PMUs, SCADA, relaying, metering and disturbance recording use a system of instrument transformers to scale the power system voltages and currents into instrumentation level voltages and currents. Standard instrumentation level voltages and currents are 67 V or 115 V and 5 A respectively. These standards were established many years ago to accommodate the electromechanical relays. Today, the instrument transformers are still in use, but because modern relays, metering and disturbance recording operate at much lower voltages, it is necessary to apply another transformation from the previously defined standard voltages and currents to another set of standard voltages of 10V or 2V. This means that the modern instrumentation channel consists of typically two transformations and additional wiring and possibly burdens. Figure 3 illustrates typical instrumentation channels.
Note that each component of the instrumentation channel will introduce an error. Of importance is the net error introduced by all the components of the instrumentation channel.
The instrumentation error can be computed by appropriate models of the entire instrumentation channel. It is important to note that some components may be subject to saturation (CTs and PTs), while other components may include resonant circuits with difficult to model behavior (CCVTs). The design should be such that during normal operating conditions these nonlinear phenomena do not occur, and indeed this is the case. Then, it is straightforward to model the instrumentation channel and compute the transfer function. The computed transfer function is integrated into the state estimation model.