Power Electronics, Smart Grids and System Integration

Introduction and Context

Renewable-energy systems are not connected to power networks simply by adding generators to existing lines. Solar photovoltaic (PV) arrays, variable-speed wind turbines, batteries, fuel cells, electric vehicles and controllable loads interact through power-electronic converters, digital controls and communication networks. Their successful integration depends on maintaining voltage, frequency, waveform quality, protection coordination and adequacy during both normal and disturbed operation.

Traditional power systems were dominated by large synchronous generators. These machines supplied active power, rotational inertia, fault current and voltage support. Modern renewable plants are increasingly connected through inverters. An inverter can respond rapidly and provide sophisticated control, but it does not automatically behave like a synchronous machine. Consequently, system operators must coordinate generation, storage, demand response, protection and network operation as one integrated system.

This chapter examines the conversion technologies and control principles that support renewable integration. It also considers grid codes, power quality, distributed generation, microgrids, hybrid systems, flexibility and stability.

Learning Objectives

After studying this chapter, students should be able to:

1. Explain the operation and application of generators, rectifiers, DC–DC converters and inverters.
2. Distinguish grid-following and grid-forming control.
3. Interpret major grid-code requirements for renewable plants.
4. Analyze power quality, protection and basic stability issues.
5. Explain intermittency, flexibility, demand response, microgrids and hybrid systems.
6. Select appropriate integration strategies for practical renewable-energy projects.

1. Renewable Generators and Electrical Interfaces

1.1 Synchronous generators

A synchronous generator converts mechanical power into three-phase electrical power. Its rotor rotates at synchronous speed, which is related to frequency by

where n_s is synchronous speed in revolutions per minute, f is electrical frequency in hertz, and P is the number of poles. The relationship assumes a balanced machine operating at steady state.

Synchronous generators can supply or absorb reactive power by changing field excitation. They also contribute physical inertia because energy is stored in the rotating mass. These characteristics are valuable for system stability, although large synchronous machines are mechanically complex and are not normally used for solar PV generation.

1.2 Induction generators

An induction generator requires magnetizing current from the grid or capacitors. In a conventional induction machine, the rotor must turn slightly faster than synchronous speed to deliver power. This slip enables electromagnetic torque production. Induction generators are robust and inexpensive, but their reactive-power demand and limited voltage control can be disadvantages in weak grids.

1.3 Doubly fed induction generators

A doubly fed induction generator (DFIG), widely used in wind turbines, connects the stator directly to the grid while a partial-scale converter controls rotor currents. Approximately 20–35% of rated machine power may pass through the converter, depending on the speed range and design. This arrangement permits variable-speed operation with a smaller converter than a full-converter system.

1.4 Permanent-magnet and electrically excited machines

Permanent-magnet synchronous generators (PMSGs) can operate over a wide speed range and have high efficiency. A full-scale converter decouples generator frequency from grid frequency. Electrically excited synchronous generators allow field control but require excitation equipment and may have greater losses. Full-converter wind turbines provide strong controllability, although their grid behavior depends heavily on converter software and current limits.

1.5 PV arrays and fuel cells

A PV array produces direct current (DC), with voltage and current depending on irradiance, cell temperature and operating point. Because the maximum-power point changes, a maximum-power-point-tracking (MPPT) controller is normally used. Fuel cells also produce DC and require power conditioning before connection to an AC network.

2. Power-Electronic Conversion Stages

A power converter changes voltage, current, frequency or waveform while controlling the direction and magnitude of power flow. Its principal semiconductor devices include diodes, thyristors, insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs) and wide-bandgap devices such as silicon carbide switches.

2.1 Rectifiers

A rectifier converts AC to DC. An uncontrolled single-phase bridge produces a pulsating DC voltage. A controlled rectifier uses thyristors or actively switched devices to regulate the average DC output. Rectifiers can introduce harmonic currents unless filtering or pulse-width modulation is used.

2.2 DC–DC converters

A buck converter reduces voltage, a boost converter increases voltage, and a buck–boost converter can perform either function depending on topology and control. For an ideal continuous-conduction boost converter,

where V_in is input voltage, V_o is output voltage, and D is duty ratio, with 0 ≤ D < 1. The expression assumes ideal switches, negligible losses and continuous inductor current. Real converters produce a lower output than the ideal value because of semiconductor conduction losses, switching losses and inductor resistance.

DC–DC converters are used for PV MPPT, battery charging, fuel-cell conditioning and voltage regulation. Their control must respect current limits, thermal limits and battery charging requirements.

2.3 Inverters

An inverter converts DC into controlled AC. Voltage-source inverters (VSIs) use a DC-link capacitor and actively switched semiconductor devices. Current-source inverters use a DC-link inductor and are less common in modern renewable applications. Pulse-width modulation (PWM) synthesizes an AC waveform from high-frequency switching pulses.

For a balanced three-phase system, apparent power is

where S is apparent power in volt-amperes, V_LL is line-to-line RMS voltage in volts and I_L is line RMS current in amperes. Real and reactive power are related by

where φ is the phase angle between voltage and current. The sign convention for Q must be stated: positive reactive power may denote injection or absorption depending on the network convention.

2.4 Filters and transformers

An L, LC or LCL filter reduces switching ripple before current enters the grid. LCL filters provide strong high-frequency attenuation but may resonate and therefore require damping. Transformers provide galvanic isolation, voltage matching and sometimes grounding-system adaptation. Filter and transformer design must consider losses, fault current, insulation, resonance and electromagnetic compatibility.

3. Inverter Control and Grid Interaction

3.1 Grid-following control

A grid-following inverter uses a phase-locked loop (PLL) to estimate grid angle and frequency. It injects a commanded current relative to the measured grid voltage. The d-axis current commonly controls active power and the q-axis current controls reactive power, although the exact signs depend on the reference-frame convention.

Grid-following converters work well when a stiff voltage waveform exists. They may become vulnerable in weak grids, where voltage distortion and rapid angle changes make PLL synchronization difficult. Several inverters simultaneously using PLLs can also interact through network impedance.

3.2 Grid-forming control

A grid-forming inverter establishes an internal voltage magnitude and angle rather than merely following an externally imposed waveform. It can use voltage-source control, droop control or virtual synchronous-machine control. Grid-forming operation is especially important in islanded microgrids, black-start applications and systems with low synchronous inertia.

A simplified active-power/frequency droop relationship is

where f₀ and P₀ are nominal frequency and reference active power, m_p is the frequency-droop coefficient, and P is measured active power. The equation represents a control law, not a universal physical law. It applies within the converter’s operating limits and assumes a defined sign convention.

Reactive-power/voltage droop may be expressed as

where V is voltage magnitude, V₀ is its reference, Q is reactive power and n_q is the voltage-droop coefficient. Droop permits decentralized power sharing but creates steady-state deviations that may require secondary control restoration.

3.3 Energy storage and bidirectional converters

A battery energy-storage system (BESS) uses a bidirectional converter to charge or discharge. Its state of charge may be approximated by

where SOC is fractional state of charge, P_c and P_d are charging and discharging powers, η_c and η_d are efficiencies, E_n is usable nominal energy, and Δt is the time step. Charging and discharging should not occur simultaneously in normal operation. The expression neglects temperature dependence, aging, self-discharge and rate-dependent capacity.

4. Grid Codes and Interconnection Requirements

A grid code is a technical rule defining how a connected generator or load must behave. Requirements differ among countries and voltage levels, but commonly include:

• voltage and frequency operating ranges;
• active-power control and ramp-rate limits;
• reactive-power capability and voltage control;
• fault ride-through during voltage disturbances;
• frequency ride-through and active-power response;
• harmonic current, flicker and unbalance limits;
• synchronization and reconnection procedures;
• communications, metering and remote dispatch;
• protection coordination and anti-islanding behavior.

4.1 Fault ride-through

A renewable plant may be required to remain connected during a temporary voltage dip rather than disconnecting immediately. This prevents a large simultaneous loss of generation. During the fault, the plant may be required to inject reactive current to support voltage, subject to converter current limits. After fault clearance, active power should recover at a specified rate.

4.2 Reactive-power capability

A converter rated at apparent power S_r cannot independently deliver unlimited active and reactive power. The capability constraint is

This is the equation of a capability circle under steady-state RMS conditions. Thermal limits, DC voltage, modulation range and grid-code rules may make the practical region smaller.

4.3 Compliance testing

Compliance is demonstrated through simulation, factory tests, commissioning tests and monitored operation. Models must represent control delays, current limits, protection actions and plant-level controllers. A model that reproduces steady-state power but omits control interactions is inadequate for transient studies.

5. Power Quality

Power quality describes how closely voltage and current remain within acceptable magnitude, frequency, symmetry and waveform conditions.

5.1 Harmonics and total harmonic distortion

For a voltage waveform with fundamental RMS component V₁ and harmonic components V_h, voltage total harmonic distortion is

The formula assumes RMS harmonic components and a nonzero fundamental. In practice, measurements use a specified harmonic range and an applicable standard. Harmonics can cause transformer heating, capacitor-resonance problems, nuisance tripping and motor losses.

5.2 Voltage variations, flicker and imbalance

Rapid changes in wind or solar output, switching events and weak feeder impedance can produce voltage fluctuations. Flicker is a perceptual effect associated with repetitive voltage modulation. Three-phase voltage unbalance creates negative-sequence currents that heat rotating machines. Mitigation includes stronger feeders, converter ramp control, dynamic reactive compensation, phase balancing and appropriate filter design.

5.3 Power factor

Displacement power factor is approximately cosφ for sinusoidal voltage and current. With distortion, true power factor is lower because harmonic current contributes to RMS current without proportionate real power. Therefore, correcting displacement alone may not satisfy a harmonic-current requirement.

6. Protection of Renewable and Distributed Systems

Protection must detect faults rapidly, isolate only the faulted section and maintain safety. Distributed generators complicate conventional protection because fault current can flow in multiple directions and inverter current is often limited to a value only modestly above rated current.

Important protection functions include overcurrent, overvoltage, undervoltage, overfrequency, underfrequency, differential protection, distance protection, reverse power and anti-islanding protection. Settings must account for minimum and maximum fault current, network topology, grounding, converter controls and reclosing procedures.

Anti-islanding protection prevents an energized local network from remaining unintentionally connected after utility supply is lost. Passive methods monitor voltage, frequency and phase; active methods introduce a small disturbance and observe the response. Excessively sensitive protection can cause unnecessary disconnection, whereas insufficient sensitivity creates safety and equipment risks. Modern grid codes increasingly require controlled ride-through rather than immediate tripping for specified disturbances.

7. Intermittency, Flexibility and Demand Response

Renewable variability occurs over seconds, minutes, hours and seasons. Variability is not identical to uncertainty: variability describes changing output, whereas uncertainty describes imperfect knowledge of future output. Forecasting, geographic diversity and aggregation reduce the operational impact.

System flexibility is the ability to change generation, consumption, storage or network flows in response to conditions. Flexibility resources include:

• fast-ramping gas or hydroelectric units;
• batteries and other storage;
• flexible industrial processes;
• thermal storage and electric water heating;
• electric-vehicle charging;
• transmission interconnection;
• curtailment and inverter-based ancillary services.

Demand response changes electricity consumption in response to price, reliability signals or direct control. It may be load shifting, peak clipping, load shedding or frequency-responsive demand. Effective programs require measurement, verification, customer protection and clear compensation.

A useful planning balance is an energy balance over an interval:

All terms are powers measured at the same instant, or consistently interpreted as interval averages. The equation states conservation of energy; it does not by itself guarantee voltage or frequency stability.

8. Smart Grids and Distributed Generation

A smart grid combines electrical infrastructure with sensing, communication, automation and coordinated control. Advanced meters, phasor measurement units, feeder automation, supervisory control and data acquisition, distributed energy-resource management systems and secure communications support improved visibility and control.

Distributed energy resources (DERs) are located near loads and include rooftop PV, small wind, batteries, combined heat and power, controllable loads and electric vehicles. DERs can reduce losses and defer network reinforcement, but high penetration may cause reverse power flow, voltage rise, protection problems and thermal overload.

Volt–var control adjusts reactive power to regulate voltage. Volt–watt control reduces active power when voltage becomes excessive. Local controls respond quickly, while supervisory controls coordinate devices over wider areas. Excessive simultaneous control action can produce oscillation or conflicting objectives; coordination and deadbands are therefore important.

9. Microgrids and Hybrid Renewable Systems

A microgrid is a group of interconnected loads and DERs with clearly defined electrical boundaries that can operate connected to the utility or in islanded mode. Its main components are generators, storage, power converters, protection, energy-management systems and controllable loads.

During grid-connected operation, the utility commonly establishes voltage and frequency. During islanded operation, at least one grid-forming resource must establish them. A hierarchical control structure is often used:

1. Primary control: fast local voltage, current and frequency response.
2. Secondary control: restores frequency and voltage to nominal values and coordinates power sharing.
3. Tertiary control: optimizes dispatch, imports, exports, fuel use and economic objectives.

Hybrid systems combine complementary technologies, such as PV–wind–battery, PV–diesel–battery or renewable generation with hydrogen production. Hybridization can reduce storage size and improve reliability. Design must consider resource correlation, load duration, reserve requirements, fuel availability, degradation and lifecycle cost.

A reliability-oriented design may define the loss-of-load probability as the probability that available supply is insufficient during a specified period. Reliability metrics must state the time horizon, treatment of outage duration and whether planned curtailment is counted as an outage.

10. Stability Requirements

Power-system stability is the ability of an interconnected system to remain in an acceptable equilibrium or recover after a disturbance. Important categories are:

• Rotor-angle stability: relevant mainly to synchronous machines.
• Frequency stability: the ability to maintain frequency after active-power imbalance.
• Voltage stability: the ability to maintain acceptable voltage after changes or disturbances.
• Converter-driven stability: interactions involving controls, filters, PLLs and network impedance.

For a simplified rotating system, the rate of change of frequency is related to the power imbalance. A commonly used aggregate approximation is

where f₀ is nominal frequency, P_m is mechanical input, P_e is electrical output, H is inertia constant in seconds, and S_b is the chosen power base. The expression assumes a coherent machine or aggregate model, small frequency deviation and neglect of damping and governor action. It should not be used as a complete frequency-response model.

Inverter-based resources can provide fast frequency response by increasing or reducing active power, if headroom or stored energy is available. A PV plant operating at maximum power cannot increase output without reserve; it may instead reduce output deliberately to create upward headroom. Batteries can respond rapidly but are limited by state of charge, power rating and energy capacity.

Stability studies use load-flow analysis, short-circuit calculations, electromagnetic-transient simulation, phasor-domain dynamic simulation and hardware-in-the-loop testing. Model validation is essential because poorly represented controls can lead to incorrect conclusions.

11. Worked Example: Battery Support for a Solar Microgrid

An islanded microgrid has a 400 kW load. PV generation is 280 kW, and a battery inverter can discharge at 150 kW with 92% discharge efficiency. Neglecting other losses, determine the battery power required and whether the inverter rating is sufficient.

The deficit is

The battery-side discharge power must account for converter efficiency:

Because 130.4 kW is less than the 150 kW inverter limit, the inverter is sufficient in steady operation. The example does not establish that the microgrid is stable: frequency control, reserve, battery state of charge, fault behavior and transient overload capability must also be checked.

12. Illustrative Case: Rural Solar–Battery Microgrid

Background

A rural community is supplied by a weak radial feeder with frequent outages. It has a 500 kW evening peak, a 300 kW daytime base load and a 600 kW solar PV plant. The feeder experiences midday voltage rise and the community wants backup capability.

Problem

The PV plant sometimes produces more power than local demand. Export is constrained by the feeder, and instantaneous disconnection during utility faults would interrupt critical medical and communications loads.

Analysis

A load-flow study identifies reverse power flow and excessive voltage at the remote feeder end. A time-series study shows that solar output falls rapidly during cloud passage. A protection review finds that inverter-limited fault current makes existing overcurrent settings unreliable in islanded operation.

Approach and solution

A 1 MW / 2 MWh battery is installed with a bidirectional grid-forming inverter. In grid-connected operation, the battery performs peak shaving, voltage support and controlled export. The PV inverter operates in grid-following mode while the utility establishes frequency. During an outage, the battery forms the island voltage and critical loads are transferred to the microgrid. Noncritical loads are shed through an energy-management system. Directional and communication-assisted protection replace unsuitable one-way overcurrent assumptions. The PV plant is curtailed when necessary, and the battery maintains reserve for cloud events and black-start support.

Outcome

The arrangement reduces feeder voltage excursions, supports critical loads during outages and permits greater solar utilization. Its success depends on periodic battery testing, accurate forecasts, cybersecurity, protection validation and a clear reconnection procedure.

Learning points

1. Energy capacity and power capacity are separate design constraints.
2. Grid-forming capability is valuable during islanded operation.
3. Protection must be redesigned when power flow and fault current are bidirectional or converter-limited.
4. Curtailment can be an economically rational reliability measure.
5. Technical performance and operational governance must be designed together.

13. Engineering, Regulatory and Ethical Considerations

System integration decisions affect safety, affordability, environmental performance and access to electricity. Engineers should use transparent assumptions about reliability, degradation, emissions and future operating conditions. Cybersecurity must protect control commands, firmware, customer data and network availability. Communication failure should lead to safe and predictable local behavior.

Interconnection standards and grid codes are jurisdiction-specific; designers must consult the current requirements of the relevant system operator, utility and national standards body. Compliance is not merely a paperwork exercise. It requires validated models, documented settings, commissioning tests and operating procedures.

Artificial intelligence can improve renewable forecasting, anomaly detection and maintenance scheduling. However, AI predictions should not replace protection logic or engineering judgment without validation. Training-data bias, false alarms, cyberattack vulnerability and lack of explainability are important risks.

14. Best-Practice Design Workflow

A practical integration study should:

1. Define load, resource, network, reliability and regulatory requirements.
2. Establish voltage, frequency, grounding and protection conventions.
3. Select generator and converter topologies according to controllability and cost.
4. Perform load-flow, short-circuit, harmonic, dynamic and electromagnetic-transient studies as appropriate.
5. Check converter ratings, thermal limits, filter resonance and capability curves.
6. Coordinate primary, secondary and tertiary controls.
7. Test abnormal conditions, islanding, reconnection, communication loss and black start.
8. Validate models against manufacturer data and field measurements.
9. Monitor performance and revise settings as the system changes.

Challenges and Limitations

No single technology solves all integration problems. Batteries have finite energy and degrade; demand response depends on customer participation; transmission expansion requires time and investment; grid-forming controls are still evolving; and detailed models may be confidential or difficult to validate. High renewable penetration can also shift constraints from energy supply to inverter controls, network strength, protection and seasonal adequacy.

A technically feasible design may be financially unattractive, and a low-cost design may impose unacceptable reliability or equity impacts. Multidisciplinary optimization should therefore include lifecycle cost, resilience, environmental impacts, safety, cybersecurity and social consequences.

Chapter Summary

Renewable-energy integration depends on coordinated generators, converters, controls, networks, storage and loads. Rectifiers, DC–DC converters and inverters condition electrical power, while MPPT, PWM, current control and grid-forming or grid-following strategies determine how resources interact with the grid. Grid codes define operating, ride-through, reactive-power, power-quality, protection and communication requirements.

Power quality includes harmonics, flicker, voltage variation and unbalance. Distributed generation changes power-flow direction and fault behavior, requiring revised protection and control. Intermittency is managed through forecasting, geographic diversity, storage, flexible generation, demand response, curtailment and stronger networks. Microgrids use hierarchical control and can operate grid-connected or islanded. Stability assessment must include frequency, voltage, rotor-angle and converter-driven phenomena. Effective design combines technical analysis with compliance, cybersecurity, safety, economics and responsible governance.

Key Terms

Power electronics: Technology for controlled conversion and conditioning of electrical power.

Rectifier: A converter that changes AC power into DC power.

Inverter: A converter that changes DC power into controlled AC power.

DC–DC converter: A circuit that changes one DC voltage level to another.

MPPT: Control that operates a PV array near its maximum-power point.

Grid-following inverter: An inverter that synchronizes to an existing grid voltage waveform.

Grid-forming inverter: An inverter that establishes a controlled voltage waveform and frequency reference.

Grid code: Technical requirements governing connection and operation of generation or loads.

Fault ride-through: Continued connection during specified voltage or frequency disturbances.

Power quality: The quality of voltage and current in terms of magnitude, frequency and waveform.

Harmonic: A sinusoidal component whose frequency is an integer multiple of the fundamental.

THD: Total harmonic distortion expressed relative to the fundamental component.

Distributed energy resource: A small-scale generation, storage or controllable-load resource connected near demand.

Demand response: Intentional change in electricity consumption in response to signals or conditions.

Microgrid: A bounded electrical system able to operate connected to or separate from the utility.

Droop control: A decentralized control method that relates frequency or voltage to active or reactive power.

Ancillary service: A service supporting frequency, voltage, reserves or reliable system operation.

Anti-islanding: Protection against unintended energization of an isolated utility section.

State of charge: The stored energy of a battery expressed relative to its usable capacity.

Converter-driven stability: Stability behavior arising from interactions among power-electronic controls, filters and the grid.

Review Questions

Short Answer Questions

1. What is the principal difference between a grid-following and a grid-forming inverter?
2. Why is reactive-power capability important for renewable plants?
3. Define total harmonic distortion and identify one of its effects.
4. Why can distributed generation complicate conventional overcurrent protection?
5. What are the primary, secondary and tertiary levels of microgrid control?

Long Answer Questions

1. Explain the complete electrical conversion chain of a PV–battery system, including the functions of MPPT, DC–DC conversion, the DC link and the inverter.
2. Discuss the principal technical requirements normally contained in a renewable-generator grid code.
3. Compare synchronous generators, DFIG wind turbines and full-converter renewable generators in terms of controllability, inertia and grid interaction.
4. Explain how intermittency can be managed through storage, demand response, forecasting, interconnection and curtailment.
5. Describe the protection and control challenges of an islandable renewable microgrid.

Analytical / Application Questions

1. A three-phase inverter supplies 500 kW at 400 V line-to-line and unity power factor. Calculate its RMS line current using the three-phase apparent-power relationship. State the assumptions.
2. A battery supplies a 90 kW load deficit through a converter with 90% discharge efficiency. Determine the battery-side power and explain whether a 110 kW battery converter is adequate in steady state.
3. A feeder has high midday PV output, voltage rise and limited fault current from inverters. Propose an integrated solution involving at least two control actions and two protection or network measures, and justify each choice.

Multiple Choice Questions with Answers

Question 1

Which device primarily converts DC power into controlled AC power?

A. Rectifier

B. Inverter

C. Transformer

D. Induction motor

Correct Answer: B. Inverter

Question 2

For an ideal boost converter with input voltage 100 V and duty ratio D = 0.5, the output voltage is approximately:

A. 50 V

B. 100 V

C. 150 V

D. 200 V

Correct Answer: D. 200 V

Question 3

A grid-following inverter normally uses which device or algorithm to estimate grid angle?

A. Phase-locked loop

B. Hydraulic governor

C. Mechanical clutch

D. Thermal relay

Correct Answer: A. Phase-locked loop

Question 4

The capability relationship P² + Q² ≤ Sᵣ² indicates that:

A. Active and reactive power are independently unlimited

B. Apparent-power rating limits the combined active and reactive output

C. Reactive power must always be zero

D. The inverter can operate only at unity power factor

Correct Answer: B. Apparent-power rating limits the combined active and reactive output

Question 5

Which action generally helps manage excess midday PV generation on a constrained feeder?

A. Increase harmonic distortion

B. Disable all voltage measurements

C. Use storage, controlled export or curtailment

D. Remove anti-islanding protection

Correct Answer: C. Use storage, controlled export or curtailment

Question 6

What is a principal purpose of fault ride-through requirements?

A. To force generators to disconnect for every brief voltage dip

B. To prevent simultaneous unnecessary loss of generation during disturbances

C. To eliminate the need for protection

D. To guarantee zero harmonic current

Correct Answer: B. To prevent simultaneous unnecessary loss of generation during disturbances

Question 7

A microgrid operating in islanded mode requires at least one resource capable of:

A. Establishing voltage and frequency

B. Producing only reactive power

C. Operating without any protection

D. Increasing feeder impedance

Correct Answer: A. Establishing voltage and frequency

Question 8

If a 400 kW load receives 250 kW from PV, the steady active-power deficit before losses is:

A. 150 kW

B. 250 kW

C. 400 kW

D. 650 kW

Correct Answer: A. 150 kW

Question 9

Which condition is most directly associated with converter-driven instability?

A. Interaction among converter controls, filters and network impedance

B. A perfectly isolated resistive load

C. A reduction in conductor temperature only

D. The use of a mechanical fuel valve

Correct Answer: A. Interaction among converter controls, filters and network impedance

Question 10

Demand response refers to:

A. Changing electricity consumption in response to signals or system conditions

B. Increasing demand without measurement

C. Replacing every generator with a synchronous machine

D. Removing all customer control

Correct Answer: A. Changing electricity consumption in response to signals or system conditions