Energy storage system is of essence for Solar PV systems?

Energy storage is a vital element in power sector particularly when a country has large volume of Renewable Energy (RE). Cheap, practical and easily available energy storage systems can address some of the critical issues associated with RE sources particularly the Solar PV systems. It will build an atmosphere of confidence in the Utility working, particularly utilities with large scale RE penetration; reduce the investment in transmission systems and development of reserve capacity. 

Widespread deployment of battery storage would mitigate the intermittency phenomenon associated with solar PV systems. In fact, in some of the countries utilities have mandated that all new PV systems should have energy storage systems to smoothen the power variations. Solar PV systems with battery storage could also help to manage unplanned voltage fluctuations, particularly in areas with high penetration of solar PV systems.

Figure 1 shows the main components of a simple Roof top Solar PV system with Battery back up.

Fig.1: Layout of a simple Roof top Solar PV system with Battery back-up

Example of Energy Storage at the Utility end:

Utilities too have promoted Energy Storage systems at their end. Duke Energy has installed a hybrid energy storage system comprising of battery bank and ultra-capacitor at its North Carolina based substation (www.technologyreview.com). In this hybrid storage system, the ultra-capacitor helps the power system during large but short duration power surges, for example when the Solar output dips during a cloudy day. Battery bank, as customary, will help to recover during large duration shortages. 

It is expected that the two storage systems along with smart electronic controls will help the power system in mitigating the power shortage over periods ranging from seconds to several hours. This hybrid storage is supposed to provide a more economical solution as compared to each of the storage systems when used separately. 

According to officials at Maxwell Technologies, the ultra-capacitor storage system can completely eliminate the output fluctuation of less than 30 seconds and provide sufficient smoothening of fluctuations of up to 5 minutes. In recent years the ultra-capacitor energy storage systems have become more accepted as high power shock absorbers for industrial and transportation applications in combination with Lead-acid batteries or advanced chemical batteries. Ultra-capacitor has high power capability due to very low internal resistance, wide operating temperature range of -40^oC to 65^oC, minimum maintenance, high cycling ability and reasonable price.

The latest utility scale battery storage technology emerged in the commercial market is the 8 MWh capacity Vanadium Redox Battery bank installed at Everett Sub-station, Washington state, and which is to be commissioned in January 2017. This battery system,  compact and concealed in container, is non-flammable and can be discharged upto 100%, i.e. the Depth of Discharge (DoD) can be 100%.    

Essence of Energy Storage to End User:

At the end user level the energy storage system will provide the user with the much needed back up during night hours when the grid is out. In countries like India the Time-of-Day (TOD) tariff is not implemented in the residential and commercial sector. I am sure that in the near future Utilities have to come up with the TOD tariff in these sectors also. Then in such a case a solar PV system with energy storage will be very beneficial. 

TOD tariff is an effective tool to reduce the peak hour kWh shortage. Shifting of some portion of the load to the storage system will be of great relief to the utilities reeling under stress. 

Fig 2: A Roof Top Solar PV system with battery back-up.

In some countries, the feed-in-tariff has been rigorously cut down or eliminated at all. In such a condition, the pay back periods of solar PV systems are highly dependent on the percentage of solar energy used for “self-consumption”. 

"Experts are in the opinion that a solar PV system is worth only if the self-consumption is about 50% or more." 

Energy storage systems enable owners of solar PV systems to increase their self-consumption. Although installing energy storage system increases the overall cost of the system and hence the electricity produced, but still it is not as expensive as many would think of. Lithium-ion batteries are considered as the most affordable and dependable energy storage systems. These batteries can discharge bigger burst of power and can eliminate the need of ultra-capacitor but are costly.

Operating Cost of a Thermal Power Plant

India is among the nations where power sector is booming up. Of its total installed power capacity more than 60% share is of coal-fired power plants. Coal-fired or the thermal power plants have a significant operating cost and a matter of concern for the agencies that care for the environment.

The optimal power system operation involves factors such as economy of operation, system security, emission of fossil-fuel fired plants, etc. All the above considerations are conflicting and hence a compromise has to be made for the optimal operation of the power system. 

The main aim of economic dispatch problem is to minimize the total cost of generating the real power of various power plants while satisfying the constraints. The operating cost is insensitive to reactive loading on a generator, and hence the manner in which the reactive load on a station is shared among various on-line generators is not going to affect the operating economy. 

The operating cost of hydro power plants is negligible, but there is limitation of availability of water in case of such power plants.   

Factors affecting the Operating Cost of a Thermal Power Plant:

The operating cost of a thermal power plant depends on the operating efficiency of generators, fuel cost, and transmission losses. In many cases the plants are located far from the load centre and hence the transmission losses to transmit the power to the major substations may be considerably high. Under such a situation, even the most efficient generator in the system does not guarantee the minimum operating cost. 

Therefore, the operating cost a thermal power plant plays a very important role in the economic scheduling of generators.

Incremental fuel-cost curve

The input to the thermal power plant is usually measured in Btu/h, and the output is given in MW. A simplified input-output curve of a thermal generating unit is called the heat-rate curve. The fuel input in Btu/h can be replaced by cost of the fuel. Normally the fuel cost of a generator is expressed as a quadratic function of real power generation. The incremental fuel-cost curve is obtained by plotting the derivative of the fuel-cost curve and the real power. This incremental fuel-cost curve is a measure of how costly it will be to produce the next MW of power.

Impact of Transmission losses

In the simplest economic dispatch case the transmission losses are neglected i.e. the model assumes that the generator and the load is at the same bus. This is the case when the transmission distance is very small and the load density is very high. As the line or transmission losses are neglected, the total demand is the sum of all the generation. When the transmission losses are neglected, the optimal dispatch of generation is obtained with all plants operating at equal incremental production cost.

However, in an actual power system, thermal plants are usually pit-heads plants located far away from the load centre. Hence, it is necessary to consider the transmission losses and in this case the generation should equal the total demand plus the transmission losses.  

Some Definitions as per the Central Electricity Regulatory Commission Regulations, 2010.

The Indian Electricity Grid Code (IEGC) 2006 is a regulation made by the Central Electricity Regulatory Commission (CERC) in exercise of powers conferred under The Electricity Act 2003. IEGC lays down the rules, guidelines and standards to be followed by the persons and participants in the system engaged in planning, developing and operating the Power system.

Further regulations were made by the CERC, called Central Electricity Regulatory Commission (IEGC) Regulations 2010, have superseded the IEGC and have come into force from May 2010.

Some of the definitions as mentioned in these regulations are:

1.      Ancillary services: Ancillary services are those services necessary to support the power system operation in maintaining power quality, reliability and security of the system or the grid. Examples are active power support for real time load following, reactive power support, black start etc.

2.    Black Start:  Black Start means the starting of a power plant or system after a partial or total blackout in the region.

3.   Available Transfer Capability (ATC): Available Transfer Capability is the Total Transfer Capability (TTC) minus Transmission Reliability Margin (TRM). ATC is the transfer capability of the transmission system, of the inter-control area, available for commercial transactions. These transactions may be through long term access, medium term open access, and short term open access in a specific direction considering the security aspects of the network.

4.      Total Transfer Capability (TTC): Total Transfer Capability is the quantity of electric power that can be transmitted reliably over the transmission system of the inter-control area under a given set of operating conditions considering the occurrence of the worst credible contingency.

5.       Transmission Reliability Margin (TRM): Transmission Reliability Margin is the margin in terms of MW kept in the Total Transfer Capability (TTC) required to ensure the security of the interconnected transmission system under a reasonable range of uncertainties.

6.      Bilateral Transactions:  Bilateral Transaction means a transaction for exchange of electrical energy in MWh between a given buyer and a seller from a given point of injection to a mentioned point of drawal for a fixed or changing quantity of power (MW) for any time period during a month. This transaction of electrical energy may be a direct transaction or through a trading licensee or power exchange.

7.      Unscheduled Interchange: For a generating station or a seller, Unscheduled Interchange means the total actual generation minus its total scheduled generation in a given time block, whereas for a buyer or a beneficiary, it is total actual drawal minus its total scheduled drawal in the given time block.

8.      Long Term Access: Long Term Access means the authority to use the Inter State Transmission System (ISTS) for a period more than 12 years, but not exceeding 25 years.

9.      Medium Term Open Access: Medium Term Open Access means the authority to use the Inter State Transmission System (ISTS) for a period more than 3 months, but not exceeding 3 years.

10.  Short Term Open Access: Short Term Open Access means the authority to use the Inter State Transmission System (ISTS) for a period upto 1 month at one time.

11.  Inter State Transmission System (ISTS): Inter State Transmission System means any transmission system for the conveyance of electricity from one state to another, across the territory of an intervening state as well as within the state, built, owned, operated and controlled by Central Transmission Utility (CTU), i.e. Power Grid Corporation of India.

12.  Control Area: Control area is an electrical system bounded by interconnecting Tie lines, metering and telemetry system which controls its generation and or load to maintain its scheduled interchange with other area and helps to regulate the frequency of a synchronously operating power system. 

13.  Congestion: Congestion is a case where the demand for transmission capacity exceeds the Available Transfer Capability.

14.  Demand: Demand means the demand for active power in MW and reactive power in MVAr.

15.  Load: Load is the MW/MWh/MVA/MVAh consumed by a utility or installation.

16.  Ex-power plant: Ex-power plant means the net active power output in MW and energy output in MWh of a generating station after deducting the auxiliary consumption of the power plant and the transformation losses.

17.  Despatch Schedule: Despatch Schedule   is the net MW and MWh output of a generating station or power plant (ex-power plant) scheduled to be injected to the grid from time to time.

18.  Spinning Reserve: Spinning Reserve means partly loaded generating capacity with some reserve margin that is synchronized to the rest of the system and is ready to provide increased generation at short notice pursuant to despatch instructions by the system operator or instantly in case of frequency collapse.

19.  Independent Power Producer (IPP): Independent Power Producer is a generating company not owned or controlled by the Government (Central or State Government).

20.  Forced Outage: Forced Outage means an outage of a generating unit or a transmission facility due to a fault or any other reasons apart from planned shutdown.

21.   Connection Point: Connection Point is a point where a plant and or electrical apparatus connect to a transmission or distribution system.

MATLAB coding for Y-bus

In recent years, the analysis and design of power system have been influenced greatly by the high end performance of personal computers. These computers can be used to perform the steady-state and transient analysis of large interconnected power systems.   

MATLAB which stands for MATrix LABoratory, is a powerful software package developed by MathWorks Inc.  This software having analysis capability, flexibility, reliability and powerful graphics is currently the main software package used by power system engineers. MATLAB provides matrix as one of the basic elements and does the basic operation as addition, subtraction, multiplication using simple mathematical operators. With hundreds of reliable and built in functions, MATLAB helps in solving a variety of mathematical problems including differential equations, linear systems, non-linear systems, optimization and many other type of engineering computations.The most appreciable feature of MATLAB is its programming capability and the several optional toolboxes for simulating specialized problems of different areas.  

In power system, nodal admittance matrix or bus admittance matrix or Y matrix or Y bus is an n x n matrix describing a power system with n buses. It represents the nodal admittance of the buses in a power system. In a real power system, each bus is usually connected to only a few other buses, hence the Y bus matrix is sparse. The Y bus is one of the data requirements needed to formulate a power flow study.

Power flow studies, commonly known as load flow, are necessary for planning, operation, economic scheduling and exchange of power between utilities. Power flow analysis is also required for transient stability and contingency studies.

Y bus is a tool that provides a method of systematically reducing a complex power system to a matrix that can be solved by a computer program. The equation used to formulate Y bus is based on Kirchhoff’s Current Law (KCL) and Kirchhoff’s Voltage Law (KVL), applied to a circuit with steady state sinusoidal operation. These laws are applied to all the nodes of a power system and elements of the admittance matrix are determined, which then represents the admittance relationship between nodes to further find the voltages, currents and power flows in the system.

The below given MATLAB program is for the formulation of bus admittance matrix or the Y bus.

The input data required for Y bus formulation is “linedata” which contains 4 columns. The 1^st column gives the branch number. The 2^nd column is the “from bus” number whereas the 3^rd column is the “to bus” number. The 4^th column is the admittance of the corresponding branch.

% Declaring function [Y] that takes the “linedata” as input and returns Y bus matrix as output.

 function [Y]=ybus(linedata)

% extracting the maximum numerical value of column 1 of the “linedata” which gives the maximum number of %branches in the network.

elements=max(linedata(:,1));

%   defining the total number of buses in the network.

buses=max(max(linedata(:,2)),max(linedata(:,3)));

Y=zeros(buses,buses);

% defining a loop for the diagonal and off-diagonal elements of Y -bus

 for row=1:elements,

i1=linedata(row,2);

j1=linedata(row,3);

Y(i1,i1) =Y(i1,i1) + linedata(row,4);

Y(i1,j1) =Y(i1,j1) - linedata(row,4);

Y(j1,i1) =Y(i1,j1);

Y(j1,j1) =Y(j1,j1) + linedata(row,4);

end

Y