With the implementation of electric energy alternatives, the large-scale application of electric energy substitution represented by air-source heat pumps has replaced traditional coal-fired heating, which is beneficial for the environment and alleviates air pollution. However, the large-scale application of air-source heat pumps has brought power quality problems such as voltage sags, harmonic pollution, and three-phase imbalance to the distribution network. This paper studies the fixed-frequency and variable-frequency air-source heat pump, introduces its working principle, analyzes the mechanism of its power quality problem. Moreover, the paper establishes a simulation model for the fixed-frequency heat pump and variable-frequency heat pump to connect to the distribution network. This research mainly studies the impact of large-scale fixed-frequency heat pumps on the depth of voltage sags in the distribution network and the impact of large-scale variable-frequency heat pumps on the harmonic content of the distribution network under different penetration rates and uses measured data to verify the reliability of the simulation results. This paper uses experimental data for the first time to verify the real power quality problems of large-scale heat pumps, which can provide a reference for determining the power quality standards for heat pumps connected to the power grid. At the same time, it can also provide a reference for the power quality management of the distribution network that is actually connected to electric heating.
In order to solve the problem of urban smog, the State Grid Corporation of China issued a power substitution implementation plan on 15 August, 2013. The plan proposes to vigorously advocate the new concept of energy consumption of “substituting electricity for coal, using electricity for oil, and electricity from afar” and fully initiate the work of electric energy substitution in the company’s business area [
After implementing electric energy substitution, heating equipment based on air-source heat pumps has been applied to rural distribution networks on a large scale
When a large-scale air-source heat pump is connected to the distribution network, the power quality problem of the distribution network becomes more and more serious. When the heating season arrives, low voltages appear in the electric energy replacement area, and even three-phase imbalance and other problems occur. Due to these problems, grid planning, power dispatching, and benefits evaluation after the large-scale integration of heat pumps cannot be carried out. Therefore, it is necessary to study the power quality problems after the large-scale application of air-source heat pumps to solve the power problems in this scenario.
The literature in [
Air-source heat pumps are classified into fixed-frequency air-source heat pumps and variable-frequency air-source heat pumps according to whether the frequency changes during operation. This article takes the fixed-frequency and variable-frequency air-source heat pump as the research objective. Later, the paper introduces its working principle and analyzes its power quality emission characteristics.
The air-source heat pump system is based on the principle of reverse Carnot cycle and is mainly composed of a compressor, an evaporator, a condenser, and an expansion valve. Its working principle is shown in
First, the low-temperature (−32–40°C) and low-pressure liquid refrigerant passes through the evaporator to absorb the heat Q2 in the air and changes from a liquid state to a low-temperature (5–40°C) and low-pressure gaseous refrigerant. The compressor converts low-temperature and low-pressure gaseous refrigerant into high-pressure, high-temperature (80–100°C) gas, and the heat converted by the compressor’s compression function is Q1. The high-temperature and high-pressure liquid refrigerant exchanges heat with water to become a medium-temperature (40–60°C) and high-pressure liquid. In this process, the refrigerant emits heat (Q3 = Q1 + Q2) to heat the water, and the hot water is placed in the water storage device. The high-pressure liquid refrigerant is decompressed through the expansion valve back to a low-temperature and low-pressure state and then enters the evaporator again to recirculate the process. Air-source uses high-level energy to make heat flow from low-level heat source air to a high-level heat source. The air-source heat pump can convert the unusable low-level heat energy (such as the heat in the air, soil, water) into usable high-level heat energy to save part of the high-level energy (such as coal, gas, oil, electricity, etc.).Therefore, it has low energy consumption, high efficiency, high speed, good safety, strong environmental protection, and can continuously supply hot water.
The compressor determines the operating performance of the fixed-frequency air-source heat pump, which can be equivalent to a single-phase asynchronous motor [
The authors in [
It can be seen that the relationship between the starting current of the fixed-frequency heat pump and the active power and reactive power is:
The voltage sag caused by the starting current on ZS is shown in
From the above analysis, it can be seen that the fixed-frequency air-source heat pump will indeed produce a voltage sag when it is started, and its size is related to the impedance of the access point, active power, reactive power, and load voltage.
The main working parts of the variable-frequency air-source heat pump can be equivalent to AC-DC-AC variable-frequency device and compressor, and its structure is shown in
It can be seen from
The literature in [
According to the current mature technologies and application practices, the “electricity substitution” policy guides village residents to use air-source heat pumps, ground-source heat pump equipment, and high-efficiency, low-emission gas heating equipment with a higher coefficient of performance (COP). The policy also promotes fixed-frequency or variable-frequency air-source heat pumps and encourages air-source heat pumps to use environmentally friendly refrigerants. For the installation of heat pumps in the whole village for heating, the municipal finance will give high subsidies to reduce the burden on households. At the same time, in the large-scale application of heat pumps, the village government often purchases the same type of heat pumps and installs them collectively. Therefore, when we study the power quality of heat pumps in large-scale applications, at the same time, we can study the power quality problems of large-scale fixed-frequency heat pumps and variable-frequency heat pumps.
The penetration rate of large-scale electric heating applications refers to the ratio of the total power of large-scale electric heating applications to the total power of all low-voltage loads in a particular area. The specific formula is shown in
where: η is the penetration rate of electric heating (%);
Take the low-voltage distribution network of a particular village as an example to study the power quality of heat pumps connected to the grid under different penetration rates. The Electric Power Science Research Institute of State Grid Jiangxi Electric Power Co., Ltd. (China) provided the village network map. The topology of the distribution network is shown in
Type | Parameter | Unit | Value |
---|---|---|---|
Power supply | Rated voltage | kV | 10 |
Rated frequency | Hz | 50 | |
Three-phase transformer | Voltage transformation ratio | kV | 10/0.4 |
Connection method | / | D1/Ny | |
Line impedance (pure resistive) | Ω/km | 0.1 | |
System short-circuit capacity | MVA | 10 |
A field test was conducted on the power quality problem in the low-voltage station area. The test helped verify whether the impact of large-scale electric heating on the power quality of the power grid is the same as the simulation results. The test points are shown in
A simulation analysis of individual heat pump equipment connected to the distribution network was carried out to study the characteristics of large-scale heat pumps connected to the distribution network. According to the fixed-frequency heat pump structure, a simulation model is built in Simulink. Observe the voltage sag when a single fixed-frequency heat pump starts under different circuit resistances, as shown in
According to the built simulation model, the average power of the air-source heat pump is set to 5.0 kW, and the average power of the ordinary electric load is 1.0 kW. There are 0, 24, 54, and 140 fixed-frequency heat pumps connected to the distribution network, respectively, and the corresponding penetration rates are 0%, 40%, 60%, and 80%. Select 0–15 nodes on the mainline, and observe the influence of the air-source heat pump starting simultaneously on the voltage sag of each node of the low-voltage distribution network under different permeability, as shown in
It can be seen from
After the fixed-frequency heat pump is started, due to the unbalanced three-phase load, it may cause the three-phase unbalance problem in the station area. Calculate the magnitude of the three-phase current unbalance at the common connection point, and the results are shown in
Permeability | 0% | 40% | 60% | 80% |
---|---|---|---|---|
Three-phase unbalance | 16.6% | 33.6% | 31.2% | 33.0% |
It can be seen from
Under test conditions, observe the fundamental power and power factor of the incoming line of the fixed-frequency heat pump in the village. Since it is impossible to start and stop the fixed-frequency heat pumps of multiple users at the same time in the test environment, three heat pumps of three phases are selected for 24 h test on the fundamental wave power and fundamental wave power factor of the inlet side, and the trend of change is shown in
It can be seen from
Test the effective value of the fundamental wave of the bus voltage, and the test result is shown in
It can be seen from
Build a simulation model in Simulink according to the structure of the variable-frequency heat pump. Observe the harmonic content generated when a single variable-frequency heat pump runs, as shown in
Harmonic order | 0 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|
Harmonic ratio/% | 0.00 | 0.00 | 16.9 | 0.00 | 12.4 |
Harmonic order | 6 | 7 | 8 | 9 | 10 |
Harmonic ratio/% | 0.00 | 9.33 | 0.00 | 4.93 | 0.00 |
Harmonic order | 11 | 12 | 13 | 14 | 15 |
Harmonic ratio/% | 2.90 | 0.00 | 1.80 | 0.00 | 1.16 |
It can be seen from the simulation results that the variable-frequency heat pump will inject harmonics into the grid side. The harmonic order is mainly an odd number, and the third harmonic ratio is the highest, reaching 16.9%. The harmonic content rate decreases with the increase of the harmonic order.
According to the simulation model built, 0, 24, 54, and 140 variable-frequency air-source heat pumps are connected to the distribution network, respectively, and the corresponding penetration rates are 0%, 40%, 60%, and 80%. Observe the harmonic content under each permeability at the common connection point, as shown in
Phase | Harmonic content/A | ||||||
---|---|---|---|---|---|---|---|
h3 | h5 | h7 | h9 | h11 | h13 | h15 | |
A | 17.58 | 14.04 | 11.67 | 7.06 | 4.86 | 3.54 | 2.66 |
B | 40.34 | 28.42 | 24.99 | 16.32 | 11.23 | 8.17 | 5.12 |
C | 65.47 | 53.23 | 42.46 | 24.91 | 17.45 | 13.43 | 6.25 |
Phase | Harmonic content/A | ||||||
---|---|---|---|---|---|---|---|
h3 | h5 | h7 | h9 | h11 | h13 | h15 | |
A | 11.49 | 9.18 | 7.63 | 4.62 | 3.18 | 2.32 | 1.74 |
B | 26.37 | 21.19 | 17.64 | 10.67 | 7.34 | 5.34 | 4 |
C | 41.72 | 33.92 | 28.33 | 17.15 | 11.79 | 8.56 | 6.41 |
Phase | Harmonic content/A | ||||||
---|---|---|---|---|---|---|---|
h3 | h5 | h7 | h9 | h11 | h13 | h15 | |
A | 6.04 | 4.8 | 3.98 | 2.4 | 1.65 | 1.2 | 0.9 |
B | 11.2 | 8.91 | 7.4 | 4.47 | 3.07 | 2.23 | 1.67 |
C | 15.12 | 12.07 | 10.02 | 6.05 | 4.15 | 3.01 | 2.25 |
Phase | Harmonic content/A | ||||||
---|---|---|---|---|---|---|---|
h3 | h5 | h7 | h9 | h11 | h13 | h15 | |
A | 3.27 | 2.15 | 1.04 | 0.42 | 0.25 | 0.06 | 0 |
B | 5.68 | 3.06 | 1.56 | 0.87 | 0.56 | 0.13 | 0.05 |
C | 9.35 | 6.01 | 4.39 | 1.34 | 0.96 | 0.41 | 0.13 |
From the simulation results, it can be seen that with the increase of the permeability of the variable-frequency heat pump, the amplitude of each harmonic current increases, and the third harmonic has the largest increase. When the penetration rate reaches 80%, the amplitude of the 3rd harmonic current generated by phase C is as high as 65.47 A, and the amplitude of the 5th, 7th, and ninth harmonic currents are also maintained above 20 A. According to the short-circuit capacity of the station area, the third harmonic current of phase C under the scenario of 80% penetration rate exceeds the allowable value of 62 A for user injection into the low-voltage distribution network harmonic current specified in the standard [
Under the test conditions, the harmonic current and harmonic distortion rate were tested at the public connection point of the village, and the changing trend is shown in
By observing the trend graph of harmonic content and total harmonic distortion at the common connection point, it can be found that there are different degrees of harmonic current in each phase. Phase C has the largest harmonic current, and the peak value of total harmonic current reaches 76.03 A. The simulation result comparison shows that the tested harmonic current is closer to the simulation result under the scenario of 80% permeability, which proves that the large-scale variable-frequency heat pump causes harmonic pollution to the distribution network.
By analyzing the mechanism of power quality problems generated by heat pump equipment, modeling and simulation are carried out to study the power quality impact of large-scale heat pump equipment connected to the low-voltage distribution network. It can be seen from the simulation results that as the load permeability of the fixed-frequency heat pump increases, the sag depth and duration increase. When the penetration rate of fixed-frequency heat pumps increased to 40%, the voltage sag of the distribution network exceeded the grid standard. With the increase in the permeability of variable-frequency heat pumps, harmonic voltage and current contents also increase. However, the increase in harmonic voltage is slight, and the content is low. The noticeable increase is in the 3rd harmonic current. When the frequency conversion heat pump penetration rate reaches 80%, the third harmonic current at the public connection point of the power grid has reached 65 A.
It can be seen from the test results that the fundamental reactive power of the fixed-frequency heat pump has an upward impact burr when the fixed-frequency heat pump is started, and the voltage drops to 0.9 p.u. several times during the test. When a large-scale variable-frequency heat pump runs, each phase has a sizeable harmonic current. The harmonic current is mainly the 3rd harmonic, and the total harmonic current of phase C is as high as 76 A.
Regarding the voltage sag caused by the start-up of the fixed-frequency heat pump, measures such as the optimization of the access position and the start-up sequence control should be taken to control it. The variable-frequency heat pump has scattered running time and large rated power, and the harmonics generated by it are mainly affected by the peak load. For the problem of harmonic pollution caused by variable-frequency heat pumps, measures to suppress harmonics should be taken in the area with a higher proportion of variable-frequency heat pumps.