TSGENCO AE common mistakes
TSGENCO AE · Understand the traps and how to avoid them.
Treating per-unit and percentage impedance values as interchangeable when combining transformer and generator impedances on a common base, then selecting a fault current from an unconverted MVA base.
Candidates memorize the fault-current formula but skip the base-change step, because the given data (transformer %Z, generator X'' in pu on its own rating) looks ready to plug in; the arithmetic then silently produces an answer that is off by the base MVA ratio.
What to do instead
Always write the chosen base MVA and base kV at the top of the working, convert every impedance with Z_new = Z_old × (MVA_base/MVA_old) × (kV_old/kV_base)², and sanity-check the result against a rough estimate (fault MVA ≈ base MVA / total pu impedance). Practise three-bus and two-transformer examples until the conversion is automatic.
Applying the equal-area criterion by eyeballing the accelerating and decelerating areas on a sketched power-angle curve instead of computing the critical clearing angle from the swing equation terms.
The concept is taught graphically, so candidates trust the picture; under exam time pressure they invert the P–δ curve or forget that the mechanical input stays constant while the post-fault maximum power changes.
What to do instead
Derive and memorize δ_cr from cos δ_cr = (P_m(δ_max − δ_0) + P_max2 cos δ_max − P_max1 cos δ_0)/(P_max2 − P_max1) for the single-machine case, and always label P_m, P_max before and after fault, and δ_0 explicitly before substituting.
Mixing up the torque–slip relationships of induction machines, e.g. stating that starting torque is maximum or that slip at maximum torque depends on rotor resistance.
The formula s_maxT = R_2/X_2 is remembered in isolation and confused with the fact that maximum torque itself is independent of R_2; candidates then pick a distractor that says adding rotor resistance raises pull-out torque.
What to do instead
Build a one-page table linking s_maxT = R_2/X_2 (varies with R_2), T_max ∝ V²/(2X_2) (independent of R_2), and starting torque behaviour with deep-bar/double-cage rotors; verify each statement against a numeric example with R_2 doubled.
Ignoring armature reaction and saturation when predicting terminal voltage regulation of a synchronous generator, and using the simple E = V + IZ_s phasor for a load power factor that is leading.
The phasor addition is done with magnitudes rather than complex quantities, and the sign of the reactance drop is flipped for leading power factor, producing an answer that looks plausible but has the wrong sign of regulation.
What to do instead
Draw the phasor diagram for lagging, unity and leading power factor separately, use complex arithmetic (E = V + I(R_a + jX_s)) with I referenced to the load angle, and check that leading loads give lower or negative regulation - a quick sign check catches most errors.
Computing the output DC voltage of a controlled rectifier with the wrong average-value formula, e.g. using the uncontrolled V_m/π for a fully controlled single-phase bridge or omitting the cos α factor for a half-controlled bridge.
All converter topologies are revised together and the formulas blur; candidates also forget that a half-controlled bridge has one freewheeling path, so its average output is V_m(1 + cos α)/π, not V_m cos α/π.
What to do instead
Tabulate average output voltage for each topology (half-wave, full-controlled bridge, half-controlled bridge, three-phase variants) alongside conduction angle and device count, and derive one case from the integral of v = V_m sin ωt over the actual conduction interval so the formula is reconstructed rather than recalled.
Confusing the measurement errors of a Wheatstone bridge with those of a Kelvin double bridge, or misidentifying which bridge is used for low, medium and high resistance measurement.
Bridge names are memorized as a list without the reason each exists; the Kelvin bridge's role in eliminating contact and lead resistance for low values is the exact point being tested.
What to do instead
Learn each bridge by the resistance range and the error it removes: Wheatstone for medium values, Kelvin double for low values (contact/lead resistance), Megger for insulation, Maxwell/Hay/Schering for inductance and capacitance. Attach a one-line reason to each name so the answer follows from the purpose.
Computing Rankine cycle efficiency from steam-table enthalpies at the wrong state points, e.g. using h_f at condenser pressure instead of h at the turbine exhaust with the correct dryness fraction.
Candidates grab the first saturated-liquid value they see in the table and skip the isentropic expansion step, so the work term is understated or the pump work is added with the wrong sign.
What to do instead
Write the four state points before touching tables: 1 boiler exit, 2 turbine exit (find x_2 from s_1 = s_2), 3 condenser exit saturated liquid, 4 pump exit. Compute W_t = h_1 − h_2, W_p = v_f(p_1 − p_2), Q_in = h_1 − h_4, and only then η = (W_t − W_p)/Q_in.
Using the wrong specific-heat ratio or forgetting the isentropic relation when calculating compressor work and pressure ratio in a Brayton cycle, and ignoring the effect of regenerator effectiveness.
The Brayton formulas look similar to the Otto and Diesel cycle expressions, and candidates substitute r (compression ratio) for r_p (pressure ratio) or use γ = 1.4 for a gas other than air.
What to do instead
Fix the notation: r_p = p_2/p_1, T_2/T_1 = r_p^((γ−1)/γ), and back-work ratio depends on r_p. For regeneration, use ε = (T_3 − T_2)/(T_4 − T_2) explicitly. Re-derive the thermal efficiency η = 1 − 1/r_p^((γ−1)/γ) once so the pressure-ratio dependence is anchored.
Applying Bernoulli's equation across a pump or turbine without the head term, or using the same velocity in both sections of a Venturi meter when the areas differ.
The energy equation is treated as a memorized formula and the shaft-work term is dropped; in flow-measurement problems the continuity equation is applied but the area ratio in the velocity-of-approach correction is forgotten.
What to do instead
Always write the full energy equation with h_pump/h_turbine and head-loss terms and cancel terms only after listing them. For Venturi/orifice problems, state A_1V_1 = A_2V_2 first, substitute V_1 in terms of V_2, then apply Bernoulli - this makes the area-ratio factor appear naturally.
Mixing limit state and working stress design partial safety factors for concrete and steel, and using the wrong modular ratio or stress-block depth in singly reinforced beam design.
IS 456 material is revised from memory and the two philosophies are learned side by side; candidates apply the working-stress modular ratio m = 280/(3σ_cbc) to a limit-state problem or forget the 0.87 f_y factor for steel in tension.
What to do instead
Keep two separate sheets: working stress design (m, σ_cbc, σ_st) and limit state design (0.36 f_ck b x_u, 0.87 f_y A_st, partial factors 1.5 and 1.15). Solve one beam by both methods and compare the steel areas so the difference in philosophy becomes concrete rather than abstract.
Skipping data-interpretation sets entirely to save time, then losing more marks through rushed last-minute guesses than the set would have cost to solve properly.
DI tables and caselets look time-consuming, so candidates defer them; with two minutes left they mark random options that have a 25 percent hit rate instead of the 80–90 percent achievable with one careful pass.
What to do instead
Budget the paper in advance (e.g. reasoning first, DI in a fixed 15-minute block) and use approximation for percentage-change and ratio questions rather than exact long division. Attempt every DI set with at least one solvable sub-question and mark only genuinely unknown items.
Answering Telangana state-focused general awareness questions from generic national knowledge, e.g. naming national schemes or rivers without the state-specific detail the question asks for.
Preparation is dominated by national current affairs, so state schemes, districts, projects and appointments are under-revised; the question wording is read as national when it is state-specific.
What to do instead
Maintain a one-page Telangana sheet updated monthly: state schemes and their launch years, major irrigation projects (Kaleshwaram, Palamuru-Rangareddy, Srisailam), districts and their formation, state institutions and recent appointments. Revise it weekly rather than reading generic national compilations.