Location

This example model can be found in the software under the category "Renewable Energy" with the file name "Combined_heat_and_power.ecf".

Description

In this example a combined heat and power (CHP) system is modeled in detail. The CHP is connected to a typical medium voltage distribution system and several tests are performed demonstrating the CHP operation in grid following and grid forming modes.

The model shows the simulation of a 10 MVA, 2.4 kV, combined heat and power system connected, via a feeder, to a simplified three-phase bus 25 kV, 60Hz distribution system. The feeders are modeled with their equivalent direct sequence and zero sequence impedances. Bus3 in the model has two loads of 5 MW and 1 MVAr connected to it. The loads and the CHP system can be islanded by opening the breaker CB2.

For more details about the CHP model refer to Combined Heat and Power (CHP).

Simulation and Results

The CHP works in either grid-connected and islanded mode. It can operate as a PV or PQ node. Also, the CHP can operate in isochronous or droop speed control mode and constant voltage control or droop voltage control.

The CHP is set initially in PV mode with an active power reference of 0.7 pu and unitary voltage reference. The purpose is to test the CHP under different modes of operation.

The sequence of the mode change is shown in the table below. The results are shown in the figure below reflecting the possible modes of operation of the CHP. The model was tested at a timestep of 50 µs.

From	To	Mode	Pref (pu)	Qref (pu)	Vref (pu)
0 s	5 s	PV	0.7	N/A	1
5 s	20 s	PV	0.6	N/A	1.02
20 s	35 s	PQ	0.7	0.1	N/A
35 s	50 s	PQ	0.8	0.13	N/A
50 s	70 s	Grid Forming	N/A	N/A	N/A
70 s	90 s	Fixed frequency/ Q control	N/A	0.13	N/A

The initial conditions of all components are set to ensure the simulation is initialized at the operating point calculated by the power flow. This allows the simulation to start in steady-state. The results show that in the first and second period from 0s to 20s, when the CHP operates in PV mode, the active power reference is followed. When the voltage reference increase by 2%, it can be observed that the reactive power changes accordingly.

From 20s to 50s the CHP operates in PQ mode and follows the active and reactive power references defined in the table above. It should be noted that the reactive power reference has a limit of +/- 0.3 pu. In this case the reference is followed as it does not pass beyond the limit.The initial conditions of all components are set to ensure the simulation is initialized at the operating point calculated by the power flow. This allows the simulation to start in steady-state. The results show that in the first and second period from 0s to 20s, when the CHP operates in PV mode, the active power reference is followed. When the voltage reference increase by 2%, it can be observed that the reactive power changes accordingly as shown in the figure below.

At 50s the breaker, CB2 opens and remains open till 90s. The CHP is also switched to grid forming mode from 50s to 70s. During this period the active and reactive power references are ignored, and the P and Q provided by the CHP is equal to the two loads connected to BUS3. The figure displaying the grid breaker currents shows that the islanding breaker has no current flowing through during the islanded period.. The frequency of the system is also maintained at 1 pu, which corresponds to 60 Hz.

At 70s, the CHP is switched to fixed frequency and reactive power control. The reactive power reference is followed. The P provided by the CHP increases to maintain the frequency stability of the system at 1 pu. The increase of active power is caused by the change in the voltage at the load, modeled as constant impedance, due to the increase in the reactive power reference.

References

[1] A. Marzoughi, H. Selamat, M. Fua’ad, H. Abdul. “Optimized proportional integral derivative (PID) controller for the exhaust temperature control of a gas turbine system using particle swarm optimization”. International Journal of the Physical Sciences. Available online in http://www.academicjournals.org/article/article1381418505_Marzoughi%20et%20al.pdf

[2] W. Rowen. “Simplified Mathematical Representations of Heavy-Duty Gas Turbines”. Published Journal of Engineering for Power. Availble in http://webm.dsea.unipi.it/barsali/materiale/Centrali/Regolazione/1983_ASME_Rowen_Simplified%20mathematical%20representations%20of%20heavy-duty%20gas%20turbines.pdf

[3] "Dynamic models for combined cycle plants in power system studies," in IEEE Transactions on Power Systems, vol. 9, no. 3, pp. 1698-1708, Aug 1994.

[4] G. Lalor, M. O’Malley. “Frequency Control on an Island Power System with Increasing Proportions of Combined Cycle Gas Turbines”. Available in https://www.researchgate.net/publication/4078318_Frequency_control_on_an_island_power_system_with_increasing_proportions_of_combined_cycle_gas_turbines

[5] IEEE, "IEEE Recommended Practice for Excitation System Models for Power System Stability Studies," in IEEE Std 421.5-2005.

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