Frequency Response of Electromagnetic Voltage Transformers

I think they are likely to be much better than you imagine. Let's have a little tutorial on transformer frequency response. We'll start with the simplified AC small-signal model of a transformer, first figure:

At the center of the model we see the perfect transformer, which steps down the voltage by the ratio N, and steps up the current by the same ratio N, and which has infinite frequency response. A second very important element in the transformer is the magnetizing inductance, L_m. The current through this inductance provides the magnetic field that energizes the core and creates a voltage in the primary that holds off the AC line voltage. This is a non-linear inductance, but that doesn't concern us here now.

At high frequencies the reactance of the magnetizing inductance L_m grows high and we can ignore it. At very low frequencies the reactance of L_mfalls to very low values and creates a substantial load current on the AC source signal, see the 2nd figure below. This causes a voltage drop across the source impedance Z_s plus the primary winding copper resistance R_p which in turn creates a drop in the secondary AC voltage. Although this current is 90° out-of-phase with the AC line (since it's an inductor), it can still be quite troublesome. That's the low-frequency-end of the frequency-response story.

In the model we see the expected primary and secondary copper-winding resistances, R_pand R_s but we also see two inductances L_lp and L_ls. These are labeled L_l (L-sub-ell) because they are leakage inductances, due to AC magnetic fields in the transformer that fail to go through both sets of windings. The windings are often interleaved to minimize this effect.

It's common to transfer all the leakage inductances to one side of the model to simplify analysis. To do this just multiply (or divide) the values by N². Now we can imagine all of the transformer's leakage inductance effectively on the secondary side of the model, and we hook the transformer up to its load, 2nd figure.

The high-frequency response dropoff will occur due to the rising series impedance of the leakage inductance against the load impedance. For the case of a light load, or a rising load impedance at high frequencies, the winding capacitance C_w will form a resonance and double-pole rolloff with the leakage inductance.

High-voltage high-power transformers are designed to be very efficient. This means in part that they will have low leakage inductances compared to the source and load impedances. As a result I expect they'll have very good high-frequency response, perhaps well beyond 100x their intended AC-line operating frequency.

Low-power transformers, often wound with split bobbins, etc., and no interleaved-windings, generally have comparatively-high leakage inductances, and therefore may not respond much beyond 5kHz. They also often show rather poor efficiencies at 50 and 60Hz.