Power balance issues in Andes simulation process

[Crazy-Bike]

Hello, power flow calculation initialization will be performed before using Andes for TDS simulation. In the andes file, I adjust the total active power p0 of the PV node of the static generator to be equal to the total active power p0 of the load PQ, and set the active power to 0 on the balance node SLACK to achieve active power balance of the system. During TDS simulation afterwards, the system frequency continued to rise, possibly due to the active power of the generator being greater than the active power of the load, and the excess active power caused the system frequency to rise.
But I have already set a balance between the supply and demand of active power, why is there still a phenomenon of frequency increase? After trying to reduce the p0 of PV nodes, the system frequency can be maintained at f0 during TDS simulation.
Thank you!
PV(p0)+Slack(p0)=PQ(p0)

PV(p0)+Slack(p0)<PQ(p0)