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!

[jinningwang]

Question
Thanks for your interests in using ANDES.

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?

No, I don't think so. We don't have composite load model in ANDES yet.

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

No, I guess some user defined functions or scripts would be the quickest way to do that.

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

Yes, it can be a helpful feature, considering composite load is gaining more and more attention for data center related work.

Two comments for your shared implementation:

ss.PQ.p0.v[pq_idx] -= motor_p
ss.PQ.q0.v[pq_idx] -= motor_q

This would not work after TDS initialization, and it is not an expected way to manipulate PQ load. Check this example https://docs.andes.app/en/latest/_examples/ex9.html

How you implement get_motor_steady_state_power? Since the voltage can change after motor start, there can be some gap between the calculated steady state power before and after it start.

[wangxiaowu-sudo]

Thank you for the quick response!

Regarding get_motor_steady_state_power

Here's my implementation:

def get_motor_steady_state_power(bus, motor_sn, motor_vn, case_path=None):
    """
    Get Motor5 steady-state power at specified bus.
    Run a short simulation to let motor reach steady state.
    """
    if case_path is None:
        case_path = andes.get_case("ieee14/ieee14.json")

    ss = andes.load(case_path, setup=False)
    ss.add("Motor5", {"idx": "M_test", "u": 0, "bus": bus,
                      "Sn": motor_sn, "Vn": motor_vn, "fn": 60})
    ss.add("Toggle", {"model": "Motor5", "dev": "M_test", "t": 1e-6})
    ss.setup()
    ss.PFlow.run()

    # Run to steady state
    ss.TDS.config.tf = 1.0
    ss.TDS.run()

    return ss.Motor5.p.v[0], ss.Motor5.q.v[0]

You're right - there is a gap because the voltage changes after motor starts. This is exactly the problem I'm facing: it requires iteration to converge, which is inefficient for parameter identification where I need to run many simulations with different motor ratios.

Regarding PQ manipulation

Thanks for pointing to the example. I'll check https://docs.andes.app/en/latest/_examples/ex9.html for the proper way to adjust PQ load.

Follow-up question

Since composite load is not yet supported, would you recommend:

1. Workaround A: Accept the small initial disturbance and just wait for steady state (e.g., run 2-3 seconds before applying fault)?

2. Workaround B: Create a custom model that inherits from Motor5 and adds a pq parameter to link with PQ load (similar to ZIP)?

3. Other approach: Any other suggestions?

For my use case (load parameter identification), I need to compare simulation results with measured PMU data during faults. A small steady-state offset might affect the identification accuracy.

Thanks again for your help!

[cuihantao]

Jinning, thanks for your reply.

Currently, Motor is included in the power flow solution like PQ. The model substitution (i.e., "borrowing") happens at the time-domain initialization step. Therefore, Motor cannot substitute PQ because both are in power flow.

I see some modeling challenge to having a Motor substitute part of a PQ. Motor's power consumption depends on the terminal voltage. With a solved bus voltage using your desired P and Q, the motor cannot be set to consume required Q (unless compensated).

The easiest way is to modify from the Motor model and introduce additional injection into the network to mimic the PQ/ZIP part.

What does the "motor percentage" mean? Is it in terms of P, Q or I?

[wangxiaowu-sudo]

In PSS/E's CLOD/CMPLDW model, the "motor percentage" typically refers to P (active power).

[jinningwang]

Option A, I didn't get it.

Option B, to the best of my knowledge, this would be the best way for your use-case. However, I have to acknowledge that developing a new dynamic model can be both interesting and challenging.

For option B, there have been some discussion about the model CLOD in an EPRI document, Technical Reference on the Composite Load Model.

You are more than welcome to model it in ANDES and post discussion if any trouble shows up. ANDES documentation has a Development section, and it can be a good starting point.

The overall idea looks straightforward:

1. Motors can be modeled with existing Motor model.
2. Discharge Motors and Transformer Saturation can be modeled with existing non-linear elements, e.g., piece-wise function.
3. Constant MVA can refer to the existing PQ model.
4. Remaining Loads can refer to the existing ZIP model.