DECLARATION 2 PUBLICATION
3.16 Utility technical challenges
The following section addresses key questions aligned to technical challenges of optimum placement of DG on meshed networks to reduce losses.
3.16.1 Critical electrical factors that determine optimal location to reduce power loss
Section 2.2 provided scholarly reference to key electrical factors that determine optimal location to reduce power loss. Network topology affects system characteristics such as voltage; power flows;
fault levels; reactive power margins for dynamic and steady state control. Transformer and line impedance defines system impedance and power flow.
Radial distribution overhead networks high R/X ratios. They are not sensitive to reactive power changes. They offer no reactive power support. They display high losses when operated at high thermal demands, resulting from I2R losses.
High Thevenin equivalent source impedance on distribution networks limit fault levels and limit DG connection to the network.
Thermal construction limits of radial distribution lines curtail evacuation of DG power. Twenty- two kV systems limit these to 15MVA.
Wind, solar PV and synchronous DG should provide reactive support on high R/X impedance ratio networks. This will reduce reactive power loss and improve network power factors. Reactive power is optimally managed when supplied at the load.
Sizing DG at the high-end of category B on radial distribution networks has a greater probability of failing the rapid voltage change during loss of generation. This is because of the lower fault levels on radial distribution lines. This is unacceptable by the grid code standard. Sizing a connection at the lower-end of category B favours DG connection, while still limited by fault level.
Conversely, transmission overhead networks have high X/R ratios. They are sensitive to reactive power changes. They offer inherent reactive power support. They display high reactive power losses when operated at high thermal demands, resulting from I2X losses.
Lower Thevenin equivalent source impedance provides higher fault levels and allows for larger DG capacity connections.
Thermal construction limits transmission lines reduce evacuation of DG capacity.
DG technologies such as wind, solar PV and synchronous machines should provide reactive support on high X/R impedance ratio networks. This will reduce reactive power loss and improve power factors. Reactive power is optimally managed when supplied at the load.
Economic viability limits sizing of category C [3] loads on transmission.
The following advantages and disadvantages [97] applies when considering “Category B” &
“Category C” [3] for rated renewable plant.
Table 3-2: Extra High-Voltage (EHV) Point of Connection (POC)
Advantages Disadvantages
Strong Fault Levels will ensure:
Minimum impact to rapid voltage change Larger size plants can be connected
Cost are highly prohibitive
Renewable energy sources may be far from load centres
Technical losses will be costly as a result of the location of the meter
Table 3-3: High Voltage (HV) Point of Connection (POC)
Advantages Disadvantages
Sufficiently High Fault Levels will ensure:
reduced impact to rapid voltage change
limited (network dependent) size plants can be connected
Substations can be established closer to source of generation
Network integration and strengthening will add to the cost
Table 3-4: Medium Voltage (MV) Point of Connection (POC)
Advantages Disadvantages
Supports smaller (less than) 10MW renewable plant connections
The connection criteria is dependent on the rapid voltage change, which is mostly the limiting factor on MV networks
Evacuating power is limited to the MV line capacity, especially on 11kV networks
Fault levels may be prohibitive and reduce size of connection Because of the issues above, network integration and strengthening will add to the cost
3.16.2 Practical power flow simulation methodology applied for placement of DG for loss optimization
Myriads of scholarly works present algorithms for optimum DG placement on radial networks and to a lesser extent on meshed networks. These were classified and discussed in chapter 2. These techniques are innovative and theoretically advanced. They lack practical application in the local environment because of inadequate data and models.
Utility engineers use a combination of power flow simulation methods to place DG [97].
Optimization techniques are limited to the closest available network. Location is dictated by large- scale integration of DG. Rapid growth in the DG landscape demands a long term optimum location;
sizing and technology mix development plan, ensuring optimally operation of the power system.
This research aims to develop a practical approach for optimal placement and sizing of DG with the aim of reducing power losses. The methodology employs DIgSILENT Programming Language (DPL) [98] in Power Factory and tests a typical utility network with time varying loads.
3.16.3 The importance of reactive power support for integration of distributed generation
Reactive power generation from DG can manage voltage control on the grid. Fast acting PV inverter technology can boost voltage levels that may suddenly become depressed. This is achieved by injecting leading reactive currents to raise the voltage at the point of connection. Grid codes may restrict DG plants to operation at unity power factors.
Transmission networks with high X/R ratios provide inherent reactive support. Voltage rises during periods of low surge impedance loading. Renewable power plant can limit the voltage rise by absorbing reactive power. Conventional control is possible to manage reactive reserve margins.
Smart grid integration with IPPs can operate their plant on Q-Control [3] to support the grid
without switching line inductors. Reactive power reserves can be supplied from Q-Controlled fully rated wind turbines.
Poor power factors loads supplied by reactive compensating DG minimize upstream reactive power reserves.
3.16.4 Penetration and concentration limits affecting losses
Ugranli proves that DG penetration levels of 30% or more increase losses as opposed to reducing losses [18]. During periods of low load, excess DG will reverse flow through the station transformer. Older technology transformers not equipped to allow reverse power flow will experience nuisance tripping. Upgrading the protection philosophy will be costly [99]. Sizing of DG at potential reverse power flow sites should ideally be relocated at higher load factors loads [100]. Integrating dispatchable DG technologies can mitigate reverse power flows especially on older protection technology networks.