Grid and Renewables

SOME JARGON

Lots of issues of integrating renewables

• Adequacy- there is simply not enough generation to back-up variable generation

• Ramping- there is too rapid a drop• Lots of solutions (demand response, real-time

optimal power flow, power electronics, smart metering, storage etc etc)

• This lecture on where to locate renewables

Where to locate renewables

• BALANCE OF THESE FACTORS• If one can locate say local wind resources close

to local demand AND if that resource diurnal and seasonal profile is well matched to the profile of demand that lowers cost

• Higher capacity factors lowers cost • If one can use existing transmission capacity

then that lowers cost • Current assignment tries to “optimize” these.

What we will not optimize for

• Reactive power• What is it and how is it addressed by grid• Some history

Why AC power

• Transforming voltages was easier • Early generation was AC • Motor loads were AC• Three-phase: more thru same wire, easier to start

motors• Why HVDC? Lower losses, long distances, but

expensive to tap power at intermediate locations, legacy T&D systems is also AC

• Less issues of optimal power flow, DC backbones?

Optimal power flow- come to this later but some intro

• Need to solve Kirchhoff’s laws for all nodes• Multiple generators, loads…• Multiple possible solutions, and to ensure

voltages within some band need to inject/remove real/reactive power at different locations.

• “optimal power flow” at any instant of time

• But loads are varying and new generation is variable, multiple possible generators

• Each generator may have it’s own price • Each load may have some price for it’s demand

response, so kind of like a generator• Orchestrating which unit needs to come on/off,

which load comes on/off, and calling in balancing, spinning reserves etc, doing this at minimum cost and maint reliability role of ISO

Some background slides

DC

Pa same as Q in figure on right

• Reactive power flow is needed in an alternating-current transmission system to support the transfer of real power over the network. In alternating current circuits, energy is stored temporarily in inductive and capacitive elements. AC connected devices that store energy in the form of a magnetic field include inductors (a large coil of wire). When a voltage is initially placed across the coil, a magnetic field builds up, and it takes a period of time for the current to reach full value. This causes the current to lag behind the voltage in phase; hence, these devices are said to be sources of lagging reactive power.

• A capacitor is an AC device that stores energy in the form of an electric field. When current is driven through the capacitor, it takes a period of time for a charge to build up to produce the full voltage difference. On an AC network, the voltage across a capacitor is constantly changing – the capacitor will oppose this change, causing the voltage to lag behind the current. In other words, the current leads the voltage in phase; hence, these devices are said to be sources of leading reactive power.

• Reactive power causes losses because it heats up the lines….

• Instant power: real component when integrated over one cycle is non-zero but integral of the reactive component is zero

• Both the act of transmitting and running motors etc leads to inductive loads

• Direct effect on system voltages

Steady-state

• Easier to deal with

transients

• Computational tools, do not scale with the size of the system today

• So one resorts to simulations and hopes that the system is either in one of stable trajectories or if not, one can do something about it

• Deviations in sinusoidal behaviors and in voltages• Solar and wind could introduce rapid transients

much faster than loads

Reactive Power Limitations

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• Reactive power does not travel very far• Usually necessary to produce it close to the

location where it is needed• A supplier/source close to the location of the

need is in a much better position to provide reactive powerY versus one that is located far from the location

of the need• Reactive power supplies are closely tied to the

ability to deliver real or active power

How Reactive Power Control Implemented

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• Regulate to control voltage to a desired nominal value

• Often, reactive power injections regulate voltage at the location of the injection

• Control effects tend to be localized• Some reactive power supply mechanisms:

Y Shunt capacitors (fixed and switchable)

Y Synchronous condensers

Y Synchronous generators Y Static VAR compensators

How Management of Reactive Power Has Changed

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• Under regulated environment, most utilities owned/controlled G&T&D in its own control areaY Provided reactive power just as it had to provide

sufficient generation and voltage• Restructuring has changed this sometimes

causing problems dealing with reactive powerY Merchant (non-utility) generation and related

financial incentivesY Transmitting power over longer distances with

multiple transactions

What has Lead to Problems

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• Regulated, electric rates based on kWh and kVA giving incentive for pf correction

• Restructuring, separation of G&T&D businesses

Y Generation: More likely kW based from non-regulated generation removing incentive for pf correction

Y Distribution: may not have significant incentive and strict budget for installation of capacitors

Y Transmission: who will own and operate and thus no incentive for improvement

• Electricity is transmitted between control areas

Y Has to be communication to properly operate the system, including adjustments to reactive power.

Y ISOs (i.e., MISO) has not yet defined any system rules concerning reactive power

• The heart of economically efficient + reliable ISO power markets is AC optimal power flow (ACOPF) problem. This problem is complex economically, electrically and computationally.

• Generators offer LBMP price/quantity pairs• System operator collects these offers and constructs cost functions at each node• Solves the optimal power flow, while balancing the loads, respecting transmission,

voltage, reliability constraints• After solving you get prices at EACH node, (through shadow prices of the

optimization prob) you pay uniform pricing at that node (can be at/above bid price)• Economically: efficient market equilibrium requires multi part nonlinear pricing.‐• Electrically, the power flow is AC, with nonlinearities.• Computationally, the optimization is a MI NLP, including both binary variables and

non-convex continuous functions, which makes the problem difficult to solve. • must be able to handle loss of any generator or transmission element, and the

system operator must make binary decisions to start up and shut down generation and transmission assets in response to system events.

• For investment planning purposes, the problem needs binary investment variables and a multiple year horizon.

• Where you put what and how much to invest• Braess’s paradox• Non-intuitive things can happen, need to careful how you expand

• Objective Function. They include minimizing generation costs, maximizing market surplus, minimizing losses, minimizing generation (equivalent to minimizing losses)…. optimal transfers.

• full ACOPF: model all constraints/controls with an objective function of minimize cost would meet the objectives of minimize (generator fuel costs, generation output, losses, load shedding, control actions).

• objective functions and constraints are not algebraic or differentiable, and that multiple solutions are likely to exist, in particular when there are many reactive power controls (such as switched capacitors, FACTS devices, or generators) in network loops.

• Bus type. ‐ In P, Q, |V|, θ space, there are four quantities at each bus: voltage magnitude (V), voltage angle (θ), real power (P), and reactive power (Q). In a power flow solution without optimization, buses are classified into three bus types: PQ, PV and slack. PQ buses generally correspond to loads and PV buses to generators. Generator buses are called PV buses because power and voltage magnitude are fixed; load buses are known as PQ buses because real and reactive power are fixed, that is, Pmin = Pmax and Qmin = Qmax; slack or reference buses have a fixed voltage magnitude and voltage angle.

• For a power flow to be solved, the slack bus needs to have sufficient real and reactive power to make up for system losses and hold the slack bus voltage magnitude at 1; for this reason, a bus with a large generator is commonly chosen as a slack bus.

• HVDC systems based on the Voltage-Source Converter (VSC) technology, which started to be deployed during the last decade, exhibit significant flexibility as they can control independently the active and reactive power. Such systems can prove helpful in the maintenance of power system security. Due to their fast response, they are able to undertake corrective control actions in order to relieve overloads and prevent voltage drops in case of contingency

Security-Constrained Optimal Power Flow including Post-Contingency Control of VSC-HVDC lines

Spyros CHATZIVASILEIADIS, Thilo KRAUSE, Göran ANDERSSON

• The EU Project IRENE-40 (www.irene-40.eu) aims to identify the appropriate transmission expansion measures in order to achieve a more secure, sustainable, and economically competitive European power system. Within this context, simulations based on different future generation scenarios are carried out. With respect to security, the objective is to identify the appropriate reinforcements in order to minimize the ‘cost of security’. Hourly generation and load data for the years 2010, 2020, 2030, 2040, and 2050 are provided for each EU-27 member state, Norway, and Switzerland

• Here, we study a single-hour snapshot of the year 2050. The generation and load data are taken from the scenario RES, which projects a high share of renewable generation in Europe (~80%) by the year 2050.

Large transformers

Where to place wind

• Example New York• Complex problem as you can see• Even if we do not take into account issues of

reactive power and control, the first order problem is that of optimizing locations so that one can ensure supply is closer to demand (both in time and space)

• That may mean even a lower capacity factor wind farm may be cost-effective