Power Planners International 

Load Modeling for Dynamic Studies

EOA Head Office, Dammam Dec. 01 – Dec 05, 2013

PSS/E Training Course for NG SA

Week 2 Day-1-C

   The  subscript o indicates the values of the respective variables at the initial operating condition     The parameters of this models are exponents a and b expressed as follows:     For constant power     a = b = 0     For constant current    a = b = 1    For constant impedance   a = b = 2 

Static Load Models

A static load model expresses the characteristics of the load at any instant of time as algebraic functions of the bus voltage magnitude and frequency at that instant . The active power component P and reactive power component Q are considered separately. Voltage dependant Load Model: It is represented by the exponential model

Where

Static Load Models (Contd.)

For composite system loads : • Exponent a ranges from 0.5 to 1.8 • Exponent b ranges from 1.5 to 6

Significant characteristic of exponent b is that it varies as non-linear function of voltage. This is caused by magnetic saturation in distribution transformers and motors. At higher voltages Q tends to be significantly higher. In the absence of specific information, the most commonly accepted static load model is to represent active power as 100 % constant current (i.e. a=1) and reactive power as constant impedance (i.e. b=2) The composite model is expressed as ZIP as follows:

Static Load Models Contd.

Frequency Dependant Load Model

Where ∆f is the frequency deviation f-fo and Kpf ranges from 0 to 3.0 Jointly, both voltage and frequency dependence can be expressed as

follows:

 BL:  Connected to Bus subsystem OW: Connected to Owner subsystem ZN: Connected to Zone subsystem AR: Connected to Zone subsystem AL: Connected to All 

 

PSS/E Load Model Library 

• Motor Load CIM5xx, CIM6xx, CIMWxx (Suffix xx is BL, ZN, AR, OW or AL)

• Induction motors and their driven loads, while not affected by rotor angle dynamics, are sensitive to changing frequency and voltage. The driven loads usually consume increased power at increased speed and hence contribute to system damping. Induction motor loads may be modelled in three levels of detail: 1. By the standard single-valued voltage/load characteristics provided by activity

CONL and by the voltage/frequency/load characteristics provided by models such as LDFRBL

2. With the dynamics of the rotating load represented by the inertial differential equation, and with the motor’s steady-state electrical characteristic represented in detail, but with the electromagnetic dynamics of the motor neglected. This level of detail is handled by model CMOTOR (Obsolete in Ver. 32)

3. With both rotating load dynamics and motor electromagnetic dynamics represented in detail. This model is handled by the family of CIM5BL, CIM6BL and CIMWBL models, as well as CIMTR2 and CIMTR4.

Induction Motor Models

Induction Motor Models

Approach 1 • is reasonable for many of the loads in a large-scale system-wide study

because the details of individual loads are often not known and the provision of detailed motor/load data for thou-sands of loads may be a difficult task.

• is not adequate for studies of events where the transient behaviour of motor loads has a critical effect on bus voltages and hence on their own, and other, loads in the system.

• It is also, of course, unusable where the induction motors themselves are of specific interest

Approach 2 • only recognizes flux linkages for which time variation consists of an equilibrium

component corresponding to operation at fixed slip and voltage influence the rotor of the induction motor. This component is a unidirectional when the reference axes are synchronized with supply frequency.

• This approach neglects the synchronizing action provided by the induction machine during transients and as well as flux and voltage decay following tripping

Approach 3 • models a transient component that is zero in the steady state but takes the form

required to change rotor flux linkages when the machine is subjected to a sudden change of supply frequency or voltage.

• The magnitude of this component is determined by the disturbance applied to the machine, and its decay is governed by the transient and subtransient time constants of the rotor winding.

Induction Motor Models

• Motor Load CIM5xx, CIM6xx, CIMWxx • The CIM5BL family of models (CIM5BL, CIM5OW, CIM5ZN,

CIM5AR, CIM5AL) can be used to model either • single-cage or double-cage induction motors including rotor flux

dynamics. • The motor is modelled in the power flow as a bus load where all of

the load at a specific load id is taken as the steady-state motor load.

• These models may be applied to an individual load or a subsystem of loads. For example, the CIM5BL model can be applied to a specific load in order to model a specific induction motor.

• The CIM5AR model can be applied to all loads in a specific area in order to model generic motor load using typical data. The load composition can be any percentage of constant MVA, constant current or constant admittance.

Induction Motor Models

Motor Load CIM5xx, CIM6xx, CIMWxx • Most common dynamic loads comprise of induction motors with following equivalent circuit

CIM5* Motor Models

1. To model single cage motor: set R2 = X2 = 0 2. When MBASE = 0, motor MVA base = PMULT x MW

load. When MBASE > 0, motor MVA base = MBASE 3. VI is the per unit voltage level below which the relay to

trip the motor will begin timing. To disable relay, set VI = 0

4. TI is the time in cycles for which the voltage must remain below the threshold for the relay to trip.

5. TB is the breaker delay time cycles 6. Under Voltage Load Shedding of Motor Loads can be

carried out by setting T1 and TB

7. The models include a relay that can be used to trip the motor for an undervoltage condition. CON(J+14) is specified as the per unit voltage level (VI) for which the relay will begin timing. CON(J+15) is the time in cycles (TI) for which the voltage must remain below the threshold in order for the relay to trip. The breaker time delay (TB) is specified in CON(J+16). The user may disable the relay by specifying CON(J+14) as zero.

8. Syntax:I, ’CIM5xx’, LID, ICON(M), CON(J) to CON(J+18)/

11383 'CIM5Bl' 1 2 0.04 0.135 4 0.04 0.08 0.011 0.05 5 0.01 7 0.02 0 1.2 0.28 0 700 3 2 0 /

CIM5xx Motor Models

1. Load torque, TL = T (1 + D)D 2. For motor starting, T=Tnom is

specified by the user in CON (J+18).

3. For motor online studies, T=To is calculated in the code during initialization and stored in VAR (L+4)

Dynamic Data Record Syntax: I, ’CIM5xx’, LID, ICON(M), CON(J) to CON(J+18)/

Complex or Composite Load Model (CLODxx)

Proportional combination of different categories of static and dynamic loads

Suffix xx is BL, ZN, AR, OW or AL

Complex or Composite Load Model (CLODxx)

Dynamic Data Record Syntax:

I, ’CLODxx’, LID, CON(J) to CON(J+7)/