What are Active Distribution Network Modeling? Sharing of Research Status at Home and Abroad

Active distribution network modeling refers to a distribution network model composed of transformers, transmission lines, and load models, and a distribution system composed of distributed power sources, loads, energy storage systems, and control devices.

It is a single controllable unit for the large power grid, which can realize the highly reliable supply of various energy forms to the load.

1. Background of Active Distribution Network

In recent years, with the increasingly prominent energy shortage and environmental problems, new energy power generation technology has developed rapidly with the strong support of national policies, and a large number of new energy power generation devices are connected in the form of distributed generation (DG) Into the grid, while relieving the pressure on the grid, it also brings challenges to the grid in terms of power quality, planning, operation, and control.

DG generally refers to a power generation system with a small capacity (generally below 10MW) connected to the active distribution network or the user side of the power grid. The types of DG mainly include micro gas turbines, fuel cells, solar photovoltaic cells, wind power generation and energy storage devices, among which photovoltaic power generation and wind power generation are the most widely used. DG is arranged near users in a small-scale and decentralized manner, so that not only the transmission loss is small, but also power compensation and power supply can be realized conveniently and reliably. Because DG has great advantages in terms of economy and environmental protection, power safety and reliability, and meeting the diverse needs of users, it has received great attention and has been widely used.

After the traditional power grid is connected to a large number of DGs, the power grid is no longer a active distribution network with only one-way transmission of power load energy, but a power grid with coexistence of power sources and power loads, and energy flows in two directions or even in multiple directions. In this way, the dispatching, control and management methods of the traditional power grid need to be updated to adapt to this new situation.

2. Characteristics of active distribution network

The characteristics of the active distribution network mainly include:

2.1 is DG with a large amount of small and medium capacity;

2.2 The power in the active distribution network flows in both directions;

2.3 The active distribution network can realize functions such as self-control, protection and management. The widely researched Microgrid can be said to be a special form of ADN.

Both academia and industry have gradually shifted their research direction from pure new energy power generation technology to the connection of new energy power generation to active distribution networks and microgrids, and established demonstration projects and experimental systems to prove the feasibility of new power distribution systems. sex and benefits.

3. Research status of active distribution network at home and abroad

Active distribution network (ADN) is a new term that has only been proposed in recent years. The earliest American Electric Reliability Technology Solutions Association (CERTS) proposed the concept of "microgrid". The microgrid is a system composed of micropower sources and loads, which can provide power and heat simultaneously. It can be said to be a special form of ADN.

The research on ADN is mainly in North America, the European Union and Japan. CERTS of the United States has successfully verified the preliminary theory of the microgrid at the microgrid test base of the American electric power company Walnut; the European Union has launched two major projects of "Microgrids" and "More Microgrids", and the microgrid laboratory built by the German Solar Energy Research Institute is the largest , with a capacity of 200kVA, the institute also designed and installed a simple energy management system on its experimental platform; Japan’s conventional energy is relatively scarce, and it has invested heavily in the development and utilization of renewable energy, and has established several micro-grids in China project, and the development of its micro-grid experimental system is also at the leading level in the world.

However, the research on ADN in my country is relatively backward compared with other countries. The research hotspots mainly focus on the control of DG itself and the planning and operation of DG. There is still a lack of grid-connected technical standards and grid-connected regulations for DG, which greatly limits Application and promotion of distributed power generation technology. However, my country strongly supports the development of renewable energy, and has established photovoltaic power stations and wind farms in the west and coastal areas. It is estimated that it will reach 20GW~30GW in 2020. As an emerging research hotspot in the power industry, most of the research on active distribution networks Concentrate on power quality (voltage and harmonics), system stability (static stability and dynamic stability), reliability, and improvement of DG control methods.

4. Active Distribution Network Modeling

ADN generally refers to a power distribution system composed of distributed power sources, loads, energy storage systems and control devices.

The active distribution network refers to the network in which the low-voltage side of the secondary step-down substation in the power system supplies power to users directly or after being stepped down by a step-down transformer. The active distribution network can be divided into high-voltage distribution network (35~110kV), medium-voltage distribution network (6~10kV) and low-voltage distribution network (0.4kV) according to the voltage level, and ADN mostly refers to the active distribution network that contains DG and is closer to the user. side of the low-voltage distribution network.

The structure of low-voltage distribution network can be roughly divided into three types according to the connection mode: radial network, tree network and ring network. The black square at the outlet of the low-voltage bus represents the circuit breaker, the black dot on the feeder represents the line node, and the arrow represents the load. Regardless of the structure, the high-voltage grid is stepped down by a step-down transformer to supply power to users in a certain area through single or multiple feeders, and users can be at any node on the feeder.

The low-voltage distribution network generally has the following characteristics: deep into the city center and densely populated areas; the transmission power; rate and transmission distance are generally not large; the power supply capacity, user nature, power supply quality and reliability requirements are different; the neutral point is not grounded. It is allowed to run for a period of time when it is grounded.

4.1 Transformer model

The ADN structure includes a step-down transformer connected to the large power grid, and its rated capacity is usually between 0.1 and 1MVA. The grade of rated capacity also determines the load capacity that can be carried in the entire distribution network. Transformers usually have a typical load regulation range of ±5% of rated capacity. Transformer load rate, also known as operating rate, is an important parameter affecting transformer capacity, number of units and grid structure, and its expression is:

In the formula, S1 is the actual maximum load of the transformer, and Ssc is the rated short-circuit capacity of the transformer. If the value of KP is large, it is called high load rate; if the value of KP is small, it is called low load rate.

The excitation current of the transformer usually accounts for a small percentage of the rated load current (normally less than 3%), so the excitation branch is often ignored in the harmonic analysis, so the transformer is generally represented by their series leakage reactance. Under the action of harmonics, the capacitance between the transformer winding and the winding turns will play a role. If the harmonic order is not too high, this effect can be ignored. Therefore, the transformer equivalent circuit can be simplified as an impedance branch connecting the primary and secondary nodes of the transformer.

4.2 Transmission line model

The transmission lines in ADN mainly include underground cables and overhead lines. The former is mainly used in urban areas with high load density, while the latter is more commonly used, and the main material is an insulated A1 core or Cu core conductor.

In power system analysis, the parameters of resistance, reactance, susceptance and conductance reflect the characteristics of transmission lines. In fact, these parameters are uniformly distributed along the line, that is, there are resistance, reactance, susceptance and conductance in any small length of the line, so accurate modeling is very complicated. Transmission line models can be divided into two categories: equivalent lumped parameter element models and traveling wave models. When it is only necessary to analyze the condition of the line port, that is, the voltage, current, and power at both ends, the distribution characteristics of the line can usually be ignored, and the lumped parameter element model can be used to simulate the transmission line; when the line is long, it is necessary to use the hyperbolic function to study the uniform distribution line of parameters.

4.3 Load model

The loads in ADN can be roughly divided into two categories: linear loads and nonlinear loads according to their characteristics. The linear load can be used as an element to suppress harmonic distortion, which can be simulated by grounding equivalent impedance. At the harmonic frequency, its reactance changes with frequency. Nonlinear loads mainly include household electronic equipment and commutation switching power supplies, etc. These nonlinear loads are equivalent to discrete harmonic sources for power distribution systems. With the large increase of nonlinear load, the distortion rate of user current will be aggravated and the waveform of supply current will be affected. However, due to the phase angle difference of the harmonic current components of different nonlinear loads, they often cancel each other out, thus reducing the effective influence of the system on the voltage distortion level.

In the same way, DG is an active nonlinear load in ADN, and there is also a phenomenon of mutual cancellation between harmonics. Studies have shown that [[32], for similar loads, low-order harmonic components (such as 3rd and 5th) have only a small offsetting effect, while the offsetting effect of high-order harmonic components is obvious. This type of cancellation effect is very important when higher harmonics need to be concerned. The phase difference between harmonics, the line impedance of the active distribution network, and the load can eliminate part of the harmonics. At the same time, the inherent single-phase load in the active distribution network makes the active distribution network asymmetrical.

4.3.1 Linear load model

At a given frequency, the equivalent impedance of the linear load is constant, and the active power and reactive power absorbed by the load are proportional to the square of the voltage of the load.

4.3.2 Nonlinear load model

Nonlinear loads mainly include some power electronic devices, such as personal computers, energy-saving fluorescent lamps, televisions and fluorescent lighting equipment. In addition to being harmonic sources, these loads cannot be represented by constant R, L, and C structures, and their nonlinear characteristics are not suitable for linear harmonic equivalent models. Nonlinear loads can be considered as harmonic injection current sources. For some nonlinear loads, as long as the actual voltage distortion is less than 1%, it can be regarded as an ideal current source.

4.4 DG model

Since DG converter is one of the main sources of ADN harmonics, in order to study the harmonic characteristics of ADN, it is necessary to study the harmonic characteristics of DG converter by establishing a reasonable model.

4.4.1 Classification of DGs

The forms of DG mainly include micro gas turbines, fuel cells, solar photovoltaic cells, wind power generation and energy storage devices, among which photovoltaic power generation and wind power generation are the most widely used. The following briefly introduces the topology and working principle of energy storage devices, photovoltaic power generation and wind power generation.

4.4.1.1 Energy storage device

Connecting energy storage devices in the active distribution network can play a good role in shifting peaks and filling valleys. When the power is in excess, the energy storage device can absorb energy; on the contrary, when the power is in short supply, the energy storage device releases energy. At the same time, in order to support the off-grid operation of ADN, an energy storage device that can quickly adjust frequency and power balance is usually installed in ADN. The energy storage device can be a battery pack, or a flywheel energy storage, etc. Its installation position is mostly at the outlet busbar of the low-voltage side of the step-down transformer.

4.4.1.2 Photovoltaic (PV)

Since photovoltaic power generation is restricted by light intensity and weather conditions, the output power is random and fluctuating, so a current-mode control strategy is usually used in ADN. The direct current generated by the solar cell module is boosted by a direct-to-direct converter, and then converted into a power-frequency alternating current by a grid-connected converter, and finally connected to the grid after passing through a filtering device and an isolation transformer. Adopting this two-stage topology structure can simplify the control method of each stage and improve the control efficiency of each stage.

4.4.1.3 Wind Power (WP)

Wind power generation (Wind Power) is a power generation technology that converts wind energy into mechanical energy by a transmission device, and then converts wind energy into electrical energy by a generator. Due to the uncertainty of wind speed, the output power of wind power generation is as random and fluctuating as PV system. Therefore, it also adopts the current mode control strategy in ADN. Take the two major models of direct-drive and doubly-fed wind power generation systems as examples. (a) It is a direct-drive permanent magnet synchronous power generation system. The electric energy generated by the synchronous generator is converted into power-frequency alternating current through an AC-DC-AC converter, and then connected to the grid after passing through a filter device and an isolation transformer. (b) It is a doubly-fed asynchronous wind power generation system. The stator side of the doubly-fed machine outputs power-frequency alternating current and is directly connected to the power grid, and the rotor side is connected to the power grid through an AC-DC-AC converter to provide controllable rotor excitation current . Compared with the double-fed unit, the direct-drive wind turbine has the advantages of simple structure, low operation and maintenance cost, high reliability and efficiency due to the omission of the gearbox.

4.4.2 DG access form

There are two ways for DG to be connected to the system: parallel connection to the grid or switching through switches.

In the parallel connection mode, when the DG power supply is interrupted, the grid can instantly make up for the load difference. Similarly, when the power grid fails, DG can also instantaneously assume the load. This way can ensure the load is always uninterrupted power supply.

In the switching mode, only one power source in the DG and the grid is connected to the load at any time, and the other only works after the switching, but the load is in a power failure state during the switching process.

Compared with the parallel connection method, the switch switching method has the following advantages:

4.4.2.1 Simple equipment and operation, fewer control and regulation loops;

4.4.2.2 Since DG generally runs only when needed, the operating cost of DG is relatively low; while in parallel operation, DG always keeps running, which will not only increase fuel and operation and maintenance costs, but also cause unit wear and tear.

4.4.3 DG converter model

Different types of DGs usually need to be connected to the grid through the interface of power electronic devices. Regardless of the characteristics of different DGs, it is considered that the intermediate DC side has been controlled constant, and only the grid-connected inverter side is concerned. Taking the most widely used three-phase voltage source type two-level PWM converter as the research object, its output current harmonics and grid voltage, PWM modulation strategy and switching frequency, control strategy and parameters, working conditions and output power, etc. There are relationships.

The most basic working principle of the three-phase PWM converter is to change the active and reactive power of the grid side by adjusting the amplitude and phase of the output voltage on the AC side of the converter on the basis of maintaining a constant DC voltage Udc.

Pref and Qref are the set values of active and reactive power output by the inverter, which can be divided by 1.5 times of the q-axis voltage to obtain the command values of the d-axis and q-axis current respectively ( and can also be given directly). After the error between the command value and the actual current of the d and q-axis currents passes through the PI adjustment link, the ud and uq obtained through decoupling are converted to the abc three-phase static coordinate system again, and are directly fed to the controllable voltage source and connected to the power grid through the inductance. Through PQ control, the output power of the inverter always tracks the given active and reactive power.

In the Average model, the change of the DC side voltage is ignored. Under the condition that the grid voltage is constant, the active power and reactive power output by the converter can be changed only by changing the active and reactive commands in the block diagram, so that the simulated Changes in DG output power such as PV and WP. At the same time, compared with the Detail model, because there is no switching process, the output voltage of the Average model does not contain high-order harmonics, but only a small amount of low-order harmonics caused by the influence of control performance. The Average model is applicable without considering the low-order harmonics generated by SVPWM.

5. Demand Response in Active Distribution Networks

With the continuous liberalization of the electricity sales market on the distribution side, the active distribution network will play an important role in future electricity transactions. The demand is usually divided into two types according to different types: the first type is the demand that will not be changed by the fluctuation of the electricity price! It is called the rigid demand; the other type is the demand that changes with the fluctuation of the electricity price. , called elastic demand.

6. Significance of Active Distribution Network

With the introduction of more and more DG into the active distribution network, its own problems are also increasingly apparent. For example, the intermittent nature of renewable energy, the uncertainty of power generation, etc. Furthermore, the grid connection of renewable energy in the form of DG usually needs to be converted into standard industrial frequency AC power supply load or grid connection through power electronic devices. Power electronic devices mostly adopt PWM control. If the harmonic current generated by it is not effectively suppressed, the harmonic problem of the power grid will be more serious. At the same time, due to the large increase of nonlinear loads in the industry, such as the widespread use of static power converter The application causes the voltage and current waveforms of the power grid to be distorted, resulting in harmonic pollution of the power grid. These have brought great challenges to the planning and operation of the power system, and the impact on the harmonic analysis of the active distribution network has gradually become one of the main issues concerned by the power industry in recent years. Therefore, a correct and reasonable analysis of the harmonic characteristics of DG, the influence of DG harmonics on the distribution of harmonics in the grid, and the interaction between DG and grid harmonics is conducive to the further development and application of DG, and has great significance for the research and development of active distribution networks. very important guiding significance.

7. Prospects for Active Distribution Networks

Future research directions can be summarized as follows:

7.1 The DG model only considers the grid-connected converter side. If the characteristics of wind power generation, photovoltaic power generation and energy storage devices are considered, the harmonic characteristics of DG output may be different.

7.2 The analysis of harmonics in the case of off-grid can be further in-depth from the aspects of load diversity, DG fluctuation and re-connection to the grid.