Control Issues and Solutions in High Renewable Penetration in High Power Micro Grid Systems

This work includes the design of new power converters, reviewing advanced power semiconductors and magnetics. Also under consideration is new energy storage devices such as super capacitors and Lithium Ion batteries.

The system comprises of the solar or wind arrays, battery, power conditioning and the diesel gensets. The central system controller has generally been used for resources scheduling, battery management and data logging, all based on one platform. These systems can be used to create a remote mini grid or a cluster approach where a number of load centres are involved.

Grid tied PV can be located along one of the feeder lines or along a dedicated line for the RE farm if there are land constraints at the power house. The communication between the components can be achieved by using wireless, optic fibre link or power line carrier. We have applied these systems in a number of large scale pilot projects in South East Asia to date. This website provides an overview of some of the issues and also reports on actual site data of an installed OPS Macro Hybrid System (MHS). Figure 1 provides a typical line diagram of the macro hybrid mini grid system.

Line diagram of the OPS Macro Hybrid Mini Grid System Optimal Power Solutions Macro Hybrid Mini Grid Line Diagram

Following extensive review on the future system types that would be required as the renewable energy market and technology grows over the future years, the following systems were earmarked for research and the MHS has to be adapted to meet the system requirements.
1. Macro Hybrid System (Ongoing and Future Developments)
2. Fuel Saving Systems (FSS)
3. Zero Daylight Diesel (ZDD)
4. Zero Daylight Diesel + 6 (ZDD + 6)

Fuel Saver System (FSS) consists of diesel energy sources combined with renewables energy. There are two scenarios under which the FSS systems can be classified. Each has its own implications on the system control strategy.

Macro Hybrid System as Fuel Saver (FSS)

These systems required the solar capacity to be less than 50% of the Solar to Load ratio. The PV capacity is to be managed to limit the impact of cloud cover upon the power quality of the systems. The maximum renewable energy penetration is calculated to be less than 50% of the smallest generator that can operate on this system.

Figure 2 below shows the MHS System modified to suit the FSS layout. The PV has to be connected via a grid tied inverter as there is no battery included in this system. The three scenarios for grid tied solar PV are:
a) PV Generation at the power house so the PV is integrated into the main AC Bus directly.
b) PV generation at the local load and LV distribution lines used to distribute the PV generation.
c) PV generation at remote sites/load and MV transmission lines used to wheel the PV generation.

MHS Adaptation for FSS with No Battery SupportMHS Adaptation for FSS with No Battery Support

Communication between the remote sites can be achieved via wireless systems, optic fibre link, power line carrier technique or other available techniques. The Station Control Module is located at the Power House and is used to monitor the renewable energy sources. Optional controlled load strategies can also be implemented.

Macro Hybrid System as Fuel Saver with Storage

With the increasing diesel fuel prices and decreasing global PV price, it is economic that the PV penetration of the FSS system exceeds the 50% Solar to Load ratio. Projects in Asia are specifying up to 70% of the annual energy to be renewable based hence the high penetration control issues come to the forefront.

In high penetration systems the power quality and system reliability of the system is put at risk. This is due to the slow response of the diesel generators to fast and large drops/ increases in PV generation during sudden cloud cover changes. The limitation of modern generators is that they can react within a few power line cycles to step changes in power up to 50% of its rated capacity. If the step load is higher than 50% the system power quality and reliability is compromised.

To minimize this problem, a battery is often included in the Fuel Saver System. The battery is connected to the Main AC bus via a bidirectional inverter. The inverter battery unit acts as a buffer (“shock absorber”) and depending on the system load demands will be discharged or charged via the bidirectional inverter to maintain the power quality and reliability of the system. Figure 3 below shows the FSS + battery layout.

MHS Adaptation of FSS with Battery SupportMHS Adaptation of FSS with Battery Support

This setup utilises the unique power conditioner (HPC) at the power station with Cloud Support control techniques implemented. The use of the HPC at the main station has the added advantage of maintaining power quality in the event of power station issues and feeder effects. The HPC is able to monitor and charge the batteries according to its set points when the Cloud Support is not required. It can also call for generators directly as needed. One variation of the system is to utilise the cloud support system located at the distributed generation sites.

Application of the Macro Hybrid System for the ZDD option

The Zero Daytime Diesel system requires the system to stop the generators within a time range in the morning and start the generators back at a specified time in the afternoon. In the event of prolonged cloud cover when the inverter battery cannot sustain the load requirements then the generator may be started as needed.

High Penetration System Control in an Macro Hybrid System

In high penetration mode it is observed that the PV to Load ratio for the new system is more than 1:1 meaning that the PV power delivery is higher than the maximum calculated load demand. These high levels of PV penetration escalate a number of inherent problems around power quality and stability of the systems. One issue is fluctuating cloud cover which could cause the PV output to vary rapidly. This drop or gain in PV will cause a high step load changes on the generator array. The generators have a specific max step load over a specified period of time that they can handle without affecting its output power quality and this level will often be exceeded in high penetration systems. The modern turbochargers that are increasingly being used on diesel engines to improve engine efficiency also mean that the GOV reaction to step load is limited which in turn affects the output frequency of the alternator.

Therefore, the high PV penetration systems have to be able to control the power quality variations introduced by cloud cover effects on the PV array. The solution is a cost effective storage unit to cater for the variations.

Objectives of Macro Hybrid High Penetration Control

The objectives of the MHS System controls are classified by the required response rates. Some controls have to be very fast to maintain power quality whereas others do not required such fast controls and are more related to the scheduling of equipment in the system. It is also essential that the controller used has suitable operation in warm, humid, noisy and dirty environments and has a proven life life without intermittent crashes or downtime.

Within the Macro Hybrid System the are three key functions; namely:
a) Very fast “high level” control that monitors power line frequency and voltage and can respond with system scheduling decisions in less than 60 to 100 milliseconds;
b) Fast scheduling control such that the various co-generation resources including load control can be scheduled and brought on line within the necessary time intervals to avoid brown outs and black outs; and
c) Overall system scheduling, storage monitoring, optimization, battery life management, operator interaction, control and supervision of distributed elements such as genset array or remote cloud support unit. Interaction with Data Acquisition sub section.

Often the general setup for an MHS system is for one piece of hardware and single board computer controllers using Linux or equivalent seem well adapted to very fast dedicated controls and also provide a very stable platform.

Aberrations are predictable where grid connect inverter are directly used. The use of a suitable PID control for the MHS controls has been adapted to address this. It is widely accepted that the use of PID controls on a Windows platform is not optimal from a system reliability perspective although this may be cost effective and acceptable for simple systems such as a FSS.

The disadvantages of the panel PC system are as follows:
i. Single point of failure for System Control and Data Acquisition;
ii. Multiple tasks to be performed by a single hardware; and
iii. Does not fit in the distributed control system architecture.

Optimised Macro Hybrid Configuration

The development of high PV penetration systems require some very fast inner control loops, within the order of 100 milliseconds or less, to maintain power quality. These controls are included in the very fast DSP controller. The cloud support system is incorporated in the DSP controller for HPC. The cloud support system is activated only when a generator is online.

The technique used is as follows:
- Measure Line Freq, factual
- If factual > (fnom + Δf) then increase battery charging to maintain fnom.
- If factual < (fnom - Δf) then increase battery discharging to maintain fnom.
- If the (fnom - Δf) < factual > (fnom + Δf) then perform battery charging duties as per HPC setpoints.

The use of a PID control and a setpoint to set the hysteresis range of the control is required in the HPC. Since the HPC is located at the power house and connected to the Main AC Bus, it can cater for power quality fluctuations other than loss of renewable energy such as loss of feeder.

Future MHS System Control including Fast Control (within 100 ms)Future MHS System Control including Fast Control within 100 milli seconds

The control of the generator minimum loading is activated and this control is used to maintain a minimum generator loading on the generators if the GEC output power is more than the load.

The control of the battery voltage is activated in this control. If the GEC output power is higher than the load, the excess solar PV power will flow into the battery. This will cause the battery to charge up. The battery charging has to be monitored and controlled to protect the battery.

Power reliability requirements incorporate the intermediate speed control loops (Within the order of seconds to minutes). These control loops may be incorporated in the Panel PC or the PLC/other unit depending on the importance of the control for system reliability.

Such control loops incorporate plant equipment scheduling such as:
i. Controlled loads or feeder control for Load Management Strategies;
ii. Short term prediction of system load requirements for more accurate battery/diesel management;
iii. Control of battery charging such as scheduling time to charge; and
iv. Scheduling of genset start/Stop sequence for ZDD or ZDD+6 system applications.

Current System Data

The team has deployed a number of MHS type systems in Asia and the USA. The sites have a 500kW MHS system operating in late 2011 on about 100 kW of load. These are both high penetration systems with more that 50% of the energy being solar PV based.

Site A Macro Hybrid System Daily Profile:

It can be seen that Gen 1 and Gen 2 are not operating during the daytime and the system uses the HPC (with battery) to achieve the desired outcome. This is a ZDD plus a 4 hour autonomy approach.

Site A Daily Load ProfileSite A Daily Load Profile

Site B Macro Hybrid System:

The data here in Figure 5 shows the system performance of each calendar month of 2011. In the best solar months the contribution is slightly more that 50%. There was a reduction in the solar output in the last two months while the main contractor undertook PV array servicing and check tests after one year.

Site B Annual performane on a monthly basisSite B Annual performane on a monthly basis

Conclusion

From the above discussion, it is argued that the increase of the level of renewable energy penetration in remote micro grid systems is inevitable. A number of independent study groups such as Pike Research have demonstrated this. The macro hybrid is evolving to meet to these changes and can be broadly adapted to several approaches:
i. The Fuel Saver System.
ii. The Zero Daytime Diesel System.
iii. The Zero Daytime Diesel + 4 hours System.
iv. The Zero Daytime Diesel + 18 hours System.

The paper provides several likely line diagrams that can be used for each of the systems for the Macro Hybrid Systems. The control requirements of the new macro hybrid system and its adaptations have also been discussed. The controls are classified based on the speed of control loop response and the power system quality of supply and reliability of supply requirements.

With the emergence of new power batteries it becomes relevant to examine how these batteries can be used in the MHS. These points are widely discussed in the literature especially the technical and cost weaknesses of Lead Acid. New power batteries could provide transient power to solve power quality problems and also meet reliability issues such as cloud support and system scheduling. This is especially the case when critical periods occur during genset synchronisation and power transfer.

The equipment decisions used for system controls must reflect the power control needs and also the investment costs in the total system. High reliability of the total MHS is always an over riding consideration.