How to properly model composite load with Motor taking a portion of PQ load power?

[wangxiaowu-sudo]

Hi, I'm trying to model a composite load (similar to PSS/E CLOD or CMPLDW) where the total load power is distributed among different components:

• ZIP load (static)
• Induction motor (Motor3/Motor5)
• Distributed PV (PVD1)

The key requirement: The motor should "borrow" a portion of the original PQ load power, not add extra load to the system.

For example, if Bus 4 has a 50 MW load, I want:

• 30% (15 MW) as Motor
• 20% (10 MW) as PVD1 generation
• 50% (25 MW) as ZIP load

Current Issue

In ANDES, when I add a Motor5 to a bus, it appears to add load to the system rather than replace part of the existing PQ load. This causes:

1. Initial disturbance: The total load increases when Motor5 is enabled, causing voltage drop
2. Incorrect steady-state: System operates at a different operating point than intended

What I've tried:

Approach 1: Manual PQ adjustment

# Pre-calculate Motor5 steady-state power and subtract from PQ
motor_p, motor_q = get_motor_steady_state_power(bus=4, Sn=15, Vn=69)
ss.PQ.p0.v[pq_idx] -= motor_p
ss.PQ.q0.v[pq_idx] -= motor_q

ss.add("Motor5", {"idx": "M1", "u": 0, "bus": 4, "Sn": 15, "Vn": 69, "fn": 60})
ss.add("Toggle", {"model": "Motor5", "dev": "M1", "t": 1e-6})

This requires running a separate simulation first to get motor steady-state power, which is inefficient.

Approach 2: Custom model with pq parameter
I tried creating a custom Motor5PQ model that links to a PQ load (similar to how ZIP uses pq parameter), but encountered memory errors during prepare().

Question

1. Is there a built-in way to make Motor models "borrow" power from an existing PQ load, similar to how ZIP model works with the pq parameter?

2. In PSS/E's CLOD model, there's a direct parameter to specify motor percentage. Is there an equivalent approach in ANDES?

3. If not currently supported, would this be a reasonable feature request? I'd be happy to contribute if there's guidance on implementation.

Environment

• ANDES version: latest
• Python: 3.12
• OS: Windows

Use Case

This is for load model parameter identification research, where we need to simulate composite loads with different component ratios and compare against measured PMU data.

Thank you for your help!