Flexible methodologies for the analysis and performance assessment of complex energy systems have been developed taking into consideration multiple energy carriers. The methodologies include technological, economic and environmental aspects, which will enable energy utilities to carry out detailed analysis of the investments as well as overall optimization of their energy supply systems. Governmental bodies will be able to do comprehensive scenario studies of local energy systems with respect to environmental impacts and consequences of different regulating regimes. The methodologies are usually based on two main levels of modeling: First, components with standard interface are combined to generate an energy network. Second, these components are represented with the necessary mathematical details, but seen from a system perspective they are all represented by standard variables. All these tools are imbedded in a specialized software package called "MODES". This package has been utilized to simulate the power system shown in figure (1) and to test the problem of voltage collapse without and with different penetration levels of DG. Table (1) gives a summary of the studied cases. The study has been carried out under variable load condition at nodes N204&N206 which simulate not a unique operating point but real system operation for almost 6 min. Nodes N204 & N206 can be viewed as the connection points of distribution networks with total active demand of 300 MW, total reactive demand of 167 MVAr ; this case was considered as the base case
without DG (case A). To study the influence of DG on the problem of voltage collapse with the above mentioned scenario, three different cases (B, C, D) illustrate different penetration levels as well as injection node. At any one of the studied nodes, if the load has zero value, it indicates that the DG is capable to supply the demand of this distribution section.

As a brief description of the software package used, program MODES enables the dynamic simulation of electric power systems. It operates as DOS application under WINDOWS (95 and higher). The MODES makes possible to simulate:

1. transient stability of generators (loss of synchronism),

2. middle-term dynamics concerned with the primary control of frequency (frequency collapse),

3. long-term stability concerned with Load Frequency Control (LFC), On-Line Tap Changers (OLTC) and the excitation limiter actions (voltage collapse),

4. transient phenomena in home consumption of the units (driven start-up, switching over to back-up),

5. steady-state stability investigation of the power system for small deviations (oscillations).

Table (1): Summary of the studied cases