Causes of PT ratio and phase errors

In an ideal PT, V_s = V_p/K_n, exactly in phase opposition. In a practical PT, the voltage drops in the winding impedances and the no-load current cause ratio error and phase angle error.

Phasor relation

 V_p = (reversed) E_p + I_p(R_p + jX_p)
 E_s = V_s + I_s(R_s + jX_s)
 I_p = I₀ + I_s/n   (I₀ = I_m + I_w, n = N_p/N_s)

So the reversed V_s differs from V_p in magnitude (ratio error) and angle (phase error).

Causes of ratio error

1. No-load (exciting) current I₀ flowing through the primary resistance and leakage reactance produces a voltage drop.
2. Load current flowing through the primary and secondary resistances (R_p, R_s) and leakage reactances (X_p, X_s) causes further drop.
3. Burden: a higher burden (more VA) means more current and larger drops, so the ratio error increases.
4. Burden power factor: at lagging pf the reactive drops add more directly to the magnitude.
5. A supply voltage or frequency different from rated changes I₀.

 Ratio error (%) = (K_n − R)/R × 100,  R = V_p/V_s

Causes of phase angle error

1. Magnetising current I_m and loss current I_w flowing through the winding impedance shift the phase of E_p relative to V_p.
2. Resistive drops (I·R) of the windings, especially at unity-pf burden.
3. Leakage reactance drops at lagging-pf burden.
4. Burden magnitude and power factor.

Methods to reduce errors

• Low flux density and good core material to reduce I₀.
• Low winding resistance and leakage reactance (thick conductors, interleaved windings).
• Turns compensation (slightly fewer primary turns).
• Keeping the burden within rating.