^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Delta transformer with delta load.

Each step up transformer is capable of 800 VA [at 600 V].

• An 800 VA 3-phase transformer can deliver \$ \frac {800}{3} = 267\ \text W\$ per phase.
• From \$ P = \frac {V^2}{R} \$ we get \$ R = \frac {V^2}{P} = \frac {600^2}{167} = 2155 \ \Omega \$.

If possible, having some way to test with load imbalance would be helpful.

2155 Ω is your minimum on any phase. You can increase to infinity (open-circuit).

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Delta transformer with delta load.

Each step up transformer is capable of 800 VA [at 600 V].

• From \$ P = \frac {V^2}{R} \$ we get \$ R = \frac {V^2}{P} = \frac {600^2}{800} = 450 \ \Omega \$.

If possible, having some way to test with load imbalance would be helpful.

450 Ω is your minimum on any phase. You can use any load between 450 Ω to infinity (open-circuit) on each phase.