5-6 D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUALOVERVIEW CHAPTER 5: SETTINGS5The basic idea of an AC source is to select a point on the power system where the voltages and currents are of interest. Toillustrate the concept of sources, as applied to current inputs only, consider the breaker-and-a-half scheme that follows.(The breaker-and-a-half scheme is used for illustrative purposes and is available on select UR products.) In this application,the current flows as shown by the arrows. Some current flows through the upper bus bar to some other location or powerequipment, and some current flows into transformer winding 1. The current into winding 1 is the phasor sum (or difference)of the currents in CT1 and CT2 (whether the sum or difference is used depends on the relative polarity of the CTconnections). The same considerations apply to transformer winding 2. The protection elements require access to the netcurrent for transformer protection, but some elements can need access to the individual currents from CT1 and CT2.Figure 5-1: Breaker-and-a-half schemeIn conventional analog or electronic relays, the sum of the currents is obtained from an appropriate external connection ofall CTs through which any portion of the current for the element being protected can flow. Auxiliary CTs are required toperform ratio matching if the ratios of the primary CTs to be summed are not identical. In the UR series of relays, provisionshave been included for all the current signals to be brought to the UR device where grouping, ratio correction, andsummation are applied internally via configuration settings.A major advantage of using internal summation is that the individual currents are available to the protection device, forexample as additional information to calculate a restraint current, or to allow the provision of additional protectionfeatures that operate on the individual currents, such as breaker failure.Given the flexibility of this approach, it becomes necessary to add configuration settings to the platform to allow the userto select which sets of CT inputs are to be added to form the net current into the protected device.The internal grouping of current and voltage signals forms an AC source. This source can be given a specific name throughthe settings and becomes available to protection and metering elements in the UR platform. Individual names can begiven to each source to help identify them for later use. For example, in the scheme shown in the preceding figure, the userconfigures one source to be the sum of CT1 and CT2 and can name this source as “Wdg1 I.”Once the sources have been configured, the user has them available as selections for the choice of input signal for theprotection elements and as metered quantities.5.2.2.2 CT/VT module configurationCT and voltage transformer (VT) input channels are contained in CT/VT modules. The type of input channel can be phase/neutral/other voltage, phase/ground current, or sensitive ground current. The CT/VT modules calculate total waveformRMS levels, fundamental frequency phasors, symmetrical components, and harmonics for voltage or current, as allowedby the hardware in each channel. These modules can calculate other parameters as directed by the CPU module.
