• RENEWABLE ENERGY INFEED

• R Herman, M Malengret and CT Gaunt

• University of Cape Town, South Africa

Technology and tariffs for renewable

energy in-feed at LV and MV

• Ron Herman :

Performance of LV feeders with DG

• Michel Malengret :

Inverters and compensators for injecting

DG into LV and MV feeders

• Trevor Gaunt :

Tariffs for DG

Voltage drop and penetration with

PV-DG

Ron Herman

Conventional Electricity Delivery

• Power source – Coal, nuclear, hydro, wind, solar thermal

• Power grid – HV, MV, LV networks: transformers & feeders

• Customer loads – Stochastic (random time variation)

– Load data, statistics

– Require probabilistic modeling approach

• Delivery constraints – Voltage limits, current limits, synchronism

Injecting DG raises issues

• Can be erratic (e.g. Solar PV, wind) – May require energy storage

• Analyses complicated by uncertainty – Require probabilistic approach with risk

• Under- and over-voltages may exist – Heavily loaded (winter) feeder without DG

– Lightly loaded (summer) with DG

• Reverse power – Islanding after load shedding, synchronization

Typical Residential Load profiles

A

B

Enhanced HB approach

• Herman-Beta probabilistic analytical tool

endorsed by NRS034 and SANS507 for passive

networks – point A

• Algorithms adapted by Gaunt to include DG on

LV feeders

• Enhanced algorithm gives feeder voltage

profiles with and without DG – point B

PV penetration limits

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1501

1.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

1.2

Vo

lta

ge

(p

.u)

% EG/ADMD

• Analytic ‘engine’ inside MCS allows DG

penetration to be evaluated

• Whole range of voltage rise with increasing

DG shown as a scatter plot

• ‘Risk’ or ‘confidence’

level can be adapted

to study’s needs

Findings

• Methods are available for design/policy and

operational studies of PV-DG on LV feeders

• Techniques and training in use of analytical tools

– available

• Research continues.

Feeding Power into a distributed

network so as to have minimal

transmission losses

Michel Malengret

A Distribution Network

A grid connected inverter to a network at

a point of common coupling (PCC)

NETWORK PCC

Power Grid

Connected

Inverter

Equivalent Thévenin circuit of network

“viewed” from a Point of coupling

(PCC)

Grid

Connected

Inverter

Power Reaching network:

PTH

IA Vth1 Rth1 + j Xth1

Vth2 Rth2 + j Xth2

Vth3 Rth3 + j Xth3

Power PCC available to inject into network

Power supplied by inverter:

PCC

Object of analysis is to find optimal current split to

achieve maximum power PTH reaching Thévenin

voltages from a specific power PCC available.

PTH Power Reaching the network

0

100

200300

400500

600700880

900

920

940

960

0

100

200300

400500

600700

940-960

920-940

900-920

880-900

Line 1 Current Line 2 Current

Calculating the optimal current

using Thévenin’s parameters

ITH = KA V’TH RTH1/2 where KA = PTH / ||V’TH||2

Conclusions

• Thévenin parameters obtained dynamically in real time

by the inverter.

• Optimal possible power PTH to destination calculated in

real time from PCC and Thévenin parameters without pre

knowledge of network characteristics.

• Needed optimal current magnitudes from the inverters

for specific power delivery calculated in real time.

• Potential Applications : 1. Reduce transmission losses.

2. Dynamically balance transmission line currents.

3. Improve network stability.

4. Deliver dynamic network characteristics to central control.

5. Improve capacity and stability.

Tariffs for DG on LV feeders

Trevor Gaunt

Tariffs for DG on LV feeders

Special RES tariffs are based on incentives

because:

• Desire technology change

• Equipment costs are too high to compete –

temporarily

• Benefits for prosumers, utility and society

• Need to share costs and revenues ‘fairly’

considering many aspects.

International models

• Many models from various countries

• Underlying mix of energy sources and public

policy regarding markets, industry structures

• Subsidies to RES are dynamic:

– Target technologies change

– Form of subsidy changes

– Subsidies reduced as policy targets reached

• RES not in markets – ‘must run’ priority

Basic approaches to LV DG tariffs

• Private responsibility – no subsidy.

• Investment subsidy – like SWHs.

• Net metering – net exchange of energy

consumed and generated.

• Feed-in tariff (FIT) – different rates for

consumption and generation.

• Time of use.

• Network charges.

Example: Tariffs in Cape Town

• Small-scale embedded generation tariff has:

Basic charge: R133.03 /day

Consumption rate: 109.17 c/kWh

Generation rate: 49.72 c/kWh

Total

cost

Consumption [kWh/month]

153.63 c/kWh for

first 600 kWh

186.81 c/kWh

thereafter

• Introduced in 2014

• Domestic tariff is inclined 2-block tariff

Choice according to consumption and size of DG

0

500

1000

1500

2000

2500

3000

3500

4000

0 500 1000 1500 2000 2500

No DG

DG.3dom

DG.5dom

DG.7dom

DG.3sseg

DG.5sseg

DG.7sseg

Consumption

[kWh/month]

Cost [R/month]

Tariffs for DG on LV feeders

• For small consumers: No advantage from SSEG tariff – advantageous to use PV simply to avoid consumption cost.

• For large SSEG consumers: Diminishing benefit from adding more PV. – Large (2100 kWh) consumer saves 2.09 R/kWh(PV)

generating 30% of consumption, or 1.56 R/kWh(PV) generating 70% of consumption.

– Can PV installed prices meet these levels?

– Consistent with incentive principles?

Conclusions

• Have good understanding of voltage rise, feeder

voltage profiles and limits of penetration with PV-

DG.

• Inverters/compensators can operate to reduce

system losses, but no standards defined yet.

• Expect dynamic tariff structures until PV prices

reach grid parity, when incentives will fall away.