This is the second part of an exciting three-part Model Monday post series highlighting RSCAD’s phase domain synchronous machine model and how it can be used for the closed-loop testing of a generator protection relay. Click here to read part one!
Last week on the blog, we discussed the phase domain synchronous machine (PDSM) model available in the RSCAD library. This week, we will discuss how the PDSM model can be used to perform the closed-loop testing of a generator protection relay using the RTDS Simulator. This post will provide an overview of the procedure for setting up a simulation case, assigning signals to the GTAO component, and scaling those signals for input to the relay. Our third post in the series, next week on Monday, August 29, will describe the process for setting the relay elements and testing the relay for various faults.
First, a simulation circuit must be developed to test different fault scenarios on simulated machines and send the corresponding signals to the relay. In this example, the circuit contains a 500 MVA, 22 kV synchronous generator which is connected to a source through a transformer. A circuit breaker separates the generator terminals from the transformer. Phase A of the machine model consists of two sub-windings, A1 and A2, and the point of connection between these two windings is available as a power system node, which can be connected to ground through a fault impedance to simulate stator-to-ground faults. Connection of this node to the machine terminals can simulate a turn-to-turn fault. The neutral point, N, is also a power system node and can be connected to other power system components such as impedances and transformers. A switch was also added to the circuit which enables the user to choose whether the excitation source for the machine is from the exciter model or from a RunTime slider, enabling the user to imitate a loss of field excitation fault. A switch is also added for the governor to imitate a loss of prime mover fault.
Once the simulation circuit has been developed, signals to be sent to the relay must be monitored and sent to the analogue output (GTAO) component. They must also be carefully scaled by adjusting the GTAO component’s scaling factors in order to ensure that the interfaced hardware receives signals at the desired levels. In this case, and generally for generator protection cases, the following signals must be sent to the relay from the simulation:
- Stator currents (GTAO Channels 1-3)
- Neutral current (GTAO Channel 4)
- Terminal voltages (GTAO Channels 5-7)
- Phase A voltage on the far side of the breaker (Bus 2), used for synchro-check (GTAO Channel 8)
- Neutral voltage (GTAO Channel 9)
- Differential current (through the circuit breaker), used for differential protection and monitored with the direction opposite to the direction of the stator currents (GTAO Channels 10-12)
Each GTAO card has 12 analogue output channels, which means that a single GTAO card is sufficient for creating this closed-loop interface. The figure on the right shows how the input signals from the simulation were assigned to the GTAO component in RSCAD.
A scaling factor must then be calculated for each GTAO channel in order to scale the analogue signals to the levels desired by the interfaced hardware. In this case, the interface to the relay is achieved using the relay’s low-level interface. This eliminates the need for a power amplifier connected between the GTAO card and the relay in order to amplify the +/- 10 V peak signals from the GTAO card to the secondary-level current and voltages usually required by the relay. The relay’s low-level interface allows us to directly connect the GTAO card to the relay inputs. The scaling factor can be calculated using the following equation:
Using the relay manufacturer’s specifications, the scaling factors can be calculated. In this case, at rated current the RMS current of the current transformer in our simulation case is 5A, which corresponds to 100 mV for the input of the low-level test interface. Therefore, the scaling factor for the current channels (Channels 1-4) of the GTAO component can be calculated as follows. The scaling factor is calculated similarly for each channel and entered into the GTAO component to scale the output.
Once the scaling factors have been set and the case is compiled in RSCAD, and prior to connecting the GTAO card to the relay, the user should measure the output voltage from each GTAO card channel to ensure it is as expected. The image below shows the user measuring the output voltage of a current channel of the GTAO (expecting 100 mV as outlined above).
Tune in next week – Monday, August 29 – for the third and final installment of this series, where we will discuss the process for setting the relay elements and testing the relay for various faults.