Modelling Photovoltaic Systems Using PSpice - 45271 08

Modelling Photovoltaic Systems Using PSpice - 45271 08

(Parte 1 de 4)

Grid-connected PV Systems

Summary

Grid-connected PV systems are starting to play a very important role in photovoltaic appkdom- This chapter describes some PSpice models for inverters and AC modules which aid the €'Spice SimOEalisag of grid-connected PV systems. Examples of sizing and energy balance for this kind of PV system are also included showing thal

PSpice can help engineers to obtain a good approximation of system behaviour using the paposed inverter models. Short- and long-term simulations validate the sizing procedures.

8.1 Introduction

Photovoltaic systems have a wide range of applications, from very small units for low powex, as will be described in Chapter 9, to large PV power plants in the MW range. Despite this wide range the most important application fields of PV systems have historically been outer- space PV systems and standalone systems in areas with poor or absent electricity supply from the public grid.

In the last few years an important growth of grid-connected PV systems has been observed, specially in industrialized countries. Several reasons lie behind this fact apart from the traditional advantages of photovoltaic electricity:

0 Utility grid-interactive PV systems are becoming more economically viable as the cost of

PV components has been significantly decreasing in recent years, in particular the average cost of PV modules and inverters.

0 Technical issues associated with inverters and interconnection of PV systems to the grid have been addressed by manufacturers and today's generation of inverters have enhanced reliability and reduced size.

0 Utility benefits. The fact that solar electricity is produced in central hours of the day can add value to the electricity. This power peak demand can be partially supplied by

216 GRID-CONNECTED PVSYSTEMS dispersed grid-connected PV systems that are able to generate power at the same place where this power is used, reducing the heavy load supported by the transmission systems and achieving benefits in distribution and line support.

0 National or international programmes promoting the implantation of gridconnected FV systems. Most industrialized countries have launched programmes offering diffemr incentives to small-scale renewable energy producers.

Even with the aforementioned benefits and the significant cost reductions achieved, these systems still cannot compete with other energy resources on a pure financial analysis without reasonable funding and promotion from public bodies. The lack of standardization interconnection requirements in different countries, for PV system components, especially inverters is also an important barrier to the market growth of grid-connected PV systems. Despite these obstacles, this market is becoming important in photovoltaic applications. Photovoltaic power generation systems are likely to become, although small compared with other power generation sources, important sources of distributed generation, intercnnmted with utility grids.

8.2 General System Description

Grid-connected PV systems, also called utility interactive PV systems, feed solar electricity directly to a utility power grid. These systems consist of

0 A PV generator, an array of PV modules converting solar energy to DC electricity.

0 An inverter, also known as a power conditioning unit or PCU, that converts DC into AC electricity.

0 System balance, including wiring and mounting structure.

0 Surge and ground fault protection and metering or other components than may be required for interconnection to the AC grid.

0 AC loads, electrical appliances.

When the sun is shining, the DC power generated by the PV modules is converied to AC electricity by the inverter. This AC electrical power can either supply the system’s AC loads, outputting any excess to the utility grid, or may completely output all the energy produced.

During dark hours, the power demanded by the loads is supplied entirely by the utility grid. The role played by batteries in standalone PV applications is replaced, in grid-connected systems, by the utility grid itself acting as energy reservoir for the system, resulting in cost and maintenance reduction. If necessary, batteries can also be included in the system in the same way as the case of standalone systems with AC output.

Figure 8.1 shows a schematic diagram of a three-phase grid-connected PV system. This schematic diagram may vary from country to country due to different national regulations, especially the circuitry involving safety and protection devices.

At can be seen in Figure 8.1, a first protection level is formed by fuses and blocking diodes between the PV array output and the main DC conductor. Surge protection elements must be

Automatic or manual line breaker Stnng fuses and diodes

Ground Figure 8.1 Grid-connected photovoltaic system schematic included at the inverter input and output, the figure shows these elem- dy ila! &e hvezter input.

an the type of inverter used; some inverters include transformers of this type inside and ahem do not. Some compulsory national regulations require an isolation transformer, in which me of the windings has a delta connection, for connection to grids of medium, high OT vay hlgh voltage. In most countries this is not required for low voltage com~tion.

If possible, utility safety devices and guard relays must be placed between the irp\ac%ller output and the switching point with the utility grid; these elements are not sham in the figure. Finally the metering can be made as shown on the figure or at the grid side.

A first classification of grid-connected PV systems can be made accmbg no dze as follows:

Isolation transformers can be optionally placed at the inveaer output

Small - Power from 1 to 10 kWp. Typical applications are: rooftops of @We hses, school buildings, car parks etc.

Medium size - Power from 10 kWp to some hundred of kWp. These kind of qskm can be found in what are called building integrated PV(B1PV) systems, in roofs OT fwades. They may operate at higher voltages than smaller systems.

Large size - Power from 500 kWp to MWp range, centralized systems. These sysaems 8te normally operated by electric companies.

8.3 Technical Considerations

Protection systems are required to prevent damage to the PV system and ah to avoid downgrading the quality of the grid electricity. Other important topics are the electrical installation procedures, electrical interference between utility grid and PV systems. EMI

218 GRID-CONNECTED PVSYSTEMS

(electromagnetic interference) and harmonics. A wide set of standards and recommendations exist in most of the EU, the USA, Japan and Australia, which have been developed by different national bodies and cover most of these topics.

As an example of these standards and recommendatiam, two US national standards can be cited here:

0 ANSUIEEE std 929-1988: IEEE recommended practice for utility interface of residential and intermediate photovoltaic (PV) systems.

0 IEEE std 929-2000: IEEE recommended practice for utility interface of photovoltaic (PV) systems.

A small overview of the most important problems associated with the connection of PV systems to the utility grid is described below.

8.3.1 Islanding protection

The continued operation of a grid-connected inverter when the utility grid, or a portion of the utility system, has been switched off or no electric energy can be delivered from the utility system, is known as the islanding mode of operation. Islanding can strongly affect the equipment and loads connected to the network, and can cause electric shocks to users or utility grid workers. For these reasons, inverters must identify a grid fault or disconnection and must immediately disconnect its output itself.

Much research has been done about islanding prevention on PV systems in the last few years [8.1-8.21, and nowadays most commercial inverters include acceptable islanding prevention capabilities obtained by a combination of different control algorithms. Detection of islanding can be achieved using active or passive methods [S.l].

8.3.2 Voltage disturbances

Appropriate voltage levels must be maintained at the customer’s input connection to the grid. Different limits for the voltage levels have been established in different countries. The fact that grid-connected PV systems can contribute to distribution line voltage variation has to be taken into account. Inverters must sense the voltage variations at the connecting point to the grid and most inverters also include the capability of disconnection and automatic reconnection after confirmation of utility stability recovery.

8.3.3 Frequency disturbances

Inverters must generally provide internal overiunder line voltage frequency shutdown. Internal shutdown should be produced if, within a few cycles, the frequency falls outside predetermined boundaries, usually ranging from f0.2 Hz to f5 Hz, around the nominal grid frequency.

TECHNICAL C0NJIDERATK)NS; 2p49 8.3.4 Disconnection

As shown in Figure 8.1 a switch for utility interface disconnect or separator, is mmwk1Iy included in grid-connected PV systems. This switch provides safety especially for psmmeb involved in maintenance or for utility workers. Taking into account the above-chuzW protections that inverters must implement, especially islanding detection and prevmtkn, @h& main disconnection switch can be considered redundant, however, personal safi%y k the most important issue concerning grid-connected PV systems [8.3], and regional regdatikms in different countries must be satisfied.

8.3.5 Reconnection after grid failure

It is important to ensure correct operation of the grid for a prudent interval of time &Em reconnecting the inverter. This task is implemented in most commercial inverters by ms of grid sensing and auto-reconnection.

Most inverters are able to reconnect themselves to the grid after observation of a nunher of grid cycles with correct values of voltage, amplitude and frequency.

8.3.6 DC injection into the grid

As has already been said, some inverter designs include transformers inside the bvaTav suppressing any DC injection into the grid. Advantages of keeping DC out of the grid me

personal safety improvement, protection of disturbances on the utility grid and satmahn effects in local distribution transformers, and finally the prevention of saturation on imh&Poe loads.

However, the number of transformerless inverters in the marketplace today, has ken increasing because of technical and cost advantages. In most countries isolation transfmers are not required for small PV grid-connected systems, but they are compulsory for medium or large size PV systems much of the time.

8.3.7 Grounding

The components of a PV system, including the inverter, must be grounded in accMdance with the applicable national regulations in each case. Grounding conductors are neceswy to conduct current when a ground fault occurs, this will minimize electrical shock hazards, fire hazards and damage to the loads and system equipment.

8.3.8 €MI

The components of a PV system, especially the inverters, are subject to varying h&h frequency noise emission/immunity requirements that limit the permissible radidon spectrum for a range of frequencies, usually between 150 kHz and 30 MHz.

220 GRID-CONNECTED PV SYSTEMS

Table 8.1 Commercial inverter characteristics

Nominal output Ouput Max. Power THD freq. Vout Efficiency

Inverter Manufacturer (VA) (%) (Hz) WAC1 (9)

Tauro

Fronius IG 5000/3000

T series

Sunmaster

QS series

Sunny Boy series

Prosine series

Atersa w.atersa.com Advanced Energy Inc.

http://www.advancedenergy.com

Fronius http://www.fronius.com/

Futronics http://www.futronics.co .uW

Mastervolt http://www.mastervoltsolar.com/ SMA http://www.sma.de/

Xantrex http://www.xantrex.com/

700-3000 <4 50f5% 220*7% 93 5000 <3 60 f 5% 120f5% 92

1300-4600 <3.5 50 230 95.5 1200-1500 <3 50/60f0.02% 230f34b 90 1500-5000 <3 50 230 94 700-2600 <4 50 180-265 93.6 1000-1800 <3 50160 f 0.05% 1201230 f 3% 90

8.3.9 Power factor

A value of unity is desired for the power factor, both at the utility grid connection as well as

at the inverter output. The regulation depends again on the country, but in general the inverter operation at power factors greater than 0.85, whenever its output exceeds 10% of its rated value, is a widely established minimum requirement.

Some of the most important inverter characteristics such as input voltage range, nominal and maximum output power, total harmonic distortion (THD) and inverter efficiency have been defined previously in Chapter 6. A wide range of inverters are now available from a number of worldwide manufacturers, with different sizes and characteristics. Table 8.1 lists some of these inverters for use in PV systems and shows some interesting characteristics. For detailed information about these inverters, the manufacturer’s web sites are also listed in Table 8.1.

8.4 PSpice Modelling of Inverters for Grid-connected PV Systems

In Chapter 6, two inverter models were described for PSpice simulation. A behavioural inverter model and a second inverter model developed for direct connection of the inverter input to a battery. These inverters are modelled as controlled voltage sources. In grid- connected PV systems, commercial inverters modelled as dependent current sources is more adequate. Moreover some of the design considerations in standalone inverter models can be neglected here, in particular the power control implemented in these models (which takes as inputs the load power demand and the battery SOC), has to be changed for inverters connected to the grid.

PSPICE MODELLING OF INVERTERS FOR GRID-CONUECIED PV lT§lEMS 221

R5 :: 1 '

A new inverter model is proposed for grid-connected PV systems. The followirrg netlist shows this inverter PSpice model, which implements the connectivity shown in Rgm 8.2. Figure 8.3 shows the schematics of the inverter model equivalent circuit.

R6 1K 7 inverter7 .cir impp node5 vmpp node61

Ll node 4 Imod nodel

+ Vmod node 3 node 3 Neutral

Figure 8.2. Schematic representation of the inverter model 6

3 Figure 8.3. Schematic of the equivalent circuit of the inverter model

*****inverter7.~ir/Inverterrnodelfor gridconnectedpvsystems

.subcktinv134 5 6Params:nf=l empp 13 value={v(6)-0.8} r6 6 3 1000 r5531 v293sin(0150Hz) eVc2 73value={(v(6)*~(5)*nf*l.41)/220} giout34value={v(7)*v(9)}

.ends inv

2 GRID-CONNECTED PV SYSTEMS

Maximum power point tracking, MPPT, capability is normally included in PV inverters to obtain the maximum power from a PV array. The model shown in Figures 8.2 and 8.3 includes such capability. In order to keep the inverter model as simple as possible, no restrictions have been considered on the maximum power tracking range and minimum inverter input DC voltage. Commercial inverters, however, implement an MPP tracking function for a given range of the DC input voltage values, and it is also necessary for the DC input voltage to be larger than a minimum value for proper operation of the inverter, as described in Chapter 6. These effects will be taken into account in a more realistic model described later in this chapter. The simplified model in Figures 8.2 and 8.3 is useful to describe some of the concepts involved.

(Parte 1 de 4)

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