Voltage Regulation

Armature Reaction Effect of the armature flux on the main flux. Three phase current in a three - phase winding - a rotating magnetic field produced (MMF = 1.5 Im Tph). UPF - cross magnetizing. Lag PF - demagnetizing. Lead PF - magnetizing.

Armature Leakage Reactance(XL) Three major components -Slot leakage reactance, end winding leakage reactance and tooth tip leakage reactance. Synchronous reactance/phase Xs = XL + Xar, where Xar is the fictitious armature reaction reactance. Synchronous impedance/phase Zs = (Ra + jXs ).

Open-circuit test The generator is turned at the rated speed The terminals are disconnected from all loads, and the field current is set to zero. Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way. It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information + Vdc If Vt

Short-circuit test Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammeters. Record the armature current Isc as the field current is increased. Such a plot is called short-circuit characteristic. A + Vdc If Isc

DC Test The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals. The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings then If the stator is Y-connected, the per phase stator resistance is If the stator is delta-connected, the per phase stator resistance is

Determination of Xs For a particular field current IfA, the internal voltage Ef (=VA) could be found from the occ and the short-circuit current flow Isc,A could be found from the scc. Then the synchronous reactance Xs could be obtained using IfA Ef or Vt (V) Air-gap line OCC Isc (A) SCC If (A) Vrated VA Isc,B Isc, A IfB : Ra is known from the DC test. Since Xs,unsat>>Ra,

Xs under saturated condition IfA Ef or Vt (V) Air-gap line OCC Isc (A) SCC If (A) Vrated VA Isc,B Isc, A IfB At V = Vrated, : Ra is known from the DC test. Equivalent circuit and phasor diagram under condition Ia Ef Vt=0 jXs Ra + Ef Vt=0 jIaXs IaRa Ia

Short-circuit Ratio Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by Ef or Vt (V) Air-gap line OCC Isc (A) SCC If (A) Vrated Isc,rated If_V rated If_Isc rated

Voltage Regulation of Alternators

Methods Of Predetermination Of Regulation Synchronous impedance method (EMF method) Magneto Motive Force method (MMF Zero Power Factor method (ZPF method) American Standards Association method (ASA method)

Synchronous Impedance Method (EMF Method) OC and SC tests conducted. Zs is calculated. Ra measured and Xs obtained. For a given armature current and power factor, Eph determined - regulation is calculated.

Magneto Motive Force Method (MMF Method) OC & SC tests conducted. field currents If1 (field current required to produce a voltage of(Vph + IaphRacos Φ) on OC) and If2 (field current required to produce the given armature current on SC) are added at an angle of (90± Φ). For this total field current, Eph found from OCC and regulation calculated.

Zero Power Factor Method (ZPF Method) OC test and ZPF test is conducted – characteristics are drawn . This is Potier triangle method From this triangle the potier reactance (leakage reactance of the alternator), XLph is obtained. The terminal voltage and the leakage reactance drop added vectorially - load induced EMF found.

Zero Power Factor Method (ZPF Method) For this load induced emf, the corresponding field current If1 obtained from OCC. The field current If2 required to balance armature reaction obtained from potier triangle. If1 and If2 are added at an angle of (90± Φ). For this total field current, Eph found from OCC - regulation calculated.

American Standards Association Method (ASA Method) The field currents If1 (field current required to produce the rated voltage of Vph from the air gap line). If2 (field current required to produce the given armature current on short circuit) added at an angle of (90± Φ).

American Standards Association Method (ASA Method) Load induced EMF calculated as was done in the ZPF method - Corresponding to this EMF, the additional field current (If3) due to saturation obtained from OCC and air gap line - If3 added to the resultant of If1 and If2 -For this total field current, Eph found from OCC and regulation calculated.

Example 1 A 200 kVA, 480-V, 60-Hz, 4-pole, Y-Connected synchronous generator with a rated field current of 5 A was tested and the following data was taken. from OC test – terminal voltage = 540 V at rated field current from SC test – line current = 300A at rated field current from Dc test – DC voltage of 10 V applied to two terminals, a current of 25 A was measured. 1. Calculate the speed of rotation in r/min 2. Calculate the generated emf and saturated equivalent circuit parameters (armature resistance and synchronous reactance)