The reader is recommended to follow a section-by-section ap-proach while building the network and to regularly test (run) the network to ensure that the network solves and that the data is entered correctly. There is nothing more frustrating than building a big interconnected network in one-step and finding at the end that something is incorrect or not working. The frustration is that it is very difficult to locate a problem or to distinguish between a decimal error in the data entered. Similarly, it is very difficult to find a line transposition error, which can cause voltage collapse on one or all phases. By using the section-by-section approach, the
Applications of PSCAD/EMTDC
reader can not only resolve problems as soon as they arise, but can also get a good feel for the characteristics and behavior of different parts of the network.
Most of the study cases found in handbooks consist of only one or two sources and loads that are inter-connected with minimum number of power lines. This is a good approach when trying to convey a specific concept or simulation technique. However, a simple network as the one described above is in reality quite rare, since most networks are heavily interconnected on various voltage levels and/or along different corridors. It is therefore important to establish the correct cut-off, or equivalence points in the network in order to produce results that accurately reflect behavior of the real network. For example, when studying power swings that result in over voltages on series capacitor banks, it is important to model both the series compensated power line and any parallel path(s) that might exist in order to get the correct flow of power. The reason for this is that the voltage across the series capacitor is directly related to the power flowing through the bank. If the parallel paths are not modeled, the simulations could give unrealistic results, i.e. the results can be either too high or too low.
As a rule of thumb, it is a good approach to terminate the net-work at a single generator, at a bus where there is a pool of gen-erators, or at a busbar that acts as a node in the network without any parallel path. When terminating the network at a generator, the approach is easy and the actual generator and generator transformer data can be entered in the PSCAD case file. When the network is terminated in such a fashion that an equivalent source is required, two methods can be used to determine the equivalent Thevenin impedance:
1) Use a Load Flow program (the equivalencing function) to automatically calculate network impedance and interconnected impedance values between different termination points, or
2) In the load fl ow, switch out all those lines that will be modeled explicitly in PSCAD, i.e. all the lines that will be retained in PSCAD. Then, obtain the short-circuit currents at all the termination points in the network and calculate the equivalent Thevenin impedance values from the short-circuit currents that were obtained. Note that this option does not accurately represent any interconnections that may exist between different termination points, however, in most cases it gives a good, adequate equivalent network.
Example
From the network shown in Figure 1, consider the line between BUS2 and BUS4 to be series compensated (see also Figure 2 below). The study to be conducted includes both switching and fault analysis. The data of the complete network is listed in Table 2 at the end of this chapter.
Step 1: Determine the points where the network will be termi-nated (equivalanced). To perform switching studies, we need to include frequency dependant models for the lines up to at least one busbar position away from the line of interest. Therefore, BUS1, BUS2, BUS4 and BUS7 (see Figure 1) must be included as a minimum. When considering the strong parallel path through BUS5, BUS5 should also be included together with the associated lines that comprise the parallel path, i.e. the lines to BUS1 and BUS7. We select the network termination points to be BUS1, BUS5 and BUS7. The generator at BUS1 will therefore have to be modeled explicitly while equivalent Thevinin sources are to be connected at BUS1, BUS5 and BUS7.
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Figure 1 - Complete network
Step 2: Determine the equivalent network impedances at BUS1, BUS5 and BUS7. PSSE has an equivalencing routine (SCEQ) that can be used to calculate the equivalent generator, load, shunt impedance and interconnected line values at the various busbars where the network is to be terminated (equivalenced), as shown in Figure 2. At BUS1, the original generator and the equivalent generator can be represented with one generator by calculating the parallel impedance. Table 3 lists the network data after apply-ing the SCEQ equivalencapply-ing routine in PSSE.
Applications of PSCAD/EMTDC
BUS1 BUS2 BUS4
BUS5 BUS7
BUS1 BUS2 BUS4
BUS5 BUS7
Figure 2 - Equivalent network
If the SCEQ equivalencing routine is not available, a simple equivalent network, as shown in Figure 3, can be manually approximated from the short circuit data. Using the load flow program, the user can switch out all the lines and generators that will be explicitly modeled in PSCAD; for example, LINE12, LINE15, LINE24, LINE47 and LINE57 and perform the short cir-cuits at buses BUS1, BUS5 and BUS7. The short-circuit values at these busbars will represent the fault current contributions from the equivalence parts of the network, and are represented as Thevenin generators with corresponding source impedance values in the PSCAD case. This approach does not always pro-duce 100% correct results in PSCAD as short-circuit currents are limited to individual sources and might need some adjustment of the source impedance values.
BUS1 BUS2 BUS4
BUS5 BUS7
BUS1 BUS2 BUS4
BUS5 BUS7
Figure 3 - Simple equivalent network