Determination of optimum operating point
There are a variety of algorithms and tools for “finding” the optimal operating point. Since the modification and optimization algorithms are currently actively working constantly publish new solutions. Commonly used algorithms to determine the optimal operating point can be classified into the following groups:
n methods “bias-monitoring;
n methods trackers “incremental conductivity;
n methods based on observation of the voltage;
n methods based on observation of the current.
Role of DC converters in the system
In real systems, photovoltaics do not connect directly to the DC load (Fig. 1). The control system uses a DC converter, which aims to convert the constant voltage output of photovoltaics to DC voltage with another – that of the bus load. Most often the DC converter is realized by raising converter whose circuit is shown in Figure 2. Typical for this type of converters is that the constant output voltage (on the RT) is higher than voltage inputs. When opening the transistor, current through the inductance starts to increase, it stacks energy. When obstruction of the transistor, EHD of inductance of inductance is summed with the voltage of photovoltaic. Logically, the condenser is charged to the sum of two voltages. During the time when the transistor is unstopped, the load is powered by energy stocks in the condenser.
Duty cycle of the converter
The ratio of time during which the transistor is unstopped, at the time, when blocked, is called. duty cycle D of the converter. Demonstrated that a change in the rate of filling of the converter is amended and its equivalent input resistance.
When the system with a photovoltaic converter is included, in practice its equivalent input impedance is a load of photovoltaics, a DC load RT – baseline resistance furnace.
In appreciating converter output voltage U0 (ie tension on RT) is associated with the input voltage Ui (ie voltage photovoltaic) with the formula: U0 = Ui / 1 – D2. For this situation, the input impedance of the inverter could be presented with the phrase: Ri = RT (1 – D) 2, where Ri is the input resistance of the converter, and RT has its output load resistance.
Controllers amend Duty cycle
Since DC converter is a burden on photovoltaics, then an amendment to the Duty cycle of the converter is changed and the load resistance of photovoltaics, which defines its work point – Figure 3. If PV is loaded directly with resistance RT, ie without dc converter, it would work at point A. It is the intersection of V-A characteristic of photovoltaic and cargo rights. In this case it would have an angular factor 1/RT. As seen from the second graph, at work in A PV has a lower power output. If you introduce a DC converter in the system operating point could be shifted to point B, where PV gives maximum power.
As already explained, the operating point with maximum power is not constant but depends on work conditions. It is important adaptive controllers to amend Duty cycle of the converter to maintain optimal system performance.
Methods “bias-monitoring”
Typical for this type of algorithms is that it causes a slight deviation from setpoint and observe the climate of giving photovoltaic power. The finding that increased power means that it is the correct direction of change and operating point, and the deviation remains the same direction. If power output is reduced after the deviation, the necessary movement of the operating point is in the other direction. If deviation in both directions, the power is reduced, thus the current operating point coincides with the maximum.
Methods trackers “incremental conductivity”
In this group of algorithms work based on the well-known dependencies. It is known that photovoltaic power, expressed by current and voltage of the module is given by P = UI. After differentiation with respect to voltage is derived dP / dU = I + U (dI / dU). At the point of maximum condition is satisfied: dP / dU = 0, whence it follows that it is in force dependence I / U = -dI/dU. If the operating point to the right of the maximum, it
dP / dU 0 and I / U – dI / dU.
Controllers working on the basis of this algorithm can quickly find the point of maximum power in rapidly changing weather conditions. The above three ways set as a condition for determining the optimum and direction of change for its search.
Methods based on monitoring of voltage and current
In contrast to the two already described the method they follow the voltage at idle or short-circuit current of photovoltaic. Make a suggestion that the operating point with maximum output voltage is equal to about 0.75 of the voltage at idle, or under a current equal to about 0.9 by the current short-circuit.
Controllers working with this type of algorithms devoted to a very short period of time the photovoltaic module from the rest of the system. Leave it at idle, when you need to measure the voltage at idle or connect it to a little value, but powerful resistance to measure current at idle. The need for separation of photovoltaic system, albeit briefly, a disadvantage of that group methods. The need for a powerful resistance to measure the current of the circuit, characterized in some systems, it is also recognized as a disadvantage.
In many cases, controllers, measurements on only one panel of the PV system, then all panels are managed in the resulting operating point. This is because it implies that they operate under identical conditions.
Methods based on monitoring of voltage and current
In contrast to the two already described the method they follow the voltage at idle or short-circuit current of photovoltaic. Make a suggestion that the operating point with maximum output voltage is equal to about 0.75 of the voltage at idle, or under a current equal to about 0.9 by the current short-circuit.
Controllers working with this type of algorithms devoted to a very short period of time the photovoltaic module from the rest of the system. Leave it at idle, when you need to measure the voltage at idle or connect it to a little value, but powerful resistance to measure current at idle. The need for separation of photovoltaic system, albeit briefly, a disadvantage of that group methods. The need for a powerful resistance to measure the current of the circuit, characterized in some systems, it is also recognized as a disadvantage.
In many cases, controllers, measurements on only one panel of the PV system, then all panels are managed in the resulting operating point. This is because it implies that they operate under identical conditions.
Difficulty study of the legal framework
“The normal functioning of our electricity system and the interests of the NEC and distribution companies are also not protected by this legislation. Companies engaged in the distribution of electricity have already gained some experience from the accession of private stations – not just solar but also wind and small hydro. In many cases there is disloyalty by the owners of plants – some say are rated and placed in service more. “This leads to unnecessary costs to resize portions of the network to those stations. Uniform shall be governed by several laws and numerous regulations, making it difficult to study the regulatory framework, say industry representatives.
“This idea for the promotion of renewable energy sources – and in particular solar power plants, not quite thought out. Currently it is implemented in a way that leads to unfair allocation of the cost of promotion. Build your own micro-solar power plant requires a major investment at this stage could be realized by sufficiently powerful business representatives. At the same time, the high selling price of photovoltaic energy in practice be offset by increased cost of electricity for the population, “commented familiar with the matter.
Difficulty study of the legal framework
“The normal functioning of our electricity system and the interests of the NEC and distribution companies are also not protected by this legislation. Companies engaged in the distribution of electricity have already gained some experience from the accession of private stations – not just solar but also wind and small hydro. In many cases there is disloyalty by the owners of plants – some say are rated and placed in service more. “This leads to unnecessary costs to resize portions of the network to those stations. Uniform shall be governed by several laws and numerous regulations, making it difficult to study the regulatory framework, say industry representatives.
“This idea for the promotion of renewable energy sources – and in particular solar power plants, not quite thought out. Currently it is implemented in a way that leads to unfair allocation of the cost of promotion. Build your own micro-solar power plant requires a major investment at this stage could be realized by sufficiently powerful business representatives. At the same time, the high selling price of photovoltaic energy in practice be offset by increased cost of electricity for the population, “commented familiar with the matter.
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