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Hybrid systems enhance reliability

Dipl.-Ing. Peter Kremer, Dipl.-Ing. Martin Erath, Dipl.-Ing.
Gerhard Aumayr, Siemens AG, Germany

Combining power generating systems from diverse energy sources offers the advantage that the strengths of each type of system can complement one another. Such hybrid systems considerably enhance operating economy and reliability, making them the ideal solution for isolated communities as well as for unreliable grids.

Figure 1. The 51 kWp PV generator in Nigeria
Click here to enlarge image

Hybrid systems are one means of providing electrical power in remote areas which are not connected to a power grid. Because these systems use two or more different sources of energy, they enjoy a very high degree of reliability compared to single-source systems. The most common energy sources are sun, wind, biomass as well as gas and diesel fuel. An instrumentation and control (I&C) system can coordinate the operation of the individual systems to match the power demand and the state of charge of the energy storage medium (batteries) as well as the available energy supply. This ensures that power is produced as economically as possible with the highest possible percentage of renewable energy.

A key feature of hybrid systems is the fact that their constituent system strengths complement one another. This provides a number of advantages, which are also determined in part by the system type:

  • Greater reliability in power supply due to the use of two or more energy sources
  • Efficient system solutions by virtue of a high degree of flexibility during the design phase and in operation
  • Low overall maintenance costs due to the short operating cycles of the motor-generator units
  • Long service life time of components.

These advantages are offset by the comparatively high purchase costs of a hybrid system, which has a direct impact on power generating economy. However, with appropriate support in the form of financing models from operators with significant capital resources such as electric utilities, the costs of investment would present no obstacle to widespread use of hybrid systems.

Hybrid solutions

Hybrid systems can be classified according to their configuration: either series or parallel. Siemens Power Transmission and Distribution Group (EV) has installed a Parallel Hybrid System (PHS) in Nigeria based on a photovoltaic (PV) system. The plant started up in May 1999, and can work either as a stand-alone system or connected to the local grid.

A PHS system has several advantages. These decentralized energy supply systems are able to satisfy the basic energy demand of rural areas using the available local energy sources in an optimised energy mix. With a PHS, energy is being produced by several closely linked generation units exploiting the different (renewable) energy sources existing in a country. In addition, generation peaks, for example resulting from high wind speeds or high solar irradiation, can be stored, and even load peaks can be managed by efficient modular designed energy storage systems e.g. AC batteries.

The power generation, energy storage, load and energy exchange functions of a PHS can be closely integrated with another closed parallel hybrid systems (PHS), and also to an existing grid by an innovative decentralized energy management system (DEMS).

DEMS will optimize resource and energy costs of a hybrid system through a microprocessor-based system that maintains the optimal balance between generation and consumption.

  • Optimal energy generation planning, taking into account weather forecasting, daily planning (short term minimises operational costs) and yearly planning (long term forecast concerning enlargement of the technical equipment)
  • Flexible load forecasting, using historical data, weather forecasting and a flexible forecast function (Kalman filter)
  • A direct influence on energy demand and load through a load manager.

A DEMS guarantees:

A DEMS is based on small, compact units with a high degree of intelligence, and can allow for flexible future expansions as the amount of different functions can be easily extended.

Socio-economic benefits

Hybrid systems allow the use of local renewable energy sources so that currency can be spent for other infrastructural investments, and their distributed nature also means that little or no additional investment is needed in energy transmission. Moreover, if the hybrid system uses biomass as a renewable resource, numerous local jobs can be created.

Overall, decentralized rural electrification projects such as hybrid plants can give developing countries the chance to compete with industrialized countries without harming the environment.

Nigeria: a pilot project

Figure 2. The innovative parallel hybrid system (PHS) concept
Click here to enlarge image

In May 1999, Siemens EV commissioned what is the largest PV hybrid power supply system in Africa. The system consists of a 360 m2 solar generator with a capacity of 51.6 kW which not only feeds batteries but also supplies electricity by means of diesel generator sets. It can also be connected to the power system with a three-phase connection by means of a solar converter.

Siemens EV, together with Siemens Automation & Drives Group, installed and commissioned the plant within 16 weeks of receiving the order. This short schedule was made possible by the high level of modularity incorporated into the design of the system.

The system consists of the following components:

  • A PV power module
  • A 60 kVA ‘Sinvert Solar’ PV power conditioner unit
  • A 60 kVA ‘Masterguard’ PV uninterruptible power supply (UPS) unit
  • Two diesel gensets, each 135 kVA
  • A battery system with OPzV 276 gel single cells 300 Ah with 160 kWh for four days operation
  • A 64 kVAR ‘Sipcon’ VAR control system
  • Low voltage distributions
  • A ‘Simatic S7’ PLC control system
  • An IPC-supported monitoring system (PPvis, PPsolar, PPbat)

The 51 kWp (STC) power module PV plant is installed on the flat roof-top of the service building, mounted at a tilt angle of 5

A weather station supplies meteorological data (irradiation and cell temperature), which are fed to the control system and are also used for visualization of the system’s data on PCs and on an annunciator board at the entrance of the main building.

The PV power condition unit, UPS and industrial PC are installed in an air conditioned electronic room in the service building. The battery system in the service building is also kept in an air conditioned room at 22

System description

In the PV plant, unframed modules known as laminates are mounted. The modules are quickly mountable through extensive prefabrication. The maximum installed power of the whole PV hybrid system is approximately 60 kVA.

In normal operation mode, the PV system will feed in parallel to the grid and supply the consumers. In case of a mains interruption, the PV energy produced is fed into the UPS low voltage distribution in the power house. Excess energy charges the battery of the UPS system.

The PV generator is totally insulated. The modules for the photovoltaic generator meet the protection class II and the standards of ISPRA-CEC 503, or IEC 61215.

The string interconnection is made by plug systems. The gas-proof plugs and tubes are safe to touch and totally insulated. The plug-in contact enable a safe module installation, so that the string cables can also be connected during the day without module covering.

The cables are double insulated and single core for ‘+’ and ‘-‘, and are installed earth-fault and short-circuit proof. All other devices (array boxes) in the PV field are also totally insulated.

The PV generator is operated in a floating DC IT system and is controlled on earth-fault. In the case of an earth-fault, the faulted partial field is located automatically, reported and switched off, if necessary. The nominal system voltage of the PV generator is at STC 630 VDC. The maximum voltage (open-circuit voltage under STC) is approximately 781 VDC.

The external lightning protection is made of four lightning masts and rods which are installed above of the PV generator. All metal equipment in the generator field are connected in the equipotential bonding. The internal lightning protection and the overvoltage protection is realised with thermally controlled varistors. The length protection with varistors is made in the array boxes.

The PV inverter, Sinvert Solar 60 kVA , changes the solar DC current into 3-phase AC. The IGBT inverter bridge works with pulse width modulation (PWM) and supplies a sinusoidal output current. The infeed is directly into the normal low voltage grid or to the UPS bus. When feeding into the UPS bus, excess energy which is not going directly to the UPS load is fed into the battery via the bidirectional inverter of the UPS and BAK (DC/DC converter for battery coupling). The battery, at 160 kWh, is capable of standalone operation for ten hours.

The control and monitoring is realised by two Simatic S7 (with CPU 315-2 DP) control systems. The communication between the CPUs and the Sinvert Solar and Masterguard S devices is via fieldbus Profibus-DP. The IPC only serves for the visualization of the system and is connected via three COM-interfaces with Sinvert Solar, Masterguard S and BAK. Remote monitoring of the system is planned.

The Simatic control system, mounted in the control cubicle, takes care of the monitoring of the whole plant. The Simatic control unit in the Sinvert solar monitors the PV generator, regulates it in its MPP and monitors the re-feed operation into the UPS bus.

Several parts of the system (strings, array box) and essential system components (PV inverter, UPS, battery) are disconnectable for maintenance or fault correction. In the case of a failure the safe disconnection of the faulty system parts is guaranteed.