A real time model of the system to be studied was developed using RSCAD. The Avon to Impala transmission lines were two 275 kV transmission lines (of length 69 km between Avon and Mersey) with the distance between the two towers being 37 meters, measured between the mid-points of each tower as shown in Figure 2.37. The transmission line and power system data was acquired from Eskom. The power system was modelled in DRAFT. Two versions of the power system were modelled: in the first model each of the two transmission lines was described using its own distinct distributed parameter model such that no mutual coupling between the transmission lines was represented; in the second model the two transmission lines were described together using a single distributed parameter model so that the full interaction and mutual coupling between the transmission lines was represented. The purpose of this approach was to examine the effect of mutual coupling against a control case. In both models all
parameters were exactly the same (with the exception of the representation of mutual coupling) such that any differences between the results that were observed can be attributed to mutual coupling alone. In the studies carried out in this chapter of the thesis, the transmission lines were assumed to be ideally transposed.
Figure 4.2: DRAFT model of the power system without mutual coupling (above) and with mutual coupling (below)
From the DRAFT model in Figure 4.2, note that the parallel transmission lines in the top part of the figure are independent from each other (two distinct distributed-parameter transmission line models) and the parallel transmission lines at the bottom part of the figure are represented using coupled-circuit transmission line tower symbols (a combined distributed-parameter transmission line model used to represent the parallel lines). In each case, the details of the tower construction, as well as the distance between the towers and other electrical and mechanical details of the conductors and ground wires need to be known so that the electrical parameters of the two sets of travelling-wave transmission line models (with mutual coupling ignored and mutual coupling represented) can be developed using the TLINE support program as shown in Figure 4.3 and 4.4.
Figure 4.3: TLINE model for one transmission line without mutual coupling
Figure 4.4: TLINE model for two transmission lines with mutual coupling
At each location A and B in the real-time model of the system in Figure 4.2, SEL 421 hardware protection relays were used to perform conventional distance protection where zone 1 was set to protect 80% of the line and zone 2 was set to protect 120% of the line. A simple POTT scheme was designed, implemented and tested. In addition, RSCAD generic software distance protection relays were run in parallel with the SEL 421 relays, with identical settings to the hardware relays, and were used to gain insight into the impedance that the hardware relays were seeing and hence to allow a better understanding of the reasons for the hardware relaysβ
responses to each fault scenario. A simple POTT scheme was also implemented using the RSCAD software relays as shown in Figure 4.7. The decision making elements of the software relays were also monitored in order to gain a proper understanding of how and why the trip signals of the hardware relays were issued. The digital and analogue inputs to the RSCAD software relays were the same inputs that were sent to the SEL hardware relays.
In order to make hardware in loop connections between the external SEL 421 hardware relays and the real time simulation, the DRAFT model of the system had to be configured to export and import digital signals between the relays and the model. Figure 4.5 shows the approach used to export analogue measurements from the DRAFT model to the external relays.
Where :
IBURx is the CT2 secondary current for input to the relay
VBURx is the VT2 secondary voltage for input to the relay
Figure 4.5: GTAO card used for exporting signals
Figure 4.6 shows the approach used to export and import digital signals between the real time model and external relays in DRAFT.
Figure 4.6: Additional logic required to import and export digital signals between the SEL relay and the RTDS model
The GTAO card shown in Figure 4.5 is a 12 channel digital to analogue converter through which analogue signals are exported and scaled before being amplified and input to the hardware relays. The signal-level analogue outputs from the GTAO card are interfaced with a high-bandwidth voltage-voltage and voltage-current power amplifier, in this case an Omicron CMS 156. It is important to note the gain of the amplifier, per signal level volt in order to configure the above DAC (Digital to Analogue Converter). The CMS 156 has the following gains [50]:
5 A/V for the voltage-current signals 50V/V for the voltage-voltage signals
This was used to calculate the scale factors required for the DAC as follows:
πΌππππ’πππ‘πππ
πΌπβπ¦π ππππ = 1 =π5
π₯Γ 5 =25π
π₯ β ππ₯= 25 for the voltage to current channels (64)
πππππ’πππ‘πππ
ππβπ¦π ππππ = 1 = 5
ππ₯Γ 50 =250
ππ₯ β ππ₯ = 250 for the voltage to voltage channels (65) Careful checking was carried out in order to ensure that the currents and voltages output from the Omicron amplifier were scaled correctly and were within the safe operating range for connection to the hardware relaysβ measurement inputs. The relays were then driven from the amplifiers, and open-loop testing was done to verify that the zones of protection of the relay were correctly set for both phase and ground faults. Closed loop testing (signals sent both to and from the simulation to the relay) was then performed with the addition of digital input and output ports as shown in Figure 4.6.
Both the RSCAD software and SEL hardware relaysβ settings were designed with mho characteristics. In the RUNTIME interface these mho characteristics and the impedances measured by the software relay models can be viewed on graphs displaying the transmission line characteristics and the zones of protection.
Figure 4.7: POTT scheme implemented using the RCSAD software relays
Once the DRAFT model, TLINE model, RUNTIME interface, relay settings and connection between hardware and software had been setup, an investigation into the effects of mutual coupling on the distance protection relay was made. Figure 4.8 shows the full hardware-in-loop connections that were used at the DUT RTDS laboratory.
Figure 4.8: Hardware-in-loop connections between the SEL 421 relays and the RTDS simulator used for the research