The Impact of Synchronous Generators Excitation Supply on Protection and Relays

Synchronous generators have two types of operational limits: thermal and stability. These limits are commonly defined in the P-Q plane and, consequently, the point of operation of a generator should not lie beyond any of these limits. The functions that prevent the generator from infringing into the forbidden zones are known as limiters and are normally embedded in the generator automatic voltage regulator (AVR). The combination of these limiters and the nature of the AVR itself will have an impact on some generator protective functions like the loss-of-field (LOF) or out-of-step protection. The purpose of this paper is not to review generator protection principles, because this has been done extensively elsewhere, but rather to revisit the basic physical and engineering principles behind the interaction between a synchronous generator AVR and its associated limiters and some of the generator protective functions. We review the technology of the limiters embedded in a generator AVR. In LOF coordination studies, the steady-state stability limit (SSSL) used most often has been traditionally based on a generator system with a constant-voltage excitation (or manual SSSL). In this paper, we discuss the impact of the excitation system with an AVR or a power system stabilizer (PSS) on the generator stability limits. A new numerical technique is introduced to determine the stability limits of a generator system where the excitation supply could be regulated using either an AVR or an AVR supplemented by a PSS. I. GENERATOR THERMAL AND STEADY-STATE STABILITY LIMITS There are three types of thermal limits [1] [2] [3] [4] in a generator: the armature current limit that is directly related to the generator rated power, the field current limit, and the end core limit. The steady-state stability limit is a direct consequence of the power transfer equation between a generator and the network that it is supplying. These different limits are reviewed in the next section. A. Generator Thermal Operational Limits In Fig. 1, the three types of thermal limits found on a generator are represented. Assuming that the power is measured in per unit (pu) values, a half-circle with unit radius represents the generator theoretical maximum capability (GTMC). This limit is caused by the armature current ohmic losses and corresponds simply to the generator MVA rating. The end core limit is a consequence of the end turn leakage flux existing in the end region of a generator. The end turn leakage flux enters and leaves in a direction perpendicular to the stator lamination. Eddy currents will then flow in the lamination and will be the cause of localized heating in the end region. In overexcited mode, the field current is high, and as a consequence, the retaining ring will get saturated so that the end flux leakage will be small. In the underexcited mode, the field current will be reduced and the flux caused by the armature current will add up to the flux produced by the field current. This will exacerbate the end region heating and will severely limit the generator output. The end core limit depends upon the turbine construction and geometry. The limitation could be particularly severe for gas turbines, yet could be nonexistent for hydro units as shown in Fig. 1; steam units would have a limiting characteristic in the middle [1]. The field and armature current limits are dependent upon the generator voltage. All three limits are dependent upon the generator cooling system. For hydrogen-cooled generators, the most tolerant limit will occur at the maximum coolant pressure (see Fig. 2).