio of R/X, the distribution network suffers from large voltage drops,

high power losses, and low voltage stability. The suffering of the distribution network is increased by rapidly increasing the demand, especially with a newly integrated load such as EVs. Recently, using distributed generators has become widespread as a solution to the problem of the distribution network, since the rapid growth of Renewable Energy Technologies (RETs) has helped to develop electricity generation and a distribution infrastructure to meet electricity demand requirements. Some of the other non-renewable energy technologies that are widely practised, such as gas turbine, fuel cell, and diesel generator, increase the development of distributed generators. Integrated distributed generators could be used to support a weak distribution network or could be operated in an island mode, which is an approach to the microgrid concept. Since the distributed generators are installed closer to the load demand, the transmission line losses are either avoided in island operation mode or reduced in connected mode.

In order to achieve the high benefits from distributed generators, their size and placement in the microgrid have to be optimised. Researchers have formulated the placement of distributed generators by using many objective functions, such as voltage profile improvement, loss minimization, improvement in reliability aspects, economic revenue, and environmental impact reduction. [47], [112], [272]–[278]. In this study, four types of distributed generators are suggested to be integrated within a distribution network: gas microturbine, fuel cell, photovoltaic cell, and diesel power generator [48], [279]. The average wind speed in the case study environment in this study, which is Baghdad city, is less than three m/s. Normally, the wind turbine is cut off at a wind speed lower than three m/s. Therefore, the wind turbine has been excluded from this study. The distributed generators are formulated according to the voltage stability improvement in two attempts: finding the lowest voltage bus bar or the most sensitive bus bar to the voltage stability (weakest bus bar). The procedure for determining the candidate bus bar in which to integrate the distributed generators is given below:

1- The power flow of the network is analysed using the Newton-Raphson method to obtain the voltage profile, line flow, total power losses (real and reactive), line losses, Jacobian matrix, and reduced Jacobian matrix.

2- According to the attempts, identifying the lowest voltage bus bar or the most sensitive bus bar to voltage stability is approached by using the participation factor based on finding the minimum eigenvalues and eigenvectors of the network.

3- The first specific type of distributed generator is integrated based on the certain capacity of active and reactive power into the candidate bus bar to enhance either the voltage level or the minimum eigenvalue of the network.

4- The system is analysed again to find the next candidate bus bar. In case the next candidate bus bar is the same as the previous candidate, the power of the distributed generator should be increased to a higher level.

5- The second generator is integrated into the second bus bar.

6- The procedure is iterated until combining the objective function that covers all demands of the system.

The flow chart for integrating the distributed generators into the most sensitive bus bar and the minimum voltage bus is shown in Figure 3-2.

The voltage stability indication increased from 0.063 to 0.446 when integrating the distributed generators at minimum voltage bus and most sensitive bus, respectively, for the same number and capacity of distributed generators. The results show that integrated microsources at the weakest bus bar improved the voltage stability much more than integrated microsources at the minimum voltage bus bar, as shown in Table 3-1. The five minimum eigenvalues of the conventional grid without distributed generators, the microgrid at island mode, and the conventional grid (smart grid) at island mode, are shown in Table 3-2.

Table 3-1: Distributed generator arrangement on buses

Uniform

distribution busses Type of sources

Power rated kW+kVAR Non-uniform distribution busses 1. 25 (62) Gas microturbine (MT) 75+35 47 2. 16 (53) Fuel cell (FC) 50+20 46 3. 10 (47) Gas microturbine (MT) 75+35 45 4. 34 (71) Photovoltaic cell (PV) 50+20 35 5. 45 (82) Fuel cell (FC) 50+20 34 6. 39 (76) Gas microturbine (MT) 75+35 33 7. 32 (69) Photovoltaic cell (PV) 50+20 32 8. 15 (52) Gas microturbine (MT) 75+35 11 9. 20 (57) Photovoltaic cell (PV) 50+20 26 10. 17 (54) Photovoltaic cell (PV) 50+20 25

11. 18 (55) Diesel power generator (DE) 250+175 10

12. 33 (70) Fuel cell (FC) 50+20 17

13. 42 (79) Photovoltaic cell (PV) 50+20 16

Minimum eigenvalue = 0.446 Minimum eigenvalue = 0.063 Total demand = 682.1Kw+328.8Kvar

Table 3-2: The eigenvalue for lowest buses

Type of connection Bus No Eigen Value

1. Traditional grid without DG

37 0.071 74 0.084 75 0.105 77 0.116 66 0.149

2. Microgrid at isolated mode

70 (33) 0.446 62 (25) 0.750 63 (26) 0.955 71 (34) 0.984 78 (41) 0.986

3. Smart grid at isolated mode

17 4.927 19 10.175 20 10.621 24 17.772 27 19.280

Start

Collecting data of network

Obtain the load flow solution for a base case of the system and get the Jacobian matrix (J)

Compute the reduced Jacobin matrix

If λ = 0 the system will collapse If λ > 0 the system is voltage stable If λ < 0 the system is voltage unstable

How close is the system to voltage instability?

Find the minimum Eigenvalue

Calculate the right and left eigenvectors

Compute the Participation factor The highest will indicate the most sensitive

bus

Installation first DG unit with certain capacity in this bus

Is the bus still most sensitive bus

Increase the capacity of DG in this bus

Print the result Is the capacity of generator

connected at PCC =0 No Yes No Yes End Install generator at PCC to

cover all demand

Reduce the capacity of generator connected at PCC

a- integrated DG at weakest bus bars attempt

Start

Collecting data of network

Obtain the load flow solution for a base case of the system

Find the Busbar that has minimum voltage

Add the DG unit with certain capacity in this bus

Is the bus still most lowest voltage bus

Increase the capacity of DG in

this bus

Print the result Is the capacity of generator

connected at PCC =0 No Yes No Yes End Install generator at PCC to cover all demand

Reduce the capacity of generator connected at PCC

b- integrated DG at minimum voltage bus bars attempt